Polymer induced depletion potentials in polymer-colloid mixtures

Abstract

The depletion interactions between two colloidal plates or between two colloidal spheres, induced by interacting polymers in a good solvent, are calculated theoretically and by computer simulations. A simple analytical theory is shown to be quantitatively accurate for case of two plates. A related depletion potential is derived for two spheres; it also agrees very well with direct computer simulations. Theories based on ideal polymers show important deviations with increasing polymer concentration: They overestimate the range of the depletion potential between two plates or two spheres at all densities, with the largest relative change occurring in the dilute regime. They underestimate the well depth at contact for the case of two plates, but overestimate it for two spheres. Depletion potentials are also calculated using a coarse graining approach which represents the polymers as "soft colloids": good agreement is found in the dilute regime. Finally, the effect of the polymers on colloid-colloid osmotic virial coefficients is related to phase behavior of polymer-colloid mixtures.

Effect of excluded volume interactions on the interfacial properties of colloid-polymer mixtures

“…The comparison between our results and the experiments of Aarts et al 21 is quantitatively better than the results of the AOV model, although de Hoog and Lekkerkerker 12 show that it is difficult to obtain accurate interfacial tension measurements. In addition, we compare our results with the predictions of the extended free volume theory plus a square gradient approximation to evaluate the tension 45 and the DFT of Moncho-Jorda et al 47 The DFT uses effective one-component pair potentials between the colloids that include the excluded volume interaction according to the approach of Louis et al 46 The predictions of the two theories are very close to each other and to our simulation results. Figure 7 shows the phase diagram of colloid-polymer mixtures confined between two hard walls with separation distances H / c = ϱ ,16,8,4,2.…”

Analytical phase diagrams for colloids and non-adsorbing polymer

“…In recent years, there have been significant theoretical developments which take into account the polymeric excluded-volume interactions [30][31][32][33][34][35][36][37][38][39][40][41]. As examples of methods that enable the prediction of the phase behavior of colloid-polymer mixtures we mention polymer-colloid liquid state theory [31][32][33][34], thermodynamic perturbation theories [12,35], density functional theory [36,37], a Gaussian-core model [14,38,39], and computer simulations of HS plus self-avoiding polymer chains [40,41].…”

“…As examples of methods that enable the prediction of the phase behavior of colloid-polymer mixtures we mention polymer-colloid liquid state theory [31][32][33][34], thermodynamic perturbation theories [12,35], density functional theory [36,37], a Gaussian-core model [14,38,39], and computer simulations of HS plus self-avoiding polymer chains [40,41]. To obtain phase lines these methods require a significant amount of numerical work.…”

“…8.3 and 8.4 these actually do appear. We note that these simulations are not exact as they are based on the Gaussian Core model [38,39]. It would be interesting to obtain α = ϕ/φ from MC simulations of HS plus selfavoiding walks [40] in the future to test the scaled-particle expression, also for other q R values.…”

Demixing in athermal mixtures of colloids and excluded-volume polymers from density functional theory

“…Both groups have adopted different approaches. To obtain the interaction potential between two colloidal spheres mediated by excluded volume polymers Aarts et al [27] have used the generalized Gibbs adsorption equation approach [30], whereas Moncho-Jorda et al [28] have used a depletion potential obtained from direct computer simulations [31]. Both groups have found that the interfacial tension is lower for interacting polymers than for the AOV model.…”