Adsorption at liquid interfaces: The generalized Frumkin isotherm and interfacial structure

The thermodynamics of adsorption of amphiphilic surface-active compounds at the interface between two immiscible liquids is considered. At the interface, these molecules are supposed to replace a few of the adsorbed molecules of both solvents. Classical isotherms of adsorption (Frumkin, Frumkin-Damaskin, Langmuir, Henry) were based on the model of non-penetrable interface, where an adsorbate can substitute only molecules of one solvent. At the interface between two immiscible electrolytes, nonpolar oil/water interfaces, and liquid membranes amphiphilic molecules can substitute molecules of both solvent and classic isotherms cannot be used. The generalization of Frumkin isotherm for permeable and non-permeable interfaces, known as the Markin-Volkov isotherm, gives the possibility to analyze adsorption in a general case. The adsorption isotherms of pentafluorobenzoic acid at the octane/water interface at different pHs were measured by the drop-weight method. The thermodynamic parameters of pentafluorobenzoic acid (PFBA) adsorption at octane/water interface were determined. From the measurements of PFBA adsorption, the structure of the octane/water interface was determined. Substitution of one adsorbed octane molecule requires approximately three adsorbed PFBA molecules. This result shows that the orientation of solvent molecules at the interface is different from the bulk. Adsorbed octane molecules have a lateral orientation with respect to the interface. Gibbs free energy of adsorption equilibrium and thermodynamic parameters of PFBA adsorption show that the adsorption of PFBA at the octane/water interface is accompanied by a reduction in the attraction between adsorbed PFBA molecules as the pH decreases to the acidic region. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 10, pp. 1194–1200. The text was submitted by the authors in English.     

Proteins at liquid interfaces: II. Adsorption isotherms

Adsorption isotherms for the three proteins β-casein, bovine serum albumin, and lysozyme at the air-water and oil-water interfaces have been determined independently using ellipsometry and surface radioactivity methods; the surface pressure and surface potential were also monitored. Saturated monolayer coverage occurs via irreversible adsorption of 2–3 mg M^?2 of protein; the resultant films generate surface pressures of about 20 mN m^?1 and are 50–60 Å thick. Molecules adsorbed in the first layer dominate the film pressures so that further adsorption causes no change in the pressure although the film thickness can increase to more than 100 Å. The molecules which give rise to this increase in film thickness are reversibly adsorbed with respect to aqueous substrate exchange. The experimental isotherm data and the Langmuir adsorption isotherm are in close agreement at low protein concentrations. However, comparison with the Gibbs adsorption equation is not valid, although reasonable agreement can be achieved if some account is taken of the fact that the protein molecules in the first layer are irreversibly adsorbed.     

Adsorption and displacement of beta-2-microglobulin at solid/liquid interfaces

Adsorption of beta-2-microglobulin from aqueous solution onto unmodified and methylated silicon wafers and subsequent displacement of the small globular protein by fibrinogen were studied by spectroscopic ellipsometry, immunosorbent assays and atomic force microscopy. The results provide evidence that hydrophobicity of the substrate increases the maximum adsorbed amount of beta-2-microglobulin and the resistance of the adsorbed protein to displacement from the interface by competing species, respectively. Further, the dynamics of beta-2-microglobulin adsorption was found to induce significant differences in the degree of displacement achieved at given conditions. The observed variations in displacement behavior of adsorbed beta-2-microglobulin were interpreted based on information on the layer structure gained by atomic force microscopy. More compact and relatively smooth protein layers were formed on the hydrophobic surface corresponding to lower displacement by fibrinogen.     

