Isosteric heat of adsorption in liquid–solid equilibria: Theoretical determination and measurement by liquid chromatography/mass spectrometry

The isosteric heat of adsorption of a pure compound dissolved in a solution in equilibrium with a solid adsorbent was determined, based on the Gibbs surface excess model. The exact isosteric heat of adsorption differs from the usual value derived from the variation of the Henry’s constant with the reciprocal temperature because this procedure assumes ideal behavior of the bulk liquid solution, which, in most cases, is only approximately so. An experimental protocol, based on the determination of the excess adsorption isotherms by combining frontal analysis (for strongly adsorbed components) and spectrometric tracer pulse chromatography (for weakly adsorbed compounds) is proposed. It allows the determination of the exact isosteric heat of adsorption provided that the activity coefficient of the compound in the bulk solution can be explicitly expressed as a function of the bulk liquid composition.     

Comparison of molecular simulation of adsorption with experiment

Experimental measurements of adsorption yield the surface excess. The Gibbs surface excess is the actual or absolute amount of gas contained in the pores less the amount of gas that would be present in the pores in the absence of gas-solid intermolecular forces. Molecular simulation of adsorption yields the absolute amount adsorbed. Comparison of simulated adsorption isotherms and heats of adsorption with experiment requires a conversion from absolute to excess variables. Molecular simulations of adsorption of methane in slit pores at room temperature show large differences between absolute and excess adsorption. The difference between absolute and excess adsorption may be ignored when the pore volume of the adsorbent is negligible compared to the adsorption second virial coefficient (VB _1s ).     

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.     

Removal of zinc metal ion (Zn) from its aqueous solution by kaolin clay mineral: A kinetic and equilibrium study

A laboratory batch study has been performed to study the effect of various physic-chemical factors such as initial metal ion concentration, solution pH, and amount of adsorbent, contact time and temperature on the adsorption characteristics of zinc (Zn²⁺) metal ions onto kaolin. It has been found that the amount of adsorption of zinc metal ion increases with initial metal ion concentration, contact time, solution pH but decreases with the amount of adsorbent and temperature of the system. Kinetic experiments clearly indicate that adsorption of zinc metal ion (Zn²⁺) on kaolin is a two steps process: a very rapid adsorption of zinc metal ion to the external surface is followed by possible slow decreasing intra-particle diffusion in the interior of the adsorbent which has also been confirmed by intra-particle diffusion model. The equilibrium time is found to be in the order of 60 min. Overall the kinetic studies showed that the zinc adsorption process followed pseudo-second-order kinetics among pseudo-first-order and intra-particle diffusion model. The different kinetic parameters including rate constant are determined at different initial metal ion concentration, solution pH, amount of adsorbent and temperature respectively. The equilibrium adsorption results are analyzed by both Langmuir and Freundlich models to determine the mechanistic parameters associated with the adsorption process. The value of separation factor, R_L from Langmuir equation also gives an indication of favorable adsorption. Finally thermodynamic parameters are determined at three different temperatures and it has been found that the adsorption process is exothermic due to negative ΔH° accompanied by decrease in entropy change and Gibbs free energy change (ΔG°).     

Comparison of molecular simulation of adsorption with experiment

Experimental measurements of adsorption yield the surface excess. The Gibbs surface excess is the actual or absolute amount of gas contained in the pores less the amount of gas that would be present in the pores in the absence of gas-solid intermolecular forces. Molecular simulation of adsorption yields the absolute amount adsorbed. Comparison of simulated adsorption isotherms and heats of adsorption with experiment requires a conversion from absolute to excess variables. Molecular simulations of adsorption of methane in slit pores at room temperature show large differences between absolute and excess adsorption. The difference between absolute and excess adsorption may be ignored when the pore volume of the adsorbent is negligible compared to the adsorption second virial coefficient (V?B _1s ).     

