Abstract
The theoretical description of the adsorption of proteins at liquid/fluid interfaces suffers from the inapplicability of classical formalisms, which soundly calls for the development of more complicated adsorption models. A Frumkin-type thermodynamic 2-d solution model that accounts for nonidealities of interface enthalpy and entropy was proposed about two decades ago and has been continuously developed in the course of comparisons with experimental data. In a previous paper we investigated the adsorption of the globular protein β-lactoglobulin at the water/air interface and used such a model to analyze the experimental isotherms of the surface pressure, Π(c), and the frequency-, f-, dependent surface dilational viscoelasticity modulus, E(c)f, in a wide range of protein concentrations, c, and at pH 7. However, the best fit between theory and experiment proposed in that paper appeared incompatible with new data on the surface excess, Γ, obtained from direct measurements with neutron reflectometry. Therefore, in this work, the same model is simultaneously applied to a larger set of experimental dependences, e.g., Π(c), Γ(c), E(Π)f, etc., with E-values measured strictly in the linear viscoelasticity regime. Despite this ambitious complication, a best global fit was elaborated using a single set of parameter values, which well describes all experimental dependencies, thus corroborating the validity of the chosen thermodynamic model. Furthermore, we applied the model in the same manner to experimental results obtained at pH 3 and pH 5 in order to explain the well-pronounced effect of pH on the interfacial behavior of β-lactoglobulin. The results revealed that the propensity of β-lactoglobulin globules to unfold upon adsorption and stretch at the interface decreases in the order pH 3 > pH 7 > pH 5, i.e., with decreasing protein net charge. Finally, we discuss advantages and limitations in the current state of the model.
Highlights
Colloid stability is a large field of physical chemistry that has been developing for centuries [1,2]
Several approaches for obtaining the adsorption isotherm and the equation of state of interfacial layers have been developed, among which perhaps the most familiar are those of Langmuir and Frumkin
As mentioned in the introduction, one of the purposes of the present work is to reconsider our previous theoretical results obtained from comparison with experimental data for behavior of β-lactoglobulin (BLG) adsorption layers at pH 7 [28] by taking into account new Γ(c) experimental data [24] and updated values of the dilational modulus, E, as measured in the linear viscoelasticity regime
Summary
Colloid stability is a large field of physical chemistry that has been developing for centuries [1,2]. Interfacial phenomena inevitably play a central role in the behavior of colloids, and the key step toward colloid stabilization is the decrease of the free energy of liquid interfaces by adsorption of amphiphilic species (commonly known as surfactants). Several approaches for obtaining the adsorption isotherm and the equation of state of interfacial layers have been developed, among which perhaps the most familiar are those of Langmuir and Frumkin These models have been further modified by specific theoretical considerations or/and toward taking account for physicochemical features of the adsorbing species and the solvent, for instance, the proper localization of the Gibbs dividing surface and assuming penetrable or nonpenetrable interfaces in the former case and, in the latter case, accounting for charge effects, molecular structure, solvent conditions, etc. Description of the adsorption of polymers with various molecular structures (random coil, block- or graft copolymer, etc.) requires reconsiderations of the available theoretical formalisms and even creation of new ones, e.g., based on selfconsistent field theories, scaling concepts and other approaches [6,7,8]
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