Equilibrium theory based design of simulated moving bed processes for a generalized Langmuir isotherm

In the frame of the local equilibrium theory of chromatography, design criteria for complete separation of binary mixtures in simulated moving bed (SMB) separations are developed, presented and discussed. These apply to systems, whose retention behavior is characterized by a generalized Langmuir isotherm. By allowing for negative terms in the denominator of the classical Langmuir isotherm, this newly introduced adsorption model captures a broad class of competitive or synergistic adsorption, including anti-Langmuir behavior for both adsorbates, and mixed cases where one species behaves in a Lagmuirian and the other in an anti-Langmuirian manner. By extending classical equilibrium theory results for the binary Langmuir isotherm, and by generalizing the approach followed earlier to derive SMB design criteria for the binary and multi-component Langmuir isotherm, exact algebraic equations for the boundary of the complete separation region in the operating parameter space are derived for all possible generalized Langmuir isotherm. The effect of changing feed composition on the shape of the complete separation region and on the position of the optimal operating point is analyzed and discussed.     

Surface vibrational structure at alkane liquid/vapor interfaces

Broadband vibrational sum frequency spectroscopy (VSFS) has been used to examine the surface structure of alkane liquid/vapor interfaces. The alkanes range in length from n-nonane (C(9)H(20)) to n-heptadecane (C(17)H(36)), and all liquids except heptadecane are studied at temperatures well above their bulk (and surface) freezing temperatures. Intensities of vibrational bands in the CH stretching region acquired under different polarization conditions show systematic, chain length dependent changes. Data provide clear evidence of methyl group segregation at the liquid/vapor interface, but two different models of alkane chain structure can predict chain length dependent changes in band intensities. Each model leads to a different interpretation of the extent to which different chain segments contribute to the anisotropic interfacial region. One model postulates that changes in vibrational band intensities arise solely from a reduced surface coverage of methyl groups as alkane chain length increases. The additional methylene groups at the surface must be randomly distributed and make no net contribution to the observed VSF spectra. The second model considers a simple statistical distribution of methyl and methylene groups populating a three dimensional, interfacial lattice. This statistical picture implies that the VSF signal arises from a region extending several functional groups into the bulk liquid, and that the growing fraction of methylene groups in longer chain alkanes bears responsibility for the observed spectral changes. The data and resulting interpretations provide clear benchmarks for emerging theories of molecular structure and organization at liquid surfaces, especially for liquids lacking strong polar ordering.     

Adsorption of detergent-solubilized and phospholipase C-solubilized alkaline phosphatase at air/liquid interfaces

To investigate the influence of a hydrophobic anchor on protein adsorption, equilibrium and dynamic aspects of the adsorption of two different solubilized forms of rat osseous plate alkaline phosphatase on Langmuir monolayers of dimyristoylphosphatidic acid (DMPA) were studied. Surface pressure and surface potential measurements at air/liquid interfaces were carried out using the detergent-solubilized form (DSAP) of alkaline phosphatase, which holds a glycosylphosphatidylinositol (GPI) hydrophobic anchor, and the glycosylphosphatidylinositol-specific phospholipase C-solubilized form (PLSAP), lacking the GPI anchor. Similar surface transitions observed for both DMPA and DMPA/PLSAP mixed monolayers indicate that the presence of PLSAP does not promote significant changes in surface packing of the DMPA monolayer. However, PLSAP interacts with the polar portion of the phospholipid even at high lateral compression. The presence of the GPI anchor increases the adsorption of DSAP at a plain air/liquid interface and also enables the penetration of the protein into the DMPA monolayers. The penetration is dependent on both time and surface pressure. Up to 20 mN/m, the surface pressure increases smoothly indicating a diffusion followed by an adsorption process. Above 20 mN/m, after a fast increase, the surface pressure slowly decays to equilibrium values quite close to the initial surface pressures. The results indicate that the molecular packing of the lipid layer drives the enzyme adsorption to the interface either through the GPI anchor or by the polypeptide moiety.     

