Sorption of binary liquid mixtures on polymer networks Part I. Determination of surface excess isotherms and free energy functions

Two groups of polymer networks (polymer resins) are investigated by selective liquid sorption fromn-propanol-water mixtures. Group 1 consists of gel polymerized polar (hydrophilic) ion exchangers which swell in the binary liquid mixture. Group 2 consists of non-polar, non-swelling, macroporous resins. The free energy isotherms accompanying the sorption processes are calculated from the excess isotherms and the bulk activities. The adsorption excess free energies reveal the differences in polarity of the polymer network.     

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.     

Surface tension and adsorption of electrolytes at the aqueous solution-gas interface

The surface tension of aqueous solutions of hydrogen, sodium, and potassium chlorides was measured. The excess adsorption isotherms for electrolyte adsorption at the liquid-gas interface were obtained. The dependence of the surface tension on the solute concentration exhibits an extremal character, with the onset surface-inactivity of electrolytes being observed at concentrations above 0.3 M. The composition of the surface layer was demonstrated to be governed by the energy of solvation, while the extrema in the surface tension and excess adsorption isotherms are associated with the behavior of individual ions in the adsorption layer of the interphase interface.     

Adsorption from binary solvent mixtures onto silica gel by HPLC frontal analysis

Summary The chromatographic technique of frontal analysis is applied to measuring adsorption from binary liquid mixtures by silica gel. The complete adsorption isotherm of a solvent mixture is obtained by measuring the break-through curves for a series of small concentration steps of the mobile phase. This method offers a direct way to determine the composition of the stationary phase in liquid-solid chromatography with mixed mobile phases. The surface excess isotherms of all binary systems formed by benzene, cyclohexane, and 1,2-dichloroethane, at the solution-silica gel interface at 25 °C are presented. The data of the three systems are shown to be thermodynamically mutually consistent.     

Thermodynamics of sodium maleate and chloride adsorption at an aqueous solution-gas interface

Surface tension of solutions of sodium chloride in water and sodium maleate in aqueous alkaline solutions in a wide concentration range at 298 and 303 K are measured. It is found that the dependences of surface tension of electrolyte solutions on concentration have extreme character. The obtained dependences are discussed from the standpoint of the effect of the nature of the solute and the processes of desolvation on the composition of surface layers and the surface activity of a solution. Total electrolyte adsorption isotherms are calculated using experimental surface tension and excess adsorption isotherms.     

The thermodynamic characteristics of adsorption of hydrogen,sodium, and potassium chlorides at the aqueous solution-gas interface

Thermodynamic models of the adsorption of ions at the interphase boundary between a solution of a 1,1-electrolyte and a gas are suggested. The experimental surface tension isotherms and the isotherms of excess adsorption of hydrogen, sodium, and potassium chlorides from aqueous solutions were used to show that the formation of the surface layer followed both the mechanism of coadsorption of the anion and cation and the mechanism of predominant adsorption of one of the ions. The calculated total adsorption isotherms were used to obtain the dependences of the heats and entropies of adsorption on the amount of the ion adsorbed. The results are discussed in terms of the solvation and desolvation of electrolyte ions in bulk solution and at liquid-vapor interfaces.     

Calculation of Adsorption Isotherms on Hard Solid Surfaces Using Measurements of Surface Tensions and Contact Angles

The adsorption excess isotherms of binary mixtures adsorbed on hard solids were calculated by means of surface tension and contact angle measurements using the Gibbs adsorption isotherm equation. The calculation procedure is described in detail using the authors' own measurements of mixtures containing ethylene glycol(1)/water(2) on Teflon and poly(vinyl chloride), and water(1)/n-propanol(2) on Teflon. On the basis of these results and also from surface tensions and contact angles on hard solids published by other authors, all types of isotherms were found as given for porous adsorbents in the classification of Schay and Nagy. In addition to those, new isotherm types are proposed.     

An adsorption calorimetry study of the adsorption of acetone on raney nickel under conditions of liquid-phase hydrogenation

Adsorption calorimetry was used to study the characteristics of the adsorption of acetone on Raney nickel from 2-propanol-water solutions under conditions of liquid-phase hydrogenation. The isotherms of adsorption of acetone on Raney nickel and the dependences of integral and differential heats of adsorption on the adsorption value were obtained. It was established that the adsorption of acetone on Raney nickel occurs by the mechanism of volume filling of micropores in the pore space of the catalyst with adsorbate solution, which consists of partially or completely desolvated acetone molecules irrespective of the concentration of 2-propanol in the solvent.     

Correlation between surface and bulk structures of alcohol-water mixtures

Adsorption isotherms of binary aqueous solutions of methanol, ethanol, 1-propanol, 2-propanol, tert-butanol, and 1-butanol are demonstrated, being calculated by using the Gibbs adsorption equation with experimental data of surface tension and vapor pressure found in the literature. For all of the alcohol-water mixtures, the maximum value in the adsorption isotherm, namely, the maximum surface excess is about that expected for the formation of a monolayer. Furthermore, the composition of the mixture for the maximum surface excess coincides with that corresponding to the minimum in the excess partial molar volume of the solutes. These results indicate that the hydrophobic hydration in bulk induces the surface excess of the alcohols and after a monolayer is formed, the hydrophobic hydration itself is no longer retained.     

