March, 1952
ADSORPTION OF HYDROPHILIC MONOLAYERS ON CARBOXYLIC ACIDS
405
THE ADSORPTION OF HYDROPHOBIC MONOLAYERS OF CARBOXYLIC ACIDS BY HAYWARD R. BAKER,ELAINE G. SHAFRIR AND WILLIAM A. ZISMAN The Naval Research Laboratory, Washington,D. C. Received February $8, 106i
The adsorption technique reported reviously has been successfully extended to the study of monolayers physically adsorbed onto platinum from aqueous soyutions of a variety of types of mono- and dicarboxylic acids. The effects have been investigated of varying the molal concentration of or anic acid over a 10,000-fold range. The condition for maximum adsorption, as evidenced by peak hydrophobic behavior$B,), has been found to lie between pH 4 and 6. Both the shape of the 8 VB. pH curve and the positions of the points of inflection were in ood agreement with the results predicted by the theoretical treatment of Merker and Zisman'; it was therefore concluded t f a t the pH effects observed were due primarily to the comy t i t i v e adsorption between the molecular and ionic species in solution rather than to micelle formation or association. rogressive increases in solute concentration for a given acid have been found to result in the asymptotic approach of e, to - _. a characteristic, limiting value, Bum. A new and convenient "thermal-gradient method" using-the pure molten compound has been found by which adsorbed, close-Dacked monolayers can be prepared and isolated on metal in tke absence of solvent. It is more.eenera1 than the oleophobic methods desciibed in pre6ois reports, since it has been found applicable to all amphipathic comcounds tried, whether straight-chain, branched or cyclic. The equivalence of the contact angle (Bwlt) obtained from films made from the molten polar compounds with that (61,) of the films adsorbed from aqueous and non-aqueous solutions has been demonstrated. The way in which the contact angle characteristic of each polar compound (Oli, or Omel%) varies with the nature of. the acid and with homology has been studied and a correlation of the results with molecular orientation, structure and packing has been attempted.
A previous study of hydrophobic monomolecular films of primary +alkyl amines adsorbed on platinum and isolated from aqueous solutiona had established an effective technique for investigating physically adsorbed monolayers of compounds which are not capable of forming oleophobic film^.^.^ I n that study, the critical concentrations for hydrophobic film formation and the degree of wettability of hydrophobic films of the alkylamines were reported as functions of the alkyl chain length, the amine concentration, and the pH of the aqueous solution. This report is concerned with the extension of the same methods to the adsorption from aqueous solution of various carboxylic acids, including representative monocarboxylic acids of the n-alkyl, cyclo-alkyl, and aryl-alkyl types, some homologous a,w-dicarboxylic acids, and a group of alkyl-substituted succinic acids. Experimental
'
The experimental procedure employed was essentially that described previously.8 The films were always adsorbed, isolated and measured at 20.0 f 0.1". The films were adsorbed upon freshly flamed dippers of carefully selected, scratch-free, commercial platinum foil, as in earlier studies. Contact of the adsorbing surfaces with the aqueous solutions of organic acids was limited to an immersion period of 30 seconds. The solutions used in the adsorption measurements were made as in our earlier paper.8 The organic acids employed represented, with the exception of those mentioned in the Acknowledgments, compounds of the highest purity available commercially; the majority of these materials were subjected to' further purification by repeated recrystallixation from appropriate solvents. The melting points of the compounds as used have been listed in the second column of Table I. The n-alkylamines were the same compounds used in previous w0rk.~.5 (1) Presented at the Symposium on Surface Chemistry in Corrosion which was held under the auspices of the Division of Colloid Chemistry of the American Chemical Society .at the 119th National Meeting (Divided) at Cleveland, Ohio, April9. 1951. The opinions or assertions contained in this paper are the authors' and are not to be construed as official or reflecting the views of the Navy Department. (2) R. L. Merker and W. A. Zisman, THISJOURNAL, 66, 399 (1952). (3) E. G. Shafrin and W. A. Zisman, J . Colloid Sci.. 4, 571 (1949). (4) W. C. Bigelow, E. Glans and W. A. Zisman, i b i d . , 9, 563 (1947). (5) W. C. Bigelow, D. L. Pickett and W. A. Zisman, +bid., I, 513 (1946).
