mz
Colloidal and Surface Phenomena GEOFFREY BROUGHTON
University of Rochester, Rochester, N. Y.
T h e extensive range of recent publications concerned with the science of matter i n the boundary state is evidence of the increasing importance of this field. N e w methods or improved old techniques for the study of colloidal solutions include extended use of the electron microscope, electrophoresis, viscosity, radioactive tracers, and ultrasonics. Studies of surface films throw light on the dependence of contact angles and frictional phenomena on adsorbed films. There were many investigations of nuclear phenomena, recognizing its fundamental importance in many chemical engineering applications; the seriousness of air pollution problems led to increased interest in aerosols. T h e behavior of macromolecules in solutions, micellar colloids, and polyelectrolytes continued to attract wide attention.
ress in modern practical microscopy has been reviewed by Barer ( I w B ) , while the need for an authoritative book in English on phase microscopy, the principles of which were first enunciated by Zernike in 1934, has been filled in a joint work by three physicists and a biologist (16B). It covers history, elementary theory, advanced theory, and method and applications of phase contrast. Electron Microscope. The electron microscope continues t o be applied to many types of problems (39B). Achieving a resolving power of 10 A., or about a hundred times better than the ordinary microscope, its utility is unquestioned (167B). During its development electron microscopy has developed a large number of techniques of its own: ultrathin sectioning, electron “staining,” shadowing, replicas, etc. New developments in these have been described ( d l B , 37B). -4mong recent electron microscope studies may be mentioned the precipitation processes of barium sulfate (130B) and of vanadium pentoxide (28B); the structures of vanadium pentoxide (B7B),gold (186B) and nickel hydroxide ( 1 0 B )sols; the mutual coagulation of oppositely charged sols (gold and vanadium pentoxide) (26%); clays (22B,54B) and their use in the petroleum industry (88B, 111B); carbon black (47B) and its dispersion in butadiene-styrene copolymer ( 9 4 B ) ; and paint pigments (42B). High polymers have also received extensive study by electron microscope techniques: polyethylene terephthalate (86B); starch (69B);keratin (114B); rubber (1S6B); collagen and elastin (161B); cellulose ( M B , it 76B, 91B), where particular attention has been paid by RLnby (14SB, 144B) among others (12SB)t o the degradation products of cellulose; and the oil soaps and greases (29B,68B, IWIB, 199B). Plasticized materials have also been studied (2B, 23B) and the films formed in mineral flotation (67B). The optical and electron microscopes have naturally been applied t o the determination of particle size. Among measurement determinations, for which the electron microscope is essential, are those of Williams and his collaborators (W06B,207B) on the tobacco mosaic virus, while determinations of the numbers and sizes of particles contained in very fine powders, suspensions, etc., have been described by Bernard, Pernoux, and Teichner (19B). Using wolframite they found good agreement between electron microscope and nitrogen adsorption methods. Ellis (6SB) investigated the effect of specimen charging, concluding t h a t i t does not significantly alter the magnification in the standard electron microscope. Fairs (67) described the use of special. graticules for microscopic determination of particle size. Surface Area Determination. Ergun (66B)reviewed briefly but critically the various methods available for surface area determination, namely, heat of wetting, adporption of dyes or other materials from solution, gas adsorption (probably the most widely used method of recent years), size analysis by sedimen-
HE increasing importance in chemical engineering of colloidal and surface phenomena (sometimes described as the science of matter in the boundary state), recognized explicitly in this survey, has been noted implicitly by the inclusion of reviews on adsorption ( 6 A ) and ion exchange ( 8 A ) in the annual “Chemical Engineering Operations Review.” These topics consequently will not be covered in this survey, which attempts to follow other main aspects of colloidal and macromolecular chemistry. Several recent books on colloid chemistry deserve mention. Alexander and Johnson ( 2 A ) have written a comprehensive text covering the classical and modern views of the colloidal state, experimental methods used for its investigation, and a concise account of the principal colloidal systems, sols, foams, gels, emulsions, and polymer solutions. Volume I of “Colloid Science,” edited by Kruyt ( 7 A ) and dealing with irreversible systems, appeared in 1952, following the treatment of reversible systems (Volume 11)published in 1949. Of more than ordinary interest is the publication ( 6 A ) of what must be regarded as a digest of the lifework of Harkins on different aspects of the physical chemistry of surface films, including his work on surface tension, monolayers on liquids and solids, soap solutions, and emulsion polymerization. Included for completeness is a chapter by Verwey summarizing the role of the electric double layer in the behavior of lyophobic colloids. Other books are a treatise, edited by Butler ( S A ) , on electrical phenomena a t interfaces and their importance in chemistry, physics, and biology and a survey of the surface chemistry of solids by Gregg ( 4 A ) . Alexander ( 1 A ) has written an elementary introduction to the principles and applications of surface chemistry while general reviews by Saunders ( I l A ) , Lawrence and Mills ( Q A ) , and Rideal (1OA) deserve mention.
Experimental M e t hods The introduction of new methods or the improvement of old techniques for the study of colloidal solutions appears to warrant a special section, particularly because so many are applicable t o varied types of lyophilic and lyophobic systems. I n visible, ultraviolet, and infrared light microscopy the introduction of phase microscopy and interference contrast by wave front reconstruction (62B)as well as the construction of practical reflecting micromopes suggests important new advances. Prog912
M a y 1953
913
ship for small spherical particles of the size range 1 to 40 microns, tation, sieving, etc., photometric or light-extinction methods, and concluding that the method is very sensitive t o the magnitude of permeability or flow methods. Noteworthy in 1951 were the the solid angle subtended by the photosensitive receiver a t the Discussions of the Faraday Society on particle size determination center of the suspension. The accuracy of the permeability ( 4 6 B ) with a thorough examination of the problems associated method, now extensively used for specific surface estimation bewith the different methods. These may be read and compared cause of simplicity, is dependent entirely upon the validity of the with an earlier symposium (192B). Schweyer (166B) also reequation used. The Kozbny equation, as modified by Carman viewed and described sedimentation procedures for determining (34B), is commonly adopted but fails a t extremely low rates of particle size and distribution. Calvet (82B) described a microgas flow through fine powders or a t high flow rates with coarse calorimeter which can be used for the determination of heat of material. To bring the specific surface results of Lea and Nurse wetting and, consequently, surface area. While reaction velocity ( 9 6 B ) more into line with determinations of other methods, Rose methods for determination of surface area have been criticized (149B) has suggested a correction factor which appears to produce from time to time, the theory has again been outlined (184B) and greater conformity. The method has been investigated using applied to particle size measurement of calcium carbonate powcalcium carbonate, zinc oxide, cement, and polyvinyl alcohol as aders (185B)in fair agreement with values obtained by other sample powders ( 7 B ) and extended to particles larger than 140 methods. I n an interesting paper (80B), the surface area of mesh (66%). Photometric determination of water turbidity can barium sulfate powders was determined by sedimentation analysis be used as a means of estimating particle size frequency, mean gravitational and centrifugal), dye adsorption, photoextinction, particle size, and suspended matter concentration (150B). air permeability, and electron microscope methods. Results of Much quantitative information concerning the size and shape the dye adsorption and air permeability methods were in good of colloidally dispersed particles or macromolecules can be ob, agreement with each other and electron microscope measuretained by a study of the intensity and polarity of light scattered ments, but were often larger than values obtained from sedimenfrom such solutions (14B, 16B,92B, lO1B). Apparatus for such tation experiments, thus emphasizing the importance of complete studies is described (62B, 9OB), while theory has been discussed *dispersionof all aggregates in any type of sedimentation analysis. and reviewed for isotropic colloidal particles ( I S S B ) , optically T h e adsorption of stearic acid from methanol has been suggested anisotropic rods (76B),charged macromolecules (48B, 1@B), and as a rapid method of measurement of the surface area of clays solutions in mixed solvents (168B). I n a study of polystyrene b(1S2B). Liang (98B) attempted t o reconcile the B.E.T. and dispersions by light scattering and electron microscope techHarkins and Jura methods for obtaining surface areas as did niques, the two methods gave excellent agreement for particle Sakiki (166B). Barrer, Mackenzie, and MacLeod (1SB)also diameter (78B). Such good agreement was not found by Atherconsidered the mathematical basis of the gas adsorption method ton and Peters ( 9 B ) in an intercomparison of electron microof measuring surface area. scope and light scattering particle size estimates for certain textile Particle Size Estimation. I n spite of uncertainties confinishing agents such as cellosolve methacrylate. The thixonected with dispersion and particle shape, sedimentation methods tropic gelation of aluminum and thorium molybdate gels has for particle size estimation are widely used because of their been followed by light scattering measurements @ $ O B ) ,as has relative ease and simplicity. Some attention has been devoted the change in size of dyestuff micelles with temperature (169B). t o the fundamental background of such methods (71B, 8SB), Diffusio.nal Measurements. Although diffusional measureKynch ( Q S B )having formulated a sedimentation theory based ments are relatively little used, several papers should be mentioned only on the assumption that a t any point in a dispersion the veloc(4OB, 2OOB). Salvinien (166B) described methods of measurei t y of a particle depends solely on the local concentration of the ment using cylindrical diffusion through a gel. He and his coparticles. I n a very exhaustive article (7OB) the physics of workers also described methods for diffusion coefficient measureparticle size measurement is reviewed with particular reference to ments by the use of radioactive indicators (157B) and by mutual fluid dynamics and Stokes’ law. Wilson (208B) showed that, precipitation of two reagents in a gel (158B). The use of opticalgiven temperature uniformity, 2.5 to 10 micron borosilicate glass interference methods (6B, 196B) has allowed solutions of extreme spheres settle in conformity with Stokes’ law. Deshpande and dilution to be employed with consequent elimination of interTelang suggested a modification leading to improved accuracy molecular interaction effects. Examples of the use of such instruand rapidity for Kelly tube ( 4 6 B ) method of sedimentation ments have been reported (164B, 197B, 198B). Concentration analysis. The Andreasan pipet technique has been used by Amdependence of the diffusion constant has been discussed (126B) stein and Scott ( S B ) for measuring particle sizes of calcined and and measurements made on cellulose xanthate (127B). Kirkhydrated alumina, aluminum powder, and various cokes and wood and Brown (86%) suggested a new method for the fractionanthracites. Centrifugal sedimentation was recommended by ation of macromolecules, diffusion-convection. In this, the Masterman ( l l O B ) for determination of the particle size of clays macromolecular components are transported horizontally in a and the necessary mathematical equations have been derived. vertical convection channel by thermodynamic interaction with The sedimentation method for particle size analysis of markedly a diffusing substance of low molecular weight, to which the chananisotropic particles, such as paper fibers, has also been discussed nel walls are permeable. The horizontal density gradient caused (128B). Light transmission measurements have been utilized by the concentration gradient of the diffusing components profor following sedimentation for particle sizes in the range 1 to 66 vides the vertical convective transport. microns (IS@), while a simple hydrostatic balance technique Work still continues on membranes and dialysis, the first prehas been described and used on kaolin suspensions (161B, 166B). parative techniques used in colloidal chemistry. Membranes are For the determination of small particle sizes the x-ray diffracnow available which are selectively permeable to anions or to tion method is much used (17B). Riley and Oster (146B)decations only, and the concentration potentials across such memveloped a theory of x-ray and light scattering, applying i t t o low branes have been investigated (l76B, 176B) and their equilibrium angle scattering data from proteins and oil-water emulsions. values found to agree well with the calculated (Donnan) values Mathematical treatments have also been developed and applied (177B). Hirsch (74B) commenced work on the behavior of for small-angle scattering from close-packed systems (138B, membranes between electrolyte solutions and Staverman (181B) 1S9B) and dilute solutions (146B). Specific applications have reported on the nonequilibrium thermodynamics of membrane been made to latex particles (97B),long-chain hydrocarbon polyand I@ iron ), processes. The Eyring rate theory has been applied to the difethylene oxide systems (16SB),clay particles (,% fusion in and permeability of membranes (89B, 2 1 l B ) , while powder (162B). I n the last case, good agreement was found surface flow through pores of molecular dimensions has been studwith the gas adsorption technique. Rose (147B, 148B) increased ied by Jones (79B). Diffusion rates across oil-water interfaces the knowledge of the extinction coefficient-particle size relationI(
