Surface Activity Coefficients Research Articles

Adsorption Isotherms for Concentrated Aqueous-Organic Solutions (CAOS)

Adsorption isotherms for concentrated aqueous-organic solutions are presented. These are calculated from the Gibbs adsorption equation, using surface tension and activity coefficient experimental data. General considerations reveal that two maxima may be expected in the adsorption isotherm. Moreover, close to phase separation, the adsorption isotherm may show singularities. Calculations for specific examples support these general predictions. The calculated adsorption isotherms are qualitatively different from existing theoretical models due to the high concentrations involved and to the nonideality of the solutions.

The influence of marine surface films on underwater sound

Essential characteristics of wind-generated ambient noise at sea depend on a few aerodynamic parameters of the sea surface–atmosphere interaction process. The film smoothing effect attributed to a reduction of the friction velocity and wind wave growth rate is shown, for model marine films, to be expressed in the corresponding changes of the sound-level intensity versus wind speed and frequency characteristics. Naturally generated ambient noise in the ocean is also created by breaking waves, resonating gas bubbles, spray, precipitation, or solid impacts, and has particular features. The spectrum of rain shows a peak at 13–15 kHz with the spectral level SL, in dB, linearly dependent on the log rain rate. The effect of a monomolecular film covering the surface exposed to splashing water droplets, uniformly distributed, and modeled as incoherently radiating dipoles, is evaluated assuming the Marangoni damping mechanism of surface disturbances by a viscoelastic film. The result is a pronounced reduction of SL beneath the film by 10 log K, 3–20 dB, for realistic systems, where the damping ratio K is a complex function expressed by the surface activity, elasticity, concentration, and diffusional coefficient of surfactants composing the film.

Concurrent Solution and Adsorption of Hydrocarbons in Gas Chromatographic Columns Packed with Different Loadings of 3-Methylsydnone on Chromosorb P

Thermodynamic properties of solution in 3-methylsydnone (3MS) and of adsorption at the nitrogen/3MS interface were gas chromatographically measured for a group of fifteen hydrocarbons at infinite dilution conditions. Retention volumes were measured at five temperatures within the range 37–52°C in six columns containing different loadings of 3MS on Chromosorb P AW. Partition and adsorption coefficients were calculated and from their temperature dependence the corresponding enthalpies were obtained, although with considerable error; infinite dilution activity coefficients of the hydrocarbons in the bulk and in the surface phases demonstrated a strong correlation. Bulk activity coefficients in 3MS were very much smaller than those previously measured for the same solutes in formamide (FA) and in ethyleneglycol (EG), and were also smaller than what could be predicted on account of 3MS cohesive energy density as estimated from the quotient σ/v1/3(σ, surface tension;v, molar volume). There was not such a large difference between the surface activity coefficients in the three solvents; furthermore, the quotients (surface activity coefficient/bulk activity coefficient) for a given solute in 3MS were twice as large as in FA and about three times larger than in EG. These results make evident the difficulties inherent in the prediction of surface phase properties from those in the bulk and cast doubts on the pertinency of employing the surface tension to compare cohesive energy densities of polar solvents with important chemical differences.

Formation of dimers in lipid monolayers

In this work, we analyse theoretically the hypothesis that zwitterionic lipids form dimers in adsorption monolayers on water/ hydrocarbon phase boundary. A dimer can be modelled as a couple of lipid molecules whose headgroup lateral dipole moments have antiparallel orientation. Properties including surface pressure, chemical potentials and activity coefficients are deduced from a general expression for the free energy of the monolayer. The theoretical model is in a good agreement with experimental data for surface pressure and surface potential of lipid monolayers. The results favour the hypothesis about formation of dimers in equilibrium with monomers, with the amount of the species depending on the area per molecule and temperature. The reaction of dimerisation turns out to be exothermic with a heat of about 2.5kT per dimer. The results may be applied to the molecular models of membrane structures and mechanisms.

Prediction of the surface tension of petroleum cuts using a modified UNIFAC group contribution method

The surface tensions of petroleum cuts were predicted using the modified UNIFAC group contribution method. Each petroleum cut was considered to be composed of three pseudocomponents: paraffinic, naphthenic and aromatic. The surface tension of the cut was predicted by calculating the bulk and surface activity coefficients, concentrations and surface tensions of the pseudo-components. The pseudocomponent surface tension was calculated from the critical properties using a corresponding state method. Crude oil cuts having boiling points ranging between 32 and 451°C were tested. Comparison of experimental and predicted surface tensions revealed quite good agreement, with a percentage absolute deviation of 4.1%.

Kinetic equations for transfer-controlled adsorption kinetics

The dynamic surface tensions of five systems showing transfer-controlled adsorption kinetics at high concentrations have been measured. The experimental results are used to investigate the kinetic equations for the adsorption processes, with the surface behaviour being not ideal or with an indifferent electrolyte added to an ionic surfactant. For non-ideal surface behaviour, the Langmuir kinetic equation is modified with surface activity coefficients based on regular solution theory, and the dynamic surface tensions are then well described. For the adsorption of an ionic surfactant with an indifferent electrolyte, a kinetic equation including the effect of the non-surface-active electrolyte is proposed and a parameter is described for relating the rate constants to concentrations. For ideal ionic systems the Langmuir-von Szyszkowski constants obtained from the kinetic surface tensions agree with those from equilibrium data. Moreover it is observed that for micellar systems in the region above the CMC the rate constant still increases up to a high concentration.

