Phase Behaviour of Colloid + Polymer Mixtures

A new treatment of the phase behaviour of a colloid + nonadsorbing polymer mixture is described. The calculated phase diagrams show marked polymer partitioning between coexisting phases, an effect not considered in the usual effective-potential approaches to this problem. We also predict that under certain conditions an area of three-phase coexistence should appear in the phase diagram. Introduction. Phase separation in colloidal suspensions, induced by the addition of nonadsorbing polymer, is a phenomenon of fundamental interest and considerable technological importance. A theoretical explanation was first advanced by Asakura and Oosawa [1], and also independently by Vrij [2], based on the exclusion of polymer from the region between two colloid particles when their surface-surface separation becomes smaller than the diameter of a free polymer coil. The resulting imbalance in osmotic pressure gives rise to an effective attractive «depletion» force between the colloid particles [3,4]. At high enough concentration of polymer this depletion force causes the suspension to separate into colloid-poor and colloid-rich phases. In the latter the particles can, depending on conditions (see below), be in either liquidlike or crystalline spatial arrangements. To predict the phase diagram of a colloid + polymer mixture, most workers to date have adopted an approach in which the depletion potential (an effective pair potential) is added to the parent interparticle potential; thermodynamic perturbation theory is then used to calculate phase stability boundaries [5,6]. Although experimental studies [6,7] show qualitative agreement with the predictions of these calculations, an important reservation () Also at University of Edinburgh, Department of Physics, Mayfield Road, Edinburgh, EH9 3JZ, UK. 560 EUROPHYSICS LETTERS has been expressed recently [8-11]. The effective potential approach assumes that the polymer concentration is the same in coexisting phases and does not allow for the occurrence of polymer partitioning between the various phases [12,13]. Here we explore the consequences of a simple statistical mechanical model [9,10] (*) for a colloid + polymer mixture which can account for and predict polymer partitioning at phase separation. In the simplest approximation the polymer is supposed dilute and at its 6-point, and the colloid particles interact as hard spheres. The only effect of the interaction between colloid and polymer is to restrict the volume in which the latter can move. Thus we treat the polymer as a suspension of freely interpenetrable coils whose centres of mass cannot approach closer than a distance à from any (nonadsorbing) surface. The centre of mass of a polymer is therefore excluded from a sphere of radius a + à around a colloid particle of radius a. Here we show that a mean-field treatment of this model does indeed predict marked partitioning of the polymer. Furthermore, under certain conditions, we find a three-phase region in the phase diagram where colloidal gas, liquid and crystal coexist. Earlier treatments of the model only considered gas-liquid coexistence [9,10]. Statistical mechanics. A convenient starting point in the calculation of the free energy in our model is to work in the grand canonical ensemble [4,10]. The integration over the translational-polymer degrees of freedom may be performed exactly, resulting in an effective interaction between colloid particles of the form W=Uc-lh(np)V{tee<rc). (D In this expression Uc is the bare colloid interaction potential, //P is the osmotic pressure of a pure polymer system expressed in terms of the polymer chemical potential ,«P, and Ffree is the free volume in which the polymer coils can move. The free volume depends on the colloid particle positions, collectively denoted by rc. The effective potential W has a many-body nature, since a full expression for Vfree can be written only in terms of the mutual overlap of the excluded-volume shells of all the colloid particles. The meaning of the final term in eq. (1) is clearly the following: if the free volume changes due to a change in the positions of the colloid particles, work has to be done against the pressure exerted by the polymer molecules. To make progress, we use a van der Waals mean field-approximation and replace Vfree(rc) by its average value in the corresponding unperturbed system of colloidal particles [15]. Thus we write Vfree = aV, where the «free-volume fraction» « depends only on the colloid volume fraction £ = 47zaNc/3V, and on the ratio à/a. The integration over the colloid degrees of freedom may now be performed, since the final term in (1) no longer contains any dependence on the colloid particle positions. Since colloidal systems are, to a good approximation, constant-volume systems [16], we present our results in terms of the Helmholtz free energy. The final expression, for the Helmholtz free energy of a system of Nc colloid particles and NP polymer coils in a volume V, can be written in the suggestive form F = Fc(Nc,V) + FP(Np,xV). (2) The free energy decouples into a term corresponding to pure colloid in a volume V, and a term corresponding to pure polymer in a volume aV. The interaction between polymer and colloid is contained solely in the dependence of a on the colloid volume fraction. C) We mention that this model corresponds to the asymmetric, nonadditive hard-sphere mixture introduced some years ago in the context of liquid-vapour transitions [14]. H. N. W. LEKKERKERKER et dl.\ PHASE BEHAVIOUR OF COLLOID + POLYMER MIXTURES 561 Since we treat the polymer as a suspension of noninteracting coils, we can write Fp (Np , aV) = kTnV log J + other terms , (3) where n = NP /V. The other terms in (3) are linear in NP and V and do not contribute to the determination of the phase behaviour. The colloid contribution to (2) can be derived by integration from the hard-sphere equation of state FC(NC,V)= -Vt d$, (4) where Z($) is the hard-sphere compressibility. For colloidal-fluid phases we use the expression for Z suggested by Carnahan and Starling [17]; for colloidal crystals (FCC lattice) we use that suggested by Hall [18]. The chemical potentials of the colloid and polymer (pc and //P), and the total osmotic pressure (//) are now obtained from the free energy by differentiation: