This chapter describes the equilibrium state of mixed protein/surfactant adsorption layers and the mechanisms of their formation by thermodynamic and kinetics theories. Measurements of the equilibrium adsorption state are surface or interfacial tension isotherms. While tensiometry allows following the formation of adsorption layers over a certain time interval, dilational rheology provides information on the response of the interfacial layer to small perturbations. There are additional studies which do not provide quantitative but qualitative information about protein adsorption layers. Surface and interfacial shear rheology, for example, probes the formation of structures within the adsorption layers. The chapter also shows that the general features of protein adsorption at the water—air surface and at water—oil interfaces are similar; however, there are significant quantitative differences. This is of great relevance as proteins and their mixtures with low-molecular-weight surfactants are frequently used for the stabilization of foams and emulsions.
ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTReorientation of Polyethylene Glycol Oxyethylene Ether in Nonequilibrium Adsorption Layers at the Water/Air Interface. Role of Molecular Weight and TemperatureV. B. Fainerman, R. Miller, and A. V. MakievskiCite this: Langmuir 1995, 11, 8, 3054–3060Publication Date (Print):August 1, 1995Publication History Published online1 May 2002Published inissue 1 August 1995https://pubs.acs.org/doi/10.1021/la00008a034https://doi.org/10.1021/la00008a034research-articleACS PublicationsRequest reuse permissionsArticle Views242Altmetric-Citations41LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts
Drop profile analysis tensiometry used in the oscillating drop mode provides the dilational viscoelasticity of adsorption layers at liquid interfaces. Applied during the progress of adsorption the dynamic surface rheology can be monitored. For β-casein solutions at the same surface pressure values, the larger the dynamic dilational viscoelasticity the longer the adsorption time, i.e., the smaller the studied protein concentration is. For β-lactoglobulin and human serum albumin, the differences in the viscoelasticity values are less or not dependent on the adsorption time at identical surface pressures. The observed effects are caused by the flexibility of BCS, while the globular proteins BLG and HSA do not change their conformation significantly within the adsorption layer.
For Langmuir monolayers at the air−water interface, an equation of state is theoretically derived which describes the main phase transition between the gaslike and the condensed phases. The theoretical treatment considers the formation of two-dimensional aggregates and describes the nonhorizontal phase transition of the surface pressure−area isotherms and its dependence on the temperature. The equilibrium between the aggregates and monomers was treated using Buttler's equation for the chemical potential of monomers and aggregates within the surface. The results predicted by the theory agree well with the experimental surface pressure−area isotherms over a large temperature range.
An improved model for the equation of state for Langmuir monolayers proposed in J. Phys. Chem. B 1999, 103, 145 is introduced for the case where two or more phase transitions occur in the monolayer. The model allows the theoretical description of the phase transition between the two condensed phases under the realistic precondition that the phase transition is accompanied by the change of molecular compressibility parameters in the condensed phases. N-Alkyl-beta-hydroxy-propionic acid amide monolayers undergo two phase transitions during the compression at low temperature, demonstrated experimentally both by the surface pressure-area (Pi-A) isotherm and the results of grazing incidence X-ray diffraction (GIXD) for the C(13)H(27) and C(14)H(29) alkyl chain lengths. The theoretical Pi-A isotherms obtained by the new model agree well with the experimental Pi-A isotherms and with the data obtained from the GIXD experiments.
The thermodynamic model of a 2D solution developed earlier for protein monolayers at liquid interfaces is generalized for monolayers composed of micro- and nanoparticles. Surface pressure isotherms of particle monolayers published in the literature for a wide range of particles sizes (between 75 μm and 7.5 nm) are described by the theoretical model with one modification. The calculations of surface pressure Π on area A provide satisfactory agreement with the experimental data. The theory also yields reasonable cross-sectional area values of the solvent molecule water in the range between 0.12 and 0.18 nm2, which is almost independent of particle size. Also, the area per particle in a closely packed monolayer obtained from the theory is quite realistic.
The semiempiric PM3 method is used to calculate the thermodynamic parameters of the formation of monomers, dimers, trimers, and tetramers of the amphiphilic melamine-type series of 2,4-di(n-alkylamino)-6-amino-1,3,5-triazine (2C(n)H(2n+1)-melamine) with n = 9-16. The most stable conformations are determined, which then are used to construct the clusters. The peculiar feature of these structures is the existence of a bend at one of the alkyl chains. Thus, the formation of infinite films becomes possible because of their spatial arrangement. From the calculation of the relative amount of various conformers in the mixture, it follows that, if the alkyl chain length is lower than 11-12 carbon atoms, the mixture is composed mainly of the monomers that do not contain any intramolecular interactions, whereas for higher alkyl chain lengths the monomers that involve such interactions prevail in the mixture. For all clusters thus considered, the thermodynamic parameters (enthalpies, entropies, and Gibbs' energies) of clusterization are calculated. It is shown that the dependencies of these parameters on the alkyl chain length either exhibit stepwise shape or are represented by the combination of a linear and stepwise function. This depends on the different number of hydrogen-hydrogen interactions in the structures considered. Five types of clusters that are capable of the formation of infinite 2D films are considered in detail. For each of these types, the dependencies of the clusterization enthalpy, entropy, and Gibbs' energy on the alkyl chain length in the constituting monomers are derived. Using these dependencies, it becomes possible to calculate these thermodynamic characteristics for clusters of any size, and also for infinite 2D films. It is shown that the spontaneous clusterization of 2C(n)H(2n+1)-melamine becomes possible if the alkyl chain length exceeds 9 carbon atoms.
