1L.I ~
jf
'nterface helps in the understandiry:
the properties of detergenh
depend primarily on adsorpInterfacial tion ' and phenomena on the composition and properties of the adsorbed films. Interfacial properties are more easily measured on liquid-gas and liquid-liquid interfaces which usually have more homogeneous surface structures than solid surfaces. This discussion deals first with some aspects of the influence of molecular structure and composition of aqueous solutions of sodium alcohol sulfates and sulfonates on the surface and interfacial tension and on minima in the concentration curves. Some examples are cited of selective adsorption at the liquid-air interface. Then, several studies are discussed on the influence of molecular structure, composition, and temperature of aqueous solutions of surface active agents, as well a8 the role of both bulk and surface viscosity, on the flow properties of films at the liquid-air interface. This paper reviews and interprets the principal aspects of these
Surface and Interfacial Tension Method of measurement of surface tension. In the studies on sodium (4,76,25), a modification of the Du Nouy ring method was used to determine the surface and interfacial tension. Scale readings were converted to surface and interfacial tension from a calibration with liquids of known surface tension (73). This method of calibration avoids the necessity of using the Harkins and Jordan correction factors (9). Special care was taken to avoid accidental contamination, both in the preparation of the solutions and in the measurements. I n all cases, the platinum-iridium ring must be completely wetted by the solution. Details of a suitable procedure (76)include the use of a 1000-ml Erlenmeyer flask which is paraffined inside and out around the neck. I n this way a relatively large surface is employed for the measurements. Surface and interfacial tension of aqueous solutions of sodium alcohol sulfates. It is now well known that the study of the relation of interfacial properties to molecular structures and compositions often necessitates the use of well-defined and pure compounds. .This requirement is particularly important when the composition at an interface is influenced by small amounts of impurities which are selectively adsorbed. VOL 60
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A systematic study of the relation of surface-active properties to molecular structures was made of a series of sodium alcohol sulfates (4, 14, 76, 25). These compounds represent alternate members of three sets of homologous sodium alcohol sulfates and one set of isomeric pentadecanol sulfates. These sulfates do not hydrolyze as salts, and ester hydrolysis does not proceed to an appreciable extent unless the solution is strongly acidic. The series of sodium alcohol sulfates will be abbreviated by a notation where the first number refers to the number of carbon atoms, and the second number to the position of the sulfate group. The surface tension of aqueous solutions of the sodium alcohol sulfates is lower for longer chain length in the symmetrical series (11-6, 13-7, 15-8, 17-9, 19-10) as well as when the sulfate group approaches the symmetrical position for the pentadecanol series (15-2, 15-4, 15-6, 15-8). The ratios of the concentrations of successive members of the sodium salts of the alcohol sulfates required to show the same surface tension are fairly constant for the secondary alcohol sulfates. These ratios have been interpreted as due to an increase in the work done when a molecule passes from the interior of the solution to the surface layer for each additional CH2 group. This behavior, which is referred to as Traube’s rule, does not apply to the salts of the primary alcohol sulfates. The interfacial tension against benzene is progressively lower as the chain length increases. The break in the curve, which corresponds to the critical micelle concentration (cmc), is much higher for 15-8 (3.5 X 10-3M) than for 15-2 where the curve breaks at 0.6 X 10-3M. At higher concentrations, the interfacial tension is lower in the symmetrical series. These values are similar to the corresponding changes in surface tension. Minima in surface tension-concentration curves.
