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PubMed2006

Sulfate adsorption and surface precipitation on a volcanic ash soil (allophanic andisol).

Sulfate strongly adsorbs on metal oxides and soils with variable charges. However, its surface precipitation has not been clearly evaluated and its adsorption mechanism has been in dispute. In the present study, an allophanic andisol, a typical volcanic ash soil having both negative and positive variable charges, was used to identify the adsorption mechanism of sulfate. Sulfate adsorption isotherms were obtained by a batch method at pH values of 4, 5, 6, and 7 in a wide range of concentrations in an Na-H-SO(4)-OH system. Theoretical isotherms were applied to the measured values for the evaluation. The surface precipitation was detected by the measured adsorption isotherms, and the BET isotherm confirmed the presence of multilayer adsorption. Stronger and weaker adsorption sites were suggested by using the Langmuir isotherm for the monolayer adsorption. The adsorption energies obtained from the Langmuir equation and recent spectroscopic analysis suggested that the stronger adsorption corresponded to an inner-sphere surface complex and that the weaker adsorption corresponded to outer-sphere surface complexation. The BET and Langmuir equations showed three types of adsorption mechanisms for the sulfate adsorption on the soil.

Adsorption↗
PubMed2006

Adsorption of aqueous alkylphenol ethoxylate surfactants by mesoporous carbon CMK-3.

Mesoporous carbon, CMK-3, was prepared using hexagonal SBA-15 mesoporous silica as the template and the adsorption of nonylphenol ethoxylates (NPE) onto CMK-3 was investigated. The adsorption process was well described using pseudo-second-order kinetics. At initial NPE concentrations of 107 and 530 mg l(-1), the adsorption rate constants were found to be 5.6 x 10(-3) and 8.7 x 10(-4) g mg(-1) min(-1), indicating that a higher initial concentration or adsorption amount resulted in a lower adsorption rate. NPE adsorption onto CMK-3 fitted a Langmuir-Freundlich model and the maximum amounts of NPE absorbed at 15, 25, and 35 degrees C were 923, 720, and 463 mg g(-1), suggesting an elevated adsorption capacity of CMK-3 for NPE with decreased adsorption temperature. In addition, increasing adsorption temperature led to the change of the adsorption model from the Langmuir-Freundlich to the Langmuir model. N2 adsorption results showed that the adsorption of NPE led to a decrease in the mesopore volume of CMK-3. However, the pore width of NPE-loaded CMK-3 was found to be identical to that of CMK-3.

Adsorption↗
PubMed2004

Removal of phenol from aqueous solutions by adsorption.

Experiments have been conducted to examine the liquid-phase adsorption of phenol from water by silica gel, HiSiv 3000, activated alumina, activated carbon, Filtrasorb-400, and HiSiv 1000. Experiments were carried out for the analysis of adsorption equilibrium capacities and kinetics. The adsorption isotherm model of the Langmuir-Freundlich type was the best to describe adsorption equilibrium data for phenol for the adsorbents studied. Results of kinetic experiments indicated that HiSiv 1000 had the highest rate of adsorption among the adsorbents studied and therefore more detailed studies were carried out with this adsorbent. The influence of particle size, temperature, and thermal regeneration on adsorption of phenol by HiSiv 1000 was evaluated. From particle size experiments it appeared that adsorption capacity of HiSiv 1000 did not change by changing the particle size, but the rate of adsorption decreased considerably by increasing the particle size. The effect of temperature on adsorption was studied by determining equilibrium isotherms for HiSiv 1000 at 25, 40, and 55 degrees C. The results showed that adsorption capacity decreased with increasing temperature. Thermal regeneration of HiSiv 1000 was performed at 360 degrees C. It was observed that adsorption capacity of HiSiv 1000 did not change after 14 regeneration cycles. Equilibrium experiments showed that the adsorption capacities of activated carbon and Filtrasorb-400 were several times higher than that of HiSiv 1000.

Adsorption↗
PubMed2004

Adsorption of lead and humic acid on chitosan hydrogel beads.