m
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are of importance in evaluating the probleinof cell wall permeabilities; measurements made by Davies (4SB) on the movement of alkali halides across a nitrobenzene-water boundary were found t o be unaffected by protein films spread a t the interface. An apparatus for measuring contact potentials of proteins and polypeptides a t an oil-water surface has been made and described (44B). The mathematics of transfer across an interface have also received consideration (166B). On the more practical side, a continuous countercurrent dialyzer for strong caustic soda (107B) and an electrodialyzer for toxicological analysis (121B) have been described. The most reliable method for measurement of particle sizes in the smaller size range (molecular weights up t o 1,000,000) remains the determination of osmotic pressure. RIoelwyn-Hughes reviewed the more important theories and concluded that thermodynamic or statistical treatments always yield the same expressions for the osmotic pressure (120B). The experimental technique is difficult and improvements continue to be reported (S6B, 55B, 95B). Viscosity. I n the use of viscosity for the determination of the size of macromolecules. a very detailed account of possible errors in the capillary viscometer and suggestions for overcoming them are given by de Wind and Hermans (209B). The retardation of flow in narrow capillaries ( 7 2 B )and the adsorption of polymer on the viscometer walls at very low polymer concentrations (41B) have also been discussed as possible sources of error. For the investigation of non-Newtonian liquids a number of constant shearing stress or constant shearing rate instruments have been described (69B, 105B, 2OSB). Viscometers with special applications for routine analysis (77B), cosmetics (106B), greases (109B), high polymers ( I S I B ) , educational purposes (73B),drilling muds ( I I S B ) , ceramic slips (dlOB), and paints ( 8 B ) have been reported. Weltmann and Kuhns (202B)made an exhaustive study of the effect of temperature upon rotational viscometer measurements. The rheology of blood (S8B) and of glass ( S S B ) with techniques for their study have been reviewed. Nielsen (125B) described a recording torsion pendulum for the measurement of the dynamic mechanical properties of plastics andrubbers. Electrophoresis. The importance of electrophoresis as a preparative and analytical technique has been emphasized in a popular review article by Gray (66B). Electrophoretic separation apparatus has been described by several workers (61B, 84B, i16B,196B) and methods of following the separations by optical means have been suggested (6B, 99B, 193%). Complexities in the movement of boundaries and spreading (187B189B) continue t o be investigated (2@, SOB, 6SB). Fractionation and determination of homogeneity of several proteins is reported (11B, S I B , 190B). Filter paper cataphoresis has undergone rapid development (49B-51B, 164B, 182B, 2OlB) leading t o a quantitative means of measurement of the diagrams obtained (116B, 117B). Techniques using electrophoresis in a glasspowder column (68B) and in agar (1S5B) should be noted, as well as a new microcell eliminating electroosmotic streaming (15SB) and a semicontinuous preparative cell (a@). The relationship between pore size and molecular weight in ultrafiltration was investigated by Grandjean (65B). A simple ultrafiltration apparatus is described by McRary (102B). Trautmann and Ambard (194B) also report on ultrafiltration and electrostriction. Radioactive Tracers. The use of radioactive tracer techniques in surface and colloid chemistry continues a t an accelerated pace. While the papers mentioned do not include all work reported, they are sufficiently representative to indicate the possibilities opened up by these materials. Diffusion coefficients in gels have been measured by following the progress of a radioactive material through a cylindrical tube of the gel (157B). Soft beta emitters, alpha emitters, and isotopes with radioactive being used for the study recoil atoms-e.g., Ss5, C14, and Ca4"are of the adsorption of dissolved substances a t the liquid-vapor
Vol. 45, No. 5
interface (4B, IOOB). The adsorption of sodium and sulfate ions a t the surface of surface active materials (Aerosol OTN) has been determined by the use of N a W 1 and NazS350a(81B). A direct application t o the problem of rate and extent of adsorption on the textile fiber of a detergent during t h e cleaning process has been made by Meader and Fries ( 11dB), who used sodium alkyl benzene sulfonate labeled with S35 and sodium palmitate labeled with C14. The amount of strontium ion required to coagulate silver iodide, mercuric sulfide, and arsenious sulfide sols was measured by tagging with Sr8: adsorption occurred nearly instantaneously, and the strontium ions were attached firmly to the coagulate (1833). Bernard, Davoine, and Hirtz (18B) also studied adsorbed layers by use of radioactive trace elements, and the adsorptions of barium and of laurate ions a t equilibrium on ground quartz have been measured with radioactive barium and radioactive carbon labeled lauric acid (GOB). Interesting applications to fields other than flotation were in aerosols (205B), rubber fillers (87B), polymerization of tetrafluorethylene (ZOB), paint technology ( I O @ ) , and portland cement ( I 7 8 B ) . Ultrasonics. The ready availability of ultrasonic wave apparatus has led t o many studies of the effect of this treatment upon colloidal and macromolecular solutions. The American Institute of Chemical Engineers devoted a symposium t o the general subject, a t which Sollner (174B) reviewed the colloidal effects of ultrasonics. One of the more promising applications of ultrasonics appears t o lie in the collection of powders from air and aerosols (I$@, ld9B, 17SB). On solutions of macromolecules the general effect of supersonic irradiation appears to be depolgmerization accompanied by a decrease in viscosity ( 6 4 4 108B, 160B, 17OB-172B). Investigations are reported on cellulose ( l B , 191B), proteins and cyclic amino acids (141B), and vinyl acetate (169B). Miyahara et al. (118B) measured ultrasonic absorption during the sol-gel transformation, finding a complex change in the case of starch (119B). Iron silicate ( l 7 9 B ) and thorium phosphate (180B) gels have also been the subject of ultrasonic studies. Kaneko et al. (82B) considered that information on the relaxation phenomena of high polymers can b e obtained by measurements of double refraction with and without the presence of an ultrasonic field. Ultrasonic vibration potentials in colloidal solutions are reported by Rutgers and Jacob (152B). Sonic and ultrasonic maves have also been investigated as aids for dyeing (25B).
Surface Films Monolayers can exist in several states-gas, vapor, liquid, and solid-as well as in some phases for which no analog exists in three-dimensional matter. A statistical mechanical theory has been developed by Sait6 (78C) for the phase transitions occurring in surface films of the oil on mater type. New techniques reported for study of the liquid-gas interface include the use of a vertical-plate Wlhelmy-type surface balance for following t h e effect of vapors on monolayers (27C), while several instances of the use of radioactive tracers for the measurement of adsorbed materials a t the liquid-air interface have been recorded (2C, SC, 54C). The behavior of mixed monolayers of fatty acids indicates t h a t the interaction of the carboxyl groups in the mixture may not be the same as that in a single-component acid film (59C); the molecular area for the mixed films was less than that of each component in any pressure region investigated. Matsuura and coworkers (66C, 68C) continued their study of films formed from nonpolar, normally nonspreading compounds by stabilization with stearic acid or cetyl alcohol. With more than 50% stearic acid the films formed are very stable. By use of substrates containing copper salts it was possible t o show the formation of chelate compounds between some azo dyes and cupric ions. I n another investigation (67C), the linear trimer of diphenylsiloxane was found to form a very stable monolayer on water with a molecular area approximately equal to that of six
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4
phenyl groups. The cyclic trimer formed no stable surface film. Films of naphthacene, when formed in the presence of oxygen, were found to be unimolecular, whereas rigid oxygen exclusion gave multilayers (S8C). Hayes and Dean ( 5 I C ) , using their surface balance, studied the adsorption of hexane on condensed stearic acid monolayers; at low hexane concentrations i t followed the Freundlich-type isotherm. Limiting adsorption appeared to be one molecule of hexane to one of stearic acid. The monomolecular layers of myristic and palmitic acids on paraffin seem to be in a “condensed liquid” state occupying less area per mole than the free liquid film on an aqueous acid surface (44C). The surface areas of two ( diastereoisomeric long-chain compounds, -( + ) - 2 ( ~ ) , 9~)-dimethyltetracosanoic acid and ( +)-2( ~),9(~)-dimethyltetracosonoic acid, were found to be identical over the whole force-area curve; the 7 keto acids, while giving identical force-area curves in the expanded state, differed in the condensed region (87C). Considering the observed molecular cross-sectional areas for “liquid” and “solid” fatty acid films, Vold (9SC) suggested that the former may correspond t o free rotation of the molecule while the latter, since the fatty acid molecule is elliptical in cross section, may correspond t o close packing. The importance of the rosin acids in paper technology has led to surface-chemical studies of abietic and dehydro-, dihydro-, and tetrahydroabietic acids on hydrochloric acid substrates (19C). Monolayers with similar pressure-area curves are formed. By introducing aluminum ions into the substrate it was possible to follow the formation of dibasic aluminum monoresinate, which formed a highly viscous, tough skin (SWC, SSC). This film had a contact angle of 108’ to water as compared with 82’ for the rosin acid film. Giles and Neustadter (41C), in some interesting work preparatory t o the use of the compounds for the study of dye photolysis and adsorption of dyes by fibers, have determined the molecular areas and orientation a t the water surface of a series of 22 aromatic azo compounds containing straight alkyl chains ( C I to ~ ~CIS),usually with one hydroxyl either ortho or para to the azo group. Condensed films were formed in all cases where the alkyl chain contained an hydroxyl group and had a t least 16 carbon atoms. If hydroxyl groups were absent, a t least two azo groups appeared to be required for the formation of stable films. Some work has also been done on partially soluble films. Desorption explains the time dependence of soluble surface layers maintained a t constant area (79C). The film characteristics of water-soluble wetting agents on sodium chloride solution, measured with the Langmuir balance, were found to conform to the equation:
.
FA =‘FJ
+ LkT
where J is the mole surface and L a dimensionless constant ( S I C ) . Studies of monolayers on solutions of electrolytes are yielding interesting results, indicating that the distribution of ions beneath a charged monolayer follows that suggested originally by Gouy (WSC, 49C, 6OC). Formation of hydrocarbon films is attributed by Gerovich, Smirnova, and Teterina (S9C) t o adsorption of the cation of the salt dissolved in the substrate and polarization of the hydrocarbon molecule. Gilby and Heymann (4OC) measured equilibrium spreading pressures and force-area curves for oleic acid on sucrose and a variety of salt solutions. The order of increase in equilibrium spreading pressure was that of the lyotropic series, suggesting that the ions of stronger hydration are less adsorbed in the surface. Wolstenholme and Schulman studied myristic acid (99C) and branched-chain fatty acid monolayers ( I O O C ) over metal salt solutions a t varying hydrogen ion concentrations, concluding that increasing molecular cross-sectional area of the fatty acid chain achieved by introduction of methyl groups in the alpha position led t o progressive liquefaction of the monolayer. In a study of the hydrolysis of a-monostearin in the monolayer i t has been shown that the molecular areas of the soap formed and the monostearin are additive ( S I C ) .