Surface tension-concentration curves often show minima. Various explanations had been offered for such curves including the effect of additive on the principal constituent, uncertainties in the methods of measurement, changes in activity coefficients, and doubts of the validity of the Gibbs adsorption equation. For one surfaceactive solute this may be expressed as in the following equation :
r = - -R1T
(du)
(dlna)
where r = surface excess of solute, and u = surface tension. Minima in surface tension-concentration curves were first shown by Miles and Shedlovsky (76) to be due to the presence of certain impurities in the principal surfaceactive agents. For example this effect appears for solutions of sodium dodecyl sulfate with as little as 0.00570 48
I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
of dodecanol or O . l O ~ o sodium hexadecyl sulfate. I n such cases, the lowering in surface tension can be from 3 to 15 dynes per cm. This work has now been frequently confirmed and extended, so that the occurrence of a minimum in the static surface tension-concentration curve is taken as evidence of the presence of one or more impurities. The absence of a minimum is now commonly used to indicate the absence of a surfaceactive constituent which is selectively adsorbed. The curves are interpreted in terms of the composition of the adsorbed layer. A minimum in the surface tension-concentration curve is attributed to the presence of two surface-active materials. At the minimum, the relative surface concentration has the largest amount of the constituent which is selectively adsorbed. This is in accord with the interpretation of anomalies in surface tension for mixed surface-active agents as special cases of salt effects. The surface tension-concentration and the interfacial tension-concentration curves (water/benzene) do not show minima for purified sodium dodecyl sulfonate (27). Pronounced minima in surface tension-concentration curves have been shown by adding dodecanol or sodium hexadecyl sulfate to solutions of sodium dodecyl sulfonate. These minima can be eliminated by selective adsorption of the additive by foam extraction or emulsion extraction at concentrations of the sulfonate near the minimum in the corresponding surface tension or interfacial tension curves. Minima in surface tension-concentration curves are obtained when the solution contains a small amount of a higher homolog or a surface-active, long-chain alcohol, or acid, which is selectively adsorbed (25, 27). Minima in interfacial tension-concentration curves are obtained only when a higher homolog is present. When a higher homolog is present in small amount, the minimum in surface tension is considered as a salt effect of the lower homolog on the selective adsorption of the higher homolog. Some theoretical aspects of this work have been discussed by Moilliet, Collie, and Black (78), Reichenberg (23), and Hutchinson (70). It is a common experience to find that the surface tension of solutions, which have minima in the concentration curves, reachcs the equilibrium surface tension slowly. On the other hand, pure sodium l-dodecyl sulfate rapidly reaches an equilibrium value in surface tension. Sutherland (30) discussed some of these changes in surface and interfacial tension of solutions with time. Since the constituent, which is selectively adsorbed, is present in very small amounts in the solution, its accumulation at the interface would be a slow process if it depended on its diffusion through the solution.
Figure I. Film-drainage apparatus
Surface tension of micellar solutions of sodium ndodecyl sulfate. Recently the surface tension curve of solutions of sodium dodecyl sulfate has been extended to much higher concentrations in the micellar region ( 5 ) . Impurities in the sodium n-dodecyl sulfate solutions were removed by repeated foam fractionations before each measurement. The concentration of the solutions was determined from electrolytic conductivity, and the surface tension was measured by a modified Wilhelmy plate method. A significant decrease of the surface tension above the cmc showed that the activity of the sodium n-dodecyl sulfate increases in this range. This limits the validity of the approximation represented by the phase separation theory of micelle formation. A calculation which depends on mass action accounts for the surface tension data. The surface tension decreases as follows: 0.0085M, 39.5 dynes per cm; 0.02M, 38.7 dynes per cm; and 0.05M, 37.5 dynes per cm. Selective adsorption at liquid-gas interface. Adsorption from solution corresponds to a higher proportion of solute to solvent in the interface than in the bulk solution. When there is more than one surface-active solute, the adsorption can be very selective, that is to say, the relative concentration of the solutes can be very different at the interface and in the solution (26). Since the properties of interfaces depend on their
