Polymer–surfactant and protein–surfactant interactions
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.
Section snippets
Preliminary considerations
The reasons underlying the use of PSS originate from practical purposes and from efforts to modulate, at the same time, the solution viscosity, adhesion, surface activity, and solubilizing capacity. Further developments showed that polymer–surfactant mixtures have more appealing applications as gelling agents, dispersion and emulsion stabilizers [50], thickeners, and regulators of matter exchange. (Note: The original definition reported in the literature is “kinetic buffers” [51], [52].)
Thermodynamics
Studies on the phase diagrams of macromolecules and surfactants in solution give important information on the nature of their mutual interactions. In some aspects, the effects induced by the polymer was originally considered as due to a sort of co-micellization.
In the 1970s and 1980s Cabane showed the occurrence of a peculiar interaction region in water–PEO–SDS systems. The observed physicochemical properties of this region were quite different from both the molecular and the micellar solutions
Liquid crystals and gels
The occurrence of liquid crystalline order was observed on mixing PE, Pr, or even small peptides, with lipids and surfactants [78]. Because of the amphipathic character of the surfactant ions, the peptide backbone (which is stabilized by an extended network of hydrogen bonds), and the surfactant hydrophobic tails separate in regions of different polarity, as schematically indicated in Fig. 8. The mesomorphic properties of the resulting complexes are ascribed to the orientation of amphiphilic
Dynamic and transport properties in polymer–surfactant systems
As a consequence of mutual interactions between the components, significant changes in dynamic and transport properties occur. Particularly relevant are changes in the hydrodynamic radii of PrSC [103], as well as in the viscosity and ionic mobility of the corresponding mixtures. Compared to the behavior observed in gel formation, the viscosity of aqueous homopolymers (PVP) only slightly increases upon addition of short-chain surfactants and remains nearly constant up to the [64], [65]. In
Conclusions
The purpose of this contribution was to focus on selected aspects of polymer–surfactant and protein–surfactant systems. In such systems, interactions between the components are relevant. The main significant differences arise from the forces responsible for the interactions between solutes. Some are electrostatic, others essentially based on vdW and weak interactions. Notwithstanding this, the phenomenological aspects of PSS and PrSS are similar. Combination of the above effects give rise to a
Acknowledgments
Without discussion and collaboration with colleagues and pupils, many aspects reported here would not have been clarified. Thanks to P. Andreozzi, M.L. Antonelli, A. Capalbi, A. Ciurleo, L. Galantini, G. Gente, K. Letizia, G. Masci, P. Michiotti, B. Orioni, A.C. Palacios, S. Pede, M. Roversi, S. Sallustio, C. Sarnthein-Graf, A. Scipioni, C. Strinati (Chemistry, La Sapienza), A. Bonincontro (INFM-CRS SOFT, Physics, La Sapienza), R. Muzzalupo (Chemistry, Calabria), O. Asaro, G. Pellizer
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2022, Journal of Colloid and Interface ScienceCitation Excerpt :It is not surprising in itself that protein and SDS interactions are pH sensitive; it has been observed that protein-surfactant systems can form large aggregates close to charge neutralization when mixing either positively charged proteins and anionic surfactant [16,62–64] or negatively charged proteins and cationic surfactants [7]. Furthermore, large aggregates around charge neutralization are a known phenomenon in surfactant/polymer systems with components of opposite charge [65–69]. These systems can be modelled in a similar way as our TlL series at pH 4.0 with components such as CS form factor, RF structure factor and a contribution from additionally larger cluster especially around charge neutralization [65–68].