Adsorption of binary mixtures of heptane and alkanols by activated carbon

The adsorption isotherm of heptane–hexanol on activated carbon at 298 K is presented and compared with our earlier work on heptane–ethanol and heptane–butanol mixtures under similar conditions. The data show that the preferred adsorption shifts from the alkane to the alkanol as the length of the alkanol chain, relative to that of the alkane chain, increases. The surface mole fraction, interfacial tension differences and surface activity coefficients have been estimated based on an approximate theoretical model (surface phase model). These results show that the minimum thickness of the layers required to achieve thermodynamic consistency between the surface phase model and the experimental data increases as the length of the alkanol chain decreases. All the systems exhibit smaller deviations from ideality in the adsorbed phases than in the bulk.

Influence of lateral interactions on preparative protein chromatography: I. Isotherm behavior

A thermodynamically consistent adsorption equilibrium model based on the stoichiometric displacement principle has been developed for non-linear protein ion exchange. The adsorbed phase is modeled as a surface solution in equilibrium with an ideal bulk liquid, and non-idealities on the surface are calculated from surface activity coefficients. The model has been used to systematically investigate the influence of lateral interactions on equilibrium isotherms. Using experimental data for the ion exchange of bovine serum albumin, it has been shown that lateral interactions are a function of the adsorbed protein concentration, and this dependence strongly influences adsorption behavior. Lateral interactions on the surface have been found to be influenced by both energy of interactions between molecules and molecular shape, with the former being dominant. The relationships between lateral interactions, effective protein charge, and modulator concentration have also been investigated. Increases in adsorbate charge may result in an increase or decrease in adsorption capacity, depending on the relative concentration of the modulator, while increases in modulator concentration decrease capacity. For a complete characterization of salt effects, it is necessary to determine the dependence of isosteric heat of adsorption on salt concentration.

Adsorption behavior of cadmium and zinc ions on oxide/water interfaces

The adsorption behavior of cadmium and zinc metals ions on silica and alumina particles has been studied as a function of concentration of these ions at 25 {degrees}C and pH 6-6.5. The range of the concentration of these ions studied was 10{sup -3}-10{sup -5} M. A modified Stern-Grahame equation has been developed to delineate the adsorption mechanism of the systems considered in this study. The surface activity coefficient values of these ions at the solid substrates were evaluated and the results were discussed. 19 refs., 7 figs., 3 tabs.

Thermodynamics of adsorption from solution. Adsorption by activated carbon of the three binary systems involving n-butanol, n-heptane and n-dodecane

Measurements of the adsorption by activated carbon of the three binary mixtures involving n-dodecane, n-heptane and n-butanol were made at 298 K. The data were tested for thermodynamic consistency by calculating the excess surface Gibbs energy of the interface between the pure liquids and the activated carbon surface. The adsorption data were also used to test the validity of the monolayer model of adsorption. This model is compatible with the data for (n-dodecane–n-heptane)/activated carbon, but a minimum thickness of two layers for (n-dodecane–n-butanol)/activated carbon and (n-heptane–n-butanol)/activated carbon is required to achieve thermodynamic consistency between the surface phase model and the experimental data. Surface activity coefficients were calculated for the last two systems and show that the adsorbed phases exhibit smaller deviations from ideality than the bulk phases.

ADSORPTION ISOTHERMS AND HEATS OF IMMERSION IN THE ADSORPTION OF BINARY MIXTURES ON ACTIVATED CARBON

The surface excess and the heat of immersion in the adsorption from binary mixtures on the active carbon surface were calculated in terms of the concentrations of adsorbed phase, the adsorption capacity ratios and the heats of immersion of pure compounds. The surface concentrations of binary mixtures were obtained from the thermodynamic model incorporating the intermolecular interactions between the surface and bulk molecules, and a model for surface activity coefficient considering the orientational states of adsorbed molecules, and the UNIQUAC equation. The calculated adsorption isotherms and heats of immersion were fair agreement with experiments of various binary isotherms.

Chronocoulometric study of electrode kinetics in the case of an adsorbed redox couple: Progesterone electroreduction on mercury from aqueous solutions

Single-potential-step chronocoulometry can be employed conveniently to study the kinetics and mechanisms of electrode reactions in the adsorbed state without complications by diffusion, provided the reactant is so strongly adsorbed as to permit the attainment of full coverage of the electrode surface at bulk concentrations as low as 10 −5 M. Under these conditions the mechanism can be disclosed in a straightforward and general way without making a priori assumptions as to the magnitude and coverage dependence of the surface activity coefficients of reactant and product, by carrying out measurements at constant surface coverage by the reactant. An application of this general procedure to progesterone electroreduction on mercury from aqueous solutions is provided. It is shown that in buffered solutions of 6 < pH < 7.5 progesterone (R) is electroreduced to the hydrodimer R 2 H 2 according to the mechanism: ▪ where AH denotes a proton donor.