composition, many of the interfacial properties are influenced by selective adsorption. consequently, a distinction should be made between pure surface-active agents and multicomponent compositions. For solutions containing sodium n-dodecyl sulfate and 1-dodecanol, the selectivity for adsorption of 1dodecanol at the liquid-air interface is shown by foam fractionation to be 30 near the cmc (37), and 285 at lower concentrations of the solution (29). The selectivity indicates the ratio of the proportion of 1-dodecanol to sodium n-dodecyl sulfate at the interface and in the solution. Suitable conditions for purification of sodium n-dodecyl sulfate have been shown by following changes in surface tension after foam fractionation (74). Concentrations of the dodecyl sulfate above the cmc do not show any fractionation because the dodecanol is solubilized in the micellar solution from which it is not selectively adsorbed. O n the other hand, at very low concentrations of the dodecyl sulfate where selectivity is high, the foam is not sufficiently persistent to take advantage of the large surface areas for effective removal of the dodecanol by selective adsorption. Other multicomponent compositions such as mixtures of homologs of sodium alkyl sulfates or partially hydrolyzed soaps show similar changes after foam fractionation. I n these cases, the longer chain alkyl sulfate or the hydrolysis products of the soap solution are selectively adsorbed. O n progressive foaming, the p H of the residual soap solution is increased. Soap solutions have minima in their surface tension-concentration curves which are attributed to hydrolysis (27, 22, 25). The surface tension is very dependent on the p H of the solution. For example, 0.1% potassium laurate solution at a p H of 7.8 has a surface tension of 23 dynes per cm. If the p H of this solution is raised to 9.5, the surface tension rises to 53 dynes per cm. This large change is due to the decrease in adsorption of hydrolysis products and to a corresponding change in the composition of the adsorbed layer.
Flow Properties of Films at liquid-Air Interface Foam properties are influenced, at first by the extent of adsorption from solution at liquid-air interfaces and by the drainage characteristics of the films, and ultimately upon the composition and properties of the adsorbed films (26). The observations of interference colors in soap films were made by Robert Hooke in 1672. Newton described “black” films of different shades and thicknesses. Plateau in 1869, introduced the concept of surface viscosity to describe the flow behavior of soap films. J. Willard Gibbs discussed his observations on soap films in detail. I n addition, his theoretical conVOL. 6 0
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tributions on adsorption, film elasticity, and flow properties are well known. A. S. C. Lawrence, in 1929, described various phenomena of liquid films including much of his work with Dewar. Mysels and coworkers (20) examined film drainage particularly with regard to the mechanism of the flow. The drainage rates of foams and films are a function of both bulk and surface viscosity (75, 77). The influence of structural characteristics of combinations of detergents and organic polar compounds on the drainage rates of films has been reported (75). Film drainage. One form of apparatus used for the measurement of the rates of film drainage is shown in Figure 1 (75). Observations of interference bands were made visually with the aid of a cathetometer and also from photographs taken at various time intervals. Another, simple, and very useful apparatus is shown in Figure 2. It consists of a rectangular frame made from a bent glass rod sealed in a test tube. The film is formed with a little more than half a tube full of solution. The tube is tilted several times to a horizontal position and back to vertical. A fluorescent bulb is a convenient source of illumination. The drainage is shown by the movement of interference bands which are due to the thinning of the film. The rate of drainage is either very fast or very slow. I n fast draining films, at the side of the frame and near the liquid interface, there are regions where colored portions swirl upward and toward the center of the film, which shows orderly progression of horizontal colored bands. The swirling is attributed to the effects of gravity and capillary suction (20). Thinner and lighter films move up replacing thicker and heavier portions, and this forms horizontal layers of colors. Because of suction at the curved meniscus in the Plateau border, there is an excess of liquid flow to the solution level. Suction is greater in thick films, due to a greater curvature and pressure drop at the surface than in thinner films. Slow-draining films when freshly formed are too thick to show bright colors. They drain about 350 times slower than mobile, fast-draining films. The flow is mostly gravitational and similar to that of liquid flow between parallel plates. This takes place when the surface viscosity is high and the surfaces of the film are nearly immobile. Both fast- and slow-draining films ultimately form very thin “black” films (50-10OA) which do not break spontaneously. However, they may break as a result of thinning by evaporation, local heating, and mechanical shock. For fast-draining films, the rate for the first-order extinction band is 3.6 cm/min and for the second order 6.6 cm/min. For slow-draining films, the first-order extinction band travels at about 0.01 cm/min (75). 50
INDUSTRIAL A N D ENGINEERING CHEMISTRY
^.
