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 exam ined as a function of surfactant concentration by measuring the hydrod ynamic radius by dynamic light scattering and adsorption isotherms by a surface tension technique. The adsorption behavior of these material s differs significantly and is related to the structures of the surfac tant ''tails''. Adsorption of TX-100 most probably occurs in an approx imately perpendicular geometry with contact between its methyl end gro ups and the partially extended polymer chains at the latex surface and is accompanied by association between the aromatic nuclei of the adso rbed surfactant molecules. The latter leads to cooperative adsorption and an S-type isotherm. The perpendicular adsorption mode leads to exp ansion of the polymer chains for steric reasons, giving a pronounced i ncrease in the hydrodynamic radius in the initial adsorption stage. Th ere 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 conf ormation. This favors bridging of neighboring polymer chains through i nteractions both between the alkyl tails and the polymer chains and be tween 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 adsorpt ion isotherm. The adsorption isotherm is two-stepped in TX-100/latex s olutions 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 unchang ed on altering other parameters, for example, the ionic strength or th e temperature. NaBr augments C(12)E(7) adsorption partly owing to salt ing-out of water from the hydration shell of the POE chain. On the oth er hand, TX-100 adsorption in 100 mM NaBr is complicated by competitio n 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 solu bility in aqueous solution deriving from a combination of changes in t he POE chain conformation and a reduction in the amount of structured water around the surfactant head group.