3828
J . Phys. Chem. 1987, 91, 3828-3829
A Comparison of Surfactant Counterion Effects in Water and Formamide J. B. Evans and D. F. Evans* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 (Received: December 29, 1986)
The colloidal forces between dihexadecyldimethylammonium bilayers immersed in formamide were measured by using the surface forces apparatus. The forces were determined as a function of added sodivm acetate and potassium bromide and found to be almost identical with those observed in water. These observations establish that the unusual counterion effects observed for amphiphiles in aqueous solution are not a consequence of the unique structural properties of water.
The self-assembly of surfactants is not unique to water but is a general feature of polar hydrogen-bonding solvents including hydrazine,1-2ethylammonium nitrate,* f ~ r m a m i d eand , ~ ethylene glyc01.~ The characterization of surfactant aggregation in these nonaqueous solvents has focused mainly on the role of solvophobicity in driving the aggregation processes. The role of counterion effects which are so prominent in water has been virtually ignored in other solvents. In this note we present surface forces measurements which compare counterion effects in formamide and water. In aqueous solution, counterions play an important role in determining the type of amphiphilic microstructure formed. For example, at room temperature dialkyldimethylammonium halides form bilayers while the corresponding carboxylate and hydroxides form spontaneous vesicles in dilute solution and spherical micelles in more concentrated s ~ l u t i o n . ~Similar types of transformations are observed with double-chained anionic surfactants such as SHBS (sodium 8-phenyl- 1-hexadecane-p-sulfonate)when the counterion interaction is modulated by the addition of cryptate complexing agents. These observations suggest that the aggregation patterns of double-chained surfactants is more subtle than previously thought. For the cationic surfactants the following story has emerged. With alkyltrimethylammonium acetate and hydroxide micelles, counterion binding is greatly reduced compared to the halide^.^ This in turn increases head group repulsion, increases curvature at the aggregate-water interface, and results in micelles with lower aggregation numbers. These observations can be rationalized in terms of theory provided that the average counterion distance for the carboxylates is displaced from the surfactant head groups by 4 A.S This implies a higher degree of hydration by water for the carboxylates compared to the halides. Extension of these ideas to double-chained surfactants suggests that substitution of carboxylates for the halides would result in similar curvature effects and thereby transform bilayers to vesicles. Further insight into the difference between the halides and carboxylates in aqueous solution comes from surface forces measurement^.^ In these experiments, dihexadecyldimethylammonium bilayers are formed on the two mica surfaces and the force vs. distance curves measured as a function of added sodium acetate and potassium bromide. With the acetates, a repulsive ( 1 ) Ramadan, M.; Evans, D. F.; Lumry, R. J. Phys. Chem. 1983,87,4538. (2) Evans, D. F.; Yamauchi, A.; Roman, R.; Casassa, E. Z. J . Colloid Interface Sci. 1982, 86,89. ( 3 ) Couper, A.; Gladden, G. P.; Ingram, B. T. Faraday Discuss. Chem. Soc. 1976, 59, 63. (4)Ray, A. Nature (London) 1971, 231, 313. (5) Brady, J.; Evans, D. F.; Warr, G.; Greiser, F.; Ninham, B. W. J. Phys. Chem. 1986, 90, 1853. (6) Miller, D.D.;Evans, D. F.; Warr, G. G.; Bellare, J. R.; Ninham, B. W.J . Colloid Interface Sci., in press. (7) Miller, D. D.; Bellare, J. R.; Evans, D. F.; Talmon, Y.; Ninham, B. W. J . Phys. Chem. 1987, 91, 614. (8) Evans, D. F.; Ninham, B. W. J. Phys. Chem. 1985, 88, 6344. (9) Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Brady, J.; Evans, D . F. J . Phys. Chem. 1986, 90, 1631.
0022-3654/87/2091-3828$01.50/0
force is detected at all distances of separation between the two surfaces. The force vs. distance curves give complete agreement with the DLVO theory assuming complete dissociation of the acetate. With the bromide, 90% ion binding is required to fit the data, and when the surfaces are brought close together, the outermost layer of each bilayer is sloughed off, and the surfaces jump into hyrophobic contact. Formamide is difficult to prepare in a pure form. Drying with molecular sieves, followed by distillation and passage over a specially prepared ion exchange column, results in formamide with a specific conductance of 1 X IO-’ ohm-’ cm-’ and 0.01 M water content.I0 Since it is almost impossible to remove all traces of water, hydrolysis results in the continual formation of ammonium formate. In our experiments, this is tolerable since both formate and acetate give very similar behavior and the bromide effects are discernible in the presence of low concentrations of carboxylates. Consequently, we used reagent grade formamide (Fisher) and passed it over Rexyn 100 mixed bed ion exchanger and through a Millipore H F 0.2 pm filter. The resin was preequilibrated by adding increasing amounts of FA to remove the residual water. After passage through the ion-exchange column, the formamide had a specific conductance of (5-9) X IO” ohm-’ cm-I which corresponds to an ammonium formate concentration of (4-9) X lo4 M. The dihexadecyldimethylammonium acetate (DHDAA) was prepared by passing the bromide over an anionic ion exchange resin in the hydroxide form, titration with acetic acid, and recrystallization from ethanol. Reagent-grade sodium acetate and potassium bromide were used as received. The surface forces measurements followed the general procudure given elsewhere.9,‘‘ Our apparatus is similar to that described by Israelachvili and Adams,” except for a number of mechanical design features, which will be detailed in a subsequent publication. The measurements were carried out in a small 30-mL stainless steel bath which attached directly to the double cantilever spring. Force measurements in FA exhibit a strong repulsive solvation force at close distances. Short-range solvation forces decrease exponentially with a decay constant equal to 2-4 8, (the diameter of a solvent molecule) and consequently are important only when the surfaces are separated by distances on the order of tens of angstroms. The repulsive force in formamide is attributable to the adsorption of ammonium ions on the surface of the mica. Evidence for this is gained from the fact that “contact” of the mica surface in the solvent is displaced outward on the order of 10 8, from the mica-mica contact in air. Consequently, the zero distance shown in Figures 1 and 2 does not correspond to the mica surfaces coming into direct contact. The measurements were started by injecting 18 mL of formamide directly into the small bath. Next, 3 mL of dihexadecyldimethylammonium acetate solution was injected into the bath to give a final concentration M. After a bilayer had formed on each surface as of 5 X evidenced by the 29 i~ 1 A increase in the contact distance (Figure (10)Thomas, J.; Evans, D. F. J. phys. Chem. 1970, 74, 3812. ( 1 1 ) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc. Faraday Trans. J 1978, 74, 915.
