Assessment of the depletion forces in mixtures of a latex and a non-adsorbing polymer by turbidimetry

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 = (4pin0/lambda0)sin(θ/2), where θ is the scattering angle, n0 is the refractive index of the medium, and lambda0 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, Mol. 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. Copyright 1999 Academic Press.

Probing Static Structure of Colloid-Polymer Suspensions with Multiply Scattered Light

Time-dependent measurements of light propagation were conducted in aqueous dispersions of 523 nm diameter polystyrene at concentrations between 0.1 and 0.4 solids volume fraction in order to assess how particle correlation is influenced by depletion interactions arising from the addition of soluble polyethyleneoxide (PEO). In the absence of polymer, the transport scattering length can be predicted from Mie scattering theory and the Percus-Yevick (P-Y) model for static structure of a dense hard-sphere colloidal solution. Depletion forces arising from the addition of PEO of varying molecular weights influenced the spatial ordering of the dispersion and caused a further increase in the transport scattering length beyond that predicted by hard-sphere static structure factor but similar to that predicted by the mean sphere approximation (MSA) to the P-Y model described by Ye et al. (1996). Onset of flocculation occurred with increased PEO addition and correlated with PEO molecular weight. Phase separation was noted by no further change in the transport scattering length, except when flocculation was induced by the highest molecular weight PEO. The use of time-dependent measurements of light propagation in dense systems provides an alternative to small-angle light, neutron, and X-ray scattering characterization of interaction potentials in dense, multiply scattering samples and promises further fruitful investigation of colloidal particle interactions in suspensions. Copyright 1999 Academic Press.

Adsorption of Anions on the Surface of Charge-Free Latex Particles as Revealed by Turbidimetric Measurements

The preferential adsorption of anions on the surface of latex particles without permanent charges is investigated. The latex consists of particles of 83 nm diameter stabilized by a thin layer (1.3 nm) of polyethylene oxide chains. The serum of the latex is adjusted to an ionic strength of 0.001 by adding different salts (KCl, KI, LiF, LiI, NaI). The structure of the latex particles has been studied by small-angle X-ray scattering and by dynamic light scattering whereas the interaction of the particles was investigated by turbidimetry. The latter method gives clear indication that the particles repel each other due to the electrostatic repulsion. The effect is much more pronounced in the presence of I- anions than in the presence of Cl- or F-. Since all other mechanisms of long-range repulsive interactions can be ruled out, the results presented herein demonstrate the preferential adsorption of anions, in particular of I- onto the latex surface. Copyright 1998 Academic Press.