Thermodynamics of adsorption of binary aqueous organic liquid mixtures on a RPLC adsorbent

The surface excess adsorption isotherms of organic solvents commonly used in RPLC with water as co-eluent or organic modifiers (methanol, ethanol, 2-propanol, acetonitrile and tetrahydrofuran) were measured on a porous silica surface derivatized with chlorotrimethylsilane (C1-silica with 3.92 micromol C1 groups per m2 of SiO2), using the dynamic minor disturbance method. The 5 microm diameter particles were packed in a 150 mm x 4.6 mm column. The isotherm data were derived from signals resulting from small perturbations of the equilibrium between the aqueous-organic solutions and the adsorbent surface. The partial molar surface area of the adsorbed components were assumed to be the same as those of the pure components. The difference sigma-sigmai* between the surface tensions of the adsorbed mixtures and that of the pure liquids was measured as a function of the organic modifier molar fraction. A simple and unique convention for the position of the Gibbs dividing surface was proposed to delimit the Gibbs's adsorbed phase and the bulk liquid phase. The activity coefficients of the organic modifiers and of water and their thermodynamic equilibrium constants between the two phases were measured. The strong non-ideal behavior of the adsorbed phase is mostly accounted for by the surface heterogeneity. Some regions of the surface (bonded -Si(CH3)3 moieties) preferentially adsorb the organic compound while the regions close to unreacted silanols preferentially adsorb water.     

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.     

Thermodynamic data for divalent cations onto new modified glycidoxy silica surface at solid/liquid interface

Ethylenediamine molecule was chemically bonded on a silica gel surface previously anchored with 3-glycidoxypropyltrimethoxysilane. This new surface was employed to adsorb divalent cation from aqueous solutions at 298±1 K. The series of adsorption isotherms were adjusted to a modified Langmuir equation from data obtained by suspending the solid with MCl₂ (M=Cu, Ni, Zn and Co) solutions, which gave the maximum number of moles adsorbed as 1.54, 0.56, 0.45 and 0.36 mmol g^-1 for Cu, Ni, Co and Zn, respectively. Suspended aliquots of the chemically modified surface were calorimetrically titrated and the thermodynamic data showed the system is favored enthalpically and by free Gibbs energy. This revised version was published online in July 2006 with corrections to the Cover Date.     

The effect of counterions on the surface adsorption and micelle formation in mixed surfactant systems of dodecyl sulfates

 The surface tension of the aqueous solutions of sodium dodecyl sulfate (SDS) and tetramethylammonium dodecyl sulfate (TMADS) was measured as a function of total molality of the surfactants at fixed composition of TMADS at 298.15 K under atmospheric pressure. The phase diagrams of adsorption and of micelle formation, the activity coefficients, and the excess Gibbs energy were calculated to estimate the deviation from the ideal mixing quantitatively. The preferential adsorption and the micelle formation for TMA⁺ to Na⁺ is attributable to some extent to the hydrophobicity of the methyl groups of TMA⁺. The composition of TMA⁺ in the micelle is larger than that in the adsorbed film at equilibrium. That is, a larger hydrated counterion is more likely to exist in the micelle than in the adsorbed film owing to geometrical benefit. The negative values of the excess Gibbs energy of the adsorbed film and of the micelle arise from the positive ones of the excess entropy greater than that of excess enthalpy. The counterions of very similar size are mixed ideally in the micelle and the size effect appears sensitively in the adsorbed film. Received: 23 May 2001 Accepted: 16 July 2001     

Preferential adsorption of cationic surfactant mixture studied by bromide ion selective total-reflection XAFS measurement

The total-reflection XAFS measurement possessing bromide ion selectivity at the interfacial region was applied to the adsorbed film of hexadecyltrimethylammonium chloride (HTAC) and dodecyltrimethylammonium bromide (DTAB) mixture. The surface compositions XjH of individual ions j ( j = HTA+, Cl(-), DTA+, and Br (-)) were evaluated by combining the surface excess concentration of Br(-) estimated from the XAFS with the surface composition of the respective surfactants from the surface tension results. It is clearly shown that HTA+ and Br(-) are preferentially adsorbed to DTA+ and Cl(-) at the air/water interface. The preferential adsorption was estimated numerically in terms of activity coefficient fi+/-(H,p) of component i and excess Gibbs energy of adsorption ?prH,E. Then, the magnitude of ?prH,E was compared with that of ?prH,E attributable to intrinsic interaction between ions.     

