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 measurements 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 angle, n(0) is the refractive index of the medium, and lambda(0) is the wavelength in vacuo) of a suspension of latex particles by weak attractions is considered 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 quantitative information on attractive forces between the latex particles. The method of analysis developed here is applied to previous turbidimetric data obtained from mixtures of a polystyrene latex and a nonadsorbing polymer, hydroxyethylcellulose (HEC) (1997, Langmuir 13, 551). Here the addition of HEC leads to attractive depletion forces between the PS particles by the nonbalanced osmotic pressure of the polymeric HEC molecules. The turbidimetric data which are not afflicted by multiple scattering can be analyzed quantitatively 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 reasons for this discrepancy are discussed. (C) 1999 Academic Press.