Surface Activity Coefficients Research Articles

SAC: Surface activity coefficient, a function to unveil surface and bulk’s ideal - non ideal behavior

The quotient between the reduced surface pressure and the amphiphile mole fraction corresponds to the surface activity coefficient which is analogous to the bulk activity coefficient under the symmetric convention. This property indicates the ideal and non-ideal behavior of the bulk and surface phases. The main advantage of the construction followed in this work is that the contributions from the bulk and surface deviations from ideality can be separated into independent contributions. The non-ideality of the surface phase is not found in the volumetric phase. The natural logarithm of the surface activity coefficient as a function of the reduced surface pressure allows to identify different configurations of bulk and surface ideality or non-ideality.Binary systems showing ideal behavior in the surface and bulk phases, non-ideal behavior in the surface but ideal behavior in the bulk phase, ideal behavior in the surface and non-ideal behavior in the bulk phase, and systems exhibiting non-ideal behavior in both regions are analyzed and compared with experimental data and classified with the presented model..

Theoretical and experimental investigation of effect of salinity and asphaltene on IFT of brine and live oil samples

Several factors influence the IFT of oil and formation water. These factors are rooted in the complex composition of oil, presence of different salts in water, water salinity, temperature, and pressure of reservoir. In the first part of this paper, effect of salinity on IFT between brine and an Iranian live oil sample has been studied experimentally. It is observed that IFT increases almost linearly with brine concentration. Also, linear increasing behavior of IFT with respect to pressure is obviously seen. Then, using thermodynamic properties such as surface excess concentration, chemical potential, chemical activity, and activity coefficient, results were analyzed and observed effect of salinity and pressure were justified thermodynamically. In the second part, the effect of asphaltene on IFT reduction has been studied. In previous works, the investigators extracted resin and asphaltene and then examined their effects on IFT in the absence of other fractions of oil phase. We believe that all fractions play a role in this phenomenon so, in this paper, the effect of natural surfactants of oil phase on IFT has been investigated in presence of all fractions of oil. Hence, SARA test was performed on all samples. Then, IFT between oil samples and brine were measured using captive drop instrument at 25 °C and 3000 psia. Results showed that neither asphaltene content nor asphaltene/resin ratio is a good indicator for effect of asphaltene on IFT, whereas colloidal instability index could be a useful tool to predict asphaltene effect on IFT.

Synthesis and surface activity of phenylselenide-1-undecyl trimthylamonium bromide

Selenium-containing surfactant has aroused considerable attention due to its unique chemical properties, especially environmental sensitivity. Here we synthesized a Se-containing cationic surfactant, phenylselenide-1-undecyl trimethylammonium bromide (PSeTAB), and studied its Krafft temperature (TK), equilibrium/dynamic surface tension, and foam properties. The results showed that with the addition of H2O2, the hydrophobic selenide group in PSeTAB was oxidized to a hydrophilic selenoxide structure, providing another polar head, and thus producing a larger geometry. As a result, PSeTAB-Ox exhibited a stronger affinity to water, lowering the TK, tendency toward micellization and adsorption (higher cmc, less negative and ), dynamic surface activity (smaller ) and diffusion coefficients, compared to PSeTAB. Macroscopically, PSeTAB showed relatively good foaming properties, while PSeTAB-Ox has hardly any foaming ability.

Ultrasonic dispersion temperature- and pH-tuned spectral and electrochemical properties of bovine serum albumin on carbon nanotubes and its conformational transition

In this paper, we first report the ultrasonic dispersion temperature- and pH-tuned spectral and electrochemical properties of bovine serum albumin (BSA) on single-walled carbon nanotubes (SWCNTs) and its configurational transition. The BSA-SWCNTs exhibit an irreversible oxidation peak at 0.793 V (vs. SCE) and a couple of tryptophan (Trp)-based surface-confined redox peaks at the formal potential of 0.154 V. The decrease of the dispersion temperature down to 283 K from 313 K at pH 7.2 leads to an increase in the surface activity coefficient (γs) and exothermic reaction rate constant (kf). The loop (L)-shaped BSA-SWCNTs with slight β-sheet prepared by controlling the dispersion temperature at 298 K (pH 7.2) show the maximum emission and minimum absorbance. The L-form of BSA-SWCNTs is transformed into basic (B)-form and fast migration (F)-form while controlling the dispersion temperature at 283 K and 313 K, respectively. The dispersion temperature and pH have a synergistic influence on the spectral and electrochemical properties of BSA-SWCNTs via the unfolding/refolding processes of β-sheet driven by ultrasonic/thermal motion and surface adsorption. The present study provides an effective approach for the preparation and evaluation of serum protein-based nanomaterials with spectral and electrochemical properties tuned by dispersion pH or temperature, as well as opens the way for optoelectronic and electrochemical applications.