.
Fiprc 2. Film-drainage tub8
Influence of composition on film drainage. Film drainage rates are affected by the structural characteristics of the surface-active agents (75). Among anionic detergents, the sulfated and sulfonated derivatives form fast-draining films even at low temperatures. Slow drainage depends both on structure of the detergent and additive. Some detergents to which certain types of organic polar compounds are added will form slow draining films. These detergents generally contain a normal saturated hydrocarbon chain with a hydrophilic group in the terminal or beta position. Some examples (A) are sodium salts of saturated fatty acids and sodium paraffin sulfates which are primary or beta types with unbranched chains. The additives which tend to give slow-draining films with t y p e 4 detergent have essentially similar hydrophobic structural characteristics as the above detergents. However, they are relatively insoluble in water, and in the presence of detergent, they form films which have a high surface viscosity. Some examples (B) are saturated fatty acids, saturated N-primary alcohols (Clz and higher). Addition of elaidyl alcohol gives slowdraining films, but the addition of oleyl alcohol, the cis-isomer, does not form slow-draining films. On the other hand, there are some types of detergents (C) which give fastdraining films when type-B additives are present. In these cases the hydrophobic part of the detergent may have a highly branched chain,
0.05
0.04
U
-i2
e E. c2
m 0
5 0.03 -= z =
c
.-cc: 20 0
0
c VI
e
0
p
30
f
e
e
0.02
a
: c .-: e E 10 i=
-0
0.01
0.2
0.4 Na lauryl sulfote g/100 g soh.
0.6
8.0
8.5
9.0 10
9.5
+ Loglo z
Figure 3. Isotherms of film-drainage transition temperature
Figure 4. Film-drainage transition temperature vs. loglo Z
a cyclic structure, or unsaturated groups (cis form). Other examples are sodium alcohol sulfates with the sulfate beyond the beta position and the oleyl derivative of methyl taurine. A variety of additives (D)do not give slow-draining films when added to solutions of detergents ( A ) . These compounds (D)have similar structural characteristics to detergents (C). Saturated and unsaturated hydrocarbons, cyclic alcohols, such as hydroabietic and lanolin alcohols, do not form slow-draining films. Among the unsaturated primary alcohols, addition of oleyl alcohol as noted above does not form slow-draining films. Film-drainage transition temperature. An increase in temperature alters slow drainage of foams or films to fast drainage. This transition is not gradual, but occurs abruptly and reversibly over a very narrow range of temperatures. A detailed examination of films formed from aqueous solutions of varying concentrations of sodium n-dodecyl sulfate or sodium n-tetradecyl sulfate containing 1-dodecanol or 1-tetradecanol shows the relation between film-drainage transition temperature and the composition of the solution (6, 8). Filmdrainage transition temperature can be used for de-
tecting as little as 0.05% of 1-dodecanol in sodium ndodecyl sulfate. A sample free of alcohol will give a fast-draining film even at 0°C. The presence of longchain alcohol will show a transition temperature which increases with the amount of alcohol present. The isotherms in Figure 3, which are drawn from the data, show a sharp break near the cmc (6). This sharp rise in the slope of the curves illustrates that much larger concentrations of 1-dodecanol are necessary to give the same transition temperature above the cmc than in the nonmicellar solutions. The shape of these lines is similar to the isothermals of the solubility of a fatty alcohol in a detergent solution. I n the micellar solution there is a sharp and progressive increase in the amount of 1-dodecanol which is dissolved. Above the cmc, the transition temperature is a function of the composition of the micelles and is independent of their number. Figure 4 represents the data as one curve with transition temperatures against log Z, where Z is the apparent micellar mole fraction of alcohol. Since higher concentrations of sodium n-dodecyl sulfate increase the extent of solubilization and, in turn, decrease the adsorption of l-dodecanol, it is implied that the alcohol is almost entirely in the micellar portion of the solution. As an extension of this work, a subsequent study was made of the effect of sodium chloride on the film-drainage transition temperatures (7). When they were related to the decreased cmc, these results were similar to those obtained without sodium chloride (8). It was suggested that at micellar concentrations, con-
AUTHOR Leo Shedlovsky is a Consulting Chemist and has published numerous papers on surface-active agents, foams, and detergents. His address is 2 1 0 West 16th St., New York,
