Counterion effects in charged monolayers

Counterion Effects in Charged Monolayers E. D. G O D D A R D , O. KAO A~D H. C. K U N G Lever Brothers Company, Research Center, Edgewater, New Jersey 1%eceived January 27, 1968 The influence of various monovalent counterions in the subsolution on the ,r - A and AV -- A characteristics of spread, long chain (R = C22), charged monolayers having the head groups -N+(CHa)a, -N+H(CH3)2, -N+Ha, and -SO4-, has been studied. Large differences in the effect of the counterions were found and the following interaction sequences established: 1%- N+(CHa)a, SCN > I > NOa > Br > C1 > F; 1%- N+H(CH~)2 and R - N+H3 , Br > C1; 1%- SO4-, Cs = Rb > K > Na > Li. The position of the tetramethylammonium ion in the last sequence could not be established unequivocally. Estimates of the energies of interaction of the various anions with the 1%- N+(CH~)3 monolayer are given. The experimental ~r - A curves are compared with those predicted from various equations of state. Agreement was generally poor and the implications of these findings are discussed. I n a recent publication (1) results were presented on the properties of charged f a t t y acid monolayers in the presence of various monovalent eounterions and the observed characteristics were compared with those predicted from various theoretical equations

of state. In view of the magnitude of the counterion selectivity effects, it appeared that agreement with theory was not be be expected unless provision for these effects were made through inclusion of specific association energy terms (or activity coefficients) in the theoretical equations. I t was also pointed out that eounterion sequence effects observed with charged monolayers were an instance of a wider phenomenon which has been extensively documented in colloid chemistry (2) and in the physical chemistry of aqueous electrolytes (3, 4) and ion-exchange resins (5). The purpose of the present work was to extend the study of the monovalent eounteflon effects to other charged insoluble monolayers. Some work of this type has been reported b y Rogers and Sehulman (6), but the results were obtained for conditions of very high ionic strength. B y use of higher chain length monolayer molecules, we have avoided the necessity of this. I t was a further Journal of Colloid and Interface Science, Vol. 27, No. 4, August 1968

purpose to estimate the magnitude of the interaction energies within a particular eounterion sequence. EXPERIMENTAL All the long chain materials were derived from behenie acid as starting material. For preparation of the tertiary and quaternary amines the pure ( > 9 9 % ) behenie acid was first converted to behenyl chloride b y reaction with thionyl chloride. The behenyl chloride was reacted with dimethylamine to yield N , N - d i m e t h y l docosanamide which was reduced to the amine with lithium aluminum hydride. The crude product was distilled twice, yielding pure N , N-dimethyl doeosylamine (DDCA) of mp 77-78°C. The pure D D C A was reacted with methyl bromide in an autoclave to yield docosyl trimethylammonium bromide (DCTAB) which was reerystallized twice from methanol to give the pure product of mp (deeomp.) 210°C. Sodium doeosyl sulfate and n-doeosylamine were derived from 1-doeosanol. The latter was prepared from behenic acid via methyl behenate as intermediate. Behenic acid was esterified with methanol,

and

the

ether

extract

was

fractionally 616

COUNTERION EFFECTS IN CHARGED MONOLAYERS distilled to yield the pure methyl behenate (bp 223°C/8 ram). The latter was converted to l-docosanol by treatment with lithium aluminum hydride. The crude product was crystallized from acetone and .distilled to give pure l-docosanol (bp 180°C/0.2 ram). Sodium docosyl sulfate (SDCS) was prepared by reacting 1-docosanol with Sulfan in an ethylene chloride, dioxane mixture. After neutralization the crude material was recrystMlized three times from ethanol and vacumn dried at 100°C. The absence of parent alcohol in the specimen was checked by differential thermal analysis. For preparation of n-docosylarnine (DCA), 1-doeosanol was converted to the bromide b y reaction with aqueous hydrogen bromide. The crude bromide was purified by distillation (bp 218-220°C/ 0.4 ram). I t was converted to the amine by refluxing with a large excess of alcoholic ammonia. The etude amine was purified by preparation of its hydroehloride, conversion back to the free amine, distillation (bp 206-207°C/2.0 mm), and two crystallizations from ethanol to yield d0eosylamine of mp 62.5-63°C. The tetramethylammonium chloride (TMAC]) was an Eastman white label specimen. The inorganic salts were of ep grade. Follow,ring earlier practice the NaC1, NaBr, N a F and NaI specimens were heated prior to use for 1.5 hours at between 200 and 300°C. Redistilled water was prepared by distillation of alkaline permanganate solution prepared from laboratory distilled water. For spreading, the D C T A B was dissolved in a mixture of 20 % ethanol and 80 % petroleum ether, both redistilled before use; the concentration was about 0.7 m g / ml. The DCA and D D C A were dissolved in 30 % ethanol and 70 % petroleum ether. The preparation of the SDCS solution was as follows: Six drops of water and 20 ml petroleum ether were added to a 25 ml volumetric flask containing 0.0175 gm SDCS. The contents of the flask were agitated vigorously in an ultrasonic cleaner for 10 minutes. Isopropanol was then

617

added drop by drop to the mark. If solution was not complete, the system was given several minutes more agitation. The film balance used was of the conventional horizontal float type. The trough was of silica. Surface potential was measured with a Keithley 610A electrometer using an air electrode coated with 226Ra and an Ag/AgC1 electrode in the subsolution. Compression of the monolayer was started 2 minutes after spreading, and the time for a complete compression was about 20 minutes. RESULTS Surface pressure-area (~r-A) and surface potential-area (AV-A) curves for doeosyl trimethylammonium bromide monolayers on 0.01M NaF, NaC1, NaBr, NaI, NaNO3, and NaSCN subsolutions are shown in Fig. 1; the latter four included 1 X 10-SM HC1, and their p H ranged from 5.1 to 5.5.

5- o.. ~n \o

~

20

II\&

40

\

°

°~-~

O

<~

-o,

60 A (A2/mofecule)

80

I00

FIG. 1. ~--A and AV-A isotherms of DCTAB on subsolutions containing various anions a* 25°G. O 1 X 10- ~ M N a F ; A 1 X 10-3MNaC1;Ell X 10-5 M NsBr, 1 X 10-5 M HC1; 9 1 X 10-~ M NaNO~, 1 X 10~5M HC1; O 1 X 10-2M NaI, 1 X 10-5 M HC1; ~ 1 X 10-2 M NaSCN, 1 X 10-~ M HC1. Journal of Cdloid and Interface Science, Vol. 27, No. 4, Augus~ 1968

618

GODDARD, KAO, AND KUNG

The addition of the hydrochloric acid was aimed at controlling and reducing the effect of bicarbonate ion from atmospheric CO~ on the surface properties of the spread monolayer. It was found subsequently that the presence of this level of HC1 in the 0.01M sodium salt subsolutions had very little effect on the surface pressure and potential of the DCTAB fihn. The DCTAB monolayers on 0.01M NaSCN and NaI, and to a lesser extent NaNO3, were rather condensed. Of the other subsolutions, the NaF gave by far the most expanded films. The overall variation in surface pressure was considerable with change in counterion, and the condensing influence of these anions, seen from the 7r values, was in the order SCN > I NOa> Br > Cl > F. The same order of influence was found on the lowering of /~V. The maximum potential in millivolts for these counterions is as follows: 380

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FIG. 3. Influence of concentration of the thiocyanate ion on the 7r-A and A V - A isotherms of DCTAB on 0.01M NaF at 25°C. O 1 X 10-2M NaF; 1 X 10-~M NaF, 2 X 10-6M NaSCN; [] 1 X 10-2M NaF, 6 X 10-6M NaSCN; ~ 1 X 10-2M NaF, 1 X 10-5M NaSCN.

~o_

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F~G. 2. Influence of anion and ionic strength on the ~r-A and A V - A isotherms of DCTAB at 25°C. O l X 10-~M NaF; A 1 X 10-~M NaBr; [] 1 X 10-aM NaBr, 1 X 10-~M HC1; ~ 1 X 10-6M NaI;

1 × 10-aM NaI, 1 X 10-~M HCI. Journal of Colloid and Interface Science,

Vol. 27, No. 4, August 1968

(CI), and 580 (F). Experiments were also carried out at much lower ionic strengths of the subsolution. The presence of highly interacting counterions could be detected down to very small concentrations. For example, on a subsolution 10-~M in B r - or I - the monolayer was found to be much more condensed than on one 10-2M in F-. See Fig. 2. However, because of the tremendous difference in ionic strength of the solutions, and hence of the ~6 potentials of the monolayer, data such as these were not employed for obtaining the relative interaction e n e r g i e s of the counterions with the monolayer. To obtain the latter it seemed preferable to minimize changes in the electrical interaction term by working under conditions of constant ~bo. This was accomplished by employing a nearly constant concentration of the least interacting ion (~-~0.01M F-)

