Complexation and coacervation of polyelectrolytes with oppositely charged colloids Ebru Kizilay a, ⁎, A. Basak Kayitmazer b, ⁎, Paul L. Dubin a a Department of Chemistry, University of Massachusetts Amherst, MA 01003, United States b Department of Chemistry, Bogazici University, Bebek, 34342 Istanbul, Turkey abstract article info Available online 8 July 2011 Keywords: Complex coacervation Polyelectrolyte-colloid interactions Coacervate Structure Polyelectrolyte-colloid coacervation could be viewed as a sub-category of complex coacervation, but is unique in (1) retaining the structure and properties of the colloid, and (2) reducing the heterogeneity and configurational complexity of polyelectrolyte–polyelectrolyte (PE–PE) systems. Interest in protein-polyelectrolyte coacervates arises from preservation of biofunctionality; in addition, the geometric and charge isotropy of micelles allows for better comparison with theory, taking into account the central role of colloid charge density. In the context of these two systems, we describe critical conditions for complex formation and for coacervation with regard to colloid and polyelectrolyte charge densities, ionic strength, PE molecular weight (MW), and stoichiometry; and effects of temperature and shear, which are unique to the PE-micelle systems. The coacervation process is discussed in terms of theoretical treatments and models, as supported by experimental findings. We point out how soluble aggregates, subject to various equilibria and disproportionation effects, can self-assemble leading to heterogeneity in macroscopically homogeneous coacervates, on multiple length scales. Published by Elsevier B.V. 1. Introduction Complex coacervation is the separation of a macromolecular solution composed of two oppositely charged macroions into two immiscible liquid phases. In order to distinguish it from the simple coacervation of a single polymer, Bungenberg de Jong and Kruyt coined the name “complex coacervation” [1]. The dense liquid phase, which is relatively concentrated in macromolecules, is called the coacervate. While the definition of “coacervation” is clear, that of “coacervate” is not since it sometimes refers to the metastable suspension of macroion-rich droplets. Here “coacervate suspension” refers to the biphasic system, while the clear dense phase is defined as “coacervate”. This coacervate phase is more viscous and more concentrated than the initial solution, and exhibits a number of unique properties. Complex coacervation was first investigated by Bungenberg de Jong for the system of gum arabic-gelatin [1]. His work was cited by Oparin who mentioned the similarity to proto-cells and coacervates, and Oparin proposed that life first formed in coacervate droplets. [3] The first theoretical model of complex coacervation was put forward by Voorn et al. [4] and following theoretical models were developed by Veis et al. [5], Nakajima and Sato[6], and Tainaka [7]. Coacervation can also take place for polyelectrolytes and oppositely charged colloids, e.g. micelles, [8,9] proteins [10] or dendrimers [11] (see Figs. 1 and 2). While it might be suggested that binding to polyelectrolytes induces micelle deformation or even disintegration, evidence for the full retention of micelle structure comes from (a) the absence of change in the solubilizing capacity of micelles regardless of whether they are free, complexed, or in coacervate [12] (b) the size of micelles within coacervates [13] and, indirectly, the strong influence of (free) micelle size and shape on the conditions for complexation and coacervation [14–16]. Colloid-PE coacervation, essen- tially its own field, has enormous potential due to the diverse functional properties of the proteins, micelles, and related colloids that replace the second PE. In this paper, it should be noted that “colloid” refers only to micelles and proteins, whose properties support a wide number of applications in foods, cosmetics, and pharmaceuticals. Polyelectrolyte- micelle systems are relevant to personal care products [17], are models for other colloidal systems since micelles have uniform shape and charge distribution). Polyelectrolyte-protein coacervates are particularly impor- tant in (i) enzyme immobilization [18], (ii) antigen delivery [19], (iii) design and production of biomaterials for cell micropatterning [20], (iv) protein purification [21], and (v) stabilization of food products [22]. More recently, a truly biological example of coacervates has been found: the mineralized tube of the sandcastle worm— formed from mineral particles glued together with cement made from coacervates of oppositely charged polypeptides (Fig. 3) [23]. The relationship between PE–PE coacervation and PE–colloid coacervation can be described in terms of similarities and differences. The identification of soluble complexes as precursors of coacervates seems to be better established for PE–protein [24] and PE–micelle [25] systems, with relatively few papers for PE–PE (e.g. gelatin A-gelatin B) systems [26]. For both types of complexes, it appears likely that large aggregates of the “primary” (intrapolymer) complexes are antecedents Advances in Colloid and Interface Science 167 (2011) 24–37 ⁎ Corresponding authors. E-mail addresses: ekizilay@chem.umass.edu (E. Kizilay), basak.kayitmazer@boun.edu.tr (A.B. Kayitmazer). 0001-8686/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.cis.2011.06.006 Contents lists available at ScienceDirect Advances in Colloid and Interface Science journal homepage: www.elsevier.com/locate/cis