Water-hydrocarbon interfaces: effect of hydrocarbon branching on interfacial structure

Molecular dynamics simulation are performed for the water/hydrocarbon system to study the effect of hydrocarbon branching on interfacial properties. The following two series of hydrocarbons are considered: (1) n-pentane, 2-methyl pentane, and 2,2,4-trimethyl pentane (constant chain length) and (2) n-octane, 2-methyl heptane, and 2,2,4-trimethyl pentane (constant molecular mass). With a simple algorithm for identification of surface sites and mapping nonsurface sites to these surface sites, intrinsic profiles were constructed with respect to the surface layer. Intrinsic density profiles for water and hydrocarbons with respect to the hydrocarbon and water surface, respectively, resemble density profiles of liquids in the presence of a wall. Order parameters were used to study orientation of molecules with respect to the surface normal and the hydrogen bond network was characterized in terms of the number of hydrogen bonds per water molecule and percentage of hydrogen bonded molecules in the first coordination shell. The corresponding intrinsic profiles were obtained. The O-H bond for surface water was found to have two preferential orientations, pointing toward the hydrocarbon phase and parallel to the interface. Hydrocarbon molecules in series 1 orient along the interface with the more branched molecule better aligned. For molecules in series 2, the larger molecular length reduces the alignment of molecules along the interface.     

Adsorption of polymers at the solution-solid interfaces

Summary Adsorption of ethyl cellulose, so-called rigid polymer, on the glass surface from various solvents having no groups to make the hydrogen bonds with solid surface was carried out. And the solvent dependence was found. The dependence is, however, small as compared with polyvinyl acetate-glass system (1). A main factor to control the solvent dependence is not competitive between solvent and polymer, but the effect of solvent to extension of polymer in solution, that is, segment density per solid surface area in poor solvent is greater than in better solvent. Maximum adsorption is independent of molecular weight of ethyl cellulose used (Mv=2×10⁴6×10⁴). It is expected that ethyl cellulose lies flat on the glass surface considering adsorption energy per segment (₀>5kT).

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Zusammenfassung Adsorptionen von Äthylzellulose als sozusagen steifes Polymeres an Glasoberfläche aus verschiedenen Lösungsmitteln heraus, die keine Gruppen für Wasserstoff bindungen an die Festoberflächen besitzen, wurden ausgeführt. Es ergab sich in der Tat Lösungsmittelabhängigkeit. Diese ist jedoch klein im Vergleich zu der beim System Polyvinylacetat/Glas. Eine Ursache für die Lösungsmittelabhängigkeit ist nicht direkt die unterschiedliche Solvatation zwischen Lösung und Polymer, sondern der Effekt des Lösungsmittels auf die räumliche Aufweitung des Polymers in Lösungen. Letzteres bedeutet nämlich, die Segmentdichte ist in schlechten Lösungsmitteln größer als in guten. Die maximale Adsorption ist unabhängig vom Molekulargewicht im Bereich vonMv=2×10⁴ bis 6×10⁴. Es ist zu erwarten, daß die Äthylzellulosc flach auf der Glasoberfläche aufliegt, wenn man die Adsorptionsenergie pro Segment betrachtet.

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With 5 figures and 4 tables     

Lipases at interfaces: unique interfacial properties as globular proteins

The adsorption behavior of two globular proteins, lipase from Rhizomucor miehei and beta-lactoglobulin, at inert oil/water and air/water interfaces was studied by the pendant drop technique. The kinetics and adsorption isotherms were interpreted for both proteins in different environments. It was found that the adopted mathematical models well describe the adsorption behavior of the proteins at the studied interfaces. One of the main findings is that unique interfacial properties were observed for lipase as compared to the reference beta-lactoglobulin. A folded drop with a "skinlike" film was formed for the two proteins after aging followed by compression. This behavior is normally associated with protein unfolding and covalent cross-linking at the interface. Despite this, the lipase activity was not suppressed. By highlighting the unique interfacial properties of lipases, we believe that the presented work contributes to a better understanding of lipase interfacial activation and the mechanisms regulating lipolysis. The results indicate that the understanding of the physical properties of lipases can lead to novel approaches to regulate their activity.     