Shape and curvature of surface tension isotherms for liquid mixtures

The different shapes of surface tension isotherms consistent with Butler’s ideal model for the planar surface phase of binary liquid mixtures are comprehensively worked out for the first time. Using the ratio of pure constituent molar surface areas and a parameter named surface factor, which depends only on pure-constituent properties, the Butler equation is expressed and solved as a function of these two independent parameters. The geometrical features for the plots of surface tension against the mole fraction of component with lower surface tension are established for every possible combination of those two parameters. Three main shape types are found, namely: concave up (positive curvature); concave down passing through an inflection point followed by concave up; and concave down (negative curvature). The range of parameter values leading to each shape type is obtained and shown graphically. It is demonstrated that Butler’s ideal model excludes the eventuality of a system presenting a shape with initial positive curvature, then an inflection point and continuing with negative curvature. All the above shapes will appear as a mirror image in the representation of surface pressure against mole fraction. Six organic–organic systems, including two ionic liquid solutions, taken from the literature are examined. It is shown that actual Gibbs adsorption isotherm shapes arise from the sum of ideal surface tension deviations with excess surface tensions, the latter being related to the equilibrium surface/bulk ratio of activity coefficients. It is also shown that this ratio is larger for the component with the greatest molar surface area. From the comparison of surface and bulk activity coefficients at the same chemical composition interesting insights into the surface phase are drawn.

Study on surface tension and surface phase of molten MCl-CeCl3 (M = Li, Na, K, Cs): A comparison of Butler's equation and a newly-proposed model

The thermodynamic optimization of NaCl-CeCl3 has been carried out through CALPHAD method, and a set of reliable parameters has been obtained to supplement the thermodynamic database. Based on Butler's equation and thermodynamic database established by CALPHAD method, the calculated surface tensions of molten salt mixtures MCl-CeCl3(M = Li, Na, K, Cs) agree quite well with experimental results reported in the literature. The model parameter β was determined to be 0.95, 1.04, 1.09 and 1.12 for LiCl-CeCl3, NaCl-CeCl3, KCl-CeCl3 and CsCl-CeCl3, respectively, by fitting the experimental data of surface tension of these mixtures, since it is out of the question to get a universal β applicable to all molten salt mixtures. The new model proposed lately by Santos et al. to describe the thermodynamics in surface phase of liquid solution has been reduced to a much simpler form which is nearly numerically equivalent to it by eliminating the derivative of molar surface area with respect to pressure. The surface compositions and surface activity coefficients from Butler's equation were compared with those from Santos model. For LiCl-CeCl3 and NaCl-CeCl3, the results from the two models are almost the same due to the similar surface tensions of LiCl, NaCl and CeCl3. Whereas for KCl-CeCl3 and CsCl-CeCl3, the mole fraction of CeCl3 in surface phase from Butler's equation is overestimated compared with that from Santos model, leading to a slight underestimation of surface activity coefficients of CeCl3 and also a slight overestimation of those of KCl and CsCl, since the surface tensions of KCl and CsCl are much smaller than that of CeCl3.

Effect of solution pH on the adsorption of BLG at the solution/tetradecane interface

Using drop profile analysis tensiometry the adsorption dynamics and the equilibrium equation of state of β-lactoglobulin (BLG) at the water/tetradecane (W/TD) interface are studied at pH 3, 5 and 7. The data are well described by a thermodynamic adsorption model using almost identical model parameters for all three pH values except for the surface activity coefficient. The surface pressure isotherms at the water/air (W/A) surface exhibit much steeper run than interfacial pressure at the W/TD interface for any of the studied cases at pH 3, 5 and 7, and the calculated adsorption isotherm data point at smaller adsorbed amounts for these pH. This seems to be in contrast to the much larger interfacial pressure changes reached at high BLG concentrations at the W/TD interface, which are almost three times higher than those at the W/A surface. The observations can be explained by a strong interaction between BLG and the oil molecules at the interface.The dynamic interfacial tensions can be adequately described by a mixed adsorption model, assuming a diffusional transport of the protein molecules in the aqueous bulk phase and an adsorption mechanism which assumes a change of the adsorption activity parameter in dependence of the interfacial coverage.