The interfacial processes between calcium—bentonite zinc ion

The interfacial reaction between calcium-bentonite zinc ion was studied by a radioisotopic tracer method. Both the quantity of zinc ions incorporated into the solid and the quantity of calcium ions leaving the solid were determined. A comparison of the two quantities and the Langmuir representation obtained suggest that other processes can occur besides ion exchange. These processes were separated; ion exchange and adsorption isotherms were constructed, and the isotherm parameters determined. The real ion exchange process was evaluated on the basis of the law of mass action as well. The equilibrium constant of the process and the surface activity coefficients of calcium and zinc ions were determined. The variation of the surface activity coefficient as a function of surface concentration is explained by supposing exchange sites of different binding energy.

Surface activity coefficients of ideal mixtures adsorbed on two-site heterogeneous surface model

Surface activity coefficients of ideal mixtures adsorbed on two-site heterogeneous surface model

Surface Activity Coefficients in Adsorption at Liquid/Solid Interface

The global surface activity coefficients characterizing nonideality of the surface solution generated by differences in molecular interactions and adsorbent heterogeneity are defined and studied. These activity coefficients may be evaluated from the excess adsorption data. It is shown that they are useful to formulate a simple unified description of liquid/solid adsorption involving effects of solution nonideality and adsorbent heterogeneity.

Surface complexation constants of the site binding model

We applied a parallel capacitors model for the site binding model. When the inner layer capacitance C 1 is strictly proportional to the fraction f of the surface covered by adsorbed ions, then it follows that in this model the slope of the p ∗Q cation int and p ∗Q anion int versus σ β plots are related to changes in surface activity coefficients of the complexated sites; not to C 1. The capacity term in p ∗Q cation int and p ∗Q anion int contributes to the values of p ∗K cation int and p ∗K anion int found by linear extrapolation to σ β = 0. Thus, these p K values are not real thermodynamic constants. The model with C 1 = fC 1 ∗ leads to expressions for ∗K cation int and ∗K anion int which remove the discrepancy between experimental and calculated σ 0 values at low electrolyte concentration, observed when the original site binding model is applied. The (outer) Helmholtz capacitance is lower than usually assumed for the series of capacitors site binding model. This is in agreement with the results of impedance measurements on semiconductor-electrolyte electrodes.

New description of heterogeneity effects in adsorption from multicomponent solutions and liquid adsorption chromatography

AbstractGeneral equations describing adsorption from solutions on solids and liquid adsorption chromatography with mixed mobile phases are formulated in terms of the bulk and surface activity coefficeints. Definition of the surface activity coefficients is extended; they describe nonideality of surface solution due to difference in molecular interactions of the components as well as nonideality of this solution generated by the adsorbent heterogeneity. It is shown that the above general equations predict simpler expressions known already in the literature.

The TiO 2/electrolyte solution interface : II. Calculations by means of the site binding model

Starting from experimental σ 0 and ζ potential values of TiO 2 in KCl and KNO 3 solution, calculations were performed in order to check whether a self-consistent description of the data in terms of the site binding model can be obtained. Two adjustable parameters were used: the Stern layer capacitance and the equilibrium constant for the association of surface hydroxyl groups with hydrogen ions (MOH + H + ↔ MOH 2 +). In the calculations, assumptions concerning activity coefficients of charged groups near the surface have to be introduced. If the quotients of surface activity coefficients of complexes and free surface hydroxyl groups ( γ MOH 2 +Cl − / γ MOH and γ MO −H + / γ MOH) are thought to be independent of the charge in the adsorption plane, the association and dissociation constant of surface hydroxyl groups turn out to be pH-dependent. This is ascribed to the influence of local deviation from the average values of the potential at a certain distance from the surface. Neglect of this effect leads to internal inconsistencies of the model.

Potentiometric analysis of amphoteric colloids

Selected potentiometric data concerning surface charge and zeta-potentials for amphoteric suspensions in water (FeOOH, Fe 2O 3, Fe 3O 4, Al 2O 3, TiO 2, ZrO 2, SiO 2, and a polystyrene latex polymerized with methacrylic acid and dimethyl aminoethyl methacrylate) as documented in the literature were reexamined by way of a Nernst-like model for simulating the proton-adsorption/desorption affecting surface potential. This model appears to fit the data in each case and allows for the extraction and general interpretation of parameters such as (i) the “slope” of the Nernst-like model, (ii) the association/dissociation constants of the proton adsorbing/desorbing sites, (iii) the mean surface activity coefficients, (iv) the position of the shear plane, (v) the density of the proton adsorbing/desorbing surface sites, and (vi) the ionic-strength dependent and independent components of the surface capacitance.

Surface and bulk activity coefficients of nonelectrolytic mixtures studied by gas chromatography

Surface and bulk activity coefficients of nonelectrolytic mixtures studied by gas chromatography