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Cellulose acetate ( I C , 7 7 C ) and proteins (24C, WK‘, W8C, Q2C) have been studied by surface film methods. Determination of the molecular weight of cellulose diacetate by the surface film method is said t o give results in good agreement with osinotic pressure and viscosity methods. With the triacetate the films are reported t o differ in properties with the spreading solvent employed. Cumper and Alexander (WSC) reviewed work on protein films a t air-water and oil-water interfaces, following this by an experimental investigation of the surface viscosity and surface rigidity of @globulin and pepsin a t such interfaces (Wac). Synthetic protein analogs have also been studied by the surface film technique (58C, SOC). Joly (Sac) reviewed non-Newtonian surface viscosity. The importance of surface films in controlling rate of evaporation in pot stills has been demonstrated in some novel work by Hickman (62C). Jones and Saunders (SSC), by measurement of the surface tension and partial vapor pressures of nitromethane solutions of the Cz to Cs fatty acids, were able to conclude that the fatty acid molecules, as a t a water-air interface, formed a unimolecular layer but arranged themselves intermediately between a horizontal orientation, with their long axes parallel in the nitromethane surface, and a random arrangement. The surface chemistry of Congo red solutions (89C) has been investigated using a simple method for the study of the variation of boundary tension with time (88C). The drop-weight method was used to study the effect on surface activity of the molecular weight of fractionated polymethyl acrylic acid (S4C). At constant concentration, surface tension decreased with increasing molecular weight and this decrease varied inversely with the square root of the degree of polymerization. Liquid-Liquid Interface. The pendant drop technique continues to be favored for the investigation of liquid-liquid interfacial tensions, Phillppovich (74C) having described a new semimicro method. It has been used (7OC) for measurement of benzene-water and n-decane-water interfacial tensions over a range of temperatures (30’ to 200” C.) and pressures (1 to 680 atmospheres) and for the measurement of surface activity in crude oils (SOC). Donahue and Bartell ( S I C ) , in a careful study of the boundary tension a t the interface of mutually saturated solutions of water and 12 organic liquids, together with data collected from the literature, concluded that the interfacial tension shows an almost linear relation with log (Ifl N t ) , where N 1 and Nz are mole fractions of the solute in the respective phases. Antonow’s rule holds only for spreading liquids that form no lenses or lenses with very small contact angles. Pendant drop measurements of the interfacial tension of mercury and water, heptane, benzene, or ethyl alcohol have also been made, the values for polar liquids changing with presence of air ( 7 C ) . Ward and Tordai (946‘) likewise used the method for determining the interfacial tensions of lauric, palmitic, and stearic acid solutions in hexane versus water. Based on the kinetics of the decrease in tension with time to a limit, characteristic for each acid, thermodynamic calculations were made (95C, SSC), leading to an estimate of the standard free energy of transference of the carboxyl group from the gas phase to the hexane. Interfacial tension measurements a t the benzene-water interface have also been made using long-chain normal alcohols, fatty acids, and esters as third components (56C). In this case no time dependence was noted. Liquid-Solid Interface. The liquid-solid interface, while less readily investigated than the liquid-liquid or liquid-gas interface, has great practical importance in problems of adhesion, waterproofing, flotation, friction, etc. Tabor (91C) summarized the thermodynamics of the wetting of a solid surface, pointing out the defects in the Dupr6 equation, first emphasized by Bangham and Razouk (SC) and Harkins and his coworkers (6A, 48C). In a more practical study (4SC), a new instrument is described whereby surface tension, adhesion tension, contact angles, interfacial forces, etc., can be determined in order to evaluate wetting.
+
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Hansen (47C),in a study of the basic concepts of surface thermodynamics, indicated that adsorption is related to the Gibbsian surface excess by factors which can be experimentally determined. Fox and Zisman (37C) measured the spreading of liquids on low energy hydrocarbon surfaces. Guastallh (48C) considerd surface forces a t solid-liquid interfaces and Benedicks ( 1 1 C ) proposed a method of determining the surface tension of solids. Some light on the dependence of contact angle and its hysteresis on adsorbed films on the solid surface and on other factors is given by a study of Bartell and Bjorklund (8C) on interfacial contact angles in the mercury-benzene-water system. Roughness, which has been shown t o cause hysteresis for some solids surfaces (86C), cannot be a factor in this case. Elton (34C, 36C) also studied contact angles and surface tensions in liquid-solid systems, verifying Antonon’s rule for water and diphenyl oxide, acetophenone, CI~H3a,and C18H38. In a series of papers, Inaba (566, 57C) investigated the wettability of solid surfaces, working initially with paraffin and palmitic acid and cetyl alcoholparaffin mixtures. Wliile the wettability of the mixtures increased with time, that of pure paraffin remain unchanged, indicating orientation or migration of the more polar acid or alcohol molecules in the interface. Theoretical calculation (5C) is said to show the contact angle of water on paraffin to be 124’48’, which may be compared to values normally obtained of the order of 110’ t o 116’. A somewhat different approach to the measurement of wettability of solid surfaces was taken by Neudert (72C), who measured the drop radius and weight of the drop on the solid surface. Bartell and R a y ( 9 C ) also measured the contact angles formed by water and organic liquids with cellulose and Majumdar ( 6 6 C ) the contact angle of pyrite. The relationship between contact angle and adsorbed films on a solid surface has been investigated extensively by Zisman and collaborators (4C, 36C, 69C, 82C) for amine, carboxylic acid, and tetratluoroethylene polymer coatings deposited on platinum or glass surfaces. Confirmation of the fact that such layers can be spread in uniform unimolecular and multimolecular layers has .been obtained by the application of interferometric techniques to fatty acid layers deposited on mica (WIG). Frictional Phenomena. The increasing knowledge of surface films on solids together with the powerful investigational aids of electron microscopy and electron diffraction have added greatly to knowledge of frictional phenomena (1.4‘2). The early view that friction is due to the passage of surface irregularities over one another is unable to account for many experimental facts, including the changes in friction caused by adsorbed films on the solid surface (IOC , I I C , 16C, 46C,7 l C , 98C). Friction between solids appears t o be due primarily t o adhesion at t h e points of contact of the solid surfaces and the necessity for shearing the “junctions” so formed before relative motion of the solids can occur. This is true also for nonmetals such as plastics (75C, 83C, 84C), powders bonded by synthetic resins ( 7 3 3 , and graphite and other forms of carbon (17C, 18C, 8OC). An interesting outgrowth of Bowden’s pioneering work has been the extension of its techniques and theories to the initiation and growth of explosions (16C). Modification of Coulomb’s law for static friction has been suggested by Ratner ( 7 6 C ) as a result of experimental work on different resins versus steel, plexiglas, and an aluminum alloy with variable applied load. Variable load experiments on copper surfaces, lubricated with one and five monolayers of stearic acid have also been reported (536). Boundary lubrication is also greatly affected by adsorbed films and is, of course, akin to the frictional phenomena discussed, allowing the use of similar techniques for its investigation ( I S C , bOC, 45C, 86C,9OC, WC). I n general, most effective lubricants form a solid boundary film having a closely packed, strongly oriented structure due to the presence of polar groups. With regard t o adhesion, it seeme likely that wetting and monomolecular layer concepts, although important theoretically, have little practical importance. The general subject in its theoretical
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and practical aspects is covered in a recent book (d9C) and a review article (91C).
Lyophobic Colloids As a guide to the radius (yC) of the smallest particle or drop im a system endowed with stability the Kelvin equation RT
p log,
P / P , = 2a M / r ,
has long been used. Particles of smaller size or nuclei are regarded as essentially unstable. It seems probable that nuclei, formed from ionic constituents endowed with strong orienting and attractive forces, require a lower number of molecules to reach a critical size, whereafter they continue t o grow, than do droplets formed from constituents between the molecules of which only secondary valence forces are operative. Thus, for barium sulfate the critical size has been suggested as seven molecules per nucleus ( 7 9 0 ) , for liquid droplets numbers of molecules from 40 to 128 have been suggested by another author (123D). Herzfeld and Reed ( 6 6 0 ) pointed out the difficulties involved in defining the thermodynamic free surface energy of very small particles. The thermodynamics of the spherical interface has received study by Buff (8OD), while Kuhrt (77D) proposed that the Kelvin equation requires correction for rotation and translation elements when the droplets are free moving. Nucleation Phenomena. At the annual Christmas Chemical Engineering Symposium held in 1951 (reported in the June 1952 the broad field issue of INnUsTRIaLAND ENGINEERINGCHEMISTRY), of nucleation phenomena was covered from every angle, including the Kelvin equation, the nucleus formation theories of Volmer, Becker and Doering, and Frankel, catalysis of nucleation, and applications t o industrial systems such as photographic emulsions. Rodebush (186D) criticized the Kelvin equation on the ground that no stable equilibrium can exist between the liquid drop and a vapor phase indefinite in extent. suggesting another approach based on the behavior of the drop as a large molecule. I n another paper, Reiss (1230) revieffed the liquid drop model as the basis of nucleation theory and suggested a statistical theory. LaMer (79D) pointed out the fundamental importance of nucleation in many chemical engineering operations and presented a comprehensive review of the present state of knowledge as applied t o the stability of colloidal dispersions, rate of nucleus formation in the gaseous and liquid states, and the preparation of colloids. Nucleation catalysis by the use of substances closely resembling the separating material in atomic arrangement and lattice spacing on certain low index planes were discussed ( 1 5 5 0 ) . Results from ice formation, casting of metals, and crystallization of salt hydrates were cited. The theory of nucleation in solids was also reviewed (3660, 1560) and experimental studies on nucleation in supersaturated inmoist air ( 1W8D), sucrose solutions (16OD), organic salt solutions (139D), bubble formation in liquids ( l l D ) , supersaturated potassium chloride solutions (1180), metals and plastics (4bD), photographic emulsions ( 1 6 4 0 ) , and the decomposition of barium azide (1480)reported. Similar discussions a t a meeting of the Faraday Society (3OD) should also be mentioned. Elsewhere, nucleation in very rapid vapor expansions ( 7 0 0 ) and the effect of formation and growth of nuclei on the decomposition of solids ( 8 9 0 ) Nras discussed. .4recent book (146D)also covered some of the above topics. Some work was reported on electron microscope studies of condensation nuclei formed from moist air ( 2 8 0 ) , which indicates that hygroscopic salts such as sodium and magnesium chloride may be the ultimate nuclei. It is clear that progress is being made in this field, although much remains to be learned. Sol Formation. An exhaustive study of the formation of gold sols by the Bredig method and by reduction of potassium chloroaurate or chloroauric acid solutions with a variety of reducing agents has been described (154D). Electron microscopy, nrphe-
May 1953
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INDUSTRIAL AND E N G INEERING CHEMISTRY