0 1987 American Chemical Society
The Journal of Physical Chemistry. Vol. 91, No. 14, 1987 3829
Counterion Effects in Water and Formamide
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2 insert), 3-mL aliquots of either potassium bromide or sodium acetate were added. A concentration range extending from 3 X lo4 to 3 X M was studied. Typical results are shown in Figures 1 and 2 for added acetate and bromide; a complete description will be given in a subsequent publication. The data were analyzed with the DLVO theory by using a nonretarded Hamaker J. This value is slightly lower than that constant of 1 X estimated12 by using the refractive index of 1.429 and a characrad s-I obtained from teristic w(UV) frequency of 1.53 X a Cauchy plot;I3 the Hamaker constant will be reduced by the presence of bilayers on the mica. Note that the theoretical curves are displaced 29 8,to the right in Figures 1 and 2 in order to make them coincide with the bilayer contact distance. With added sodium acetate, the force curves are repulsive at all distances down to bilayer contact, the same result as for water. The solid lines represent the theoretical curve assuming a constant charge density of 0.23 C m-2 (70 A2 per head group) and no counterion binding. The Debye length ( K - ~ ) used in these calculations is the sum of the concentration determined from K-I required to fit the bilayer force curves (Figure 2 insert) plus added salt. The Debye lengths for the formamide and for bilayers are ~
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Horn, R. G.; Israelachvili, J. N. J . Chem. Phys. 1981, 75, 1400. (13) Hough, D. B.;White, L. R. Adu. Colloid Interface Sci. 1980,14,3.
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Figure 1. Force measured between DHDAA bilayers adsorbed on mica with 3.4 X M potassium bromide in formamide. After the bilayers come into contact, increasing the force results in the surfaces reproducibly jumping 20 A into solvophobic contact. The solid line represents the DLVO fit to the data assuming constant charge of 0.03 C m-* (Le., 87% Br- binding) and a nonretarded Hamaker constant of J. The dashed curve is the DLVO calculated value assuming no counterion binding. The Debye length of 48 A was calculated from the concentration of KBr plus the concentration obtained by fitting the bilayer data shown in the insert of Figure 2. The existence of a solvation force in formamide associated with adsorbed ammonium ions (with no surfactant) is shown in the insert of Figure 1.
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Figure 2. Measured forces between DHDAA bilayers with 1.8 X lo-’ (0)and 1.4 X lo-* (A)sodium acetate. The arrows indicate which axis the curves refer to. The solid line gives the DLVO fit assuming a charge density of 0.23 (70 A2per head group, Le., no counterion binding) and Debye lengths of 60 and 33 A. The insert shows bilayer formation upon addition of 5 X lo4 M DHDAA as evidenced by a 29 f 1 A increase in the contact distance over that observed with only formamide (insert Figure 1). The fitted curve corresponds to a charge density of 0.03 C m-2 and a Debye length of 80 A.
smaller than usually encountered in solvents due to the presence of the ammonium formate. For potassium bromide, the force curves at all distances are considerably reduced compared to sodium acetate. When the surfaces are at a distance corresponding to bilayer contact and the force is increased, the surfaces reproducibly jump 20 A into solvophobic contact. In water the jump distance is 27 A. These results suggest that the bilayer thickness in formamide (-20 A) is less than in water (-27 A). The theoretical curve is calculated by using a charge density of 0.02 C m-2 (600 A2 per head group). This corresponds to 80-90% counterion binding. For comparison, the calculated curve (0.23 C mW2)corresponding to no counterion binding is also shown in Figure 2. These observations establish that the unusual counterion effects observed for surfactant aggregates in aqueous solution cannot be ascribed to the unique structural properties of water but must be a more general feature of hydrogen-bonding solvents. In one sense this is reassuring. The solvation effects associated with specific counterions located near charged interfaces must be due to a short-range interaction arising from hydrogen bonding and dipolar effects and consequently must involve only a few solvent molecules. How the subtle properties of water affect and direct amphiphilic aggregation remains elusive.
Acknowledgment. This work was supported by N I H (Grant GM 34341). We thank Dr.G. G. Warr for assistance in writing computer programs.
