Measures to determine the hydrophobicity of colloidal polymers

Abstract

The role of latex dispersions as well as water-soluble polymers in industrial and pharmaceutical applications (i.e. analytical and drug delivery systems) is growing fast. An important attribute of polymer particles is the high adsorption capacity for organic moieties (e.g. of molecules of biological origin). It is important that the biologically active molecules remain fully hydrated during the adsorption by the colloidal system; therefore, it is essential to have a measure for the hydration of both adsorbent particles and adsorptive molecules and to correlate its value with other properties of the polymeric material. We have estimated the hydrophobic/hydrophilic balance with adsorption isotherms of polymethine dyes (surface probes). The adsorption of the anionic dyes is 10 times stronger on positively charged polystyrene particles compared to the adsorption of the corresponding cationic dyes on negatively charged particles. We found a redshift in the absorption maximum of the anionic dyes after adsorption onto the positively charged particles. A blueshift was observed for cationic dyes bound to negatively charged particles. We deduce from our measurements that the nature of the surface charges present at the particleliquid interphase alters the surface characteristics of the particles. We suppose that cationic charged polymer particles are more hydrophobic than particles from the same matrix with negative surface charges. In a qualitative model, we illustrate the governing role of hydration forces on particle aggregation.