lometry, and slit-ultramicroscopy were used t o determine the number of gold nuclei present and the auric ion concentration was also followed. Growth of the nuclei was found to be exponential and the size distribution of the final sol particles was controlled by gold concentration and the nucleation process. Miura ( 1 0 6 0 ) also used the electron microscope to investigate gold sols formed by various reducing agents; a relationship between color and particle size was established. T h e formation of aluminum hydroxide sols from aluminum chloride and ammonium hydroxide solutions, followed by dialysis, has been studied with the electron microscope (80). A transition from spheres in fresh sols t o microcrystals of hydrargillite over a few weeks was observed. The stabilizing influence of glycerol on barium sulfate sols has led t o the development of an interesting method for the rapid determination of sulfate ion by nephelometric means (147D). The effect of the presence of various neutral electrolytes during the formation of hydrophobic sols has been the subject of work by T e i a k and his collaborators ( 9 7 0 , 1030, 1040, i S 7 0 , 1400, 14WD-i&D), using mainly silver halide sols, although some work on barium sulfate ( i 4 l D ) has been described. As a result, Matijevic and Teiak ( 9 8 0 ) , in a review and discussion of coagulation theories as related to their measurements on sols in the nascent state, concluded that Wo. Ostwald’s views are confirmed. Similar experimental work has been done on ferric hydroxide sols ( W D ) . The general subject of the electric double layer and colloid stability continues to be active ( S i 0 )with opinion divided over the Gouy and Stern views of the charge distribution. The former theory treats the counterions as point charges while the latter allows for their finite dimensions. I n both theories the surface is considered as a uniformly charged, impervious film, although this is probably far from true. Verwey and Overbeek (1630) treated the interaction between the two flat double layers and also that between spherical particles, finding that, in accordance with the Hardy-Schulze law, the coagulating concentration of added electrolytes should vary inversely as the sixth power of the valency. A general discussion by the Faraday Society (1600) should be consulted for recent views on the double layer, as should a review article by Hauser (600). The Stern theory has been developed by Levine in a series of papers (810-860), while Loeb (87D) applied an interionic attraction theory to the diffuse layer around the colloid particles. Electrophoresis. The theory of electrophoresis has been reviewed very fully by Overbeek ( 1 1 2 0 ) but further progress has been made. The original formula of Henry connecting electrophoretic velocity and the electrokinetic potential ( 6 6 0 ) has been modified to correct for surface conductance ( 1 7 0 , 640) and for fluid rather than solid particles (140, 670). Booth also considered the effect of surface conditions on the electrophoresis of solid particles (160) and derived a new equation for the electroviscous effect in suspensions of solid, spherical particles (160). The newer equation, since i t predicts much smaller effects than the original Smoluchowski equation, is in better agreement with the experimental facts. The potential a t a charged water-oil or water-air interface has been shown to be dependent upon the ionic strength of the aqueous phase. This potential has been determined by four independent methods (W6D), vkifying the Gouy equation in solutions of low ionic strength. The fact that when a colloid is centrifuged some separation of the charges occurs gives rise to a potential difference which may be used to measure the potential (360, 660, 660). The potentials thus determined on arsenious sulfide sols after the addition of various electrolytes are somewhat less than those measured by electrophoresis. The method has also been used for silica suspensions ( 8 4 0 ) . More light on the properties of the electric double layer is also given in a series of papers by Mackor (91D9401, in which the double layer on silver iodide in water and water-acetone mixtures is studied. Up to 30 to 40 volume yo
acetone stabilized the sol to flocculation by sodium, barium, and lanthanum perchlorates; a t higher concentrations stability was lowered rapidly. Cataphoretic measurements have been made on freshly prepared CY- and y-FeO(0H) (1320); r-AIO(OH) and a- and yA1(OH)3(1510); lampblack, ferric oxide, and powdered lignite in anion-active detergents ( T 6 0 ) ; and clays (60, 1OD). Electrokinetic potentials, ( C and h,determined by cataphoresis and electroendosmosis, respectively, were found to be almost identical for hydrogen-mica, sodium-mica, and sodium-bentonite suspensions in different electrolytes ( 4 8 0 ) . Several papers indicate an interest in electrophoresis in organic media (180, 19D, 4 1 0 , 630, 1OOD) and the stability of organic dispersions (680, 74D). Schreiner (1SOD) found that the sedimentation rate of cupric oxide, nickel oxide, and zinc oxide powders was much lower in organic liquids, the vapors of which were irreversibly adsorbed by the pigment. Much valuable information with regard to stability, formation, and other factors important in colloidal behavior has been gained by study of monodispersed sulfur hydrosols, initiated by Zaiser and LaMer (1680). Their formation is one of selfnucleation; in dilute solution the supersaturation is relieved so effectively and suddenly by nucleus formation that no new nuclei are formed and uniform growth can occur by diffusion to a highly monodisperse preparation (800). Dinegar and Smellie ( 2 9 0 ) investigated the stability of such sols, while their formation and growth has been considered theoretically (124D) and the measurement of their particle-size distribution by the polarimetric analysis of scattered light described ( 7 3 0 ) . The structure of arsenious sulfide ( 4 4 0 ) and vanadium pentoxide sols (9D, 3.90) has also received study. Dispersions of tetrafluoroethylene resins, which are made up of negatively charged particles 0.05 to 0.5 micron in diameter, behave as lyophobic sols (880). Coagulation Theory. The Smoluchowski theory of time change of size distribution of colloidal particles during coagulation has been extended to include various initial conditions besides uniform initial distribution (1.530). The relationship between concentration, c, of coagulating electrolyte and time to coagulate, 1, was determined for a series of arsenious sulfide sols and various electrolytes to be of the empirical form: c = a
+ b/t
where a and b are constants (IZD,780). The coagulation of silver thiocyanate sol ( 4 D ) and silver chloride and cyanide sols ( 3 0 ) have been studied using measurements of light absorption, while fumaric and maleic acids have been shown to behave differently in the coagulation of ferric hydroxide sol (990). Coagulation of the latter by anions used as corrosion inhibitors has been shown not t o be a significant factor in the corrosioninhibition mechanism (136D). The stabilizing action of ferric chloride on nondialyzed ferric hydroxide sols is reduced by presence of sodium chloride ( 8 2 0 ) . One study of the effect of fission products on lyophobic colloids has been made (49D). Aqueous sols of positively charged copper, and negatively charged silver and gold, prepared by Rredig’s method, were irradiated with x-rays from radium bromide, gamma rays and neutrons from radium bromide and beryllium, and with fast neutrons from cyclotron deuterium and beryllium. Invariably the copper sol was coagulated, probably through intermediate formation of hydroxyl ions which would affect the sol double layer. In nearly all cases thci silver sol was stabilized. Krishnamurti and Karbelkar (76D) disclosed that the coagulating power of an ion adsorbed in a colloidal sol on an oppositely charged suspensoid is greater than that of the same ion in normal solution. This is ascribed to the close approach of the double layers of the colloidal particles. It is well known that suspensoids become unstable and coagulate when frozen. I n a series of papers (1660-1690) the freeze-coagulation of gold, platinum,
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INDUSTRIAL AND ENGINEERING CHEMISTRY
and arsenious sulfide sols has been btudied with particular regard t o the protective action of such materials as alcohols, urea, and sugar. I n another study (ZQD), organic liquids, such as ethyl acetate, amyl alcohol, and methyl ethyl ketone, added to arsenious sulfide sol were found to increase its stability t o coagulation by barium chloride. I n considering the stability of suspensoids t o coagulation, a return to the concept of critical electrokinetic potential has been urged by Weiser and Merrifield (1650). The unusual nature of the Liesegang ring phenomenon was attributed by Rastogi t o coagulation rather than t o supersaturation (1200-122D). A description of the Liesegang rings formed by the simultaneous precipitation of potassium chromate and bichromate in gelatin medium was given by Veil ( 1 6 1 0 ) . The peptization to a sol of mixtures of negatively charged iron and chromium hydroxides with agents such as sugar and glycerol points to possible formation of a complex (25D). Mutual gelation, rather than coagulation, can be obtained by mixing a negatively charged nickel hydroxide so1 with positively charged copper hydroxide, stannic hydroxide, or ferric phosphate sol ( 1 1 6 0 ) . The process of mutual gelation can be followed by light extinctio? measurements ( 1 1 7 0 ) . The mutual coagulation of ferric hydroxide and antimony sulfide sols has been attributed to two factors: the coagulating power of the particles themselves and that of the intermicellar electrolytes ( 2 0 ) . The protective action of high molecular weight compounds such as rubber or cellulose on tungsten sols has been investigated with the ultramicroscope and electron microscope (109D). Aggregates were observed to form. An interesting suggestion has been made by Hounam ( 5 9 D ) that dusts producing silicosis and pneumocosis form negatively charged hydrophobic sols when dispersed in water or 50% ethyl alcohol solutions. Harmless dusts form positively or very slightly negatively charged sols. Bedos (6D)investigated the electrical properties of industrial silver sols in an alternating, variable-frequency field. The presence of a Debye effect in colloidal solutions has been shown for arsenious sulfide sols b y Derouet and Denizot (27D). Inorganic Gels. While many inorganic gels, either nonreversible or thixotropic, have considerable industrial importance their structure is still not completely certain, though progress has been made. Thiele and Kienast (146D), following the gelation of anisometric colloids such as vanadium pentoxide, mercury sulfosalicylic acid, and ferric oxide by chemical and electron microscope methods, concluded that the steps occurring are ion diffusion, alignment, ion exchange, and gel formation. Silica gel continues to be investigated (63D, 6 4 0 , 7 l D , 108D) and it seems probable t h a t its formation may be by polymerization or condensation leading t o gels of very different physical properties ( 1 6 6 0 ) . The acceleration or retardation of the gelation of silicic acid by fluoride ( 6 2 D ) and chromate ( 6 1 D ) ions, respectively, has been studied by Iler, as has the association between polysilic acid and polar organic compounds (COD). Mixed silica-alumina gels have been investigated with respect to acidity (149D) and surface structure ( 5 2 0 ) , while alumina (48D), zirconium hydroxide (69D), sodium thiohydroxyferrate ( 3 2 D ) gels, and colloidal metaphosphates ( 9 5 D ) have also received attention. ITiscosity measurements made during the sol-gel transformation of many colloids indicate an exponential relationship between viscosity and time ( 1 1 4 0 , 1150). Clays. The colloidal behavior of clays, which may be regarded as forming suspensoid-type sols, gels, and pastes, is of the greatest importance in ceramics (580,l l O D ) ,drilling muds(lOID), and soil science (450, 1 2 9 0 ) . Transactions of international congresses in ceramics, held in 1948, and in soil science, held in 1950, are now available and contain many valuable papers concerning the properties and behavior of clays (47D, 1510, 1 5 2 0 ) . The crystal structure and water adsorption of clay minerals have been discussed using all available data and the results applied to lattice stability, changes in viscosity, electrophoretic mobility,
Vol. 45, No. 5
shear modulus, etc., of plastic clays (S7D). The adsorption isotherm of water on montmorillonite has been found to be depende n t on the initial water content, but the desorption isotherm is reproducible (106D). Exchangeable ions, univalent and bivalent, were found t o influence adsorption according t o their size and charge (107D). Similar results were obtained with kaolinite (72D). Ion activities in sodium clay suspensions have been measured by the Donnan equilibrium method (167D) and the base-exchange capacity of quartz, mica, silica gel, kaolinite, and montmorillonite have been determined by reacting the hydrogen clays with saturated potassium chloride ( 5 8 0 ) . Heteroionic and homoionic clays have been investigated using this technique (S9D). The adsorptive and bleaching properties of Indian clays were studied ( 4 7 D ) and also their electrochemical properties ( 1 2 7 0 ) . Van Olphen ( 1 1 1 0 ) has given attention t o the charge distribution on clay particles in relation to the rheological behavior of their sols. The viscosity of clay slips also continues to receive notice (1350, 16dD). Thixotropic behavior, with particular reference to clay gels, has been reviewed (57D), while the effects of heat, alcohol, and caustic soda on the thixotropy of bentonite suspensions were studied (1SD). Anionic wetting agents were found t o decrease the drying shrinkage and porosity of clay mixes but cationic and anionic agents had no effect ( 1 1 9 0 ) . Research on organic derivatives of clays continues (lSl+D, 1S6D), and a review of work done in this field has been published ( 5 1 0 ) . Toluene and xylene were found to give a rose to red color with certain acid activated clays (1D). Miscellaneous. Some miscellaneous papers may be noted. Flocculation of pigment suspensions has been measured by several methods (74D), particle structure of lithopone wa9 found to change greatly paint properties (1O%D),and Mardles ( 9 6 D )issued a summary report on the viscosity of paints and suspensions from the standpoint of the Einstein equation. Chadrvick ( 2 1 0 ) covered somewhat similar ground but prefers the Arrhenius to the Einstein equation. Brownian movement measurements on sphalerite particles indicate many complicating factors due to ionic type and concentration of the suspension medium, cell type, etc. ( 7 D ) . Mackor ( 9 0 0 ) suggested a stability model for colloidal dispersions in hydrocarbons. Properties and uses of colloidal carbon have been reviewed (1S8D), and the rates of settling of carbon suspensions were compared after the addition of variouq fatty acids or their calciuni salts ( 4 0 0 ) .