N. Y.
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stant transition temperatures correspond to an essentially constant composition of the nonmicellar aqueous solution. Among nonionic detergents, many ethylene oxide condensates of alkyl phenols with branched chains do not produce slow-draining films when fatty alcohols or fatty acids are added. When lauryl alcohol is added to polyoxyethylene- (12) and polyoxyethylene- (23)-lauryl alcohol, the transition temperatures of the slow-draining films are similar to the extrapolated values for sodium n-dodecyl sulfate with 1-dodecanol. The values reported for these nonionic detergents were obtained mostly from measurements of surface viscosity ( I ) . The transition temperature for compositions with a fatty alcohol appear to depend only on the mole ratio of the alcohol and nonionic compound. Intermolecular compounds of sodium dodecyl sulfate a n d dodecanol. Two intermolecular compounds or adducts of sodium n-dodecyl sulfate and 1-dodecanol have been isolated as crystalline precipitates and analyzed (6-8). These precipitates have a composition which contains 1 mole of sodium dodecyl sulfate to 1 mole of dodecanol or 2 moles of sodium dodecyl sulfate to 1 mole of dodecanol. Adducts have also been prepared by a dry melt method and from aqueous or aqueous-ethanol solutions of 1-dodecanol and sodium n-dodecyl sulfate with a composition of 2 moles of sodium dodecyl sulfate and 1 mole of dodecanol ( 7 7 ) . This interaction was studied by differential thermal analysis, infrared, and X-ray techniques. Slow-draining films are probably due to a n ordered surface structure with rigid films and high surface viscosity. Furthermore, this property is related to the interaction of detergent and long-chain polar compound. The transition from slow to fast drainage is comparable to a two-dimensional melting process to form fast-draining films. Such interactions will occur as long as the alcohol and sulfate do not have very different chain lengths. From measurements of surface viscosity of these compositions, including low concentrations of sodium n-dodecyl sulfate where persistent films could not be maintained, it was found that the transitions in film and foam drainage correspond within about 0.2OC to transitions in surface viscosity (24a). These sharp and reversible transitions, observed by raising the temperature, are closely related to the transitions which occur upon compressing monomolecular films of insoluble long chain alcohols on water. I n the presence of dissolved 1-dodecanol in solutions of sodium n-dodecyl sulfate, definite transitions in surface tension have been observed at temperatures which agree with the values found from film drainage and surface-viscosity measurements for the same compositions (24b). Determination of composition of thin films. The properties of thin detergent films (50-1 OOOA) have been studied in some detail, particularly by Derjaguin, Overbeek, Sheludko, and others. This deals with the 52
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
thickness of the black film which is governed by the balance between van der Waals attraction and electrostatic repulsion between the surfaces. Many features of these continuing studies have been discussed recently (28). However, most of this work does not include a systematic investigation of both composition and dimensions of the films. Some estimates of the composition of drained thin films have been made by using radioactive materials for some of the film components ( 2 , 3 a , 3 b ) . Film thickness was obtained from film reflection coefficients, and the water content was leduced from infrared absorption. An optical reflectance method for measuring film thickness has been described elsewhere (72). I t is deduced that black detergent films possess a well-defined sandwich structure which consists of two monolayers of detergent enclosing a n aqueous core ( 2 )* For the system sodium n-dodecyl sulfate and l-dodecanol, the composition of the film was in reasonable agreement with that of the adduct ( 3 a , 3 b ) (2 moles of sulfate to 1 mole of alcohol) which had been isolated as a precipitate (8). From the film analyses (34 3 b ) , it is considered that sodium n-dodecyl sulfate and 1-dodecanol form a close packed monolayer which enhances the stability of the film by restricting the evaporation of water. This suggests a n additional factor which contributes to the maintenance of foams made from solutions of high surface viscosity. REFERENCES (1) Becher, P., and Del Vecchio, A. J., J.Phys. Chem., 68,3511 (1764). (2) Clunie, J. S., Corkill, J. M., and Goodman, J. F., Discurrionr Faroday Soc., (42), 34 (1966).
