Langmuir 1996, 12, 4111-4115
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Adsorption of Dipolar (Zwitterionic) Surfactants to Dipolar Surfaces Patricia Chavez,† William Ducker,†,‡ Jacob Israelachvili,*,† and Kathryn Maxwell‡ Materials Research Laboratory, College of Engineering, University of California, Santa Barbara, California 93106, and Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand Received September 22, 1995. In Final Form: May 2, 1996X The adsorption of the neutral but dipolar (zwitterionic) surfactants, dodecyldimethyl(3-sulfopropyl)ammonium hydroxide and diheptanoic-sn-glycero-3-phosphocholine to the silicon nitride/water interface has been studied via contact angle measurements. Adsorption is highest in the pH range where the Si3N4 surface is also electrically neutral, but dipolar. The adsorption decreases sharply above the isoelectric point of Si3N4, when the surface is negatively charged, and is more complex below the isoelectric point, when the surface charge is positive. Qualitatively similar results were obtained with alumina (Al2O3) surfaces. The results indicate the importance of discrete or individual charge-charge interactions, rather than the net or smeared-out surface charge interaction in driving the binding of zwitterionic surfactants to amphoteric surfaces that carry both positive and negative charges.
Introduction The physi-sorption of surfactant from solution to an interface depends on the intermolecular forces between the surfactant and substrate, for example, the electrostatic, van der Waals, hydrophobic and steric forces. Adsorption thus depends on the concentration, type and geometry of the chemical groups on the surfactant, the solvent, and at the interface. Here we examine the adsorption of two zwitterionic surfactants to two “amphoteric” surfaces that contain both positive and negative charges, with the aim of determining which factors are most important in the electrostatic interaction. We stress, however, that contact angle measurements do not provide the thermodynamic adsorption isotherms nor the actual surfactant structures adsorbed on the surfaces, although they do provide qualitative insights into both. The adsorption of zwitterionic surfactants to ceramic surfaces has wider interest because of its potential use in the processing of ceramics. Before a solid ceramic is sintered from colloidal particles, it is desirable to obtain a high density colloidal suspension that is cohesive but weak enough to allow the particles to rearrange.1 In principle, such properties should occur when the interparticle potential has a minimum deeper than several kT at some small but finite surface separation, with a stabilizing short-range repulsion or force-barrier closerin. If the barrier is wide and high enough to adsorb applied strains and stresses, then the particles should be able to rearrange to obtain maximum density and also be able to resist fracture. Adsorbed zwitterionic surfactants with their headgroups facing the solution meet this idealized force-distance behavior through five simultaneously occurring mechanisms:2 (1) the presence of a thin hydrocarbon layer on each surface decreases the otherwise strong van der Waals attractive force between the ceramic particles across water; (2) the strong steric-hydration * To whom correspondence should be addressed: phone, (805) 893 8407; fax, (805) 893 4731; e-mail,
[email protected]. † University of California. ‡ University of Otago. X Abstract published in Advance ACS Abstracts, July 15, 1996. (1) Russel, W. B. J. Rheol. 1980, 24, 287. Lange, F. F. J. Am. Ceram. Soc. 1989, 72, 3. Horn, R. G. J. Am. Ceram. Soc. 1990, 73, 1117. Velamakanni, B. H.; Chang, J. C.; Lange F. F.; Pearson, D. S. Langmuir 1990, 6 , 1323. (2) Ducker, W. A.; Clarke, D. R. Colloids Surf. A 1994, 94, 275.