TABLE I HYDROPHOBIC CONTACT ANGLES4 FOR MONOLAYERS OF CARBOXYLIC ACIDSADSORBEDON PLATINUM Films prepared from a ueous soyution, Acid Caproia Caprylic Capric Lauric Myristic Palmitic Stearic Behenic
Nb 6 8 10 12 14 16 18 22
M.P., 'C. 0iim n-Alkyl monoacids
.. .....
16 31.5 42-43 52-53 61-62 68-69.5 80.0-80.3
Films prepared from molten compound by thermal radient metgod Omelf
@meit
Receding Advancdrop ing drop 490 59 66 71 75 79 81 89
530 64 70 75 78 83 88 96
..
44 65
52 76
..
66 64 64
76 74 75
46 45
48 49
24 35 52 62 72
26 40 58 65 76
69 71 72
73 76 79
52' 62 70 76
.. .. .. ..
Cycloalkyl monoacids Cyclohexylcarboxylic 7 29.5-30.5 53 Cyclohexylacetic 8 28.5 Cyclohexylpropionic 9 15.0 64 Cyclohexylbutyric 10 29 Cyclohexylcaproic 12 >32 66 Aromatic monoacids Benzoic Phenylbutyric
7 123-125 10 49-50
53
..
.
u,w-Alkane dicarboxylic acids 4 185-186 29
Succinic Adipic Sebacic Dodecanedioic Tetradecanedioic
6 10 12 14
Octylsuccinio Decylsuccinic Octadecylsuccinic
12 14 22
151 129 125-126 124.5-125.5
40 54
.. ..
n-Alkyl succinic acids 88-89 92.5-95 104-106
71 72 77
a Contact angle measurements made at 20.0 i 0 .1'. N = Total number of carbon atoms per molecule.
Measurements of the hydrophobic contact angle (0) on films isolated from aqueous solutions were made, as before, with drops of the generating solutions at the same concentration and pH at which the individual films were formed. Since conditions of solution equilibrium obtained, dissolution of the film into the superjacent drop was avoided. Changes in p H were effected by the dropwise addition of
406
HAYWARD R. BAKER, E L A I N E G. S H A F R I N
AND
WILLIAMA. ZISMAN
b
CAPROIC ACID (CR) - - I 5x10-1 MOLE~/LITER
VoI. 56 CAPRYLIC ACID (CB)
-- 5.8x 10-3 MOLE/LITER @ --5.9 x 10-1
8 --isxia;2 $--15xlo-3
@ - - 5 9 x 10-5
e -- I 5 X
WET-- 5.9 X
WET - - I 5 Y K5
J
I
I
I
CAPRIC ACID (Clo) - - 2 ~XIO-~MOLES/LITER
C
0-2
d
LAURIC ACID (CIJ --4ax10-4 MOLES/LITER @ --4 9 X
9x10-4
@--29X10-5
A
W E T - - Z 9 X10e6
00
0
2
4
6
10
12
PH.
Fig. 1.-Effect
PH
of p H on hydrophobicity of n-alkyl monocark)oxylic acids: a n-caproic acid ((36); 'c, n-capric acid ((310); d, n-lauric acid (C12).
aqueous HC1 or KOH to the acid solution with continuous The presence of electrolytes is known to influence stirrin marke3ly the adsorption of hydrophobic films on minerals,B the critical concentration for micelle formation in aqueous soap solutions,' and the surface tension of solutions.* Precautions were therefore observed to keep the concentration
.
(8) I. W. Wark, "Prinoiplea of Flotation," Australasian Institute of Mining and Metallurgy. Melbourne, Australia, 1938. (7)F. A. Long, G. C. Nutting and W. D. Harkins, J . A m . Chen. doc., OS, 2197 (1937). (8) J. Powney, T~ana.Faraday Soc., 31 1510 (1935).