Measurement of Gas Mixture Adsorption Equilibria of Natural Gas Compounds on Microporous Sorbents

Physisorption equilibria of multicomponent gases on microporous solids like zeolites or activated carbons are considered. In view of lack of reliable and simple methods to calculate mixture adsorption isotherms from pure component data, experiments are still indispensable. An overview of classical and new methods to measure multicomponent gas adsorption equilibria is given. Some of the basic concepts like the Gibbs excess mass and the absolute mass adsorbed underlying these methods are discussed. Experimental data and a class of new adsorption isotherms for inhomogeneous microporous adsorbents of fractal dimension will be given in another subsequent paper (ADSO 635-98) by the same group of authors.     

Adsorption of Methylene Blue from Aqueous Solution on High Lime Fly Ash: Kinetic,Equilibrium, and Thermodynamic Studies

Kinetic, equilibrium, and thermodynamic studies were performed for the batch adsorption of methylene blue (MB) on the high lime fly ash as a low cost adsorbent material. The studied operating variables were adsorbent amount, contact time, dye concentration, and temperature. The kinetic data were analyzed using the pseudo-first order and pseudo-second order kinetic models and the adsorption kinetic was followed well by the pseudo-second order kinetic model. The equilibrium data were fitted with the Freundlich, Langmuir, and Dubinin Radushkevich (D–R) isotherms and the equilibrium data were found to be well represented by the Freundlich and D–R isotherms. Based on these two isotherms MB is taken by chemical ion exchange and active sites on the high lime fly ash have different affinities to MB molecules. Various thermodynamic parameters such as enthalpy of adsorption (ΔH°), free energy change (ΔG°), and entropy change (ΔS°) were investigated. The positive value of ΔH° and negative value of ΔG° indicate that the adsorption is endothermic and spontaneous. The positive value of ΔS° shows the increased randomness at the solid–liquid interface during the adsorption. A single-stage batch adsorber was also designed based on the Freundlich isotherm for the removal of MB by the high lime fly ash.     

Nonideal mixing of alkylammonium chloride and decyldimethylphosphine oxide surfactants in adsorbed films and micelles

Miscibility and interaction of decyldimethylphosphine oxide (DePO) with ammonium chloride (AC), hexylammonium chloride (HAC), and dodecylammonium chloride (DAC) in adsorbed films and micelles were studied by surface tension measurements. Phase diagrams were drawn for the mixed adsorption, mixed micelle formation, and equilibrium between adsorbed films and micelles. Nonideal mixing of DAC and DePO was characterized by a negative excess Gibbs free energy and positive excess area of adsorption and negative excess Gibbs free energy of micelle formation. It is concluded that the interaction between DAC and DePO in adsorbed films and micelles is larger than those between the same surfactants alone due to two factors: ion-dipole interactions between the head groups of DAC and DePO and alkyl-chain/alkyl-chain interactions.     

Characterization of Liquid-Solid Adsorption Systems by Using TLC Data

Abstract

The thin-layer chromatography (TLC) method is used to measure the excess adsorption isotherms for different binary liquid mixtures on silica gel. These isotherms are interpreted by means of the Langmuir-Freundlich equation, which involves the adsorbent heterogeneity and the difference in the molecular sizes of the components. This equation makes possible the evaluation of the surface phase capacity, equilibrium constant and heterogeneity parameter, which characterize liquid-solid adsorption systems.     

Adsorption of Metanil Yellow Azoic Dye from Aqueous Solution onto Mg‐Fe‐NO3 Layered Double Hydroxide

We report the preparation and characterization of a layered double hydroxide (LDH) adsorbent for azoic dye, metanil yellow (yellow GX; YGX) removal. The nanoparticles of Mg‐Fe‐LDH‐NO₃ adsorbent were prepared with Mg/Fe molar ratio of 3:1 by a hydrothermal process and coprecipitation method at pH 9.5 and were characterized by X‐ray powder diffraction (XRPD), thermal gravimetric analysis (TGA), elemental analysis, and Fourier transform infrared spectroscopy (FT‐IR). The size and morphology of nanoparticles were examined by scanning electron microscopy (SEM). The XRD patterns indicate that the intercalation of YGX between the LDH layers has not occurred and surface adsorption has happened. In the adsorption experiments, the Gibbs free energy ΔG⁰ values, the enthalpy ΔH⁰, and entropy ΔS⁰ was determined. The isotherms showed that the adsorption of YGX by Mg‐Fe‐LDH‐NO₃ was both consistent with Langmuir and Freundlich equations.     

