Coadsorption of Steroids and Nonionic Surfactants on Polystyrene Latex Particles from Aqueous Solutions

The effect of the adsorption of two nonionic surfactants, Triton X-100 and a copolymer, poloxamer 188, on the coadsorption (adsolubilization) of three steroids, hydrocortisone, testosterone, and progesterone, on latex polystyrene nanoparticles has been investigated. It is shown that the formation of surfactant aggregates on the nanoparticles induces the uptake from the aqueous solution of up to 50 wt% of the total concentration of those steroids which are not adsorbed at the solid/liquid interface in the absence of surfactant. Above the equilibrium surfactant critical micelle concentration, the solutes are distributed between the adsorbed surfactant aggregates at the solid surface and the free micelles. The most hydrophobic molecules, progesterone, at all concentrations, and testosterone, at higher concentrations, are adsorbed on the particles in the absence of surfactant. The influence of steroid concentration, the hydrophobicity of the solutes, and the effect of added ethanol were investigated, especially in the case of hydrocortisone. Although, in absolute values, the steroid coadsorption is larger at higher total concentrations, the relative solute uptake is larger at lower concentrations. The addition of ethanol has a small effect on the adsorption of Triton X-100 but a large effect on the coadsorption of hydrocortisone. An attempt has been made to evaluate the solutes partition coefficient between the adsorbed surfactant aggregates and the aqueous solution. Adsorption of Triton X-100 or poloxamer 188 aggregates onto the polystyrene latex particles induces solute partition coefficient values on the same order of magnitude when expressed on the molar scale. For all systems studied, the coadsorption partition coefficients are larger than the corresponding values for the classical micellar solubilization effect.