COSMO-3D: Incorporating Three-Dimensional Contact Information into the COSMO-SAC Model

The COSMO-SAC model for the calculation of activity coefficients has been improved by incorporating three-dimensional geometric information on molecules. This information is added to the model by means of the effective contact area of the molecules. We define a procedure to compute this contact area by using a probing sphere. The probing sphere rolls around the COSMO surfaces of two contacting atoms to define a spherical cap on each atom. The segments of the COSMO surfaces within the spherical caps are marked, and the summation of their areas defines the contact area for a pair of contacting molecules. The effective contact areas are used to compute the σ-profiles and the Onsager screening energies in the calculation of the surface activity coefficients, allowing us to remove one parameter from the COSMO-SAC model. The σ-profiles and effective areas need to be calculated only once and can be stored in a database. We show that the new model, named COSMO-3D because some three-dimensional information (the po...

Activity coefficients in the surface phase of liquid mixtures.

A novel equation for evaluating surface activity coefficients is obtained from a recent thermodynamic formalism describing the surface phase of liquid mixtures. The input quantities are the surface tension, bulk activity coefficients and pure constituent thermophysical properties. It is demonstrated thermodynamically that the order of magnitude of each component surface and bulk activity coefficients must be the same. This order is intrinsically associated with the sign of excess surface tension. Reliable activity coefficients of ethanol and water in the surface phase of their mixtures are computed and reported for the first time, by using literature data for the required input quantities. It is shown that the so-called transferring method for estimating surface activity coefficients is severely flawed, because it leads to contradictory values of predicted excess surface tensions depending on which component this prediction is based.

Application of surface complexation models to anion adsorption by natural materials

Various chemical models of ion adsorption are presented and discussed. Chemical models, such as surface complexation models, provide a molecular description of anion adsorption reactions using an equilibrium approach. Two such models, the constant capacitance model and the triple layer model, are described in the present study. Characteristics common to all the surface complexation models are equilibrium constant expressions, mass and charge balances, and surface activity coefficient electrostatic potential terms. Methods for determining parameter values for surface site density, capacitances, and surface complexation constants also are discussed. Spectroscopic experimental methods of establishing ion adsorption mechanisms include vibrational spectroscopy, nuclear magnetic resonance spectroscopy, electron spin resonance spectroscopy, X-ray absorption spectroscopy, and X-ray reflectivity. Experimental determinations of point of zero charge shifts and ionic strength dependence of adsorption results and molecular modeling calculations also can be used to deduce adsorption mechanisms. Applications of the surface complexation models to heterogeneous natural materials, such as soils, using the component additivity and the generalized composite approaches are described. Emphasis is on the generalized composite approach for predicting anion adsorption by soils. Continuing research is needed to develop consistent and realistic protocols for describing ion adsorption reactions on soil minerals and soils. The availability of standardized model parameter databases for use in chemical speciation-transport models is critical.

The surface chemical potential from a surface equation of state versus Butler's equation

In the last years, our research group has been committed to the development of bulk–surface equilibrium models that relate surface tension, bulk composition, and bulk activity coefficients; in this paper, we focus on two of them. The starting premise is the equality of bulk and surface chemical potentials. Instead of the common practice of expressing the surface chemical potential analogously to the bulk, in the first model, surface chemical potential is obtained from the integration of Gibbs adsorption equation coupled to Volmer's equation of state, and written as a function of surface pressure shown to be a measure of surface activity. In the second model, Gibbs adsorption equation was evaluated in the surface saturated region and then coupled to the bulk chemical potential avoiding the use of an explicit expression for the surface. Both models are equivalent in meaning and structure to the widely accepted Sprow and Prausnitz equation (Trans. Faraday Soc. 62 (1966) 1105), originally derived by Butler (Proc. R. Soc. A 138 (1932) 348), where it is necessary to introduce non-thermodynamic approximations for the calculation of partial molar areas, surface activity coefficients and surface composition. On the contrary, the parameters of the two proposed models are attained from the fitting to the experimental data. Obeying a monolayer model, Butler arrived to the conclusion that bulk and surface chemical potentials are not equal, but it is demonstrated that in Gibbs adsorption equation, the equality of chemical potentials is valid and that the models are thermodynamically consistent.