b, n-caprylic acid
((38);
of added electrolytes as low as possible, consistent with the adjustment of the pH. For the more soluble compounds and for low concentrations, the pH response was investigated for two separate portions of the solution, to one of which was added HC1 only and to the other KOH only. For the more insoluble acids the initial addition of KOH was sometimes required to facilitate the dissolving at higher concentrations; in such cases it was impossible to avoid the presence of neutral salts. Periodic checks to determine the effect of excess electrolytes on the reproducibility of the v5. pH curves, with one exception, revealed no significant differences. Attempts to retrace the acidic portion of a
March, 1952
ADSORPTION OF HYDROPHILIC MONOLAYERS ON CARBOXYLIC ACIDS
curve for benzoic acid solution from p H 2 to its peak, in the presence of an unavaidably high concentration of HCl, were only partially successful. Although the general shape of the curve was repeatable, subsequent values of the peak angle (e,) were never as high as the original value. The curve was more nearly reproducible when approached from the alkaline direction despite the presence of excess KOH. Measurements of hydrophobic contact angles on films isolated directly from melts of pure compounds necessitated the use of distilled water, thus unavoidably increasing the probability that films of the more soluble organic compounds dissolved into the test drops. Because of the experimental difficulties involved in transferring a drop of water onto a highly hydrophobic surface, it was frequently difficult to ensure the initial existence of an advancing contact angle. The introduction of additional liquid into a drop already in contact with the monolayer was found t o increase the contact angle from a somewhat low initial value to a reproducible, limiting, maximum value equivalent to the advancing contact angle.
Monocarboxylic Acids.-The change in contact angle (e) with p H for each of several concentrations ( W ) of four n-alkyl monocarboxylic acids in aqueous solution is shown (Figs. la,b,c,d) for caproic (C,), caprylic (C,), capric ((31,) and lauric acids (C12), At least four different concentrations, representing a 10,000-fold increase in solute, were investigated over a pH range of 2 to 13 a t intervals of 0.5- to 1.0 pH unit. Each curve is the locus of points obtained from solutions having the same concentration. The families of curves for the four acids followed the same general pattern: The individual curves rose sharply from some low value between pH 2 and 3 to a peak value (e,) in the region pH 3-6, then slowly declined (revealing an occasional secondary, plateau-like feature) until they intersected the 0 = 0" axis between pH 8 and 12. The extreme insolubility of lauric acid precluded the completion of the low pH portion of the high-concentration curves; the presence of precipitate or solution cloudiness has been indicated by dashed lines over the appropriate pH range. The curves for successively less concentrated solutions of a given acid covered a smaller span of pH values, extended less into the alkaline pH range, and exhibited lower peak contact angles. Eventually a concentration was reached so dilute that no signs of hydrophobic film formation were obtained a t any pH within the allotted, 30-second immersion period. The existence of a critical minimum concentration (W,) necessary for formation of a solution repellent film had been established in previous studies, 34 where W o had been shown to be a measure of the total number of molecules a.vailable to an adsorbing surface within a given period of time, and thus to be a function of such experimental conditions as concentration, volume and rate of stirring of the solution. A plot of 0, as a function of W for each of these acids (Fig. 2) shows an initial, sharp rise for concentrations just above Wo,followed by an asymptotic approach (indicated by dashed lines) to some The regularity of the change limiting angle (eli,). in elim in the homologous series of acids is evident in the third column of Table I. Proof that 81im is characteristic of the composition of the monolayer and independent of the method of film formation (providing maximum adsorption occurs) is afforded by a comparison of 81im with the values listed for the angles exhibited by drops of distilled water in
407
contact with the corresponding monolayers adsorbed from the molten, pure compound by the thermal-gradient method described later. Values of ernelt (advancing) have been indicated by bars a t the extreme right of Fig. 2. There is good agreement between the several values despite the limitations imposed by the diverse experimental techniques. Since the molecular interstices of a film isolated from aqueous solution must be saturated with water,s Blim is the same as a receding contact angle. In determining Bmelt, however, both receding and advancing contact angle measurements are possible. The duplication of emelt for the more soluble homologs under conditions corresponding to receding contact angles is complicated by the lack of solution equilibrium between the films and the test drops. This tendency toward dissolution is eliminated in determining 81im since test drops of the generating solution are available.
CONCENTRATION (W) MOLES/LITER. Fig. 2.-Correlation of eak contact angle (e,) with concentration for n-&kyl mgnocarboxylic acids.