(3a) Corkill, J. M., Goodman, J. F., Haisman, D. R., and Harrold, S. P., Tronr, Faroday Soc., 57,821 (1961). (3h) Corkill, J. M., Goodman, J. F., and Ogden, C. P., ibid., 61, 583 (1765). (4) Dreger, E. E., Keim, G. I., Miles, G. D., Shedlovsky, L., and Ross, J., IND. ENC.CHEM.,36, 610 (1944). (5) Elworthy, P. H., and Mysels, K. J., J. ColloidInlerfac. Sci.,21, 331 (1966). J . ColloidSci., 8, 5 0 (1753). (6) Epstein, M. B., Ross, J., and Jakob, C. W., (7) Epstein, hi.B., Wilson, A., Gershman, J., and Ross, J., J . Phyr. Chem., 60, 1051 (1756). (8) Epstein, M. B., Wilson, A., Jakob, C. W., Conroy, L. E., and Ross, J., ibid., 5 8 , EGO (1954). (9) Harkins, W. D., and Jordan, A. F., J.Amer. Chem. Soc., 52,1751 (1730). (10) Hutchinson, E., J . ColloidSci., 3,413 (1948). (11) Kung, H . C., and Goddard, E. D., J . Phys. Chem., 67, 1965 (1763); 68, 3465 (1964). (12) Lyklema, J., Scholten, P. C., and Mysels, K. J., ibid., 69, 116 (1765). (13) hlacy, R., J . Chem. Ed., 12, 573 (1935). (14) Miles, G. D., J. Phys. Chem., 49, 7 1 (1745). (15) Miles, G. D., Ross, J., and Shedlovsky, L., J. Amer. O i l Chemirlr’ Soc., 27 268 (1950). (16) Miles, G. D., and Shedlovsky, L., J . Phys. Chem., 48,57 (1944). (17) Miles, G. D., Shedlovsky, L., and Ross, J., ibid., 49, 93 (1945). (18) Moilliet, J. L., Collie, B., and Black, W., “Surface Activity,” 2nd ed., Van Nostrand, New York, N. Y., 1761, pp. 79-83. (17) Mysels, K. J., J.Phys. Chem., 68,3441 (1764). (20) hlysels, K. J., Shinoda, K., and Frankel, S., “Soap Films,” Pergamon Press, New York, N. Y.,1759. (21) Powney, J., Trunr. Faraday SOL.,31, 1510 ( 1 9 3 5 ) . (22) Powney, J., and Addison, C. C., ibid., 33, 356, 372 (1737). (23) Reichenberg, D., ibid., 43, 467 (1747). (24a) Ross, J., J . Phys. Chem., 62, 531 (1958). (24b) Ross, J., and Epstein, M . B., ibid., p. 533. (25) Shedlovsky, L., Ann. N . Y . Acad. Sci., 46, 427 (1746). (26) Shedlovsky L “Foams” in “Encyclopedia of Chemical Technology,” Vol. 9, John Wiley aAd Sbns, New York, N.Y., pp. 884-701,1966. (27) Shedlovsky, L., Ross, J., and Jakoh, C. W., J. Colloid Sci.,4, 25 (1749). (28) Sheludko, A., “Colloid Chemistry,” Elsevier, New York, N. Y.,1766. (27) Shinoda, K., and Nakanishi, J., J. Phyr. Chem., 67,2547 (1963). (30) Sutherland, K . L., Aurt. J. Chem., 12, 1 (1958). (31) Wilson, A., Epstein, M. B., and Ross, J., J. CoNoidSci., 12, 345 (1957).
