1OG8
0. LEVINEAND W. A. ZISMAN
Whereas the radius of the pure detergent micelle is calculated to be 24.4 A., the radius of the mixed micelle would be 36.4 A. (if it were a sphere). Hence it is suggested that mixed micelles of sodium dodecane-1-sulfate and n-dodecanol-1, where the additive has undoubtedly been incorporated into the palisade layer, are larger (or less isometric) than the pure detergent micelle.
Vol. G 1
Acknowledgment.-The authors wish to acknowledge the help of Miss June Hughes and Mr. Sherwood Beckley in the experimental phases of this investigation. The foam stability data21 have been furnished by Dr. W. M. Sawyer of these laboratories, to whom the authors express their thanks. (21) W.M.Sawyer and F. M. Fowkes, to be published.
PHYSICAL PROPERTIES OF MONOLAYERS ADSORBED AT THE SOLID-AIR INTERFACE. I. FRICTION AND WETTABILITY OF ALIPHATIC POLAR COMPOUNDS AND EFFECT OF HALOGENATION1 BY 0. LE VI NE^ AND W. A. ZISMAN Georgetown University and U.S. Naval Reskarch Laboratory, Washington, D. C. Received March 8 , 1067
Condensed monolayers of polar paraffinic compounds were adsorbed a t the interface of olished glass and air. These films were prepared by retraction of the glass from solution, by retraction from the melt, a n t b y a vapor condensation method. The state of orientation and molecular packing of each monolayer was readily controlled and measured by the contact angle (e) with methylene iodide. Measurements of the frictional force were made on a loaded stainless steel ball sliding at a uniform speed of 0.01 cm./sec. on the monolayer-covered glass late. A plot of the frictional force us. the load was always a straight line passing through the origin for each monolayer. t h e kinetic coefficient of friction (pk) and the maximum contact angle (Omax) were each plotted against the number of carbon atoms in the principal chain of the compound. Homologous series of fatty acids, alcohols, primary amines, quaternary ammonium halides, perfluoroalkanoic acids, and telomers of tetrafluoroethylene, as well as a number of miscellaneous chlorinated and brominated acids were studied. Graphs of pk vs. N and emaxus. N for each homologous series consisted of two straight lines. The portion of each curve where N is greater than 14 is a horizontal straight line. Both the friction and wettability results were shown to be independent of the method used for isolating the condensed adsorbed films. The asym totic minimum in the pk us. N curve corresponded to an asymptotic = 14 marked the transition point a t 25" in most of the homologow maximum in the Omax us. N curve. The value of series between monolayers which are solid and those which are li uid condensed. Much evidence is given for concluding that the ability of a paraffinic derivative to adsorb as a solid monoqayer is the result of considerable intermolecular cohesive energy between the paraffinic chains; dramatic examples of this effect are made evident by the substitution of fluorine, chlorine and bromine for hydrogen. Large differences are found in with the position of substitution of the halogen atom, and the results are explained by the effect of molecular geometry on intermolecular cohesion in the monolayer. Finally, it is concluded that the transition from a solid to a liquid condensed film takes place a t lower values of N the greater the energy of adhesion of the polar group to the glass substrate.
2
Introduction Since the pioneer work of Hardy and co-workers, a host of investigators have studied the effect of films of polar organic compounds in decreasing the coefficient of friction of rubbing solids. Langmuir4 first demonstrated that a monomolecu1a.r film of fatty acid deposited on glass could decrease the coefficient of friction to only 0.1 and years later6 proved that a multilayer of calcium or barium stearate did not reduce the friction significantly more than did a monolayer; however, the multilayer was much more durable and greatly decreased the rate of wear of the underlying solid. Similar investigations leading to essentially the same conclusions have been reported since by Sameshima and co-workers,6 Bowden and Leben,' Frewing,* Hughes (1) Presented at the 130th Meeting of the American Chemical Society, Division of Colloid Chemistry, Atlantic City, N. J., September. 1956. (2) A portion of a thesis submitted to the Graduate School of Georgetown University in partial fulfillment for the requirements of the Ph.D. in Chemistry. (3) W. B. Hardy, "Collected Scientific Papere," Cambridge Univereity Press, 1936. (4) I. Langmuir, Trane. Faraday Soc., XV,pt. 3,62 (1920). (5) I. Langmuir, J . Franklin Inst., 218, 143 (1934). (6) fa) H. Akamatu and J. Sameshima, BulE. Chem. Soc. Japan, 11, 791 (1936); (b) J. Sameshima, H. Akamatu and T. Isemura, Rev. Phgs. Chem. Japan, 14, 55 (1940). (7) F. P. Bowden and L. Leben, Phil. Trans. Roy. SOC.,8398, 1 (1940).