Chitosan hydrogel beads were studied for the adsorption of lead ions and humic acid from aqueous solutions to examine the adsorption behaviors and mechanisms. The experiments were carried out at room temperature with solution pH ranging from 5 to 7.5 (in near neutral pH range). Three types of batch adsorption experiments, including single species adsorption, sequential adsorption of one species after another and co-adsorption of both species, were investigated. The results show that: (1) adsorption of either species mainly results from the complexations between adsorbate and functional groups at the surface of the hydrogel beads; (2) previously adsorbed species can either act as additional binding sites for, or occupy the same binding sites as the subsequent species to be adsorbed, resulting in enhanced or retarded adsorption of the subsequent species; and (3) for co-adsorption, metal-organic interactions play a very important role in determining the extent of adsorption. It is concluded that multi-species adsorption can be significantly affected by adsorbate interactions and the understanding of these interactions needs great attention in adsorption study in the future.

Adsorption↗
PubMed2001

Adsorption--from theory to practice.

Adsorption at various interfaces has concerned scientists since the beginning of this century. This phenomenon underlies a number of extremely important processes of utilitarian significance. The technological, environmental and biological importance of adsorption can never be in doubt. Its practical applications in industry and environmental protection are of paramount importance. The adsorption of substrates is the first stage in many catalytic processes. The methods for separation of mixtures on a laboratory and on an industrial scale are increasingly based on utilising the change in concentration of components at the interface. Moreover, such vital problems as purification of water, sewages, air and soil are involved here too. On the other hand, many areas in which technological innovation has covered adsorption phenomena have been expanded more through art and craft than through science. A basic understanding of the scientific principles is far behind; in part because the study of interfaces requires extremely careful experimentation if meaningful and reproducible results are to be obtained. In recent years, however, considerable effort has been increasingly directed toward closing the gap between theory and practice. Crucial progress in theoretical description of the adsorption has been achieved, mainly through the development of new theoretical approaches formulated on a molecular level, by means of computer simulation methods and owing to new techniques which examine surface layers or interfacial regions. Moreover, during the last 15 years new classes of solid adsorbents have been developed, such as activated carbon fibres and carbon molecular sieves, fullerenes and heterofullerenes, microporous glasses and nanoporous--both carbonaceous and inorganic--materials. Nanostructured solids are very popular in science and technology and have gained extreme interest due to their sorption, catalytic, magnetic, optical and thermal properties. Although the development of adsorption up to the 1918s has been following rather a zig-zag path, this arm of surface science is now generally considered to have become a well-defined branch of physical science representing an intrinsically interdisciplinary area between chemistry, physics, biology and engineering. This review presents in brief the history of adsorption and highlights the progress in theoretical description of the phenomenon under consideration. The paper deals with the above problems critically, showing the development of adsorption, presenting some of the latest important results and giving a source of up-to-date literature on it. Moreover, in this paper the most important aspects are overviewed referring to today's trends and visions in application of adsorption science in industry, environmental protection and in environmental analysis. The relationship between development of adsorption theory and adsorption practice is pointed out. Current understanding and perspectives pertaining to applications of adsorption phenomena on laboratory and on industrial scale as well as environmental protection are discussed and illustrated by means of a few spectacular examples.

Adsorption↗
PubMed2003

Adsorption of Cr(III) on ozonised activated carbon. Importance of Cpi-cation interactions.

The adsorption of Cr(III) in aqueous solution was investigated on a series of ozonised activated carbons, analysing the effect of oxygenated surface groups on the adsorption process. A study was carried out to determine the adsorption isotherms and the influence of the pH on the adsorption of this metal. The adsorption capacity and affinity of the adsorbent for Cr(III) increased with the increase in oxygenated acid groups on the surface of the activated carbon. These findings imply that electrostatic-type interactions predominate in the adsorption process, although the adsorption of Cr(III) on the original (basic) carbon indicates that other forces also participate in the adsorption process. Thus, the ionic exchange of protons in the -Cpi-H3O(+) interaction for Cr(III) accounts for the adsorption of cationic species in basic carbons with positive charge density. Study of the influence of pH on the adsorption of Cr(III) showed that, in each system, the maximum adsorption occurred when the charge of the carbon surface was opposite that of the species of Cr(III) present at the pH of the experiment. These results confirmed that electrostatic interactions predominate in the adsorption process.

Adsorption↗
PubMed2003

Examination of hydrophobic contaminant adsorption in mineral micropores with grand canonical Monte Carlo simulations.

