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In mixed solutions of anionic and cationic surfactants, called catanionics, ion pairs are formed which behave like non-ionic surfactants with a much higher surface activity than the single components. In equimolar mixtures of NaCnSO4 and CmTAB, all surface-active ions are paired. For mixtures with n + m = const, the interfacial properties are rather similar. Catanionics containing one long-chain surfactant and one surfactant with medium chain length exhibit a strong increase in surface activity as compared with the single compounds. In contrast, catanionics of one medium- and one short chain surfactant have a surface activity similar to that of the medium-chain surfactant alone. Both the Frumkin model and the reorientation model describe the experimental equilibrium data equally well, while the adsorption kinetics of the mixed medium- and short-chain surfactants can be well described only with the reorientation model.

The dynamic and equilibrium properties of adsorption layers of poly (oxyethylene) alkyl ether (CnEOm) can be well described by the reorientation model. In its classical version, it assumes two adsorption states, however, there are obviously surfactants that can adsorb in more than two possible conformations. The experimental data for C14EO4 and C14EO8 (dynamic and equilibrium surface tensions and surface dilational visco-elasticity as measured by bubble profile analysis tensiometry) are used to verify if a reorientation model with more than two possible adsorption states can better describe the complete set data of CnEOm adsorption layers at the water/air interface. The proposed refined theoretical model allows s different states of the adsorbing molecules at the interface. The comparison between the model and experiment demonstrates that, for C14EO4, the assumption of s = 5 adsorption states provides a much better agreement than for s = 2, while for C14EO8, a number of s = 10 adsorption states allows an optimum data description.

The adsorption behaviour of linear poly(oxyethylene) alkyl ether (CnEOm) is best described by a reorientation model. Based on a complete set of experimental data, including the adsorption kinetics, the equilibrium surface tension isotherm and the surface dilational visco-elasticity, the thermodynamic and kinetic adsorption parameters for some CnEOm at the water/air interface were determined. For the study, six CnEOm surfactants were selected (n = 10, 12 and 14 and m = 4, 5 and 8) and were studied by bubble profile analysis and maximum bubble pressure tensiometry. A refined theoretical model based on a reorientation-adsorption model combined with a diffusion-controlled adsorption kinetics and exchange of matter allowed us to calculate the surface layer composition by adsorbing molecules in different orientations. It turns out that at larger surface coverage, the adsorption rate decreases, i.e., the apparent diffusion coefficients are smaller. This deceleration can be explained by the transition of molecules adsorbed in a state of larger molar surface area into a state with smaller molar surface area.

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 &gt, pH 7 &gt, pH 5, i.e., with decreasing protein net charge. Finally, we discuss advantages and limitations in the current state of the model.

Rutile decomposition by sulfuric acid, including the formation of two salts, Ti(SO4)2/TiOSO4, is thermodynamically modelled. It is shown that TiO2 can spontaneously dissolve in H2SO4 solutions. However, titania is considered as an inert (ballast) phase component of titanium-containing raw materials due to the decelerated separate nature of such chemical transformations. It is concluded that the hampered related kinetics of dissolution can be attributed to the lability of Ti(IV) cations/the specific engineered features of the hierarchical crystalline structure. It is suggested that the breaking of Ti&ndash, O&ndash, Ti bonds without additional mechanical strains in crystal lattice geometry becomes more advantageous when smaller negative anions/fluoride ions can be used. The analysis of sulfate-fluoride extraction leaching of titanium confirmed that a decrease in the Gibbs energy in the presence of F occurs. It is indicated by kinetic research studies that the addition of corrosive sodium reagent (NaF) reduces the activation by 45 kJ/mol, which results in intensification. A mechanism is proposed for the interactions involving the Ti&ndash, Ti cleavage on the surface/the H2SO4-induced Ti dioxide degradation on the sites of defects. Moreover, F acts as a homogeneous/heterogeneous bifunctional catalyst.