COUNTERION

EFFECTS

IN CHARGED

whilst maintaining the overall ionic strength constant at 0.0100. Various amounts of the sodium salts of other anions were then added to NaF solution, and the 7r-A and AV-A relations of the DCTAB film were measured. The amounts of anion needed to depress the pressure a small but definite value (0.5-1.0 dyne/era) at a conveniently chosen area (75 A~/molecule), were as follows: 2 X 10-~M e l - , 5 X 10-L~r ]~r-, 3 )K 10-~M N Q - , 8 X 10-~M I - and 2 X 10-6M S C N - . Typical data for this purpose are shown in Fig. 3 for the S C N - / F - system. The results for doeosylamine and dimethyl doeosylamine spread on subsolutions 0.1M NaCI and NaBr, each containing 0.001M HC1, are presented in Fig. 4. The D D C A monolayer on NaC1 was m a r k edly more expanded than t h a t on N a B r but had a m u c h lower collapse pressure; the surface potentials for the two subsolu-

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FIG. 4. z-A and AV-A isotherms of doeosylamine and dimethyl docosylamine on NaC1 and NaBr subsolutions at 25°C. DCA: A 0.IM NaC1, 0.001M HCI; O 0.1M NaBr, 0.001M HC1. DDCA: 0.1M NaBr, 0.001M t/C1; [] 0.1M NaCI, 0.001M HC1. Journal of Colloid and Interface Science, Vol. 27, No. 4, A u g u s t 1968

MONOLAYERS

619 [00

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FIG. docosyl 25°C.O LiCl; []

5. ~r-A and AV-A isotherms of sodium sulfate on 1 X 10 -3 M salt solutions at 1 X 10-3M (CH~)4NCI; Zk 1 X 10-3M 1 X 10-3M NaC1; O 1 X 10-~M KC1.

tions were relatively close with A V ~ c l > Films of docosylamine on 0.1M NaC1 and N a B r solutions, 10-3M in HC1, were relatively condensed above 40A2/molecule. Pressure values were slightly higher on 0.1M NaC1, but the limiting area was lower. Surface potentials of the monolayer on the NaC1 subsolutions were appreciably higher than those on NaBr. The 7r-A and AV-A characteristics of a sodium docosyl sulfate monolayer spread on 0.001M salt solutions are shown in Fig. 5 and on 0.0001M salt solutions in Fig. 6. It is seen from Fig. 5 that the sequence of condensation of SDCS films on 0.001M alkaline metal ion solutions is K > Na > Li. On the tetramethylammonium chloride subsolution the monolayer was most expanded in the lower area range, but at areas above 60A2/mo]ecule the 7r values on this subsolution lie between those of LiCl and NaCI. The surface potentials of the SDCS films on the solutions are negative at most areas. The maximum AV values

620

GODDARD, KAO, AND KUNG

are + 3 3 mV (TMA), + 1 7 (K), - 6 7 (Na), and - 150 (Li). Fig. 6 presents a more complete cation series for 10-4M salt subsolutions. In general, the changes observed in the 7r-A and AV-A relationships were small on changing the salt concentration to 1 X 10-4M: ~r values were slightly higher in the high area regions and AV generally lower, although AAV values do not attain the value of 59 mV predicted by the Davies treatment (7). The values above areas of 30A2/molecule on 0.0001M KC1, RbC1, and CsC1 were low and close to one another. It is to be noted that the curve for RbC1 is slightly, but reproducibly, the mos~ condensed for the whole area range investigated, especially for area values around 25A2/moleeule. Over most of the area range the sequence of AV values is Li < N a < K < Rb = Cs < TMA. The AV values for SDCS on 0.001M NaC1 are close to those reported b y Mingins and Pethiea (8) for oetadecyl sulfate monolayers at comparable areas. tOO 0