A molecular level understanding of the interactions between hydrophobic organic contaminants (HOCs) and sediments is needed in order to assess contaminant fate in the environment. Grand canonical Monte Carlo simulations were performed to investigate water and trichloroethylene (TCE) adsorption in slit micropores confined by charged and uncharged silica surfaces. Gas-phase single-sorbate simulations with water or TCE were performed as well as mixture simulations of bulk water containing TCE at 1% of its saturation concentration. Gas-phase isosteric heats for water adsorption in the uncharged pores ranged from -40 to -52 kJ/mol, and the densities of the adsorbed water phases were always less than that for bulk water. Gas-phase isosteric heats for water adsorption in the charged pores ranged from -79 to -170 kJ/mol, and the densities of the adsorbed water phases were close to that for bulk water. The isosteric heats and water densities indicated that the uncharged pores were mildly hydrophobic, and the charged pores were very hydrophilic. In mixture simulations of adsorption from solution, the presence of water promoted TCE adsorption in uncharged pores with widths between 14 and 20 A. The isosteric heats for TCE adsorption from solution ranged from -14 to -27 kJ/mol in the uncharged pores and from -9.3 to -50 kJ/mol in the charged pores. Strong attractions to the pore surfaces were significantly diminished after adsorption of the first two monolayers of either adsorbate. Aqueous-phase TCE at a concentration equal to 1% of its saturation concentration was able to completely displace adsorbed water in uncharged pores. Even in highly hydrophilic pores, TCE at this concentration was able to displace up to 50% of the adsorbed water. Apparent differential enthalpies of adsorption determined from the temperature dependence of TCE adsorption isotherms underestimated the magnitude of the true isosteric heats of adsorption by up to 30 kJ/mol. This shows that HOC adsorption enthalpies determined from the temperature dependence of their adsorption isotherms underestimate the true strength of HOC-adsorbent interactions.

Adsorption↗
PubMed1999

Adsorption of benzalkonium chloride by filter membranes: mechanisms and effect of formulation and processing parameters.

The purpose of this study was to elucidate the mechanism(s) of adsorption of benzalkonium chloride (BAK) by filter membranes and to determine the effect of formulation and processing parameters on the adsorption of BAK. Six different sterilizing grade filter membranes were used in this study. Adsorption was monitored by passing an aqueous solution of BAK at pH 6 through a 47-mm (14.2 cm2 effective filter area) disk filter membrane and measuring the UV absorption of the filtrate with a UV micro flow cell. Hydrophobic and/or electrostatic forces were chiefly responsible for adsorption. The adsorption properties of each filter membrane could be related to its composition. Membranes that were hydrophilic and nonionic or hydrophilic and cationic adsorbed little BAK. However, membranes that were hydrophobic or anionic exhibited significant BAK adsorption. Adsorption followed the Langmuir equation and adsorptive capacities ranged from 116 to 429 micrograms/47 mm membrane (8.17-30.23 micrograms/cm2). Formulation factors studied included the concentration of BAK, the presence of a tonicity-modifying agent (sodium chloride, mannitol, glycerin), and the presence of a chelating agent (edetate disodium). The greatest increase in adsorption of BAK occurred when sodium chloride or edetate disodium was used with membranes that contained cationic sites. The effects of flow rate, temperature, autoclaving, interrupting the flow, and presaturating the filter membrane were the processing parameters studied. The rate of adsorption was inversely related to flow rate and the extent of adsorption was inversely related to temperature. Presaturation of the filter membrane with BAK was an effective method to reduce the adsorption of BAK during filtration.

Adsorption↗
PubMed2005

Anode glow discharge plasma treatment of titanium plates facilitates adsorption of extracellular matrix proteins to the plates.

