ELECTRICAL DOUBLE-LAYER PROPERTIES OF HEXADECYLTRIMETHYLAMMONIUM CHLORIDE SURFACES IN AQUEOUS-SOLUTION

The effect of aqueous KCl concentration on the electrical double layer properties of hexadecyltrimethylammonium chloride (CTAC) surfaces has been investigated. For self-assembled CTAC bilayers adsorbed onto amo rphous silica surfaces, flat plate streaming potentials (zeta(s) poten tials), electrostatic potentials obtained from the electrophoretic mob ility of colloid microspheres (zeta(e) potentials) and diffuse layer p otentials (psi(d)) derived from the force versus separation curves for the interaction of a colloid microsphere with a flat plate (measured with an atomic force microscope) have been analysed. Trends and differ ences in the zeta(s) and zeta(e) potentials have been attributed to in complete electrical neutralisation of the silica surface by the inner adsorbed monolayer of CTAC, and differences in the silica surface char ge density for the colloid and flat plate. It is considered that deriv ed psi(d), values highlight the problem created by surface roughness f or atomic force microscopy force curve analysis. CTAC miceliar surface potentials (psi(0)), which have been determined by a solvatochromic a cid-base indicator technique, are greater than the zeta potentials for the planar adsorbed CTAC surfaces. Analyses in terms of a site-bindin g model and classical electrical double layer theory suggest that the degree of counterion dissociation from the surfactant headgroups (alph a) is significantly different for highly curved and planar surfaces; a is in the range 0.10-0.15 for the adsorbed CTAC bilayers and in the r ange 0.41-0.66 for spherical CTAC micelles when activities are used in the calculation. The CTAC critical micelle concentration and the mini mum area per CTAC molecule at a planar interface as a function of KCl concentration have been ascertained from air/aqueous solution surface tension curves. The hydrophobic contribution to the free energy of CTA C micellisation has been calculated on the basis that CTAC micelles be have as Nernstian objects in KCI solution with the surfactant monomer acting as a potential determining ion. The results for the CTAC/KCl sy stems have been compared with the results obtained for similar hexadec yltrimethylammonium bromide/KBr systems.