ADSORPTION OF MIXED CATIONIC AND NONIONIC SURFACTANTS AT THE HYDROPHILIC SILICON SURFACE FROM AQUEOUS-SOLUTION - STUDIED BY SPECULAR NEUTRON REFLECTION

Specular neutron reflection has been used to determine the structure a nd composition of the mixed cationic and nonionic surfactants of hexad ecyltrimethylammonium bromide, C(16)TAB, and hexaethylene glycol monod odecyl ether, C12E6, adsorbed at the hydrophilic silica solid/water in terface. Measurements have been made at two different values of pH (2. 4 and 7.0), where the relative affinity for the adsorption of the two surfactants at the interface is dramatically altered. At high pH (pH = 7.0) the composition of the adsorbed bilayer is dominated by C(16)TAB and the structure of the bilayer is nonuniform: that is, the surfacta nt layer immediately adjacent to the solid silica surface is richer in C(16)TAB. At the lower pH (pH = 2.4), where the affinity for the surf ace of C12E6 is much greater relative to that of C(16)TAB, the surface composition is closer to the bulk solution composition and the struct ure of the adsorbed bilayer is essentially uniform.