A study of interfacial dilational properties of two different structure demulsifiers at oil-water interfaces

The interfacial dilational viscoelastic properties of two demulsifiers with straight chain (SP-169) and branched chain (AE-121) at the oil-water interfaces were investigated by means of the longitudinal waves method and the interfacial tension relaxation method, respectively. The results obtained by the longitudinal waves method showed that the dilational viscous component for AE-121 and SP-169 also passed through a maximum value with increasing concentration. It was found that the maximum value appeared at different demulsifier concentrations during our experiment frequency; and the higher is the dilational frequency, the lower is the concentration. The influences of AE-121 and SP-169 on the dilational viscoelastic properties of the oil-water interface containing surface-active fraction from Iranian crude oil have been measured. The results clearly stated that both demulsifiers could obviously decrease the dilational elasticity of oil-water interface containing surface-active fraction. At low concentration, because of stronger adsorption ability, SP-169 has stronger ability to decreasing the dilational modulus than AE-121. We also found that the dilational modulus of the interface contained surface-active fraction passed through a minimum value with increasing demulsifier concentration for both demulsifiers. This result indicated the dosage of demulsifier had an optimum value. The results obtained by means of interfacial tension relaxation method showed that the slow relaxation processes involve mainly rearrangement in the conformation of the molecules appeared with increasing demulsifier concentration.     

Water structure at interfaces: the present situation

This paper discusses the structure of adsorbed water at interfaces. It begins with a review of the development of the research, then examines and compares some of the most important models in this field. The results of recent spectroscopic work, especially those of IR spectroscopy, are discussed and applied to the selection of the most probable model.     

Electrochemistry at micro- and nanoscopic liquid/liquid interfaces

In this tutorial review, we will briefly introduce the history and basic concepts of micro- and nanoscopic liquid/liquid interfaces (size from nm to μm) in electrochemical studies of charge (electron and ion) transfer reactions at soft molecular interfaces. Their advantages and problems are usually compared with those of conventional liquid/liquid interfaces (size from mm to cm); and with solid/electrolyte interfaces. Three methods of fabrication of micro-liquid/liquid interfaces and one approach to support a nano-liquid/liquid interface are surveyed. The experimental and theoretical aspects are discussed along with possible approaches to characterize these micro- and nanoscopic liquid/liquid interfaces, and the methods to modify them with new functionality. Unique examples of applications of electrochemistry at micro- and nanoscopic liquid/liquid interfaces are provided. Some novel and potential research interests in the future in this field are discussed.     

Adsorption of microstructured particles at liquid-liquid interfaces

The solid particles are adsorbed at interfaces and form self-assembled structures when the particles have suitable wettability to both liquids. Here, we show theoretically how the microstructure on the particle surface affects their adsorption properties. The physical properties of the interface adsorbing a particle will be described by taking into account the surface roughness due to the microstructure. The microstructure on the surface changes drastically the wettability and the equilibrium position of the adsorbed particle. Therefore, the contact angle of the particle at the three-phase contact line shifts with the particle surface area, because the surface roughness enhances the interfacial properties of the particle surface. Moreover, the range of the interfacial tensions at which the particle is adsorbed becomes narrower with the increase of the surface roughness. The effect of the particle shape on the adsorption properties is also studied. In the case of disk-shaped particles, the energy changes discontinuously when the plane surface of the particle contacts the liquid-liquid interface. The adsorbing position does not change with the surface roughness. The orientation of a parallelepiped particle at the liquid-liquid interface is governed by the aspect ratio and the surface area of the particle. On the other hand, the particle which is partially covered with the microstructured surface is adsorbed firmly at the interface in an oriented state. We should consider not only the interfacial tensions but also the surface structure and the particle shape to control the adsorption behavior of the particle.     

Surface pressure isotherm for the fluid state of Langmuir monolayers

The equation of state for the monolayer comprised of the molecules of different sizes (water and biopolymers) is modified to describe the fluid (liquid-expanded and gaseous) state of the insoluble molecules monolayer. In contrast to the equation of state derived previously in ref 1, this equation does not involve either the Gibbs' adsorption equation or the differential equation for the chemical potential of the insoluble component, but it is based only on the equations for the chemical potential of the solvent in the bulk and in the surface layer. The results calculated from the proposed equations are in perfect agreement with the experimental Pi-A isotherms of the liquid-expanded state obtained for Langmuir monolayers of various types of amphiphilic compounds. The values of molecular areas of amphiphilic molecules estimated from the fitting of experimental data to the proposed equation are found to be quite similar to those measured in independent grazing incidence X-ray diffraction (GIXD) experiments.     

