Polymer–surfactant and protein–surfactant interactions

Abstract

The phase behavior and some physicochemical properties of homopolymers (HP) and hydrophobically modified (HMP) polymers, as well as of polyelectrolytes (PE) and proteins (PR), in the presence of aqueous surfactants, or their mixtures, are discussed. Mixing the above components gives rise to the formation of organized phases, whose properties are controlled by polymer and/or surfactant content, temperature, pH, and ionic strength. Depending on the nature, concentration, and net charge of both solutes, molecular solutions, polymer–surfactant complexes, adsorption onto micelles and vesicles, gels, liquid crystalline phases, and precipitates are observed. Such rich polymorphic behavior is the result of a complex balance between electrostatic, excluded volume, van der Waals, and other contributions to overall system stability. It is also modulated by the molecular details and architecture of both the polymer and the surfactant. Different experimental methods allow investigation of the above systems and getting information on the nature of polymer–surfactant interactions (PSI). Surface adsorption and thermodynamic methods, together with investigation of the phase diagrams, give information on the forces controlling PSI and on the existence of different phases. Conductivity, QELS and viscosity allow estimating the size and shape of polymer–surfactant (protein–surfactant) complexes. Optical microscopy, cryo-TEM, AFM, NMR, fluorescence, and relaxation methods give more information on the above systems. Use of the above mixtures in controlling gelation, surface covering, preparing dielectric layers, and drug release is suggested.

Introduction

The interactions occurring between intrinsic and association colloids are subjects of significant interest in the past few years [1], [2], [3], [4]. The reasons for focusing on this research line are manifold. In particular, many peculiar physicochemical properties of mixtures containing macromolecules and surfactants (or lipids) are relevant and of widespread use in technological applications [5], [6]. Formulation procedures based on the above mixtures may be used to prepare gels for controlled drug release; this is one among many of their appealing applications [7].

The phase behavior of polymer–surfactant, PSS, or protein–surfactant, PrSS, systems and the interactions observed in such mixtures (indicated, in the following, as PSI and PrSI, respectively) may be clarified by ad hoc investigation. Fundamental studies are helpful to clarify still unknown aspects of the complex colloid systems formed by mixing the above substances [8], [9], [10]. They focus, essentially, on the adsorption of macromolecules onto micelles, on the occurrence of polymer–surfactant complexes, PSC, and/or on the formation of stable gels. Surface and interfacial effects are relevant too [11], [12].

PSS and PrSS are also important in a better understanding of biochemically and physiologically related questions. They may clarify the behavior and functionality of lung surfactants [13], the role of surfactants and lipids in DNA transfection [14], and the synergistic effects of albumin and bile salts in hepatic, or gall-bladder, bile [15], to mind but a few. From a fundamental viewpoint, in fact, there is no significant difference in the observed phase behavior of PSS when proteins, or DNA, replace polymers. As concerns many physicochemical properties, the above biological substances may be considered peculiar block polyelectrolytes. Thus, some properties of PrSS, such as precipitation and redissolution or their affinity toward selected surfactants (or lipids), may be clarified by keeping in mind this incontrovertible fact. The presence of lipoplexes (complexes of proteins and lipids) in biological fluids may be clarified if occurrence of PrSC is accounted for.

Mixing macromolecules and association colloids (surfactants or lipids) gives rise to rich polymorphic behavior, with the presence of molecular or micelle-like solutions, complexes and lipoplexes, precipitates, liquid crystals, and gels [16], [17], [18], [19], [20], [21]. The first studies along this line go back to the 1940s, when Dervichian gave evidence on the occurrence of PSI [22]. Later on, relevant contributions were reported by Tanford, who studied the phase behavior of some protein–surfactant mixtures and developed the rigid-rod model of the resulting complexes [23], [24]. More recent studies performed by Shirahama and co-workers [25], [26], [27], Kwak and co-workers [28], [29], [30], Zana and co-workers [31], [32], [33], Kabanov and co-workers [34], [35], and Lund's group [36], [37], [38], [39], [40], [41], focused on selected aspects pertinent to PSS and PrSS.

A huge number of such systems have recently been investigated, as can be inferred from available literature findings. Excellent reviews on the same subjects have been issued by Goddard and co-workers [42], [43], [44], by Lindman and co-workers [45], [46], and in specialized books [47], [48], [49]. It is not easy to avoid partial overlapping of the subjects reported here with the above contributions. That is why a few selected items will be dealt with. Similarities and differences in the phase behavior of surfactant systems containing water-soluble homopolymers or hydrophobically modified ones, polyelectrolytes, and proteins are considered in more detail. The concepts of binding, cooperativity, and selectivity are briefly dealt with. Some aspects inherent in the phase behavior of PSS and PrSS are also considered. Spectroscopic, transport, and rheological properties of some selected systems will be summarized and discussed in more detail. Considering with due care the currently available information may help to clarify still open questions and some controversial aspects inherent in both PSS and PrSS systems.