COMPARISON OF TECHNIQUES FOR MEASURING THE ELECTRICAL DOUBLE-LAYER PROPERTIES OF SURFACES IN AQUEOUS-SOLUTION - HEXADECYLTRIMETHYLAMMONIUM BROMIDE SELF-ASSEMBLY STRUCTURES AS A MODEL SYSTEM

The effect of aqueous KBr concentration on the electrical double layer properties of two types of self-assembled hexadecyltrimethylammonium bromide (CTAB) surfaces has been investigated. A solvatochromic acid-base indicator has been used to determine the surface potential (psi(0)) of CTAB micelles. Three different techniques have been employed to examine self-assembled CTAB bilayers adsorbed on amorphous silica surfaces. Flat plate streaming potentials (zeta(s) potentials), electrostatic potentials obtained from the electrophoretic mobility of relatively large colloid spheres (zeta(e) potentials), and diffuse layer potentials (psi(d)) derived from the force versus separation curves for the interaction of a colloid sphere with a flat plate (measured with an atomic force microscope) have been analyzed. zeta(s) and zeta(e) potentials were found to be equivalent over a wide range of aqueous HBr concentration. psi(d) values in accord with zeta(s) and zeta(e) potentials were only derived in CTAB systems with a relatively low KBr concentration. At relatively high KBr concentrations (> 10(-3) mol dm(-3)), psi(d) values were less than zeta(s) and zeta(e) potentials. This discrepancy has been attributed to the influence of surface roughness on the AFM force curve analysis. Micellar psi(0) values were larger than zeta potentials. Analyses in terms of classical electrical double layer theory suggest that the degree of counterion dissociation from the surfactant headgroups (alpha) is markedly different for highly curved and planar surfaces; alpha is in the range 0.05-0.11 for the adsorbed CTAB bilayers and in the range 0.29-0.43 for spherical CTAB micelles. Air/water surface tension curves have been employed to illustrate the effect that KBr has on minimum area per CTAB molecule at a planar interface and to provide values for the critical micelle concentration as a function of KBr concentration. CTAB micelles behave as Nernstian objects in KBr solution with the surfactant monomer acting as the potential determining ion. On the basis of this Nernstian behavior, a simple method has been employed to calculate the hydrophobic contribution to the free energy of CTAB micellization.