New Conceptualization and Solution Approach for the Ideal Adsorbed Solution Theory (IAST)

Despite the widespread use of the ideal adsorbed solution theory (IAST) to predict competitive adsorption of hydrophobic organic compounds from aqueous solutions, the underlying principles of the model are not widely understood. A new conceptualization is presented that treats the adsorbed phase as gas-like with a total surface pressure equal to the sum of the partial surface pressures of the adsorbed species. In this conceptualization, the key assumption of the IAST is reformulated as indicating that the partial surface pressure exerted by an adsorbate depends only on its adsorption density and a surface activity coefficient that depends on the total surface pressure. The parallels of the model parameters and assumptions with those in gas/liquid equilibrium facilitate a more intuitive understanding of the model concepts. This understanding, in turn, clarifies the reasons underlying the predictions of the IAST for limiting conditions where one adsorbate dominates the surface phase. Finally, a new solution algorithm is introduced that allows the model equations to be solved using a simple spreadsheet analysis in which onlytwo independent parameters need be evaluated regardless of the number of adsorbates in the system or the characteristics of the single-solute isotherms.

Electrophoresis, surface charge and surface activity coefficient of ionic adsorbates at interfaces

Historical development of measurements of electrophoretic mobilities of solid particles and proteins using moving boundary techniques have been discussed in brief. Results on micro-electrophoretic methods of mobility measurements of solid particles adsorbed with proteins and other biopolymers have also been elaborated and features of electrophoretic separation of biomolecules using various types of zone electrophoresis techniques have been mentioned. The corrections required in connecting electrophoretic mobilities with electrokinetic or zetapotential using Smoluchowski relation discussed by Henry, Booth, Ghosh and others have been mentioned. The pictures included in the electrical double layers proposed by Helmholtz, Gouy-Chapman and Stern have been discussed. From the adsorption experiments of surfactants at oil-water interfaces, the charge densities (σ) and diffuse double layer potentials (ψ δ ) at a given concentration have been evaluated. The data are observed to fit the Stern layer model with counter-ion binding at the charged surface. ψ δ is found to be significantly higher than zetapotential. From the boundary tension measurement, the surface tension lowering or surface pressure II for soluble monobasic and dibasic acids have been calculated at different values of area A per adsorbed molecule. The electrical pressure II e obtained for these acids in the monolayer does not fit the Gouy model of double layer but using Stern model, the extents of counter-ion binding in the monolayer can be calculated. Using the Langmuir surface balance experiments, II-A curves for insoluble monolayers of different behenate salts have been obtained. At pH 12.0, values of IT e depends upon extents of counter-ion binding, hydration and other factors. A new concept of surface activity coefficient based on Raoult's law, and Hennry's law have been introduced to explain deviation of II from their ideal values.

Behavior of sulfatide/cholesterol mixed monolayers at the air/water interface

The monolayer properties of sulfatide and cholesterol binary system have been investigated with surface pressure–mean molecular area isotherms measurements and atomic force microscopy (AFM). The thermodynamic analysis indicates that the obtained negative deviation of the excess molecular area (Δ A (ex)) and surface excess Gibbs energy (Δ G (ex)) from the ideal behavior at various molar ratios, suggesting an attractive interaction between sulfatide and cholesterol in the monolayers as compared with the pure components monolayers. Meanwhile, the compression modulus ( C s − 1 ) vs. surface pressure ( π) and activity coefficients ( f 1 and f 2) of mixed films dependencies for mixed monolayers are drawn at different mole fractions. The AFM images for the mixed sulfatide/cholesterol monolayers deposited on the mica at 15 and 30 mN m −1 show the stronger molecular attractive force to form condensed structure. The behavior of sulfatide is thought to be involved in lateral domain formation in biological membranes. Therefore, the interaction between sulfatide and cholesterol becomes more important in mimicking “lipid rafts” domains.