and Whitti~lgham,~Hutchinson, loa Greenhil1,'Ob and Isemura and Nakagawa." The mono- or multi-layers studied were always limit#ed to only those substances capable of being floated on water as insoluble monolayers and then transferred like a carpet t o the solid surface by the well-known B1odgett technique. I n relating the mechanical properties of these monolayers to those protective films developed from lubricants in practice, there always has been uncertainty about the equivalence of the two kinds of films with respect to molecular packing and composition. From the studies of numerous investigators of the preparation and properties of Langmuir-Blodgett multilayers, it appears that these types of films cannot be produced by any other method; they are artifacts which neither occur in nature nor in technological practice. A new method of investigating the properties of monomolecular films adsorbed on solids has resulted from our many years of resehrch on the (8) J. J. Frewing. Proc. Roy. SOC.(London), Al81, 23 (1942). (9) T. P. Hughes and G . Whittingham, Trans. Faraday Soc., 38, 9 (1942). (10) (a) E. Hutchinson, ibid., 43, 439 (1947); (b) E. Greenhill, ibid., 48, 631 (1949). (11) T.Isemura and R. Nakagawa, Mem. Inst. Sei. I n d . Res. Osaka Uniu., 12, 129 (1955). (12) K. Blodgett, J . A m . Chsn. Soc., S I , 1007 (1935).
August, 1957
PROPERTIES OF MONOLAYERS ADSORBED AT SOLID-AIRINTXRFACE
1069
mechanisms involved in the wetting and spreading the residual film by making comparisons with a of liquids on solids. It is based upon an apparently film prepared by the reliable “retraction” method. We have found that the state of packing of an general technique for adsorbing from solution under equilibrium conditions a monomolecular adsorbed film can be identified and controlled by film of any polar-non-polar organic compound on a the contact angle measured with a drop of some clean, smooth, solid surface and for completely suitable standard liquid. This gives us a convenseparating the resulting film-coated solid from the ient and general method of preparing and studying ~ o l u t i o n . ~This ~ - ~procedure ~ has been referred to adsorbed organic films under conditions of closest as the “oleophobic film,” the “withdrawal” or the molecular packing. Since the contact angle of a “retraction” method. Early limitations of this liquid of high surface tension is sensitive to small method relative to the types of monolayers capable differences in the orientation, packing, and comof being isolated and the types of liquids suitable positioii of the molecules in the surface phase, profor use as the solvent phase have been removed16~16vided the solid substrate is smooth and non~ ~ state , ~ ~ of each organic film studied as we have learned more about the effect of chem- p o r o u ~ ,the ical constitution on wetting. Any polar-non- has been recorded by measuring on each of them polar organic compound may be used as the solute, the contact angle exhibited by methylene iodide and any liquid niay be used as the solvent if it is (surface tension 50.8 dynes/cm. at 20’). This less adsorptive than the polar solute and has a sur- choice of liquid was made because of its large face tension sufficiently greater than the critical molecular diameter, high boiling point (180’), and surlace tension (rc). This method of preparing low viscosity a t ordinary temperatures. The first adsorbed films a t the solid/air interface is simple in two properties are especially important in decreaspractice and leads to highly reproducible films; ing the tendency of the molecules of the liquid drop jt has already proved a valuable way to prepare and from diffusing through the pores which exist bestudy monolayers of a variety of straight-chain, tween molecules of the film, thus minimizing (or branched, and cyclic polar-non-polar structures. eliminating) the formation of mixed films as well as Another effective and reliable method for isolat- the tendency for the advancing and receding coning adsorbed monolayers of paraffinic derivatives tact angles to differ. 15--20 is that of adsorbing the vapor on the smooth solid Polished glass surfaces were used as the adsorbsurface under conditions inviting the formation of ing solid in this investigation since this is a concondensed films. This is done by exposing the venient way to obtain clean hard surfaces which are clean solid to the vapor of the polar compound in a smooth enough to avoid problems caused by the closed glass container placed in a small, constant- well known variation of the apparent contact angle temperature oven whose temperature is adjusted with surface roughness. Values of the contact high enough to obtain an adequate vapor pressure angle (e) of methylene iodide with a variety of orbut low enough to permit the molecules to adsorb ganic surfaces and also the values of e correspondas a close-packed film. After the solid has been ing to the maximum packing of adsorbed films (deallowed to cool to room temperature, any excess noted