Abstract
The adsorption of cationic and anionic surfactants on three hydrophilic silicas and one hydrophobic (methylated) colloidal silica was studied in the presence of organic solvents. Cationic surfactants (alkylpyridinium chlorides and alkyltrimethylammonium chlorides) are strongly bound by hydrophilic silicas in a weak polar solvent (CHCl3). While pyridinium and ammonium as the head group have no marked influence upon adsorption, the hydrocarbon chain length (in the range C4-C10) determines the surface areas occupied by adsorbed surfactant molecules. Homologs with longer hydrocarbon chains (C12-C18) all occupy equal areas in the saturated adsorption layer. Polar organic solvents (alcohols) reduce the adsorption of cationic surfactants on hydrophilic silicas. In the presence of dimethyl sulfoxide, no adsorption is observed. Maximum adsorption of dioctyl sodium sulfosuccinate is attained in the presence of carbon tetrachloride. Solvents with hydrogen-bonding abilities (dimethyl sulfoxide and dioxane) prevent adsorption of anionic surfactants (dioctyl sodium sulfosuccinate and sodium alkyl sulfates). It is suggested that the silanol groups on the surface, which are capable of hydrogen bonding and electrostatic interactions, are the active adsorption sites of the silica. If their number is reduced, the adsorption is diminished to the same extent.
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