The theoretical description of the adsorption of surfactants at interfaces between aqueous solutions and oil was based over a very long time on models derived for the solution/air interface. Thus, most of the experimentally observed peculiarities could not be specifically considered but were merely interpreted in terms of a penetration of oil molecules into the alkyl chain layer of the adsorbed surfactant molecules. These penetrating oil molecules enhance the surfactant adsorption as compared to the water/air interface. Later on, for the special situations at water/oil interfaces a competitive adsorption of surfactant and oil molecules was postulated, allowing a much better description of experimental data. This picture, however, was unable to explain why the interfacial tension of the water/oil interface decreases very quickly when extremely small amounts of surfactants are added to the water. This effect cannot be of competitive nature, but a cooperativity of surfactant and oil molecules forming a mixed adsorption layer is required instead. This cooperative effect means that already few surfactant molecules adsorbed at the interface can induce a significant ordering of oil molecules in the interfacial layer. This new interfacial structure, in turn, attracts further surfactant molecules to adsorb. Improving the theoretical description of experimental data was finally achieved by applying suitable adsorption models for the two adsorbing compounds, i.e. a Frumkin adsorption model for the oil molecules and a Langmuir, Frumkin, or reorientation model for the adsorbing surfactant molecules. Here, the progress in modelling surfactant adsorption at water/oil interfaces is discussed mainly for the homologous series of the cationic surfactants C(n)TAB, of the anionic surfactant SDS, and members of the homologous series of the non-ionic surfactants CnDMPO at water/alkane interfaces. (C) 2020 Elsevier B.V. All rights reserved.

Drop profile analysis tensiometry is applied to determine the distribution coefficient of a nonionic surfactant for a water/hexane system. The basic idea is to measure the interfacial tension isotherm in two configurations: a hexane drop immersed in the surfactant aqueous solutions at different bulk concentrations, and a water drop immersed into a hexane solution of the same surfactant. Both types of experiments lead to an isotherm for the equilibrium interfacial tensions with the same slope but with a concentration shift between them. This shift refers exactly to the value of the distribution coefficient.

In drop profile analysis tensiometry, the ratio of drop surfaces area S to volume V is large, i.e., S/V >> 1. In such a case, the concentration of a surfactant within the drop bulk decreases due to adsorption at the drop surface. In contrast, in bubble profile analysis tensiometry, we have S/V << 1 so that depletion due to adsorption is negligible. A protocol is presented to determine the correct adsorption parameters of surfactants from surface tension data measured by bubble and drop profile analysis tensiometry. The procedure is applied to experimental data measured for selected surfactants of different adsorption activities: C10OH, CTAB, Tween 20, and the equimolar mixture SDS + DoTAB. The results show that for surfactants with higher surface activity, the differences between the surface tensions measured with the drops and bubbles profile analysis tensiometry, respectively, are larger, while for less surface-active surfactants, such as SDS, the results obtained from drop and bubble profile experiments are very close. The correction procedure is based on the same set of adsorption parameters used to fit both the experimental data obtained from drop-based measurements (which involve the depletion effects) and those data measured in a way that depletion effects are negligible.

Two oppositely charged surfactants in an aqueous solution interact with each other via electrostatic interactions and produce surfactant complexes-catanionics. However, their quantitative influence on the interfacial tension at the solution/air and solution/oil interfaces is still unknown. We have measured the interfacial tension of sodium dodecyl sulphate (SDS) and dodecyltrimethylammonium bromide (DoTAB) aqueous mixtures using the du Nouy ring tensiometer and drop profile analysis tensiometry (PAT-1). For the oil phase, we used hexane. The obtained kinetic and equilibrium experimental results were fitted with the common theoretical Frumkin adsorption model. We found that SDS + DoTAB complexes show much higher surface activity than the single surfactants. At solution/air and solution/oil interfaces, SDS and DoTAB produce ion pairs with low water affinity and relatively low oil solubility. SDS + DoTAB partition coefficient between hexane and water phases is 0.32. Such surfactant complexes with advanced surface characteristics may be beneficial in different industrial branches nowadays. Refereed/Peer-reviewed

Protein adsorption is one of the most complex and intriguing phenomenon in interfacial science. This phenomenon has been addressed in this paper for a short time range using maximum bubble pressure tensiometer. This study presents the issue of negative surface pressure of proteins which has been much of a speculation, with the thermodynamic model. Here beta-lactoglobulin has been studied at different pHs and the experimental results agree with the theoretical model presented. Refereed/Peer-reviewed