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DISCUSSION The results presented indicate there is a marked influence of the counterion on the surface pressure and potential of the monolayers. Aside from one or two exceptions there is generally conformity between the AV and 7r results in as much as lower pressures are associated with lower potentials. (Here potential refers to values in a positive sense for amine monolayers and a negative sense for the alkyl sulfate.) According to ideas developed b y Davies (7), both of these effects correspond to higher interaction of the counterion with the monolayer. It is to be noted that a major change in the degree of expansion of a charged monolayer can be effeeted merely b y changing the counterion in the subsolution. In a previous publication (1) we examined the applicability of various equations of state to ionized monolayers of fatty adds. Each was found to overestimate the pressure of the monolayers spread on aqueous solutions of alkali metal chlorides. Simple inspection shows that this is also the ease with the data reported here for the docosyl sulfate monolayers. On the other hand, since monolayers of D C T A B in certain cases are markedly more expanded than anionic counterparts of the same chain length, it seemed appropriate to examine the applicability of the theoretical equations to the data obtained for this cationic monolayer, especially when spread on solutions containing the least interacting anions. The first two equations, derived respectively by Fowkes (9) and by Lucassen-Reynders and van den Tempel (10), are again used in their "ideal" form; and the third employs the 100 A2/moleeule cohesion energy "cutoff" proposed b y Davies (11).

kT

(

= T , In

zA~

)

~_ _ A0 + 1

~r = --(2k, T/Ao) In (1 - Ao/A) IO0

FIG. 6. ~r-A and AV-A isotherms of sodium docosyl sulfate on 1 X 10-4 M salt solutions at 25°C.O 1 X 10-4M (CH3)4NC1; A 1 X 10-4M LiC1; [] 1 X 10-SM N a C 1 ; ~ I X 10-4MKC1; ~1 X 10-~M RbC1; A 1 X 10-4M CsC1. Journal of Colloid and %nterface Science, Vol. 27, No. 4, August 1968

[11

[2]

~r = kT/(A -- Ao) + 6 . 1 % / c {cosh sinh-0 (134/AV'c) - 1 } -

[3]

8800/A~/2

where A0 is a co-area term, A~ is the area per molecule of H20 in the surface, c is the

COUNTERION EFFECTS IN CHARGED MONOLAYERS TABLE I

concentration of monovalent eounterions in the subsolution, z is the number of ions into which the surfaetant (electrolyte) dissociates and the remaining terms have their usual significance. We examine first the two t h e r m o d y n a m ically derived equations using values for A~ and z, in the Fowkes equation, of 10A2/ molecule and 2, respectively. B o t h equations seriously overestimate the surface pressure in the high area region even with A0 given the favorably low value of 20A2/ molecule for which value Eqs. [1] and [2] then lead to the same 7r-A curve. See Fig. 7. B y placing z = 1, i.e., assuming ion association between head-groups and counterion, the fit of the Fowkes curve improves at high area but is iIldifferent at lower areas. This is also the ease over the whole area range for D C T A B on subsolutions of the more strongly interacting chloride ion. The Davies equation with A0 = 30A2/molecule some-

5C

~)ERIVATION OF INTERACTION ENERGY~

¢), OF ANIONS WITh DCTAB

\ ) R "'

x, r

40

~

i

60

i

i

,

80 .~ (A2/molecule)

i

I00

i

120

FIG. 7. ~--A isotherms of DCTAB on 0.01M NaF and 0.01211 NaC1 subsolutions at 25°C. TheoretieM curves are included:-----Lueasse~aReynders (Ao=20), Fowkes (A0=20, z = 2 ) ; - - Davies ( A 0 = 3 0 ) ; - - - - F o w k e s (A0=20, z=l); - - - Fowkes (Ao=35, z=l). © 0.01M NaF; A 0.01M NaC1.

