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-b ase indicator has been used to determine the surface potential (psi(0) ) of CTAB micelles. Three different techniques have been employed to e xamine self-assembled CTAB bilayers adsorbed on amorphous silica surfa ces. Flat plate streaming potentials (zeta(s) potentials), electrostat ic potentials obtained from the electrophoretic mobility of relatively large colloid spheres (zeta(e) potentials), and diffuse layer potenti als (psi(d)) derived from the force versus separation curves for the i nteraction of a colloid sphere with a flat plate (measured with an ato mic force microscope) have been analyzed. zeta(s) and zeta(e) potentia ls were found to be equivalent over a wide range of aqueous HBr concen tration. psi(d) values in accord with zeta(s) and zeta(e) potentials w ere only derived in CTAB systems with a relatively low KBr concentrati on. At relatively high KBr concentrations (> 10(-3) mol dm(-3)), psi(d ) values were less than zeta(s) and zeta(e) potentials. This discrepan cy has been attributed to the influence of surface roughness on the AF M force curve analysis. Micellar psi(0) values were larger than zeta p otentials. Analyses in terms of classical electrical double layer theo ry suggest that the degree of counterion dissociation from the surfact ant headgroups (alpha) is markedly different for highly curved and pla nar surfaces; alpha is in the range 0.05-0.11 for the adsorbed CTAB bi layers and in the range 0.29-0.43 for spherical CTAB micelles. Air/wat er surface tension curves have been employed to illustrate the effect that KBr has on minimum area per CTAB molecule at a planar interface a nd to provide values for the critical micelle concentration as a funct ion of KBr concentration. CTAB micelles behave as Nernstian objects in KBr solution with the surfactant monomer acting as the potential dete rmining ion. On the basis of this Nernstian behavior, a simple method has been employed to calculate the hydrophobic contribution to the fre e energy of CTAB micellization.