Role played by the head size of some non‐ionic surfactants on their adsorption onto montmorillonite clay

Sorption behavior of polyoxethylene(n)monooleate series [Ol(EO)_n] onto montmorillonite clay was studied at 25°C to investigate the influence of the surfactant's head size on the sorption process. All the tested surfactants exhibited L‐shaped isotherms that means a strong interaction between the adsorbent and the adsorbate. Also, all the obtained isotherms ended with a drastic increase in the isotherm slope at nearly constant equilibrium concentration. This abrupt increase reflected the fairly high affinity of the tested surfactants to the clay surface at high bulk concentration region. The maximum amount adsorbed at the plateau region, Γ_max, was calculated according to the Langmuir adsorption theory and followed the order: Ol(EO)₁₄ > Ol(EO)₂₀ < Ol(EO)₄₀ < Ol(EO)₈₀. In case of short ethylene oxide (EO) chain, Γ_max decreased with the increase in the chain length; but a reverse result was obtained in case of surfactants with longer EO chain length (20 to 80 units). The free energy of adsorption, ΔG°_ad, had negative values indicating the spontaneous adsorption of surfactant molecules onto clay. The values of ΔG°_ad increased with increasing EO units from 14 to 20 units and decreased with further elongation in the EO chain from 20 to 80 units. Copyright © 2004 John Wiley & Sons, Ltd.     

Adsorption and thermodynamic behavior of uranium(VI) on <Emphasis Type="Italic">Ulva sp</Emphasis>.-Na bentonite composite adsorbent

Summary The algae-clay composite adsorbent was tested for its ability to recover U(VI) from diluted aqueous solutions. Macro marine algae (Ulva sp.) and clay (Na bentonite) were used to prepare composite adsorbent. The ability of the composite adsorbent to adsorp uranium(VI) from aqueous solution has been studied at different optimized conditions of pH, concentration of U(VI), temperature, contact time. Parameters of desorption were also investigated to recover the adsorbed uranium. The adsorption patterns of uranium on the composite adsorbent followed the Freundlich and Dubinin-Radushkevich isotherms. The thermodynamic parameters such as the enthalpy ΔH, entropy ΔS and Gibbs free energy ΔG were calculated from the slope and intercept of lnK_d vs. 1/T plots. The results suggested that the Ulva sp.-Na bentonite composite adsorbent is suitable as sorbent material for recovery and biosorption/adsorption of uranium ions from aqueous solutions.     

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.     

Surface adsorption and micelle formation of aqueous solutions of polyethyleneglycol and sugar surfactants

The surface tension of aqueous solutions of tetraethyleneglycol octyl ether (C8E4) and octyl-β-d-maltopyranoside (OM) mixture was measured as a function of the total molality of surfactants and the composition of OM under atmospheric pressure at 298.15 K by drop volume technique. The results of surface tension measurements were analyzed by originally developed thermodynamic equations, then phase diagrams of adsorption and micelle formation were constructed. From the analysis of the surface tension data, it was found that the C8E4 and OM molecules interact attractively in the adsorbed film and the excess Gibbs energy of adsorption can be compared with those observed in typical cationic–nonionic surfactant systems; nevertheless, they are mixed almost ideally in the mixed micelle. Judging from a negative excess surface area calculated by differentiating the excess Gibbs energy by the surface tension, we concluded that the attraction between C8E4 and OM molecules is a short-range one originated in the hydrogen bonding between them which favors the planar configuration.     

Equation of State for Adsorption of Gases and Their Mixtures in Porous Materials

Desorption functions (G, H, S) are useful for adsorbent characterization, phase equilibria, and enthalpy and entropy balances. Adsorption isotherms, enthalpy, and entropy are temperature and pressure derivatives of the free energy, so that G(T, P) is an adsorption equation-of-state which contains complete thermodynamic information about the system. The free energy of desorption is the minimum isothermal work necessary to regenerate the adsorbent. The free energy of desorption also determines the selectivity of an adsorbent for different gases. The ideal enthalpy of desorption for a mixture (H = _i n _i h° _i ) is a simple function of the enthalpies of desorption for the individual components. Sample calculations of the free energy, enthalpy, and entropy desorption functions are provided for pure components and mixtures.