A proposal for the estimation of binary mixture activity coefficients from surface tension measurements throughout the entire concentration range

A method to estimate the concentration dependence of the bulk activity coefficients of both binary mixture components from experimental surface tension data covering the whole concentration range is presented. To this end, high-quality experimental surface tension data as a function of the concentration, paying special attention to the diluted regions, are needed. The method is based on the application of equilibrium conditions to the surface chemical potential arisen from the Volmer equation (the simplest non-ideal surface EOS) and to the bulk chemical potential coming from the three-suffix Margules equation for activity coefficients. It can be applied provided that positive aneotropy is not present and the second derivative of the surface pressure as a function of the mole fraction is not positive in any composition region, which means rather unrestrictive conditions. In order to test the method, a compilation of 25 systems for which surface tension and activity coefficient data are simultaneously available in the literature was performed, turning out that most of them involved water. It was found that surface tension data with enough concentration coverage and good quality are really scarce, which makes the development of this kind of methods difficult.

Influence of Zn(II) adsorption on the photocatalytic activity and the production of H2O2 over irradiated TiO2

The photocatalytic activity of semiconductor oxides, in particular TiO2 powders or colloids, is a complex function of bulk (light absorption and scattering, charge carrier mobility and recombination rate) and surface (structure, defects and reconstruction, charge, presence of adsorbate, surface recombination centers) properties. Among surface modifications, the inner sphere surface complexation of metal cations can change the surface charge of the metal oxide, thus changing the surface activity coefficient of ionic substrates, the band edge positions, as well as the mechanism and kinetic of interfacial electron transfer by blocking surface trapping sites for photogenerated carriers (≡Ti−OH). In this work we show that in anatase/water systems under band-gap irradiation, both the organic substrate (formate) oxidation initiated by photogenerated valence band holes and the formation of hydrogen peroxide from O2 reduction (by conduction band electrons) is strongly influenced by the presence of Zn2+ cations. Depending on the pH, the formate oxidation rate can be enhanced or nearly completely inhibited. The observed result can be rationalized by considering the fraction of ≡Ti−OH surface sites blocked by inner sphere complexation of Zn2+ as a function of pH. When this fraction is low, the more positive surface charge favors formate oxidation, whereas when the fraction is high the almost complete blockage of ≡Ti−OH surface sites by Zn2+ stops almost entirely formate oxidation. Interestingly, the surface complexation of Zn2+ is accompanied by an increasing production of H2O2 during formate degradation in the presence of O2. Zn(II) cations are not complexed by peroxide/superoxide species derived from O2 reduction. When ≡Ti−OH sites are blocked by Zn2+, the complexation on the TiO2 surface of peroxide/superoxide species is inhibited, hindering their further transformation. The results presented demonstrate that the combined effect of pH and surface complexation of redox inert cations greatly influences both the oxidative and reductive processes during the photocatalytic process over TiO2.

Classic adsorption isotherms incorporated in modern surface complexation models: Implications for sorption of actinides

Computer-aided surface complexation models (SCMs) are widely used in describing mineral-water interface reactions such as sorption of actinides onto (hydr)oxides and clays. Most SCMs are built upon balances of surface binding sites. In fact, such models can reproduce only competitive Langmuir isotherms, plus optional electrostatic corrections. Hence, it may be difficult to extend such SCMs to some real-surface phenomena (site heterogeneity, lateral interactions, surface precipitation) described in isotherms other than Langmuir. In this contribution, a new approach to overcome this difficulty is presented which does not use surface site balances, is not restricted only to Langmuir isotherms, and can be implemented within the Gibbs energy minimization solvers of chemical (adsorption) equilibria. It is suggested to define the ideal behavior of a surface species using a linear isotherm only. Conversely, an adsorption isotherm equation of choice can be split into a linear term (involving the standard-state density and the equilibrium constant referenced to infinite dilution) and a non-linear surface activity coefficient term (SACT, involving the site density parameter). A simple method is proposed for deriving a SACT for the monodentate surface binding from the Langmuir isotherm. This method is then applied to obtain SACTs for mono-, bi-, tri- and tetra-dentate binding from the “quasi-chemical approximation” isotherm, as well as a SACT accounting for non-ideal lateral interactions between surface species from the Frumkin isotherm. The new approach is illustrated by modeling UVI adsorption on goethite at ambient conditions using a GEM SCM. It is shown that SACTs are useful at moderate- to high surface coverage, but only when the site density parameter is fixed from independent (crystallographic) data and not adjusted to fit the titration data. Ways to enhance SCMs with BET, Freundlich and other isotherms of interest for retention of actinides and fission products on mineral surfaces are also discussed.