Glow discharge plasma (GDP) supplied to an anode (GDP+) promotes calcium phosphate adsorption onto titanium better than that supplied to a cathode (GDP-). However, the adsorption of extracellular matrix (ECM) proteins is crucial for cell adhesion to titanium. Since GDP+ induced both inorganic adsorption and cell adhesion, we hypothesized that the inorganic adsorption in a culture medium might affect the adsorption of the ECM proteins. In this study, ECM proteins adsorbed on titanium with and without GDP were examined by x-ray photoelectron spectroscopy. After 1 hr of incubation, increasing sodium adsorption on GDP+ specimens induced adsorption of ECM proteins as shown by NH(+) and COO(-) signals without calcium adsorption. In contrast, since no specific adsorption of sodium on GDP-specimens was detected, GDP-did not contribute to the adsorption of ECM proteins. Thus, promotion of sodium adsorption of GDP+ was effective, at least in the initial ECM protein adsorption on a titanium surface.

Adsorption↗
PubMed1983

[Adsorption characteristics of organic solvent vapors on the fixed bed of activated carbon].

Many different sizes of activated carbon fixed beds are widely used for adsorption of organic solvent vapors and hazardous gases, such as air cleaners, gas masks and gas sampling tubes, but only limited information is available for the comprehensive relationship between adsorption characteristics of the fixed bed and practical factors; the amount of activated carbon, the gas flow rate, the vapor concentration and bed diameter. The authors investigated experimentally the effect of these factors on the adsorption zone length, the effective adsorption capacity and the breakthrough time. The following results were obtained; The adsorption zone length was approximately independent of the amounts of activated carbon packed in a column of a definite diameter. Therefore, the effective adsorption capacity increased with the increasing activated carbon amount, and the breakthrough time was nearly proportional to it. The adsorption zone length increased and the effective adsorption capacity reduced with the increasing gas flow rate. The adsorption zone length was little affected by the vapor concentration, but the effective adsorption capacity increased with the increasing concentration. In case of the same amount of activated carbon, the adsorption zone length, the effective adsorption capacity and the breakthrough time reduced with the increasing bed diameter.

Adsorption↗
PubMed2007

Adsorption of 2,4-dichlorophenoxyacetic acid by an Andosol.

To identify the important soil components involved in 2,4-dichlorophenoxyacetic acid (2,4-D) adsorption on Andosols, 2,4-D adsorption on a surface horizon of an Andosol was compared with that on hydrogen peroxide (H2O2)-treated (soil organic matter [SOM] was removed), acid-oxalate (OX)-treated (active metal hydroxides and SOM were removed), and dithionite-citrate-bicarbonate (DCB)-treated (free and active metal [hydr]oxides and SOM were removed) soil samples at equilibrium pHs ranging from 4 to 8. Although the untreated soil contained a large amount of organic C (71.9 g kg-1), removal of SOM had little effect on 2,4-D adsorption. Active surface hydroxyls, which were attached to the active and free metal (hydr)oxides and metal SOM complexes, were identified as the most important soil functional group for 2,4-D adsorption. The dominant mechanism of the 2,4-D adsorption was a ligand exchange reaction in which the carboxylic group of 2,4-D displaced the active surface hydroxyl associated with metals and formed a strong coordination bond between the 2,4-D molecule and soil solid phase. The ligand exchange reaction reasonably accounted for the selective adsorption of 2,4-D over Cl-, competitive adsorption of phosphate over 2,4-D, reduction in plant-growth-inhibitory activity of soil-adsorbed 2,4-D, and the high 2,4-D adsorption ability of Andosols. Although a humic acid purified from the soil did not adsorb 2,4-D, the presence of the humic acid increased 2,4-D adsorption on Al and Fe, probably by inhibiting the hydrolysis and polymerization of Al and Fe resulting in the preservation of available adsorption sites on these metals. The adsorption behavior of 2,4-D on soils could be a good index for predicting the adsorption behavior of other organic acids in soils.

2,4-Dichlorophenoxyacetic Acid↗
PubMed2002

[Adsorption behavior of reactive X-3B red dye in soil environment and relevant influencing factors].

The determination of rates of soil adsorbing reactive X-3B red dye opened out that the adsorption of the dye by soils could be divided into four stages: high-speed adsorption, slowdown adsorption, slow-speed adsorption and zero adsorption. There was a great difference in adsorptive rates between various soil types during these different stages. The research on adsorptive isotherms of reactive X-3B red dye pointed out that adsorptive behavior of the dye tallied with the Langmuir equation. The calculation on the basis of the adsorptive isotherms indicated that the maximum adsorption of the dye was in the sequence paddy soil > red soil > brown earth > cinnamon soil. As for relevant environmental factors influencing this adsorption, contents of organic matter and water in soils, soil pH and air temperature were involved. When soil organic matter or air temperature was reduced and soils were at neutral, runny or unwatered conditions, the adsorption of the dye on soils was decreased, thereby inhibiting the accumulation of the dye in soil environment.