The 0 os. pH curves for cyclohexylcarboxylic (C,), cyclohexylpropionic (C,) and cyclohexylcaproic (C12)acids adsorbed from aqueous solution are essentially identical with those shown in Fig. 1; differences occur only in the concentrations investigated and in the relative hydrophobicity (ie., the magnitude of e) of the acid films. The limited solubility of cyclohexylcaproic acid in water prevented completion of the curves for the higher concentrations since extensive precipitation occurred before 0, could be determined. An indication that the highest value of eP obtainable experimentally (63 ") approximated elim lay in the value of 64" observed for erneit under conditions leading to a receding contact angle. The results obtained with benzoic acid, including the behavior to changes in pH and concentration as well as the value of Blim, were nearly the same as those obtained for the corresponding saturated cyclic acid, In the plot of Op us. W for the four cyclic acids (Fig. 3), the asymptotic approach to Blim is indicated by a dashed line and the appropriate value of by a bar a t the right of the figure. Dicarboxylic Acids.-The general behavior of the graph of 8 us. pH for three homologous a,w-dicarboxylic acids (succinic (Cd),adipic (C,) and sebacic ((31,) acids) ie similar to that previously observed (9) H. R. Baker and W. A. Zisman, Ind. Eno. Chem.,40, 2331 (1948).
*
HAYWARD R. BAKER,ELAINEG. SHAFRIN AND WILLIAM A. ZISMAN
408
e 0
80’
e
e 60’
S I -
- 0 SEBACIC
ACID (CIo)
0 ADIPIC ACID (Cd @ SUCCINIC ACID (CI)
e
-
o
f
60’-
0
----------- -
-
40’s 0 a s
-
20’-
z
-
Ln
I
I
I
The 8 vs. pH curves for decylsuccinic (C,,)and octadecylsuccinic ( G 2 ) acids were similar to those shown in Fig. 1 over the alkaline region in which they were determinable; the acidic side of the curves could not be obtained owing t o the extreme insolubility of these compounds. Indeed, the addition of ethyl alcohol was required for all pH values below 7.8 in order to determine 8, for the most concentrated solution of octadecylsuccinic acid. In the complete curves obtainable for the more soluble octylsuccinic acid ((312) a reproducible, well-developed “shoulder” occurred on the acidic side of the peak. 100‘ ~ - - O C T L O E C I L S U C C I N I C LClO IC221
-
Thermal-Gradient Method.-Previous success in isolating adsorbed monolayers by withdrawing the platinum dipper from a pool of the molten polar compound4 had been limited to those essentially paraffinic or unbranched types of compounds meeting the requirements of structure and orientation developed earlier5 for formation of oleophobic films. It .became possible, by this “isothermal method,” t o prepare monolayers of those lower series members which were not sufficiently oleophobic to permit their isolation from oil solution. This method was not effective, however, in isolating monolayers of highly-branched or cyclic compounds; even for straight-chain compounds, its effectiveness in preparing monolayers completely free of residual bulk compound was limited by the working range of temperature between the melting point of the compound and the temperature, T, at which the monolayer became wetted by the melt. The need for a method of preparing monolayers of any type compound in the absence of solvent led to much experimenting with ways other than the isothermal method of separating films from the melt. The result was the “thermal-gradient method.” A small amount of pure compound was placed on a piece of platinum foil held with its plane slightly inclined from the horizontal. After the uppermost edge of the platinum was gently heated, the organic material melted and wet the metal surfaae. As the heating continued, the molten liquid suddenly receded to the lower, cooler edge of the platinum, exposing a dry area in a band immediately adjacent to the liquid. This occurred at a temperature slightly below that required for visible vaporization of the organic compound from the hot, upper edge of the foil. If the foil was cooled t o room temperature (20’) while still tilted, the organic material remained a t the lower edge where it either solidified or, if liquid, drained off upon contact with filter phper. Hydrophobic contact angle measurements taken a t various positions over the surface of the foil were large and reproducible only over the “dry band” immediately adjacent to the bulk material. Lower and erratic contact angles were observed near the uppermost edge of the foil where the higher temperature may well have led to the vaporization of some of the organic film. Proof that this thermal-gradient method gives rise t o monolayers in the “dry band” identical with those prepared by earlier methods is afforded by a comparison (Table 11) of the hydrophobic contact angles observed on monolayers of several n-alkyl pri-amines isolated from the melt by the isothermal and thermal-gradient methods with those obtained on monolayers adsorbed from dilute solution in cetane or water. The agreement between the several values is good, considering the experimental limitations. These include the impossibility of obtaining true advancing angles on films isolated from aqueous solution and the inability to duplicate conditions of solution equilibrium for films obtained from the melt. For those longer chain compounds for which solubility effects become negligible, the direct comparison of the values listed for the receding contact angles is warranted.