Aerosols The simplest application of the Kelvin equation is perhaps to aerosols and i t has proved to be of importance in the understanding of aerosol and mist formation. The reader is referred t o the discussion of the previous section. More specifically, L a M w and Gendron ( 1 5 E ) reviewed recent advances in homogeneous aerosols, while the increasing importance of aerosols in meteorology is signalized by many papers. A comprehensive review of cloud seeding has appeared ( 1 0 E ) . Kramer and Rigbv (14E) compiled a bibliography covering 1895 to 1950 on cloud seeding and other meteorological aspects of aerosols. Mathematical expressions for the thermal force on an aerosol particle in a temperature gradient ( 2 l E ) and for the growth of a charged water droplet have been derived and tested experimentally ( 4 E ) . The formation of an aerosol b y condensation of a dilute vapor in an inert gas also has been investigated mathematically ( 5 E ) . Johnston and Manno (12%') describe the formation of Liesegang rings of ammonium chloride, which can occur only in presence of water . The proceedings of the U. S. Technical Conference on Air Pollution, now available in book form (17E), deal with aerosol precipitation and analysis, instrumentation, diffusion and, of course, effects of atmospheric pollution. A symposium on air pollution was also held at the X I I t h International Congress of Pure and Applied Chemistry, the proceedings of which have also been
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INDUSTRIAL AND ENGINEERING CHEMISTRY
reported (11E). Monographs have also been devoted to the meteorological aspects of air pollution ( I 8 E ) and the general properties of aerosols (d7E). Aqueous fogs interact with fogs prepared by dispersing 10% aqueous calcium (8E). The use of a vibrating membrane t o disperse dusts in the 1t o 30 microns size range, together with their evaluation by light iscattering has been reported ( 2 E ) and a new method for making aerosols of relatively low boiling materials described (86E). A4erosolsof sodium chloride can be prepared (SE,7 E ) . Aerosols for air pollution studies have been produced in a smog chamber ( 1E ) . From the fundamental viewpoint the monodispersed systems of spherical aerosols (W4E, b7E) are of great importance. They have demonstrated the difficulties met in the absolute measurement of concentration and size distribution of aerosol systems. Such analysis, reviewed by Kay (ISE),is receiving much study. The use of the cooling effect on an electrically heated filament due to evaporation has been suggested for the detection and measurement of aerosol particles of water and other liquids (28E). Ranz and Wong (%)E, H E ) proposed the collection of dust and smoke particles by jet impaction. Other methods for particle size determination were reported (QE,10E). Edge and variable compression filters for aerosols were described by Silverman and First (ISE). The collection and analysis of sulfuric acid mists have been reviewed but experiment indicates that a sintered glass filter gives satisfactory results (16E).
Foams and Emulsions Papers on foam formation in binary (88F, 8QF) and ternary (408’)organic systems were reported. As found frequently in the past, pH plays an important part in foaming. Sodium oleate solutions show maximum foaminess in the range of pH 9 to 10 (S6F). The properties of soap foams, with particular reference to their use in fire fighting, have been reviewed by ohman ( I 7 F ) . A generator for such foams has been described by Fry and French (108’)and their stability to radiant heat has been measured (QF). Foam consistency measurements were made by adaptations of the Gardner mobilometer and the Brookfield viscometer and by a third new device (348’). I n agreement with many investigators ( I F , S F ) high consistency foams were found to be associated with slow drainage rates and high viscosity. A new apparatus for carrying out laboratorx determinations of the volume and stability of lather has been proposed by Schlachter and Dierkes (dBF). A rotational viscometer device was also used to study foams formed from naphthalenate sulfonate derivatives, saponin, and other similar materials (318’). A surface viscometer for the study of foams was devised and measurements indicate a close correlation between this property and foam life (16F). Stability of foams to mechanical destruction was found to be better when freshly foamed, using falling mercury stream technique (@). In breaking a molasses foam, Shkodin ( 3 6 F ) found that a glass stirrer coated with paraffin wax was effective, presumably by allowing wetting of the stirrer to occur, although an uncoated stirrer had no effect. This may indicate an important consideration in agitation of some foams during breaking. Continuing a series of papers on inhibition of foaming, Ross and Young (SOF) found that foam inhibitors usually show a large positive spreading coefficient and a low viscosity. Drainage and evaporation are the chief factors which influence the thickness of a liquid film and hence the life of the bubble, the former being the more important (44F). In a device for measuring the efficiency of defoamers, clean measured air is passed through a standard mixture of yeast, molasses, and the test defoamer at a controlled temperature (d6F). Antifoamers, having a proper hydrophil-hydrophobe balance within the molecule, were the most effective. The foaming properties of a number of proteins a t various pH’s have been given and suggested as an aid t o characterization of soluble proteins (98F), it was again pointed out that certain substances could be concentrated from solution by foaming ( 6 F ) .
919
Emulsions. Emulsions, as well as being one of the oldest colloid systems, form one of the most important. A book by Manegold @ I F ) covers their properties in considerable detail. New emulsification apparatus have been described (ICF, 19F) and dependence of droplet radius on emulsion concentration and interfacial tension has received attention ( 1 5 F ) . Varnish oils can be dispersed in water by ultrasonic vibrations (42F). Interfacial tension measurements on oil and glue emulsions indicate that addition of a solid, litharge, to the oil increases the value threefold (84F). Although interfacial tension is important, i t affects only the work necessary for dispersion and stabilization of emulsions must occur through the droplets’ surface layer rendering collisions due to Brownian motion ineffective (IOF). The potential energy barrier which prevents coalescence may be due to electrical or mechanical causes, or differential wetting effects. Stability involves the factors of charge, surface rigidity, and viscosity-particularly in the case of water-in-oil emulsions (33F)and elasticity as well as surface tension effects. Their interplay is complex and not yet too well understood. The behavior of water-boil emulsions using wool wax and its constituents as emulsifiers ( 4 1 F ) forms an interesting case in point. The free alcohols present in the wax were found to be the active emulsifying agents because their long hydrocarbon chains could form a rigid film by lateral adhesion, reinforced in some cases by hydrogen bonding between the polar heads when close approach between a carbonyl oxygen atom and a hydroxyl group was made possible by the presence of esters. With emulsions stabilized by soap, calcium ions can be powerful breaking agents and alkali lignin, which can act as a sequestering agent, has been recommended as a stabilizer for slow-break asphalt emulsions ($?IF). Measurement of emulsion stability has been used as confirmatory evidence for the structure of diphenylsiloxane surface films (67C). A study ( 4 S F ) of spontaneous emulsification of sodium alkyl aryl sulfonates in white mineral oil suggests as a possible mechanism penetration of water between the sulfonate layers in the plate-shaped layers of the wetting agent micelles, followed by splitting of the micelles and formation of oil-water interfaces. In the case of spontaneous emulsification of oleic acid-paraffin oil mixtures in alkaline solutions, it has been proved that while the interfacial tension is very small, negstive values ( S 7 F ) do not occur and streaming away of oleic acid into the alkaline phase is the cause of emulsification (28F). Francis ( 8 F ) , in an interesting paper, demonstrated that emulsions formed from colorless liquid phases having equal refractive indices but different optical dispersions exhibit brilliant colors when viewed a t a light barrier with transmitted light. This phenomenon under suitable conditions can be used for the determination of tbe refractive index of liquids. The damping coefficient of sound in emulsions is about 100 times as great as in pure liquids, a phenomenon studied by Isakovich ( 1 1 F ) . The homogenization of emulsions by passage through capillaries under low pressures has been investigated with a variety of emulsion concentrations and stabilizers (18F, 19F). High pressure expulsion of cod-liver oil against a crash plate under water was found to give emulsions and their particle size distribution has been studied (17F). Jelinek ( 1 S F ) described the physicochemical aspects of the emulsification of plastomers. Rate of creaming has been used by Cockbain (48’) to follow the reversible agglomeration of soap-stabilized benzene or paraffin hydrocarbon emulsions. Multimolecular adsorption is thought to occur a t soap concentreationsgreater than the critical micellar value leading to agglomeration. The breaking of emulsions by a variety of methods was considered in several papers ( 7 F , 12F, W F ) and a method ( 6 F ) has been recommended by the Institute of Petroleum Cutting Oils Panel for the determination of the oil content of soluble oil emulsions.