Adsorption↗
PubMed2005

[Adsorption behavior of cadmium in phaeozem and burozem].

The study on the thermodynamics and kinetics of Cd2+ adsorption in phaeozem and burozem showed that within the test range, Cd2+ adsorption was increased with its increasing initial concentration, and phaeozem had a much higher Cd2+ adsorption capacity than burozem. When the Cd2+ concentration in adsorption equilibrium was 20 mg x kg(-1), the amount of adsorbed Cd2+ was 1 485.2 mg x kg(-1) in phaeozem, but 700.6 mg x kg(-1) in burozem. The adsorption isotherm of Cd2+ in the two soils could significantly conform to Langmuir, Freundlich and Henry equations, while Temkin equation was not applicable to describe the isothermal adsorption process of Cd2+ in phaeozem and burozem. The maximum adsorption of Cd2+ in phaeozem and burozem was 5939.3 and 2 790 mg x kg(-1), respectively, according to Langmuir formulation. The Cd2+ adsorption in test soils was a rapid reaction, with 90% of Cd2+ adsorption in the first 2 min and reached equilibrium in 15-30 min. First-order dynamic equation was the optimal model to describe the kinetics of Cd2+ adsorption, while the other models in point were Elovich and Two-constant equations. The adsorption rate of Cd2+ in test soils was increased with its increasing initial concentration, but decreased with its increasing adsorbed amount. Under low concentrations of Cd2+, the decrease of its adsorption rate with time was more quickly in phaeozem than in burozem.

Adsorption↗
PubMed1977

Adsorption of naturally occurring polymers onto calcium oxalate crystal surfaces.

The adsorption of proteins and mucopolysaccharides on calcium oxalate crystals was measured by solution depletion. Anionic protein adsorption was found to be sensitive to calcium ion concentration. Adsorption of fibrinogen was anomalously large in the presence of 0.01 M Ca2+. Adsorption of cationic protein (histone) was sensitive to oxalate ion concentration. A small alteration in adsorption of protein as a result of pH or temperature change was also observed. Plots of adsorption versus concentration were interpreted in terms of a Langmuir adsorption isotherm. The derived Langmuir adsorption parameters were then used to investigate the contribution of protein, by physical adsorpti, to the quantity of matrix in urinary stones. It was concluded that physical adsorption can account for the deposition of part but not all of the matrix in calcium oxalate stones. It was also concluded that physical adsorption of mucopolysaccharides by calcium oxalate crystals could explain the inhibition of growth and aggregation of calcium oxalate crystals seen with in vitro precipitation systems. Recalculation of published data indicates that adsorption of protein on dental enamel (calcium hydroxyapatite) results in approximately the same extent of surface coverage as adsorption on calcium oxalate crystals, but protein has a much lower affinity for dental enamel than for calcium oxalate crystals.

Adsorption↗
PubMed1995

[Studies on the method to quantify PPD and its adsorption onto glass-surface].

