Analysis of attractive interactions between latex particles in the presence of nonadsorbing polymers by turbidimetry

The analysis of attractive forces between latex particles by scattering mea surements is reconsidered. The change effected on the structure factor S(q) (q = (4 pi n(0)/lambda(0))sin(theta/2), where theta is the scattering angl e, n(0) is the refractive index of the medium, and lambda(0) is the wavelen gth in vacuo) of a suspension of latex particles by weak attractions is con sidered in terms of the HMSA-integral equation theory as presented recently by Bergenholtz et al. (1996, Mel. Phys. 87, 331). Model calculations using this approach show that turbidimetry is highly suitable to obtain quantita tive information on attractive forces between the latex particles. The meth od of analysis developed here is applied to previous turbidimetric data obt ained from mixtures of a polystyrene latex and a nonadsorbing polymer, hydr oxyethylcellulose (HEC) (1997, Langmuir 13, 551). Here the addition of HEC leads to attractive depletion forces between the PS particles by the nonbal anced osmotic pressure of the polymeric HEC molecules. The turbidimetric da ta which are not afflicted by multiple scattering can be analyzed quantitat ively in terms of an Asakura-Oosawa interaction potential between the latex particles. The osmotic pressure derived from this analysis is much smaller than the actual osmotic pressure exerted by the dissolved HEC. Possible re asons for this discrepancy are discussed. (C) 1999 Academic Press.