Rapid Measurement of Adsorption Isotherms of Emulsifiers on Latex Particles

Abstract

The adsorption of emulsifier on latex particle surfaces often determines the stability of the latex. However, relatively few adsorption isotherms have been published because these experiments are difficult and time-consuming. This paper describes the determination of adsorption isotherms using a new serum replacement technique, which was developed to characterize latex particle surfaces by removing adsorbed emulsifier from the latex particle surfaces and solute electrolyte from the aqueous phase. The latex is confined in a cell with a Nuclepore filter membrane, and doubly-distilled, deionized water is pumped through the latex to replace the serum. This method allows complete recovery of the serum, as well as determination of the concentration profile of the desorbing species in the effluent stream. To determine adsorption isotherms, a known amount of emulsifier is added to a cleaned latex and the concentration profile of the emulsifier in the effluent stream is measured. A material balance between added emulsifier and desorbed emulsifier (from the concentration profile) gives the adsorption isotherm. This adsorption isotherm is correlated with the predictions of a mathematical model of the process. The adsorption of technical-grade sodium lauryl sulfate, Aerosol MA, and Aerosol OT on a Dow monodisperse polystyrene latex of 190 nm diameter determined at 23 ± 2° follows a Langmuir adsorption isotherm. The mole-r culaç areas at saturation are 42 - 43 A² , 39 - 45 A², and 85 -100 A², respectively. The adsorption of sodium lauryl sulfate on a monodisperse polymethyl methacrylate latex of 330 nm diameter also follows a Langmuir absorption isotherm and gives a molecular area at saturation of 57 A². The adsorption of sodium lauryl sulfate on a monodisperse polyvinyl acetate latex of 165 nm diameter does not follow a Langmuir adsorption isotherm, but instead gives a curve characteristic of porous surfaces. The molecular area at saturation is 30 A².