621

co

Concn. NaF

2 X 10-3M C1 5 X 10-SM Br 3 X 10-~M NO~

8 X 10-3M 1 X 10-2M 1 X 10-2M

8 X 10-6M I

1 X 1O-~M

2 X 10-GM SCN

1 X ll-2M

¢

1 4.5 5 6.5 8

kT kT kT let kT

what underestimates the pressure of the experimental fluoride curve over most of the area range and wauld lead to serious overestimates of the pressure of monolayers spread on all the other subsolutions. T h e above results reinforce our previous remarks (1) regarding the need in theoretical equations of state for terms which account for specific ion-counterion interaction forces, or, in thermodynamically derived equations, for the inclusion of activity coefficients which, a priori, would not be expected to be unity (1). These ideas will be developed further in subsequent papers. Interaction of a counterion with a charged monolayer m a y be considered to be governed b y two energy terms: the first, a specific interaction energy designated 0, and the second a nonspecific electrical energy, e~o, for a monovaient eounterion. A measure of the concentration ca of added counteranion in the "surface region" of an ionized monolayer can be obtained from the expression, cl = co exp (e¢o ÷ ¢)/kT, where co = bulk anion concentration. We turn now to consideration of the effect of incorporating different anions, but maintaining a constant ionic strength, in a 0.01M N a F solution underlying a D C T A B monolayer. (The N a F molarity was somewhat lower in solutions containing CI-. See Table I). For a small and constant change of ~r-A brought about b y the addition of an anion, we assume as a first approach t h a t ca will have the same value for all the anions. The provision of constant ionic strength in subsolutions predominating in the most weakly interacting fluoride ion ensures virtual constancy of ¢/0 as was verified b y experiment. I f we arbitrarily assign a value ¢ = l k T to the chloride ion, then a set of rela-

Journal of Colloid and Interface Science, VoI. 27, No. 4, August 1968

622

GODDARD, KAO AND K U N G

tire values can be assigned for the group of Sehulman (6) for monolayers of oetadecyl counteranions examined. See Table I. sulfate spread on strong salt subsolutions} Davies (11) has calculated the specific The rise in ~r for the Cs curve versus those interaction energy of F, C1, and I with an of Rb and K at A = 30A2/molecule may oetadeeyl trimethylammonium monolayer well be connected with a reorientation in from surface viscosity measurements. The the monolayer at this area. See Fig. 6. This values obtained, at an area per OCTAB is supported by the drop in AV at the same molecule of 85A2/moleeule, viz., 2.1, 3.0, area. The position of TMA in the series is and 3.9kT, respectively, are considerably clearly dependent on the area per molecule lower than those in Table I. of SDCS. So far as the AV sequence is conThe anion sequence shown in Table I is cerned, viz., TMA > Cs = Rb > K > in harmony with the surface pressure results Na > Li, the position of TMA would at of Rogers and Schulman (6), viz., I < first sight indicate maximum interaction. C1 < F, for monolayers of the octadeeyl However, this conclusion concerning the homolog, and, except for the position of TMA ion is uncertain since the influence of NO3, with the AV results of Mine and the TMA dipole on AV is unknown. Koezorowski (12), viz., SCN < I < Br < SPECIFIC COUNTERION EFFECTS C1 < NO3 < F, for spread films of the For the DCTAB monolayers the order of hexadecyl homolog at the water/decane interface. (The latter results are for 1M interaction of the anions is SCN > I > salt solutions; for lower concentrations, the NOn > Br > C1 > F. This, with minor exceptions, corresponds to the anion seion sequences were less certain.) For the doeosylamine and dimethyl quence reported for interaction with various docosylamine monolayers the effect of positively charged colloids (2), to the order eounterions was investigated only on sub- of CMC lowering of quaternary ammonium solutions of 0.1M NaC1 or NaBr including surfactants (14), to the order of decreasing 0.001M HC1. The order of influence on interaction of halide ions with strong base surface pressure and potential in the more ion exchangers (15-17) and of increased expanded region was Br > C1 in both eases, activity coefficients of solutions of tetrasimilar to that observed for DCTAB and, alkylammonium (TTA) halides (18, 19).2 An by Rogers and Schulman (6), for dimethyl explanation of the sequence in the latter oetadecylamine at near neutral pH. Under ease has been put forward by Diamond (20) the same conditions, these authors found no who points out that, although a tetraalkyl anion effect upon monolayers of oetadeeyl- ammonium ion introduced into water reamine. It is to be noted that the primary mains largely unhydrated, the ordering of and tertiary amine monolayers are very the water structure it promotes around much less expanded than the quaternary itself makes it more difficult to introduce homolog, with pressures at all areas much further TAA ions into the solutions. In lower than those predicted by the theoretical this way, for TAA salts containing the relaequations. We believe that, even at the low tively highly hydrated chloride ion, positive subsolution pH of 3, a contributing factor is deviations from unity of the activity coincomplete ionization of the monolayer (1) efficient result. On the other hand, if the especially in the case of the primary amine eounterion is also large and relatively unhydrated, e.g., iodide, the water tends to which is the weaker base. A combination of the effects of the alkali minimize the disturbance to itself by forcing metal cations on the surface potential and the two ions together in the same cavity by pressure of sodium docosyl sulfate mono1 See also Matijevie, E. and Rogers, J., Nature, layers at the air/water interface indicates 180, 560 (1957). The effect of various anions, viz., C1 < Br, the order of interaction to be Cs = Rb > NO, < SCN, in increasing the micellar size of a K > Na > Li. This agrees with the results cationic surfactant (Cohen, I. and Vasilliades, T., of Weil (13) for adsorbed dodeeyI sulfate J . Phys. Chem., 65, 1774, 1781 (1961) is another monolayers and with those of Rogers and instance of the same sequence. Journal of Colloid and Interface Science, Vol, 27, No. 4, August 1968