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I N D U S T R I A L A N D ENGINEERING CHEMISTRY
Lyophilic Colloids Lyophilic colloids differ from lyophobic colloids in many ways, the two most outstanding of which are their stability t o electrolytes and their spontaneous and reversible dispersion in suitable media. I n a review article, Johnson (9JG) emphasized the difference in heat content and entropy changes during dispersion for nonpolar and polar lyophilic systems. Rubber in benzene, representing one extreme type, shows a small heat content and large entropy change, while agar-agar in water, representing the other extreme, shows a large negative heat content and small entropy change, due to interaction of the polar groups. Such free energy considerations control swelling and dispersion into single molecules or ions or into aggregates or micelles (SSG, 7OG, 185G, 186G). The properties of high polymer solutions have been reviewed by Bawn (11G). Other review articles by Merrill (116G), Muller (1,98G), and Klevens (105G) are also of interest. Stauff (187G) considered t h e thermodynamics of stable, associated colloids while Ekwall (44G) studied their surface and colloid chemical properties. Huggins et d . (86G) reported on nomenclature in the field of macromolecular chemistry. A book by Frey-Kyssling (60G) deals with deformation and flow in biological systems. Polar-Type Systems. Turning t o work on specific systems of the more polar type-e.g., polysaccharides, proteins, etc.Alexander and Stacey (SG), using light scattering studies, found that a number of azo dyes in aqueous solution formed micelles when electrolyte was present. Urea and phenol prevented aggregation in several of the dye solutions. The stability of a very different type of lyophilic sol was investigated by Hamdi (71G ) , who coagulated dialyzed graphitic oxide sols prepared by oxidizing graphite with concentrated sulfuric and nitric acids and potassium chlorate. The Hofmeister series was found t o hold. Nutting (l46G) found t h a t addition of electrolytes t o potato starch pastes decreased their viscosity as did factors depressing ionization. General developments in the field of starch chemistry were covered by Bourne (16G, 16G), and Nussenbaum and Hassid (146G) proposed a new colorimetric method, rapid in execution and utilizing little material, for determining the molecular weights of amylodextrins, amyloses, and amylopectins. Good agreement with osmotic pressure measurements was claimed. Agar gels have received attention from h’akagawa et al. (J6G, 132’G-1SCG), a Couette-type apparatus being used t o measure their viscoelastic properties. The gel appears t o have a tangled lattice structure, while viscosity measurements in dilute solution indicate that the agar molecule in solution is nonspherical. Grover (67G) re-evaluated the data of Hinton (80G) on pectin jelly formation. The reaction between polyvinyl alcohol and borax which gives a rigid gel has been subjected to x-ray diffraction study (147G). Other studies of lyophilic-type gels have been made by Hirai (81G-8SG). It seems probable that in gels of this type the dispersed particles become bonded sufficiently at their points of contact to give the whole system rigidity. Herrent (77G) offered evidence to support such a structure for cellulosic gels. Kast (IOOG), in a study of cellulose gels using x-ray orientation measurements, applied similar thoughts to the stretching process. Hirai (8JG) calculated the increases of internal energy and entropy for the dissociation of such contact points in the gel structure and established a correlation between the square root of the reciprocal of the gold number of the gelling material and the heat of dissociation of the gel network. This suggests an interesting inverse relationship between protective action and gelling power. Proteins. I n the case of proteins, their solubility depends upon a complex interrelationship between chain length, end groups, polarity, and length of side chains. Much progress has been made in the last few years in the elucidation of the fundamental structure of the natural proteins (6G). Rydon (166G), in a n excellent review, summarized the evidence for the Pauling
Vol. 45, No. 5
and Corey helical coil (15SG) and “pleated sheet” (161G) structures for globular and alpha forms of fibrous proteins and beta forms of fibrous proteins, respectively. The work of Bamford el al. (7G, 8G) on synthetic polypeptides also throw8 additional light on protein structure. Since structure of the solid proteins is still not completely understood, it is perhaps not surprising that much work remains t o be done on solutions. Depending upon their structure, proteins may be insoluble or soluble. Those containing a large proportion of neutral amino acids or disulfide bridges tend t o be insoluble or only to swell in water, while in the soluble and corpuscular proteins the polypeptide chain is folded regularly on itself and stabilized by electrostatic and nonpolar linkages. I n this folding, certain of the side chains may become submerged within the molecule, making the prediction of molecular properties difficult. Moelwyn-Hughes (IWSG) considered certain types of equilibrium in protein solutions but the limitations necessary were great. Interaction between proteins is of great biological interest and Oncley et d . (149G) have made a n important study of such behavior. Robinson and Bott (16SG), on the basis of optical rotation and chain folding in synthetic polypeptides and gelatin, proposed a gelling mechanism for gelatin. Gelation appears to be dependent upon the guanidine group; removal of some of the arginine by treatment with sodium hypobromite or ultrasonic vibration destroys the gelling power (IS5G). Urea appears to induce gelation by breaking hydrogen bonds, thus enabling -SH groups to react with intermolecular disulfide linkages, each of which becomes an intermolecular bridge liberating a new S H group (86G). This eventually gives birth to a disulfide cross-linked framework. I n some ways this may be connected with denaturation, which may be regarded as the freeing of the main chain of the protein from ita stabilization with consequent freedom to take up any one of a number of positions (dSG). Fricke (61G) measured the kinetics of the thermal denaturation of x-rayed egg albumen. Directed aggregation in colloidal systems and the formation of protein fibers were reported by Rees (161G). Much work continues to be done on gelatin because of its availability and industrial importance. I n a n interesting paper, Deryagin, Levi, and Kol’tsov (38G) measured the viscosities of 4 t o 10% gelatin solutions with surface active dyes present. They concluded that the gelatin particle in solution was an elongated chain coil, which upon sorption of the dye, stretched out to about twice its original length. Pouradier and Venet continued their work on gelatin by a n examination of demineralized gelatins of different origins (158G), demonstrating t h a t these have different structures regardless of isoelectric point. I n further work ( 159G), they found that gelatin solutions undergo degradation even in absence of bacteria or enzymes and at the isoelectric point. Measurements of the viscosity, rigidity, and elasticity of gelatin gels continue t o be made (SSG, 34G, 97G, 118G); it is suggested that only a few of the ionized side chains contribute to the electrostatic forces that cause rigidity. Specific volume-temperature measurements on gelatin gels were also reported (1S9G, 140G). Nobel has begun a series of investigations on the influence of cations on gelatins; thus far their effect on melting point (14dG), swelling ( I W G ) , and elasticity ( f & G ) have been reported. Gelatin films have also been subjects of research with respect t o their crystalline structure (19fG, 193G), swelling in water ( H G ) , and strength and mechanical properties (17G, 18G, 106G). Kutyanin (109G), in a study of the thermal stability of dry collagen, found that it was unaffected by tanning. Hall (26G)reported on the energy and entropy effects in a study of the elasticity of collagen fibers. Macromolecules in Solution. The behavior of the lyophilic polymer macromolecules in solution will be largely dictated by two factors-their molecular shape, usually expressed as a n axial ratio a / b , and their solvation. The former is again influenced by the “flexibility” of the polymer chain. Cellulose and its derivatives are relatively stiff and unbending, rubber rela-
May 1953
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
tively flexible. Thus, we have to envisage not only relative motion of the macromolecule and the solvent but also motion of the segments of the polymer chain within itself. I n a (‘poor” solvent a flexible polymer will tend to fold itself in an irregular manner into a coil, while in a “good” solvent the chain will be extended. Hence, the colligative and flow properties of the polymer solution depend to a great degree on interaction with the solvent; much work has been directed in the last few years to evaluating this interaction, using so-called p values of the Huggins-Flory theory ( I i G , 188G). Several papers dealing with the mathematical aspects of this picture should receive mention. I c h i u r a (80G) considered the interaction of the solvent and polymer molecules from statistical theory when heat of mixing cannot be neglected. A statistical method was used by Grimley (66G) to calculate the average dimensions of a high polymer chain and the theory was applied with good results t o experimental data on polystyrene-butanone, polystyrene-toluene, and polyisobutylene-cyclohexane systems. A generalization of the statistical theories for high polymer solutions had been made by Miinster (119G) and satisfactory agreement claimed with polystyrene, polyvinyl, and nitrocellulose solutions. Because of the variety of positions that the chain segments can take up as a result of rotation about the individual bonds, a statistical basis must be used and it has become customary to express the dimensions of a chain molecule in terms of root mean square end-to-end distance, R. Tchen (IOSG) continued such calculations using the more general model of random flight with multiple partial correlations. The data of Doty and Mishuck (4WG) for polyvinyl chloride have been recalculated by Takeda (198G). Since in a real polymer two segments of its chain cannot simultaneously occupy the same site, the random-flight treatment must be modified leading to so-called excluded volume effect calculations (STG, 5SG, 69G, 197G, 108G), which seek t o modify the value of R. Cragg, Dumitru, and Simkins (SWG) found good agreement between experimentally measured viscosities of polystyrene solutions in a variety of solvents and values predicted by the Fox-Flory theory (516, 656, 66G). Tompa (WOOG) derived an expression for the isothermal entropy of mixing of solvent and polymer molecules, while Oyama (15OG) applied statistical thermodynamics to the phase separation of high polymer solutions. Experimentally, the Flory-Huggins equation was found valid for the sydtem polyisobutylene-cyclohexane by der Minassian and Magat (119G) and for polystyrene-toluene by Guggenheim and McGlashan (SSG).. Jung (98G) determined the second virial coefficient for solutions of polystyrene, Schulz and Doll (177G) for polymethacrylic acid ester in a variety of solvents; similar work on nitrocellulose and polystyrene is said by Lang and Miinster (11OG) to indicate that the coefficient is proportional to the reciprocal of the molecular weight. Kawai (IOSG),in a mathematical paper, suggests a theoretical basis for the Schultz osmotic pressure equation. Hitchcock (84G) reviewed osmotic pressure and molecular weight relations and Wales (208G) reviewed sedimentation equilibria in concentrated high polymer solutions. G r a l h and Lagermalm (66G), in a unique study, investigated fractionated and unfractionated polystyrenes by ultracentrifuge techniques. Molecular weights were determined and the degree of linearity of the chains estimated. Peterlin (164G, 1666) also reported sedimentation and diffusion measurements on polystyrene, polychloroprene, and a number of cellulose derivatives. Sedimentation-diffusion studies on carrageenin indicate the presence of two components and molecular weights of from 110,000 to 530,000 (SOG). Molecular Weight Determinations. Considerable light on the shape factor of high polymers in solution can also be obtained from viscosity measurements. Higgins and Hayes for example, applied such measurements to casein and modified caseins. The starting point for such investigation of the viscosity
(roc),
921
of suspensions and macromolecule dispersions was the Einstein equation postulated for spherical particles. Three experimental investigations of this law were reported (41G, 486, I l d G ) , though it has long been realized that i t has definite limitations and must be modified for chain-type polymers. An interesting, more fundamental study of individual particle motion in sheared suspensions has been published by Manley and Mason (IIWG). The last decade ( I I G , 16OG) has seen extensive work upon the theoretical basis of the Staudinger equation, linking intrinsic viscosity, 171, and molecular weight, M , and also upon the modification, now generally used, suggested by Mark, [?] = KM“. The exponent Q! can assume values between 0.60 and 1.00. Frank (58G) discussed this equation. Thurmond and Zimm (19OG) attempted to estimate the effect of chain branching using copolymers of styrene and a minor proportion of divinylbenzene. As might be forecast, branched higher molecular weight material showed lower intrinsic viscosity. A similar investigation for polybutadiene was made by Johnson and Wolfangel (01G). As concentration of a polymer solution is increased, interaction and tangling of the polymer chains with each other becomes more pronounced and allowance must be made for this (185G, WIWG) in relating viscosity with concentration. A thorough investigation ( I S G ) of the viscosity of GR-S rubber solutions in concentrations from 0.25 to 90%, using a capillary viscometer up to 10% and the McKee worker-consistometer for concentrations above this figure, indicated no agreement with any of the conventional correlations between viscosity and concentration. In work on polyvinyl acetates of three different molecular weights in a variety of solvents, it was found by Ferry et al. (5OG) that the func d( log ? ) / d ( C,)l/z, where C , is the volume concentration, was rly constant with a value approximately equal to that observed with other polymers and solvents. Weber and von Ker6kj&rt6 (WIIG) found the Arrhenius equation to be valid for a number of protein solutions providing their relative viscosities do not exceed 2.5 to 3. Stawita (IOOG) found this equation inapplicable to the cellulose ethers. Non-Newtonian flow or structural viscosity is also a consequence and mathematical-theoretical analysis of the relation between viscosity and velocity gradient have been made (WOG, 87G, 88G, IWOG, IWIG, WO5G). Hermans and his collaborators investigated the non-Newtonian flow of dilute polymer solutions, using cellulose nitrate in acetone (W16G) and sodium carboxymethylcellulose in aqueous solutions of sodium chloride (WG). Elastic effects may also make their appearance (WWG, 67G), and Ferry and his collaborators continued work on the non-Newtonian flow and stress relaxation of high polymer solutions (JSG, 51G, I75G). Various mechanical models for viscoelastic systems have been proposed and the mathematical equations for them derived (87G, 28G). Harper, Markovitz, and De Witt (72G),in addition to measuring dynamic viscosities and rigidities, determined falling ball viscosities on polyisobutylene and silicone oils and were able to show that in all cases the dynamic viscosity extrapolated to the falling ball viscosity a t zero frequency. Scott-Blair (I70G) and Scheele (ISSG,160G) discussed the general relationship between stress, strain, and time of deformation for lyophilic systems. The variation of viscosity with rate of shear can naturally vitiate measurements of molecular weight (156G), and a study of the intrinsic viscosities of cellulose as affected by this variable has been made by Conrad, Tripp, and Mares (26G). Akkerman Pals, and Hermans (WG)have made a similar study on sodium carboxymethylcellulose solutions, finding that shearing stress has a large effect on intrinsic viscosity. Anomalous flow also rendere valueless normal fluid flow calculations made by the engineer using friction factor, Reynolds number plots. Katchalsky and Sternberg (102G) attempted to extend Poiseuille’s equation t o non-Newtonian liquids, obtaining an equation- involving only one additional constant. Tyabin (WOdG)and Shchipanov ( 1 8 1 6 )considered both streamline and turbulent flow in pipes. Several
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INDUSTRIAL AND ENGINEERING CHEMISTRY
922
other approaches to the problem in circular pipes have also been made (4G, 76G, 1i7G, 123G), and Bayer and Towsley (14G) even considered flow in rectangular channels. The extrusion of elastomers varies roughly as a power of the extrusion pressure. Determinations of molecular weight of polymers by osmotic pressure, viscosimetric, and other methods are noted in Table I.