International audience; The adsorption of members of the homologous series of alkyl trimethylammonium bromides (CnTAB) is studied at water/alkane interfaces by drop profile analysis tensiometry. The results are discussed in terms of a competitive adsorption process of alkane and surfactant molecules. A thermodynamic model, derived originally for the adsorption of surfactant mixtures, is adapted such that it describes a competitive adsorption of the surfactant molecules from the aqueous phase and alkane molecules from the oil phase. This new model involves the interspecies attraction coefficient, which mutually increases the adsorption activities of the alkane and CnTAB. The effects of the alkyl chain length n of CnTABs and the influence of the number of C atoms in the alkane chain are discussed, and the physical quantities are compared to those determined at the aqueous solution/air interface. The new theoretical model for aqueous solution/oil interfaces is also compared to a theory that does not consider the adsorption of alkane. The proposed new model demonstrates good agreement with the experimental data.

International audience

The analysis of the dynamic behaviour of an oscillating bubble or drop provides the amplitude- and phase-frequency characteristics which take into account the dependency on the viscoelastic properties of the fluid-fluid interface. Corresponding experiments can be organized in different ways depending on the type of the measuring cell (open or closed), generation of bubble and drop oscillations (either by external pressure variations or volume variation in the cell), and registration of the measured signal (meniscus volume or pressure changes in the cell). All experimental schemes can be considered in the framework of a common theory based on the application of Fourier transforms. The method demonstrates a high sensitivity of the measured signal to changes in the surface elasticity and viscosity, which depends on the presence of surfactants in the solution and on the mechanisms of surface relaxations. The good qualitative agreement with experiments underlines that the theory is adequate to describe the main properties.

The application of new thermodynamic adsorption isotherms allow to improve the description of surfactant adsorption kinetics based on a diffusional transport. While the consideration of interfacial reorientation corrects apparently too high diffusion coefficients, interfacial aggregation avoids too small diffusion coefficients or the assumption of adsorption barriers. The adsorption kinetics of alkyl dimethyl phosphine oxides is influenced by interfacial reorientation. While the lower homologues (C8–C12) follow the classical diffusion model, the higher homologues (C13–C15) yield diffusion coefficients several times larger than the physically reasonable values. Assuming two different adsorption states, the resulting diffusion coefficients agree with those expected from the geometric size of the molecules. The model also works well for oxyethylated non-ionics, such as C10EO8. As a second example, a good theoretical description is obtained for experiments of 1-decanol solutions when a mean surface aggregation number of n=2.5 is assumed. The same n was obtained from the description of the equilibrium adsorption isotherm of 1-decanol. Assuming that the transition from one into the other state is controlled by a rate constant (change in orientation, formation or disintegration of two-dimensional aggregates) significant changes in the kinetics curves can result. The use of additional rate constants yields an improved fitting to experimental data.

The adsorption of water and other polar substances on to the surface of graphitized carbon black and macroporous silica modified by polymethylhydride silicone has been studied using gas chromatography. The adsorption heat values of the substances studied were determined and the concentration of surface hydrophilic centres estimated. Conclusions have been drawn concerning the number of water molecules adsorbed on to hydrophilic centres at low surface occupation values. In the case of water adsorption on the graphitized carbon black surface, the chromatographic data for the adsorption heat have been compared with the energy of water molecule interaction with the surface clusters modelling the micropores existing on the basal surface of carbon black particles as calculated using quantum chemical methods.

A molecular statistical method has been used to describe both specific and non-specific sorption of hydrocarbons on the surface of a non-swelling layer silicate (kaolinite) modified with octadecylammonium chloride. Sorption of n-hexane on this surface appears to be determined primarily by dispersion attraction to the modified overlayer, while specific interaction has been shown to play a major role in the case of benzene sorption. The energy characteristics of the benzene—RNH3+ adsorption complex have been estimated and the assumption that the benzene molecule is partially delocalized on the modified kaolinite surface shown to be reasonable.