Correlation between excess adsorption data measured for solvent mixture/adsorbent systems and RM values obtained for different solutes chromatographed in binary mobile phases

This paper concerns the application of excess adsorption isotherms, measured for solvent mixture/adsorbent systems, to the characterization of TLC data. For this purpose the excess adsorption isotherms for three liquid mixtures: cyclohexane/ benzene, benzene/acetone, and carbon tetrachloride/ethyl acetate on silica gel at 20°C have been measured. These mixtures have been used as binary mobile phases in TLC measurements. It has been shown for a given solute in binary mobile phase that the quantity R_M is a simple function of the excess adsorption. Parameters of this function have been used to characterize chromatographic systems with binary mobile phases.     

Adsorption from Ternary Liquid Mixtures on Partially Dehydroxylated Silica Gel

The formation of mixed adsorbed layers has been tested for ternary mixtures containing a specifically adsorbed component—acetone and binary solvent benzene +n-hepane. The specific excess adsorption isotherms from the liquid phase were measured on silica gel samples partially dehydroxylated. The competition of liquid components for silica surface is discussed on the basis of changes in the mixed solvent composition.     

Experimental observations on the scaling of adsorption isotherms for nonionic surfactants at a hydrophobic solid-water interface

The self-assembly of nonionic surfactants in bulk solution and on hydrophobic surfaces is driven by the same intermolecular interactions, yet their relationship is not clear. While there are abundant experimental and theoretical studies for self-assembly in bulk solution and at the air-water interface, there are only few systematic studies for hydrophobic solid-water interfaces. In this work, we have used optical reflectometry to measure adsorption isotherms of seven different nonionic alkyl polyethoxylate surfactants (CH3(CH2)I-1(OCH2CH2)JOH, referred to as CIEJ surfactants, with I = 10-14 and J = 3-8), on hydrophobic, chemically homogeneous self-assembled monolayers of octadecyltrichlorosilane. Systematic changes in the adsorption isotherms are observed for variations in the surfactant molecular structure. The maximum surface excess concentration decreases (and minimum area/molecule increases) with the square root of the number of ethoxylate units in the surfactant (J). The adsorption isotherms of all surfactants collapse onto the same curve when the bulk and surface excess concentrations are rescaled by the bulk critical aggregation concentration (CAC) and the maximum surface excess concentration. In an accompanying paper we compare these experimental results with the predictions of a unified model developed for self-assembly of nonionic surfactants in bulk solution and on interfaces.     

Application of spline functions in calculating the surface excess isotherm according to the gibbs isotherm

Spline functions are proposed for computing the surface excess isotherms for the adsorption of 2-hydroxy-5-alkyl-benzophenone oximes at toluene/water and octane/water interfaces. The proper choice of boundary conditions, sub-intervals and type of the functions used to compute the first derivative is discussed. The spline functions give the values of the surface excess which can then be used in describing the kinetics and mechanism of copper extraction by hydroxyoximes.     

Adsorption from ternary liquid mixtures on silica gel and aluminium oxide

Formation of mixed adsorbed layers was tested for ternary liquid mixtures containing methanol or acetone and the binary solvent benzene+n — heptane. The specific excess adsorption isotherms from the liquid phase were measured on silica gel, silanized silica gel and aluminium oxide. The experimental adsorption data are discussed on the basis of the changes in the mixed solvent composition.     

The Prediction of Surface Tension and Thermodynamic Analysis of the Surface in Mixtures of Cryogenic Liquids

The surface tensions of 42 binary cryogenic mixtures at low temperature are correlated using the Shereshefsky model and excellent results are obtained. The average percent deviation is about ～ 1.08%. The Gibbs energy change in the surface region is calculated and is used to obtain the excess number of molecular layers in the surface region. Furthermore, the model is used to derive an equation for the standard Gibbs energy of adsorption. The experimental standard Gibbs energy of adsorption is obtained from surface tension data and compared with calculated data. The agreement between experimental and calculated data is found to be very good. The magnitude of the Gibbs energy change in the surface region and the standard Gibbs energy of adsorption are discussed in terms of nature and type of intermolecular interactions in binary mixtures.     

Interfacial Activity of Trioctyloamine in Hydrocarbon/Water Systems with Nonorganic Electrolytes

Interfacial tension and surface excess isotherms for trioctylamine (TOA) were determined and interpreted. Despite its high hydrophobicity, TOA adsorbs at the hydrocarbon/water interfaces and decreases effectively the interfacial tension, especially in systems containing acidic aqueous phase. Interfacial activity of TOA rises with an increase of the aqueous phase acidity. The effect of amine protonation is clearly observed. Interfacial tension isotherms obtained experimentally can be well matched with the Szyszkowski equation. The interfacial activity of TOA is affected by the type of the organic diluent and the composition of the aqueous phase, i.e., the kind and concentration of nonorganic electrolyte present in the system. Copyright 2001 Academic Press.     