Table I.
Determination of Molecular Weight
Polymer Polyethylene
Polytrifluorochloroethylene Polybutadiene Polystyrene Polyvinyl acetate Polyvinyl chloride
Polyvinyl alcohol
Methods Vis. and b.o. elevation Osmotic piess., vis., b.p. elevation Via.
Osmotic press. Vis. Osmotic press. Vis. a n d f.p. depression Vis. Osmotic press. Osmotic press. a n d vis. Osmotic press. a n d vis. Vi*. O&otic press. and vis. Vis. Vis. Vis. Osmotic press. and vis Osmotic press.
!E% (1570)
YiS.
Polymethacrylic acid Polymethyl acrylate Polymethacrylate Polybutyl methacrylate Polyvinylpyrrolidone
Vis. Osmotic press. and vis. Vis. Oamotia press. and vis. Osmotic press. and light spattering Vis. and ultracentrifuge Vis.
Cellulose triesters Vis. Cellulose acetate Vis. Cellulose Osmotic press. and Bencylcellulose Cellulose triphenylmethtyl ethers Osmotic press. a n d Osmotic press. a n d Methylethylcellulose Osmotic press. and Nitrocellulose Vis. Wool Amylose acetates Vis. Ultracentrifuge Wool
vis. vis. vis. vis.
Schulz ( l 7 6 G ) made a critical comparison of three absolute methods, osmotic pressure, light scattering, and ultracentrifuge, and one relative method, viscosity, for the measurement of the molecular weight of high polymers. T h e temperature dependence of the intrinsic viscosity of polyvinyl chloride in three solvents was investigated (194G). Molecular Weight Distribution. There is much activity not only in molecular weight and shape estimation but also in molecular weight distribution determination by fractionation (SgG, 2096). Mathematical treatments of polymer fractionation continue t o be presented ( I G , 24G, 107G, lZ6G). New methods for fractionating polymers have been proposed (6SG, l04G, 147G, I70G, 189G), while Vallet (206G) suggested a method for establishing a relationship between a physical property of a polydisperse high polymer solution and molecular weight without the necessity of making a fractionation. Experimental fractionations of polyisobutylene (296G), polystyrene (IS6G),polyacrylonitrile (78G), polymethyl methacrylate (74G), polyvinylpyrrolidone (91G), cellulose nitrate (ISOG, ISSG), and cellulose acetate ( 165G)have been made. Additional information on particle shape and chain flexibility is given by flow birefringence measurements. The magnitude of the birefringence should be proportional t o the velocity gradient, u p to a point where the latter becomes great enough t o extend the coils or segments of the molecule, thus lengthening i t and increasing the molecular birefringence. These and other considerations were taken up by Joly (94G-96G) and Scheraga, Edsall, and Gadd (173G). Scheraga (171G) also published a graph relating molecular size and extinction angle. A special photoelectric circuit for the measurement of flow birefringence and dichroism has been designed by Zucker, Foster, and Miller (226G), and the first-named authors have applied i t to the study of the amylose-iodine complex (54G) and fractionated amylose itself (66G). Flow birefringence measurements have also been
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reported upon bentonite hydrosols (184G), polyvinyl pyridinium bromide (64G, 164G), n-hexadecyltrimethylammonium bromide ( 172G), sodium thymonucleate ( 178G), detergent micelles (6G), polystyrene (182G), and nitrocellulose (182G) solutions. I n general, the best solvents for polymers are those giving the highest intrinsic viscosities. Treloar (2OlG) considered the application of the statistical theories of solution to the mechanical and swelling properties of nonpolar polymers. Daoust and Rinfret (S6G) examined 16 solvents for polymethyl methacrylate and polyvinyl acetate t o determine the best for use in light scattering studies. A number of liquids have also been examined for their action on polyvinyl chloride, polyvinyl acetate, and polyvinyl acetate chloride (141G). Fair correlation with physical properties of the solvents was obtained. Swelling measurements can be used to calculate mean chain segment lengths in rubber (162G). Edwards and Goldfinger (4SG) obtained a definite increase in the viscosity of polyphenyl under the influence of a magnetic field. Latices. Latices cannot be regarded as true lyophilic colloidx but, dependent upon their stabilizers, do sometimes behave somewhat similarly to emulsoids. Zwicker (217G) described in considerable detail the properties of synthetic latices including stability and viscosity. Murphy (131G) reviewed recent progress in latex technology, both natural and synthetic. Maron and his collaborators have investigated flow equations for latex suspensions (108G), the concentration dependence of flow in GR-S latices ( I I S G ) , and the concentration of such latices by a varieti, of methods (214G).
Soaps, Micellar Colloids, and Solubilization Aqueous solutions of soaps and their physicochemical behavior have been the subject of several excellent surveys (27H, 28H, 49H). The occurrence of micelles in soap solutions is now generally accepted but discussion still continues as to their shape; indeed, much of the book by Moilliet and Collie (55H), “Surface Activity,” is devoted t o this topic. Brady ( 8 H ) , as a result of x-ray scattering measurements on sodium dodecyl sulfate solutions, concluded t h a t a spherical micelle is probable. Nalragaki (57H-5.9H), in a study of surface chemical data, found that similar results are obtained whether a disk or spherical shaped micelle is assumed. On the other hand, Debye and Anacker ( 1 S H ) have shown that in the presence of high salt concentrations the micelles can cause measurable dissymmetry of the scattered light, calculations from which indicate a rodlike shape. Double refraction measurements can throw light on the micellar form of soap solutions as described by Thiele (75H). l’hilippoff (64H) reviewed the situation for soap-water-electrolyte systems above the critical concentration for micelle formation. H e concluded t h a t the micelle consists of 50 to 100 molecules, partly ionized and partly hydrated, arranged in a double layer and so oriented that they give a n x-ray diffraction pattern. Winsor ( 8 S H ) has attempted to explain systematically x-ray diffraction data from a number of sources by assumption of different types of micelles. Brown et al. (QH) suggested a bubble pressure method for determining critical concentrations of micelle formation where other methods are unusable. Calculations of the critical concentration for micelle formation have been made by Oshilra ( 6 2 H ) and compared with the experimental data, Using a special “matched” thermistor technique for measuring vapor pressure lowering, Huff, McBain, and Brady ( S S H ) determined at 30” and 50’ C. the osmotic coefficient for a number of detergent solutions, thus allowing the calculation of their thermodynamic properties. The law of mass action appears to hold not only for single micellar species but also for their distribution (81H). The law also leads to the equation that the logarithm of the critical concentration for micelle formation decreases linearly with the logarithm of the total concentration of oppositely rharged ions (8bH).
May 1953
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I N D U S T R I A L AN'D E N G I N E E R I N G C H E M I S T R Y
Got6 and his collaborators (20H-26H) used the classical methods of this field t o make a thorough investigation into the physicochemical properties of polyoxyethylene glycol alkyl ethers. Work on alkylbenzenesulfuric acid derivatives ( 8 5 H ) and magnesium n-alkane sulfonates ( 5 0 H ) is also reported. Cook (11H, 1 2 H ) worked on the hydrolysis behavior of aqueous soap and sodium lauryl sulfate solutions. The changesin electrical conductivity with time of sodium oleate solutions in water, with and without benzene present, have been measured under controlled conditions, and the results are interpreted as showing acid soap formation and adsorption of oleic acid to the interior surface of the micelle (44H). Electrical conductivity and solubilization methods were used by Ekwall ( 1 8 H ) to folIow micelle formation in sodium cholate solutions. Sodium pinabietate was also found to act a s a soaplike material, behaving in dilute solutions as a univalent electrolyte and forming micelles in 0.02 M solution (29H). Materials of this type are important in paper sizing, and Back and Steenberg ( d H , S H ) considered their properties from this viewpoint. The coagulation of rosin sols by electrolytes has been investigated by Mitra (5SG,54H). The requirements for a successful detergent is the subject of a thought-provoking article by Lawrence (48H), while the American Society for Testing Materials ( I H ) considered the evaluation of soaps and detergents. Synthetic detergents sometimes can give trouble from the standpoint of sewage treatment; a comprehensive paper ( 7 0 H ) deals with this topic. Cationic surface active agents and their applications have been described ( 1 0 H ) as have the properties of a new class of surface active agents (S1H). The study of detergency by the use of radioactive tracer techniques was discussed in the section on experimental methods ( B ) . Space does not permit consideration of the many technical papers published. Solubilization Studies. Solubilization, or the ability of soaplike compounds t o dissolve organic materials insoluble, or only sparingly soluble, in water, is another active field for research with much industrial importance. Klevens (105G) reviewed experimental results up t o 1950. Many dyes are solubilized in the presence of soaps; Yurzhenko and Kucher ( 8 6 H ) found that the solubility of Sudan I11 in aqueous alkylbenzene sulfonic acid derivatives was greater the larger the micellm weight, in agreement with similar observations on other materials. Ross ( 7 1 H ) found that the solubilization of dyestuffs was less dependent upon their nature than upon the behavior of the surface active agent. Kolthoff and his collaborators continued work on solubilization. It was suggested ( Q S H ) that solubilization increases with micelle size, and that polar compounds are solubilized at a different locus from that of nonpolar. I n later papers (46H, 47H), a new type of solubilization, t h a t of a n inorganic salt of a detergent in solutions of freely soluble salts of the detergent, is recorded. Jackson and Strauss ( 3 7 H ) determined solubilization values for iso-octane and dodecane in aqueous solutions of a polysoap prepared by partial quaternization of poly-2-vinyl pyridine with dodecyl bromide. The reverse effect, solubilization of water in nonpolar solvents, has been studied by Palit and Venkateswarlu (6SH), using fatty acid salts of longchain aliphatic primary amines. Under favorable conditions nearly 30 moles of mater per mole of soap can be solubilized. Soap Coacervates. The viscosity of the water-sodium oleatealcohol system has been measured by Rose ( 7 H ) for alcohols from propyl to amyl, with and without the prefience of sodium chloride. The results are explainable if the alcohol molecules orient themselves between the oleate ions with their polar heads pointed toward the water. D e Jong and his coworkers continued to investigate the elastoviscous properties of soap systems, recent papers dealing with systems containing potassium chloride (4OH, QIH), carbonate (S8H), or organic substances ( 4 2 H ) and experimental details of equipment radius, slippage, etc. ( S Q H ) . Soap coacervates also continue to be investigated ( 5 H , 6 H , 43H); they are particularly interesting because they appear t o simulate
923
biological structures. Coacervates have also been observed in a study of phase equilibria in cellulose acetate solutions (16H), in polyvinyl acetate solubility studies (84H), and tobacco mosaic virus and bentonite solutions (S6H). The general theory is t h a t coacervates consist of aggregates with large quantities of solvent trapped in the chains, which, because of the poor solvent, are strongly coiled up. Evidence is given to support this view ( 4 H ) . Soap Hydrates. I n a continuation of work on soap hydrates (62H), dehydration isobars have been determined and indicate the existence of calcium-palmitate monohydrate. or-Sodium palmitate and stearate exist as hemihydrates. Anhydrous sodium stearate has been prepared (74H). Some of the heavy metal soaps are of particular interest because of their gelling properties in organic solvents. Infrared absorption spectrum studies indicate that the aluminum soaps do not exist as the trisoaps but that the mono- and disoaps are actual chemical compounds (27H). Nelson and Pink ( 6 1 H ) believe ferric laurate t o be the neutral trisoap, dispersing in toluene as single molecules or at most dimers. Other Investigations. The heat of wetting of sodium stearate and butyrate in a number of hydrocarbons is said to indicate formation of a chemical compound with one molecule of hydrocarbon (14H). Micelle formation occurs in hydrocarbon solutions of soaps; depolarization of the fluorescence emitted from a dye absorbed by the micelle has been used b y Singleterry and Weinberger ( 7 S H ) as a method for estimation of micelle size. Comparison with osmotic pressure measurements on similar solutions indicated the method t o be satisfactory as long as the product of the gram micellar volume by the solvent viscosity was not too great. Pink and his coworkers (60H, 7 8 H ) find t h a t peptizing agents for metallic soaps in hydrocarbons decrease micellar size. The most effective peptizers are those with powerful coordinating groups; the fatty acids themselves were found to be ineffective. Dielectric constant and conductivity curves have been used to follow the transition from the liquid crystal phase to a solid crystal network in soap-cetane systems, a change influenced by water (16H). Inoue and Iida (SCH, S 5 H ) explained the viscous and dielectric properties of aluminum stearates in benzene and toluene by the formation of association micelles through hydrogen-bonding. Flow properties of aluminum naphthenate in decahydronaphthalene (76H, 7 7 H ) are found to be those of a Bingham body over part of the range of shearing stresses, although some flow always occurs even a t the smallest shearing stress. The syneresis of soap gels in various organic solvents has been investigated by several workers (30H, 65H-69H, 72H). There appear to be two processes occurring, exudation from within interfibrillar spaces followed by bound liquid exudation. No syneresis takes place at the gelation temperature. The fatty acid soaps are important in grease formulation and several studies of a more or less fundamental nature directed t o this purpose may be mentioned. Mardles and Puddington (51H ) considered that the characteristic properties of lubricating greases are traceable to high yield values, 10,000 dynes per square cm. or greater, produced by aggregates of highly anisometric particles. The rheological properties of greases indicate complex nowNewtonian behavior (56H, 79H, 80H), thixotropy being observed. Evans, Hutton, and Matthews ( 1 9 H )used a differential calorimeter to determine the phase transformations occurring in lithium soap greases, the fiber structure of which has also been investigated using the electron microscope (32H).