/e I:--
CONCENTRATION (W) MOLES/LITER. Fig. 4.-Correlation of peak contact angle (e,) with concentration for a,walkyl dicarboxylic acids.
I
n olo-l
Vol. 5G
(0.6
h Y‘
I
10.5 IO.’ C O N C t H l R l l l O H (WI IN Y O L I S / L I T E R .
I 10.2
10.1
Fig. 5.-Correlation of peak contact angle (e,,) with concentration for alkylsuccinic acids.
qno
ADSORPTION OF HYDROPHILIC h~ONOLAYlCRSON CARBOXYLIC ACIDS
Mnrch, 1052
TABLE I1 COMPARISON OF HYDROPHOBIC FILMSOF AM~NES
n- Alkyl
aniine
NO
Films prepared from Molten compound Cetane soln. By thermalBy isothermal by oleophobic method b methodb gradient methodb Advancing Reoeding Advancing Receding Advancing Receding
..
7
Water soln. by hydrophobic method thim
51' .. 52 O' 48 550, 4 55 O Butylamine 69 73 O 68 O 69' 67 74 Octylamine 8 81 83 89 83 85' 83 89 Dodecylamine 12 90 87 90 86 86" 84 92 14 91 Tetradecylamine 89 96 89 89" 87 96 Hesadecylamine 16 90 91 101 90 !IOb 89 102 Octadecylamine 18 102 Measa N = total number of carbon atoms peromolecule. * Measurements were made a t temperature 20.0 *0.lo. urements were made a t temperature 25 *l These data were reproduced from Table 11 in reference 3.
.
I
Because of the difficulty encountered in contacting drops of water to highly hydrophobic surfaces, the method of increasing the size of the drop was employed to ensure development of advancing angles. Since this precaution was not taken in the previous study of amine monolayer^,^ the contact angles reported earlier for water on amine films need not correspond to advancing angles, as do the values listed in Table 11; the divergence is greater for the longer chain, more hydrophobic homologs. A similar qualification is believed applicable to the other contact angle values reported for water on highly hydrophobic monolayers.4*5 Possible mechanisms for the thermal-gradient method may involve one or more of the following factors: (a) the preferential evaporation of all the organic molecules except those strongly adherent ones adsorbed in direct contact with the metal surface; (b) a gravitational diffusion of all molecules except those anchored in the monolayer, involving, perhaps, the ease of slippage of the molecules in the liquid with respect to the external plane presented by the oriented array of adsorbed molecules; and (c) the existence of a relatively narrow range of temperatures over which the oleophobic property of the liquid and the adsorbed film can become operative. Definitive experiments are in progress to determine the actual mechanism. By the use of the thermal-gradient method, monolayers have been obtained from the melt for every acid of interest in the present study. Data for the hydrophobic contact angles (O,,lt) have been listed in Table I for comparison with the values of Olim obtained for films from aqueous solution. The maximum hydrophobicity obtainable from an adsorbed close-packed monolayer can evidently be measured quickly by the thermal-gradient method, eliminating the more laborious evaluation of Olim based on Op vs. W data. Moreover, this method of film preparation makes possible the measurement of advancing angles, whereas receding angles only are obtainable for films adsorbed from aqueous solution. In addition, a fuller correlation of hydrophobicity with chain length and acid type becomes possible since there is no longer any restriction to the lower, more water-soluble members of the series. Thus, for the n-alkyl monocarboxylic acids, for which Oli, data are available only for the homologs up to N = 12, it has been possible, by this new method, to obtain monolayers and to measure the effect of chain length on hydrophobicity for the series through N = 22 (curve A, Fig. sa).
TABLEI11 COMPARISON OF EXPERIMENTAL CURVESOF t L l t vs. N FOR SEVERAL HOMOLOGOUS SERIES Slope (degrees/corbon increment)
Homologous series
%Alkyl pri-amines %Alkyl monoacids Cycloalkyl monoacids Aryl-alkyl monoacids a,w-Alkane dicarboxylic acids n-Alkyl succinic acids
2.0" 2.2 0 0
4.9 0.5
e for N
=
10
80 70 .75 49 56 73
In Figs. 6a and 6b are presented data showing the effect on Ornelt of the length of the alkyl chain. Within each series the data for all but an occasional lowest homolog are well represented by a straight line. Changes in the hydrocarbon portion of the molecule or in the number .and type of functional groups in going from one homologous series t o another are reflected in the slopes and positions of the lines. These slopes are listed in Table 111; the relative positions of the lines have been indicated by listing the arbitrarily chosen intercepts of the lines with the ordinate N = 10. The implications of these data will be more fully developed in the next section.
