Polydispersity of a Non-ionic Surfactant as Related to Its Adsorption Characteristics on Porous Silica Particles in Water.

The effect of particle porosity on the adsorption of the oligomers of a polydispersed non-ionic surfactant of the alkylpolyoxyethylene series has been investigated at a silica/water interface using a HPLC technique. This surfactant displayed a high degree of polydispersity relative both to alkyl chains and to polar chains. The results obtained are compared to previously determined adsorption isotherms for the same surfactant on a nonporous silica. It is observed that qualitatively, the shape of the isotherms are similar on both silicas with minor differences below the adsorption plateaus. However, quantitatively, the adsorption density is about four times lower on the porous material as compared to the nonporous one. These findings are interpreted using available information on the porosity and on the size of the primary particles. The composition of the adsorbed micellar-like aggregates is shown to be independent upon the structure of the surface particles. Preferential adsorption of the oligomers with the shorter ethoxychains is displayed on both silicas. The polydispersed surfactant used did not follow a Poisson distribution with respect to the ethoxychains and the presence of polydispersed alkyl chains did not influence the shape of the polar chains distribution. Copyright 1998 Academic Press.