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forces between the headgroups across water provide a short-range (100 mM) where the Debye screening length (λD ≈ 1 nm) becomes comparable to the distance between the charges on the headgroups and/or surfaces, the electrostatic binding between the headgroup and surface should be enhanced, as observed. However, the enhanced ionic binding to surfactants and surfaces at high ionic strengths is also known to increase their hydration, which in turn also affects their steric-hydration interactions.11 In summary, at low pH and high bulk Cl- concentrations, since the concentration of Cl- is high both in the diffuse (∼1 nm) double-layer and adsorbed on the surface, this will decrease the repulsive interaction between the cations on the surface and surfactant, thus promoting adsorption. Effects of Surface Chemistry: Results on Alumina Surfaces. To establish the generality of our results, some experiments were also conducted with C12SOB solutions on an alumina surface, which has a higher PZC (6-9.5) than silicon nitride, but which also contains positive and negative sites. The results are shown in Figure 8. The similarity with the results of Figure 6 suggests that similar adsorption mechanisms occur on these two chemically different but electrically similar surfaces. Summary and Conclusions We have examined the effects of electrostatic interactions on the adsorption of two dipolar surfactants onto two dipolar surfaces by varying the ionic strength and pH which alter the charge-charge interactions of the headgroups and surfaces. Adsorption of zwitterionic surfactants to silicon nitride is high when both the surface and surfactant are overall electrically neutral but contain both positive and negative charges, i.e., when they are dipolar or zwitterionic. Thus, consideration of only the net charge of each species does not lead to an accurate prediction of adsorption or binding; it is necessary to consider the individual charge-charge interactions. A similar conclu(11) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1991; Chapters 13 and 18. (12) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1991; Chapters 3 and 4.
Adsorption of Dipolar Surfactants
Figure 8. Contact angles of aqueous solutions of 1.1 × 10-4 M C12SOB on alumina Al2O3 surfaces (pKS or IEP between 6 and 9.5) as a function of pH.
sion was arrived at in a recent study of the pH-dependent adsorption of gelatin (an amphoteric polyelectrolyte) to mica surfaces,13 and the importance of “electrostatic complementarity” is a well-known factor in the binding of biological macromolecules.14 In the case where the surfactant is zwitterionic and the surfaces carry a net charge, two limits can be distinguished. A high pH, when the surface is net negative and the surfactant is zwitterionic, adsorption is very low even though binding could occur between the negative groups on the surface and the positive groups on the surfactant (cf. Figure 1A). In contrast, at low pH, when the surface is net positive and the surfactant is zwitterionic, adsorp(13) Kamiyama, Y.; Israelachvili, J. N. Macromolecules 1992, 25, 5081. (14) Dean, P. M. Molecular Foundations of Drug-Receptor Interactions; Cambridge University Press: Cambridge, 1987; Chapter 7.
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tion is high. These findings show (a) that it is not sufficient to treat the interaction as if it were between a freely rotating dipole and a charge, which is always attractive.12 and (b) that the binding is dominated by the positive groups (e.g., silylammonium) on the surfaces and the negative groups (e.g., sulfonate) on the surfactant. Possible reasons for the different behaviors at high and low pH are specific ion size and hydration effects and/or competitive binding of other ions; for example, at high pH hydrated sodium ions may compete successfully for the negative sites on the surfaces and OH- ions for the positive charges on the surfactant, whereas at low pH protons and Cl- ions may not compete so effectively. Finally, when both the surfactant and the surface bear the same charge (e.g., C7C7PC at low pH) adsorption is low, as expected. Thus, for zwitterion surfactant adsorption, it is necessary to consider the discrete charge-charge interactions rather than the smeared-out charge, as is commonly done when considering surface interactions (e.g., based on the double-layer forces). The fact that the electrostatic binding does not appear to be affected when the order of the + and - sites on the surfactant heads are reversed suggests that the strongest binding is due to discrete ion-pair bonds, or dipole-dipole bonds, as illustrated in parts B and C of Figure 7. Both of these interactions are known to give rise to strong binding energies of similar magnitude.12 The ability to manipulate surfactant adsorption to ceramic surfaces should enable better control of the rheology and colloidal interactions of concentrated colloidal suspensions and slurries, as required, for example, during colloidal processing. Acknowledgment. This work was carried out in the Materials Research Laboratory at UCSB which is supported by a grant from the National Science Foundation (DMR-9123048). LA950793G