here as Omax) have been reported in other condensed material above that in the primary publications of our laboratoryL5 -20; when such monolayer readily is removed by vigorously wiping data were not available for the films to be disthe surface with clean, grease-free, tissue paper or cussed, measurements of O us. the concentration of absorbent cotton. Experiments reported here on the polar solute were made to ottain Onlax. In the preparation of films by both the “retraction” short, we have used emaxas a convenient parameter and vapor-condensation methods using many dif- to characterize the state of each condensed film jmferent compounds did not reveal any significant mediately before measuring the coefficient of fricdifferences in the frictional and wetting properties. tion. Other experiments on the durability of these Experimental Procedures monolayers under repeated sliding and high load The methylene iodide used in the wettability measureconditions (to be reported shortly in Part 11) aleo ments was the Eastman “White Label” grade. This liquid show that they possess identical properties. The and all the other organic solvents used were freed from surlatter measurements reveal, too, that these mono- face-active impurities by percolation through adsorption layers are so durable that only a prolonged wiping columns packed with “Florisil” and activated alumina. Each monolayer studied was deposited on a freshly cleaned action with much pressure on a grease-free tissue slide of soft flint, soda-lime glass (Fisher “Non-Corrosive” paper can damage the primary monolayer. This brand). This glass has a hardness number of 450 by the method of preparing the film is not new it1 prin- Knoop method using a 1 kg. load. Profilometer measureciple, since many early investigators deposited ments with a type PAC Profilorneter (manufactured by Research Co.) indicated a surface roughness of films on clean solids by either condensing the Physicists 0.5 microinches (root mean square). Contact angles weie vapor or smearing the molten compound over the measured by the sessile drop method using a small telescope surface and then wiping off any excess with grease- with a goniometer eyepiece.13 The precision of measurefree cotton or cloth. However, we have been able ments was f 2 ’ for small contact angles and =!=lofor conto understand and control this process better since (17) H. R. Baker, E. G. Sliafrin and W. A. Zisnlan, Tiits JOURNAL, we can now verify the monomolecular nature of 56. 405 (1952). (la)
W. C. Bigelow, D. L. Pickett and W. A. Zisman, J. Colloid Sci., 1, 513 (1946). (14) W.C.Bigelow, E. Glass and W. A. Zisman, ibid., a, 563 (1947). (15) H.W.Fox, E. F. Hare and W. A. Zisman, ibid., 8, 194 (1953). (16) W.A. Zisman, “The Relation of Chemical Constitution to the Wetting and Spreading of Liquids on Solids,” to be published.
(18) E. F. Hare and W. A. Zisman, ibzd., 59, 335 (1955). (10) H. W. Fox, E. F. Hare and W. A. Zisman, abzd., 59, 1097 (1955). (20) E.G.Shafrin and W. A. Zisman, “Hydrophobic Monolayers nnd Their Adsorption from Aqueous Solutions” in “Monomolecular Layers,” edited by H. Sobotka, pnbl. b y Amer. Assoo. Adv. Sci., Washington, D. C., 1954.
1070
0. LEVINEAND W. A. ZISMAN
Vol. 61
TABLE I WETTABILITY AND FRICTION DATAFOR MONOLAYERS OF FATTY AMINESAND THEIRDERIVATIVES (Data at 25" unless otherwise stated) 8mx
Method of prepn. of film
(degrees)
Retraction from aqueous soln.; concn. S 1 X lo-' moles/].; acidic pH Retraction from aqueous soln.; concn. S 1 X 10-4moles/l.; acidic pH Retraction from aqueous soln.; concn. S 1 X loW4 moles/l.; natural pH Retraction from aqueous soln.; concn. E 1 X lo-' moles/l.; natural pH Retraction from aqueous soln.; concn. S 5 X lo-' moles/l.; natural pH Retraction from melt a t 70" 1, vapor phase adsorption a t 85" 2, retraction from n-cetane soln.; concn. E 3 X 10-4 moles/l. 3, retraction from nitromethane soln. (satd.) 4, retraction from melt at 70" Retraction from melt at 55" Retraction from melt at 50" Retraction from melt at 45" Retraction from melt at 40" Retraction from melt at 35" Retraction from melt at 30" Retraction from melt at 30" Retraction from aqueous soln.; concn. S 1 X moles/l.; natural p H Retraction from aqueous soln.; concn. G 1 X moles/l.; natural pH Retraction from aqueous soln.; concn. S 5 X moles/l.: . . natural DH a Test drop consisted of a dilute solution of the bulk film material in methylene iodide.
Mk
69 69 69 69 64" 69 69 70 69 69 68-69 68 65" 62" 58" 53" 66 59 59 58
0.06 .06 .04 .06 .07
.04 .04 .05 .05 .05 .06 .04 .06 .06 * 09 .10 .05 0.05 0.06 0.06-0.07
TABLE I1 WETTABILITY AND FRICTION DATAFOR MONOLAYERS OF FATTY ACIDS (Data a t 25" unless otherwise stated) Compound
n-CzaH5iCOOH n-CzsH,iCOOH n-CziH43COOH n-ClgHa,COOH n-CigH3gCOOH n-CnH3sCOOH n-Ci7HasCOOH n-CisH31COOH n-CisHzTCOOH n-CiaHz7COOH TL-C~ZH~~COOH n-CizHzsCOOH n-CiiHz3COOH n-CgHiDCOOH n-CrHisCOOH a Test drop consisted of
Method of prepn. of film
1, vapor phase adsorption a t 125" 2, retraction from n-cetane soln. (satd.)
