Comparative study of the adsorption of nonionic surfactants: Triton X-100 and C(12)E(7) on polystyrene latex particles using dynamic light scattering and adsorption isotherm measurements

The adsorption of the nonionic surfactants Triton X-100 and C(12)E(7) On predominantly hydrophobic polystyrene latex particles has been examined as a function of surfactant concentration by measuring the hydrodynamic radius by dynamic light scattering and adsorption isotherms by a surface tension technique. The adsorption behavior of these materials differs significantly and is related to the structures of the surfactant ''tails''. Adsorption of TX-100 most probably occurs in an approximately perpendicular geometry with contact between its methyl end groups and the partially extended polymer chains at the latex surface and is accompanied by association between the aromatic nuclei of the adsorbed surfactant molecules. The latter leads to cooperative adsorption and an S-type isotherm. The perpendicular adsorption mode leads to expansion of the polymer chains for steric reasons, giving a pronounced increase in the hydrodynamic radius in the initial adsorption stage. There is a larger adsorbed amount of TX-100 compared with the C(12)E(7). C(12)E(7) has a straight dodecyl chain and an extended molecular conformation. This favors bridging of neighboring polymer chains through interactions both between the alkyl tails and the polymer chains and between the POE chain and the terminal sulfate groups. Bridging leads to aggregation of the polymer chains and reflects the strong interaction between C(12)E(7) and the polymer chain which gives an L-type adsorption isotherm. The adsorption isotherm is two-stepped in TX-100/latex solutions but is single-stepped for C(12)E(7)/latex suspensions, except in 100 mM added NaBr and at 35 degrees C. The characteristic types of adsorption isotherms observed with the two surfactants remain unchanged on altering other parameters, for example, the ionic strength or the temperature. NaBr augments C(12)E(7) adsorption partly owing to salting-out of water from the hydration shell of the POE chain. On the other hand, TX-100 adsorption in 100 mM NaBr is complicated by competition between adsorption and micellization in aqueous solution since there is a pronounced tendency for TX-100 to aggregate in bulk solution at higher ionic strengths. Enhanced adsorption of both surfactants occurs on increasing the temperature, owing to a decrease in surfactant solubility in aqueous solution deriving from a combination of changes in the POE chain conformation and a reduction in the amount of structured water around the surfactant head group.