INTERACTIONS BETWEEN SODIUM DODECYL-SULFATE AND STYRENE-BUTADIENE COPOLYMER LATEX-PARTICLES WITH CARBOXYL GROUPS STUDIED USING DYNAMIC LIGHT-SCATTERING AND ELECTROPHORETIC MOBILITY MEASUREMENTS

The adsorption of the charged surfactant SDS to hydrophobic styrene-butadiene copolymer latex particles, surface-modified with carboxyl groups, has been examined as a function of the SDS concentration by determination of the particle hydrodynamic radius (RH) using dynamic light scattering and the particle mobility by electrophoretic mobility measurements. Adsorption isotherms were also determined by using a surface tension method. The observed maxima in RH and the mobility as a function of added salt, SDS concentration, and pH are the result of competing processes sited at the surface ''hairy'' layer of the latex particle. Adsorption is strong at zero and low levels of added salt (1 mM of low molecular weight electrolyte) on the partly extended polymer chains at the surface of the latex particle. Close to the critical micelle concentration of SDS, micelles are formed which are anchored these chains. At a higher salt level (32 mM), a further increase of the size of the primarily adsorbed SDS micelles is observed. Increased adsorption was also observed at pH < 4. A possible explanation is hydrogen bonding between protons of the carboxyl groups at the latex surface and the polar groups of the SDS molecules with the formation of a double surfactant layer. In order to facilitate interpretation of the adsorption phenomena, the properties ofthe latex particle in the absence of SDS were also examined.