Omax
(degrees)
71 69 71 Retraction from n-cetane soh. (satd.) 1, vapor phase adsorption a t 100" 71 2, retraction from n-cetane soh. (satd.) 71 70 1, vapor phsse adsorption a t 100" 2, retraction from nitromet,hane soln. (satd.) 70 ' 70 Vapor phase adsorption a t 100" 70 1, vapor phase adsorption at 100" 2, retraction from nitromethane soln. (satd.) 69 67 1, vapor phase adsorption a t 50" 2, retraction from nitromethane soln. (saki.) 66 62-63" Retraction from melt at 50" 58-59" Retraction from melt at 40' 57" Retraction from melt at 25" a dilute solution of the bulk film material in methylene iodide.
Pk
0.04 0.06 0.05 0.05 .05 .06; 0.05 .05 .05 .05 .05 .06 .06 . l o ; 0.11 .13; 0 . 1 4 .17
tact angles of 30" or more; this was always adequate for machine was built at our laboratory from blueprints kindly our purposes. When the adsorbed film on the glass sub- made available by Arthur E. Underwood of the General strate was significantly soluble in the sessile dro of methyl- Motors Research Laboratory. All friction measurements ene iodide, film depletion was avoided during t i e measure- reported were made a t 25" and between 30 and 50y0 R. H. on boundary friction and adhesion, seri~ ~experiments ~~~~ ment of the contact angle as in previous i n v e s t i g a t i o n ~ ~ 6 ~ ~In hy using a drop of a dilute solution of the polar compound ous problems arising from the contamination of the rubbing surfaces are always encountered. Such difficulties were in methylene iodide. Measurements werc made of the force of friction ( F ) be- avoided by taking extraordinary care in cleaning the steel tween a '/z" diameter stainless steel ball sliding a t the ball and glass plate and in maintaining them so during s eed of 0.01 cm./sec. while pressing against the coated glass each measurement of friction and wettability. Each glass pyate under a total load ( W ) . The ball was made of 440 C slide was scrubbed just before use with a degreased, soft camel's hair brush and an aqueous solution of the deterstainless steel, had a Knoop hardness number of ap roxi inately 800, and was obtained from the Strom Steer Ball gent "Tide." It was then soaked in a concentrated nitricCompany. The value of W could be varied from a very few sulfuric acid solution for a few hours, washed with hot greasegrams up to 9000 grams. The ratio F/W was constant free tap water, then washed with hot distilled water, and and as usual is denoted here as the kinetic coefficient of fric- finally dried at 120" in an electric oven lined with greasetion (pk) for the sliding velocity used. Friction measure- free stainless steel. Just before the friction measurements ments were made with a Bowden-Leben machine as modi- the steel ball was degreased in a Soxhlet extractor using hot fied recently by Goodzeit, Hunnicutt and Roach.22 This ACS grade benzene. I n all of these experiments the force of friction a t a given load was observed during a single traverse of the clean steel (21) E. G. Shafrin and W. A. Zisman, J . Colloid Sci., 7,1613 (1952). ball over the freshly coated glass plate. When that coating (22) C. L. Goodzeit, R. P. Hunnicutt and A. E. Roach, l r a n s . A . S . d f . E . , 7 8 , 1669 (1956). was a condenAed monolayer of any of the compounds re-
1071
PROPERTIES OF MONOLAYERS ADSORBED AT SOLID-AIR INTERFACE
August, 1957
TABLE I11 WETTABILITY A N D FRICTION DATAFOR MONOLAYERS OF FATTY ALCOHOLS (Data at 25" unless otherwise stated) Method of prepn. of film
Compound
w
Bmsr (degrees)
69-70" Retraction from melt at 90" ~CzsHss0H 69" n-CzoH4iOH Retraction from melt at 76" n-CisHnOH 1, vapor phase adsorption at 75' .. TL-C~~H~~OH 2, retraction from melt a t 70" 68-69" n-Ci~Hss0H Retraction from melt at 65' 69" 1, retraction from nitromethane soln. (satd.) .. n-CisHsiOH 68-69" n-CisHs10H 2, retraction from melt at 55" 1, retraction from nitromethane soln. (satd.) 66-67" dhHzg0H 67-68" 2, vapor phase adsorption a t 60" n-C~Hzg0H 67" *CirHzoOH 3, retraction from melt at 45" n-CizHz60H Retraction from melt at 35" 63" n-CioHziOH Retraction from melt at 25" 60" 56" n-CsHi?OH Retraction from melt at 25" 0