Prediction of multicomponent liquid adsorption using excess quantities I. Statistical thermodynamic derivation of the basic equation of excess formalism

The utilization of excess quantities as the basis of a thermodynamic approach can simplify the prediction of multicomponent liquid adsorption from binary data. From statistical thermodynamics, the fundamental equation is derived for the prediction of ternary or higher order data from adsorption data for the constituent binary mixtures. An additive expression is obtained for the double Gibbs free excess energies, valid for adsorption on liquid mixture/air interfaces as well as liquid mixture/solid interfaces.     

Interfacial energetics of protein adsorption from aqueous buffer to surfaces with varying hydrophilicity

Adsorption isotherms constructed from time-and-concentration-dependent advancing contact angles thetaa show that the profound biochemical diversity among ten different blood proteins with molecular weight spanning 10-1000 kDa has little discernible effect on the amount adsorbed from aqueous phosphate-buffered saline (PBS) solution after 1 h contact with a particular test surface selected from the full range of observable water wettability (as quantified by PBS adhesion tension tauoa=gammaolv cos thetaoa; where gammaolv is the liquid-vapor interfacial tension and thetaoa is the advancing PBS contact angle). The maximum advancing spreading pressure, Pimaxa, determined from adsorption isotherms decreases systematically with tauoa for methyl-terminated self-assembled monolayers (CH3 SAM, tauo=-15 mN/m), polystyrene spun-coated onto electronic-grade SiOx wafers (PS, tauo=7.2 mN/m), aminopropyltriethoxysilane-treated SiOx surfaces (APTES, tauo = 42 mN/m), and fully water wettable SiOx (tauo=72 mN/m). Likewise, the apparent Gibbs' surface excess [Gammasl-Gammasv], which measures the difference in the amount of protein adsorbed Gamma (mol/cm2) at solid-vapor (SV) and solid-liquid (SL) interfaces, decreases with tauo from maximal values measured on the CH3 SAM surface through zero (no protein adsorption in excess of bulk solution concentration) near tauo=30 mN/m (thetaa=65 degrees). These latter results corroborate the conclusion drawn from independent studies that water is too strongly bound to surfaces with tauo>or=30 mN/m to be displaced by adsorbing protein and that, as a consequence, protein does not accumulate within the interfacial region of such surfaces at concentrations exceeding that of bulk solution ([Gammasl-Gammasv]=0 at tauo=30 mN/m). Results are collectively interpreted to mean that water controls protein adsorption to surfaces and that the mechanism of protein adsorption can be understood from this perspective for a diverse set of proteins with very different amino acid compositions.     

A generalized scale of free energy of excess adsorption of solute and absolute composition of the interfacial phase

Moles of a surfactant (gamma2(1)) absorbed per unit area of the solid-liquid interface estimated analytically from the difference of the solute molality in the bulk phase before and after adsorption have been quantitatively related to the absolute compositions deltan1 and deltan2 of the solvent and solute forming the inhomogeneous surface phase in contact with the bulk phase of homogeneous composition. By use of isopiestic experiments, negative values of gamma2(1) for the adsorption of inorganic salts onto a solid-liquid interface have been calculated in the same manner. From the linear plot of gamma2(1) versus the ratio of the bulk mole fractions of the solute and solvent, values of deltan1 and deltan2 have been evaluated under a limited range of concentrations. For the adsorption of the surfactant and the inorganic salt respectively onto the fluid interface, gamma2(1) values have been evaluated from the surface tension concentration data using the Gibbs adsorption equation. Gamma2(1) based on the arbitrary placement of the Gibbs dividing plane near the fluid interface is quantitatively related to the composition of the inhomogeneous surface phase. Also, the Gibbs equation for multicomponent solutions has been appropriately expressed in terms of a suitably derived coefficient m. Integrating the Gibbs adsorption equation for a multicomponent system, the standard free energy change, deltaG degrees, per unit of surface area as a result of the maximum adsorption gamma2(m) of the surfactant at fluid interfaces due to the change of the activity alpha2 of the surfactant in the bulk from zero to unity have been calculated. A similar procedure has been followed for the calculation of deltaG degrees for the surfactant adsorption at solid-liquid interfaces using thermodynamically derived equations. deltaG degrees values for surfactant adsorption for all such systems are found to be negative. General expressions of deltaG degrees for negative adsorption of the salt on fluid and solid-liquid interfaces respectively have also been derived on thermodynamic grounds. deltaG degrees for all such systems are positive due to the excess spontaneous hydration of the interfacial phase in the presence of inorganic salt. Negative and positive values of deltaG degree for excess surfactant and salt adsorption respectively have been discussed in light of a generalized scale of free energy of adsorption.