Effects of ionizable groups on the adsorption of surfactants onto latex particle surfaces

Abstract

Soap titration of polymeric latices can be used both for the determination of an average latex particle size and as a method for investigating the adsorption of surfactants onto hydrophobic surfaces. This investigation has studied the adsorption of anionic and nonionic surfactants onto polystyrene latex particles in order to develop an understanding of the relationship between surfactant structural characteristics (chain length and end-group) and the area occupied by an adsorbed molecule. As expected the area occupied by a surfactant molecule increased as the ethylene oxide chain length increased for a series of octaphenoxy ethoxylate nonionic surfactants. The results also indicated that in addition to structural factors the adsorption area of a surfactant molecule is also dependent upon the surface charges present on the particle surface. For the nonionic surfactants studied the adsorption area per molecule increased as the number of charges chemically attached to the particle surface increased (increasing surface polarity). For sodium dodecyl sulfate, however, just the opposite trend was observed. The increase in the amount of sodium dodecyl sulfate adsorbed with increasing surface charge may be due to a more extended configuration for the adsorbed surfactant molecules which gives rise to more surfactant—surfactant interactions. These surface charges, which arise from the initiator, may vary widely from one latex to another depending upon the polymerization method and recipe and may be responsible for some of the scatter in previous soap titration results.