Polyelectrolytes Polyelectrolytes continue to attract much attention (61,82), partly because many of them appear to simulate biological structures. Kuhn and Hargitay (261), for example, suggested the importance of the polymer of acrylic acid cross linked with polyvinyl alcohol as a model for muscle myosin. When under a load,
924
INDUSTRIAL AND ENGINEERING CHEMISTRY
such a polymer expands and contracts reversibly according to whether the solution is alkaline or acid. Macromolecules, which are freely flexible along their chain, normally take up a random configuration corresponding to maximum entropy. If, however, they become ionized, owing to suitable ionizing groups distributed along the chain, the mutual repulsion of the charges causes the particle to take up a more extended form with a resultant great increase in specific viscosity. Hill (151)considered such changes. The titration behavior of a polyelectrolyte acid can therefore be expected t o differ from that of a normal electrolyte b]. the work expended in removal of the hydrogen ion from the field of the ionized groups and also by work done by electrostatic forces as the polymer molecule is extended. Calculations on such effects were reported by Kataoka (191))6sawa and Imai (RW), Katchalsky et al (201, 211), Kuhn (241), and Kimball et nl. (231). Suzuki ( 4 1 2 ) calculated the viscosity-concentration relation for linear polyelectrolyte solutions by application of the Debye-Huckel theorv. The counterion distribution in solutions of rod-shaped polyelectrolytes has been treated mathematically (91,131). Katchalsky, Lifson, and Eisenberg (221) extended the mathematical theory to the swelling of polyelectrolyte gels. A test of Smoluchowski’s electrical theory of viscosity has been made by Dobry (71), who found certain anomalies with silicon carbide suspensions. However, the viscosities of dilute solutions of polyacrylic acid in presence of added salts appear to be in agreement a t low concentrations (261). At higher concentrations of added salt the folding chain theory indicated above appears to be operative. As would be expected, solutions of polyelectrolytes in which the molecule is fully extended are very shear-dependent (11). Strauss and Fuoss (401) found that the apparent viscosities of aqueous solutions of poI3‘-4-vin3‘1-A--buty1pyridinium bromide are linear in average rate of shear in the capillary viscometer. The theory of light scattering bv solutions of polyelectrol\-tes has heen worked out hy Dotv and Steiner (SI),who have continued their work and extended it to include solutions of such materials ar charged proteins and polymeric electrolytes (101). In bovine serum albumin the effective diameter of the macroion appears inversely proportional to the cube root of the concentration. I n a further paper, 0 t h and Doty (281) studied the light scatterine;, viscosity, potentiometric titration, and electrical conductivity of two fractions of polymethacrylic acid as a function of neutralization and concentration. The root-mean-square separation of the ends, a measure of the extension of the chain, increased from about 170 A. in the unionized state to 1180 A. when degree of ionization was about 70%. Natural Polymers. 9 number of natural polymers are no\$ recognized as polyelectrolytes. Basu, DasGupta, and Sircar (51) discussed the properties of gum arabic as a polyelectrolvte. It appears to have an unusually high degree of chain branching. Schmidt (351) described proteins as heteropolar, high molecular weight, partially esterified galacturonic acids and determined their viscosity under a number of conditions. Pals and Hermans (991-311), in a series of papers, have measured the viscosities, osmotic pressures, and dissociation constants ( b y potentiometric titration) of the sodium salts of pectin and carbox]-methylcellulose. Some disagreement was observed between the dissociation constants observed and those calculated from poll-mer dimensions and charge, as estimated from the osmotic pressure and viscosity measurements. Sodium carboxymethylcellulose was also investigated n ith respect to viscometric behavior by Basu and DasGupta (411, who stated that the folding chain theory of polpelectrolvte~e ~ p l a i n s all the results. Sodium thymonuclente x a s reported to behave as a typical polyelectrolyte in aqueous solution (31). Sodium alginate has been the subject of viscofiity determinations by Harkness and Wassermann (141), who found that the alginate chains are not fully extended. Several Japanese chemists also reported much work on properties of this natural polyelectrolyte,
Vol. 45, No. 5
including osmotic pressure ( 1 1 I ) , specific volume (.?SI), hcat of swelling ( S 6 1 ) , electrical conductivity (371-391, @I, viscosity (111), and rate of hydrolysis (441, 461). Sodium celluloseglycolates have also been examined as polyelectrolytes using light scattering (161)?specific viscosity (161), electrical conductivity (161, 1 7 I ) , and osmot’icpressure (181) methods. Synthetic Materials. Studies on eynthetic materials include dye acids (461)) copolymers of maleic acid (121), polyvinylpyridinium bromide (321), and an amphoteric copolymer of vinylpyridine and acrylic acid (471). It is also of interest to note that polymers can show polyelectrolyte behavior in nonaqueous solution as evidenced by rvork of Schaefgen and Trivisonno (331, 341) on the behavior of polyamides dissolvcd in anhydrous formic and sulfuric acids.
Bib I iog ra phy General
Alexander, A . E . , “Surface Chemistry. An Introduction to Its Principles and Application S e w York, Longmans, Green & Co., 1951.
Alexander, A. E., and .Johnsol>, E’., “Colloid Science,” Vols. I and 11,England, Oxford University Press, 1949. Butler, J. A. V., ed., “Electrical Phenomena at Interfaces in Chemistry, Physics and Biology,” London, Nethuen Co., 1951.
Gregg, S.J., “The Surface Chemistry of Solids,” Kew York, Reinhold Publishing C o r p , , 1951. Harkins, W. D . , “Physical Chemistry of Surface Films,” ISew York, Reinhold Publishing Corp., 1952. Harris, I3. L., ISD.ENG.CHEM.,45,24-31 (1953). Kruyt, H. R . , ed., “Colloid Science. Irreversible Systems,” Vol. I, Houston, Tex., Elsevier Press, 1952. Kunin, R., and XIcGawey, E’. X., IND.ENG.CHEN.,45, 83-8 (1953).
Lawrence, A. 9. C . , and Mills, 0. S.,Ann. Repts. on, Progr. Chem. (Chem. Soc., London), 4 8 , 7 8 4 6 (1951).
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Algar, Wr.H., and Gierte, H. W., Svensk Pnpperslidn., 54, 693-700 (1951).
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Aniansson, G., J . Phys. Le: Colloid Chem., 55, 1286-99 (1951). Antweiler, H. J., Chem.-Ing.-Tech., 24, 284-8 (1952). Antweiler, H. J., Mikmzhemie, 36/37, 561-73 (1951). iirakawa, M., Arakawa T., and Suito E., B u l l . Inst. C‘hem. Research, Kyoto Univ., 2 9 , 7 8 (1952). Asbeok, W. K . Laiderman, D. D., and Van Loo, AI., J . Colloid Sci., 7, 306-15 (1952). Atherton, E., and Peters, R. H., .J, Tertile I n s t . , 43, T 1TBX37 (1952). Bagno, O . , Longuet-Escard, J . , and Mathieu-Sicaud, A , , Compt. rend., 232, 1350 (1951). Baldwin, R. L., Laughton, P. AI,,and Alberty, R. A., KolloidZ . , 119, 50 (1950); 6.Phys. ck Colloid Chem., 55, 111-25 (1951). Barer, R., J . R o y . M i c ~ o s c o p .Soc., 71, 307-37 (1952). Barrer, R. M.,llackeneie, N., and RlacLeod, D., J . Chem. Soc., 1952, pp. 1 7 3 6 4 4 . Bawn, C. E. H., Ann. Repts. o n Progr. Chem. (Chein. SOC. London), 47,88-91 (1950). Bender, M., J . Chem. Educ., 29, 15-23 (1952). Bennett, ii. H., et al., “Phase Microscopy,” Kew l o r k , John Wilev & Sons. 1952. Bernal, J. D., and Carlisle, C . H., Discussions Faraday Soc., 11, 227-9 (1951). Bernard, R.. Davoine. F.. and Hirtz. J., Compt. rend., 232, 1826-8 (1951). Bernard, R., Pertioux, E., arid Teichner, S.,J . chim. phys., 49, 147-56 (1952). Berry, K. L., and Peterson, J. H., J . Am. Chem. Soc., 73, 5195-7 (1951). Blois, &I. S., Jr., Science, 114, 175-7 (1951). Boselli, A., L a Ceramica, 7, N o . 4,59-60 (1952). Bradford, E. B., J . Appl. Phys.. 23,609-12 (1952).
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May 1953
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(1D) Balduin, H., and Wieden, P., I