Surface complexation modeling of zinc sorption onto ferrihydrite

A previous study involving lead(II) [Pb(II)] sorption onto ferrihydrite over a wide range of conditions highlighted the advantages of combining molecular- and macroscopic-scale investigations with surface complexation modeling to predict Pb(II) speciation and partitioning in aqueous systems. In this work, an extensive collection of new macroscopic and spectroscopic data was used to assess the ability of the modified triple-layer model (TLM) to predict single-solute zinc(II) [Zn(II)] sorption onto 2-line ferrihydrite in NaNO 3 solutions as a function of pH, ionic strength, and concentration. Regression of constant-pH isotherm data, together with potentiometric titration and pH edge data, was a much more rigorous test of the modified TLM than fitting pH edge data alone. When coupled with valuable input from spectroscopic analyses, good fits of the isotherm data were obtained with a one-species, one-Zn-sorption-site model using the bidentate-mononuclear surface complex, (FeO) 2Zn; however, surprisingly, both the density of Zn(II) sorption sites and the value of the best-fit equilibrium “constant” for the bidentate-mononuclear complex had to be adjusted with pH to adequately fit the isotherm data. Although spectroscopy provided some evidence for multinuclear surface complex formation at surface loadings approaching site saturation at pH ⩾6.5, the assumption of a bidentate-mononuclear surface complex provided acceptable fits of the sorption data over the entire range of conditions studied. Regressing edge data in the absence of isotherm and spectroscopic data resulted in a fair number of surface-species/site-type combinations that provided acceptable fits of the edge data, but unacceptable fits of the isotherm data. A linear relationship between log K (FeO)2Zn and pH was found, given by logK ( FeO)2 Zn at 1 g/l =2.058 (pH)–6.131. In addition, a surface activity coefficient term was introduced to the model to reduce the ionic strength dependence of sorption. The results of this research and previous work with Pb(II) indicate that the existing thermodynamic framework for the modified TLM is able to reproduce the metal sorption data only over a limited range of conditions. For this reason, much work still needs to be done in fine-tuning the thermodynamic framework and databases for the TLM.

Nonideal electrochemical behavior of biomimetic iron porphyrins: interfacial potential distribution across multilayer films.

The electrochemical behavior of multilayer films formed by iron porphyrins deposited on an edge plane graphite electrode has been examined under anaerobic conditions. In the scan rate interval (1-250 mV/s) where the electrode reaction is reversible, CV diagrams of these films demonstrate substantial deviations from ideality in broadening and separation of the peaks. A model that describes the observed behavior is proposed by taking into account the potential distribution at the electrode/film interface and the concentration dependence of surface activity coefficients. The peak separation is described in terms of the electric double layer that affects the potential difference driving the electrode reaction. The effective potential difference deviates from the applied value due to the potential distribution across the film. The interfacial potential distribution depends on the ionic concentration inside the film. When different ionic concentrations are assumed for oxidation and reduction, different shifts from the applied potential lead to a hysteresis of the peaks. The peak broadening is modeled by using the lattice theory expression for the surface activity coefficients. The model shows that the midpoint potentials of the redox centers depend on the ionic concentration inside the film. At low ionic concentrations, they are remarkably close to the midpoints of the cytochrome c oxidase heme a3/CuB site.

Study on absorption and transport of carbon in steel during gas carburizing with rare-earth addition

Abstract The effect of rare-earth (RE) as a catalyst adding to carburizing atmosphere on absorption and transport of carbon is presented in this paper. The processing were carried out at 900°C for different time in a decomposition atmosphere of kerosene and methanol with and without RE addition. The compositions of carburizing atmosphere such as CO, CH4, etc. and the average carbon concentrations in 75 μm thick 08F steel foil were measured by infrared and chemical analysis, respectively. The surface activity and boundary transfer coefficient of carbon have been calculated based on the measured data. The results show that introducing RE into carburizing atmosphere increases the volume fraction of CO, CH4, etc. degree of dissociation of kerosene, carbon potential, surface activity and boundary transfer coefficient of carbon.