Interactions in mixtures of latex particles and polymers as revealed by turbidimetry

Interaction potentials, structural ordering and effective charges in dispersions of charged colloidal particles

As colloidal dispersions of charged particles exhibit a wide variety of commercial, technological and scientific applications, a considerable theoretical effort has been devoted to finding an effective interaction potential from primitive models. The forces derived from this potential should justify the spatial ordering experimentally observed under certain conditions. This paper reviews the advances in these theoretical studies as well as some experiments (based on the mentioned order) that try to corroborate them. Special attention has been paid to the Derjaguin-Landau-Verwey-Overbeek (DLVO) potential. Nowadays, many of these theoretical investigations suggest that it could be applied if some of its parameters are renormalized. Nevertheless, to achieve a renormalization procedure in a strict way (from a primitive model) is a difficult task as a result of the size and charge asymmetries between small ions and macroions. Thus, several procedures for computing renormalized charges in a simple way have been developed. However, the notion of effective charge has also been widely used (as a adjustable parameter) in order to justify results found for several kinds of colloids (like solid particle dispersions or micellar systems) by means of quite different experimental techniques. Renormalization (as well as ion condensation) approaches, experiments and the controversial relationship between theoretical and phenomenological effective charges are also reviewed in this work.

Probing Electrostatic Forces in Colloidal Suspensions through Turbidity Data

This paper deals with the use of turbidimetry to obtain information on forces between negatively charged polymeric particles at very low ionic strength. The specific turbidity of deionized colloidal suspensions is measured and compared with the values obtained for suspensions of noninteracting particles. From these data, the integrated structure factors are experimentally obtained for several latex samples. These functions are fitted using the repulsive part of the DLVO potential, the Ornstein-Zernike equation, and the hyper-netted chain closure, which enables us to interpret the results in terms of an effective charge. The reliability of these values is discussed. In addition, a comparison between turbidimetry and light scattering data is presented. Copyright 1999 Academic Press.

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.

Investigation of static structure factor in dense suspensions by use of multiply scattered light

Near-infrared, frequency-domain photon migration measurements of phase shift are used to derive accurate values of isotropic scattering coefficients in concentrated, interacting suspensions of aqueous polystyrene microspheres with volume concentrations ranging from 1% to 45% by solids and mean diameters ranging from 135 to 500 nm. Under conditions of high ionic strength, the isotropic scattering coefficient can be quantitatively predicted by the Percus-Yevick model for hard-sphere interactions and Mie theory. In addition, the attractive interactions between scatterers arising from the addition of soluble poly(ethylene glycol) with molecular weights of 100 and 600 K cause hindered scattering. The increases in static structure and decreases in isotropic scattering coefficient agree with that predicted by Mie theory and the depletion interaction model developed by Asakura and Oosawa [J. Chem. Phys. 22, 1255 (1954)]. These results demonstrate the success of monitoring interaction between particles by use of multiple-scattered light and the necessity of incorporating models for these interactions when predicting scattering of dense, concentrated suspensions.