Adsorption isotherms of associating asphaltenes at oil/water interfaces based on the dependence of interfacial tension on solvent activity

In the Gibbs adsorption equation, the application of solvent activity for the calculation of the surface/interfacial excess is proposed for nonideal or associating or pseudocomponents such as asphaltenes. For the aforementioned systems, only the mass-based phenomenological interfacial excess can be determined based on interfacial tension versus activity data. The use of the mole fraction is compared to the use of the activity when the adsorbed amount of associating asphaltenes is calculated at a water/toluene interface. Langmuir-type isotherms describe the adsorption of asphaltenes at toluene/water interfaces. Asphaltenes were treated to remove the resins and natural surfactants using cyclic precipitation and dissolution of asphaltenes at a fixed aliphatic/aromatic ratio. Different fractions of asphaltenes were obtained by changing the aliphatic/aromatic ratio of the precipitating solvent. The limiting molar masses of asphaltenes measured by vapor pressure osmometry are different for fractions precipitated at different heptane to toluene ratios. The mass-based adsorbed amounts at the water/toluene interface, at a 0.1 asphaltene-to-toluene mass-ratio, varied in the range of 0.8-2.8 mg/m(2), depending on the molar mass of asphaltenes.     

Adsorption of hydrophobin proteins at hydrophobic and hydrophilic interfaces

The surface activity of two hydrophobin proteins, HFBII and SC3, at the solid–liquid, liquid–liquid and liquid–vapor interface has been investigated. Hydrophobins are fungal proteins that are known to adsorb and affect the physico-chemical properties of an interface. In this study, the surface activity was determined by measuring the interaction of hydrophobin molecules with various liquids, solid particles and films that are commonly used or produced in industrial processes. We found that a very low concentration of hydrophobin is required to facilitate the wet-in of hydrophobic solid particles, such as Teflon^®, into aqueous solutions. It is also capable of stabilizing aqueous dispersions of Kevlar^® nanopulp, reversing the wettability of hydrophobic films and stabilizing polyunsaturated fatty acid (PUFA) oil-in-water emulsions.     

Adsorption and adhesion processes in microbial growth at interfaces

By considering bacteria as living colloidal particles it is possible to define, at least in part, the physicochemical and biological factors involved in the adhesion of bacteria to apparently inert surfaces. Adhesion is considered in terms of the attraction and repulsion forces operative as the organism approaches a substratum surface, as well as in terms of the surface free energies of the bacterium, the substratum and the liquid phase. Bacterial adhesion at a surface is preceded by molecular adsorption, a process that may alter the substratum surface properties and may provide a concentrated source of nutrients allowing limited growth of the bacteria at the surface.     

Analysis of application of Langmuir isotherm to heterogeneous systems: high-pressure conditions

In this paper, an adsorption model that is based on the statistical mechanics approach was applied to study the saturation phenomena in adsorption to calculate the minimum pressure needed to attain the complete surface coverage on a physical adsorption. The fundamental integral equation, Theta(T)(Q)=integral N(Q)Theta(Q)dQ, for the calculation of the coverage degree of the surface was developed for an exponential distribution function, N(Q)=(m/RT)exp(-mQ/RT), and the representation of local adsorption sites is given by the Langmuir expression, Theta(Q)=b(0)C exp(Q/RT)/(1+b(0)C exp(Q/RT)). At high values of the pressure C, a solution of the fundamental integral equation was obtained by imposing the condition b(0)C>1. The expression for the saturation condition, b(0)C>m/(m+1), was obtained; that is, the saturation phenomenon is dependent on two parameters, correlated with the energetic heterogeneity and adsorption energy of the system. The pressure in the analysis of the cited expression shows that, for low m values (more heterogeneous systems), saturation is attained for b(0)C>m, while for m congruent with 1 (more homogeneous systems), the saturation is attained for b(0)C>0.5.