Discussion Numerous investigators have described the effects of pH variation on the surface-chemical properties of aqueous solutions of the carboxylic acids and their salts. These include effects on surface and interfacial t e n ~ i o n , 7 , 8 * ~detergency113 0 , ~ ~ , ~ ~ flotation of minerals,B,' 4 and stability of foams. 15, In most cases the variations observed were attributed to the relative proportions, solubilities and surface activities of the carboxylic acid anions, the hydrolytic free acid, the undissociated soap molecules, the acid-soaps, and the colloidal micelles or other aggregates. In general, maximum surface activity was obtained at those pH values which corresponded to partial or complete hydrolysis of the soap, indicating the molecular form of the acid to be (10) L. Chromy, Roctniki Chem., 18, 434 (1938). (11) H. L. Cupples, I n d . E n s . Chem., 29, 924 (1937). (12) J. K. Davis and F. E. Bartell, THIS JOURNAL, 47, 40 (1943). (13) W. W. Niven, Jr., "Fundamentals of Detergency," Reinhold Publishing Corp., New York, N. Y.,1950. (14) H. H.Kellogg and H. Vasquez-Rosas, Mining Technology, 9, 1 (1945). (15) C. L. Baker, Ind. Ene. Chem., 23, 1025 (1931). (18) G. D. Miles and J. Ross, THISJOURNAL,48, 280 (1944).
HAYWARD R. BAKER, ELAINE G. SHAFRIN AND WILLIAM A. ZISMAN
410 110'
d Y LD
HOMOLOGOUS MONOFUNCTIONAL SERIES
c 0-- --ARYL-AlKYL
~a-
---n-ALKYL
MONOACID! PRI-AMINE!
Vol. 56
in decreasing the CMC.17p1s From these it is concluded that the concentrations of acids and soaps used in this investigation in the more dilute solutions were too low to permit micelle formation. The relative importance of the various ions and molecules of organic acid present in aqueous solution and their influence on the adsorption of hydrophobic monolayers are being reported separately in a related paper by Merker and Zisman.2 As a result of a preliminary report of the experimental observations related here, they mere led to attempt the correlation of hydrophobic behavior with composition of the adsorbed films. Curves representing the concentration in solution of the various adsorbable entities present a t different pH values were derived by them from the application of the law of mass action. By postulating an adsorption equilibrium in which, as a first approximation, the same relative proportions of the various species were maintained in the adsorbed film as in the generating solution, they were able to conclude that the sharp decrease in hydrophobicity noted for films prepared a t both extremes of t.he pH scale is due to the competitive adsorption of the appropriate hydrophilic aqueous ions (Hf or OH-) whereas the hydrophobic character of films adsorbed in the intermediate pH range is due to the adsorption of the hydrophobic carboxylate ions and neutral acid molecules. The peak angle was shown to occur a t the pH favoring the adsorption of a film composed primarily of the undissociated acid. This pH value was shown by them t o be predictable, given the dissociation constant for a weak acid. Good agreement was obtained between the theoretically derived curves and subsequent experimental observations. From its definition, it is evident that Blim is characteristic of monolayers in the closest packed arrangement obtainable by adsorption from aqueous solution. Provided the solubility of the organic compound is sufficient, Oli,,, will also correspond to the maximum packing obtainable from solut,ion in any solvent; the only exceptions to be expected occurring under conditions of adsorption of mixed films of solvent and solute. Hence it, is not surprising that essentially the same values of 0 have been reported for saturated films prepared by adsorption from water, ethanol-water, benzene, dicyclohexyl or hexadecane. Since in the limit a solution of inmessing concentration will approach in behavior that, of the pure molten acid, el;,,, should equal Bmelt, t'he exceptions occurring where there are marked effects on the orientation or packing of the adsorbed film due (a) to t'he higher temperature often needed to produce monolayerti from the melt, or (b) to any differences in the films because of mixed film formation. This agreement of &, and is well exhibited in Tables 1and 11. The thermal-gradient method is valuable in making available data on the relative hydrophobicity of closest packed monolayers not otherwise obtainable. Since t,his method requires less experimental manjpulat