COUNTERION EFFECTS IN CHARGED MONOLAYERS a process described as water structureenforced ion-pairing. This leads to a negative deviation of the activity coefficient. Such a mechanism would seem plausible to explain results obtained with the DCTAB monolayers. The sequence of interaction of the alkali metal ions with monolayers of docosyl sulfate, is virtually the same as that known as the Hofmeister series for floeeulation of negatively charged colloids, viz., Cs, Rb, K, Na, Li; it has also been reported (in some cases partially) for the lowering of CMC (21) and the solubility of aikyl sulfates in water, for the activity coefficients of toluene-(22) and alkane-(23) sulfonates and other strong acid anions (3, 4) and for interactions with strong acid ion exchange resins (5, 24). The above sequence is, however, the opposite of that reported for cation interactions with ionized fatty acid monolayers (1). Known as the "inverse" Hofmeister series, the latter sequence is usually observed with anions derived from weaker acids (3, 4, 5, 25) and this factor is considered in the Robinson-Harned-Stokes (3, 26) hypothesis of "localized hydrolysis" involving highly hydrated cations such as lithium. Several other hypotheses to explain eounterion sequence effects have been proposed, for example, the "order-producing" and "order-destroying" classifications of Gurney (27), the predominating influence of the field strength around the anion as advanced by Eisenman (28), and the importance of the relative polarizabilities of water and the anion as proposed by Teunissen and de Jong (2, 5, 29). We believe, however, that improved knowledge of the properties of positive and negative ions in an aqueous environment, which classification includes monolayer-, miceHe-, resin-, and polyelectrolyte-counterion systems, awaits more fundamental knowledge of the relative and absolute role of factors such as electrostatic interaction, ion hydration and water structure, and specific ionic interaction in these systems (30). We consider finally the interaction of quaternary ammonium ions with long-chain anions. Work based on CMC data of lauryl sulfate has indicated that TAA ions react

623

more strongly with the aikyl sulfate ion than do alkali metal ions. Recently, this subject has been reviewed extensively by Mukerjee, IVIysels, and Kapauan (30) who show that a series of TAA ions fall on a different line from the alkali metal line in a plot of CMC versus counterion size. Interaction increased with increased size of the TAA ion. These results run parallel to results on ion binding by ion exchange resins (31), although it is now known that interactions in the latter case are influenced by the degree of cross-linking of the resin (5). The increased interaction with size of the TAA may indicate increased participation of the alkyl groups of the TAA groups in structure-enforced bonding, but as Boyd et al. (32) point out, this cannot explain the opposite effect noticed in the interaction of TAA with carboxylate resins (or fatty acid monolayers (1)). In this connection it is of interest that, by criteria of monolayer contraction, the results in the present paper do not indicate the association of the TMA ion with an alkyl sulfate monolayer to be particularly strong. At monolayer areas > 60A2/molecule the TMA curves lie between those of Li and Na ions. These results thus appear to be in good agreement with activity coefficient data on methane- and ethane-sulfonates of Gregor et al. (23). On the other hand, with bulky counterions such as TAA ions, it is difficult to separate the opposing effects on surface pressure of counterion penetration into the monolayer and reduction of the 60 potential. We believe that in the TMA system counterion penetration and reorientation effects are in evidence at lower areas as suggested by the maximum in the A V - A plot. ACKNOWLEDGEMENT The authors wish to thank Dr. A. H. Gilbert and Mr. W. F. Pease of the Organic Section for preparing the long-chain materials, Miss Carole Vissers for assistance with the measurements, and the Lever Brothers Company for permission to publish this paper. REFERENCES 1. GODDARD,E. D., KAO, O., ANn KUNG, H. C., J. Colloid and Interface Sci. 24, 297 (1967). orournal of Colloid and Interface Science, Vol. 27, No. 4, A u g u s t 1968

624

GODDARD, KA0 AND KUNG

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Journal of Colloidand Interface Science. Vol. 27. No. 4, August1968