The potency of PPD is very stable when kept as a lyophilized preparation or as a solution of high concentrations, but, as is well known, when diluted to the practical concentrations such as used in skin-testing for human, its potency diminishes rather rapidly. It has been considered that such decrease in potency is not due to denaturation of PPD, but due to its adsorption onto the surface of glass- or plastic-container. The adsorption of PPD had been studied by several authors with radiolabeled PPD or by the potency test in experimental animals or human. However, either method is not satisfactory because they are not quantitative and reproducible. Consequently, we developed a new method basing on ELISA to measure such low concentrations of PPD as used in skin-testing in human. Using this new method, we analyzed the adsorption of PPD onto glass-surface. The major characteristics of the PPD adsorption revealed by the present studies are as follows; (1) The adsorption of PPD takes place very rapidly immediately after the distribution of PPD solution to a new container and then proceeds in slower rates. (2). The rate of adsorption is influenced by both temperature and concentration of PPD solution. The rates of adsorption are almost the same at -80 degrees C and at 6 degrees C, much higher at 22 degrees C and further higher at 37 degrees C at any concentration tested. At -80 degrees C and 6 degrees C, there is no difference among the adsorption rates at three different concentrations, but at 22 degrees C and 37 degrees C, the rate of adsorption at the lowest concentration (0.25 microgram/ml) is much higher than those at higher concentrations. (3) The amounts of PPD adsorbed are determined by the surface area of glass in contact with PPD solution. (4) The larger the volume of PPD solution distributed, the bigger the absolute amount of PPD adsorbed, but the smaller the rate of adsorption. (5) So-called "volume effects" are evidenced. Commercially available Japanese PPD preparations have been produced and adjusted their potency taking such adsorption phenomena into consideration. But, if undue adsorption is induced by unappropriate handling and/or usage, the potency might be diminished. Especially, the single-dose preparation may be affected most strongly by so-called "volume-effects". Standard handling and usage recommended by the manufacturer are strictly required.

Adsorption↗
PubMed1997

Removal of Pb(II), Cd(II), Cu(II), and Zn(II) from Aqueous Solutions by Adsorption on Bentonite

Removal of Pb(II), Cd(II), Zn(II), and Cu(II) from aqueous solutions using the adsorption process on bentonite has been investigated. In order to find out the effect of temperature on adsorption, the experiments were conducted at 20, 35, and 50degreesC. For all the metals, maximum adsorption was observed at 20degreesC. The rate of attaining equilibrium of adsorption of metal ions follows the order Zn(II) > Cu(II) > Cd(II) > Pb(II). Equilibrium modeling of the adsorption showed that adsorption of Pb(II), Cd(II), and Cu(II) were fitted to a Langmuir isotherm, while the adsorption of Zn(II) was fitted to a Freundlich isotherm. Dynamic modeling of the adsorption showed that the first order reversible kinetic model was held for the adsorption process. The overall rate constant k', the adsorption rate constant k1, the desorption rate constant k2, and the equilibrium constant Ke for the adsorption process were calculated. From the results of the thermodynamic analysis, standard free energy DeltaG0, standard enthalpy DeltaH0, and standard entropy DeltaS0 of the adsorption process were calculated.

Journal Article↗
PubMed1997

Heteroenergetics of Bovine Serum Albumin Adsorption from Good Solvents Related to Crystallization Conditions

Total internal reflection fluorescence (TIRF) was applied to study adsorption of bovine serum albumin (BSA) from a thermodynamically good solvent commonly used as a buffer for BSA crystallization. BSA adsorption is shown to exhibit heteroenergetics, which is possibly a general property of protein interfacial behavior. For example, BSA adsorption on quartz, SnO2 , and alkyl-modified surfaces demonstrates broad continuous distributions of energies of adsorption activation and protein-surface binding. For heteroenergetic adsorption, the type of the isotherm and kinetics depends on the type of the energy distribution functions and the type of correlation between the energies. In the case of BSA adsorption on quartz and SnO2 surfaces, protein interfacial behavior is consistent with a model suggesting rectangular distributions both of activation energies of adsorption and energies of protein-surface binding. In the case of an alkyl-modified surface, BSA adsorption kinetics is interpreted in terms of a heteroenergetic model suggesting initial adsorption at selected adsorption sites with maximum rate constants. The model also suggests that lateral diffusion quickly redistributes the molecules from the initial adsorption sites to the sites with higher binding energies. The combination of TIRF spectroscopy with an electrochemical system was used to study the effect of surface charge on protein adsorption on a transparent SnO2 electrode. BSA behavior on the SnO2 electrode was found to correlate with surface hydrophobicity and indifferent to charges both of the surface and the protein. Hydrophobic interactions seem to be the principal driving force in determining the behavior of BSA at the solid-liquid interface and likely plays an important role in protein crystallogenesis.

Journal Article↗
PubMed2000

On the Behavior of Nonionic Surfactants at the N-Heptane/Silica Gel Interface: Influence of the Presence of Interfacial Water Inferred from Adsorption Isotherms and Calorimetric Data.

The behavior of two polydisperse nonionic surfactants, poly (oxyethylene) glycol alkylphenyl ether TX-35 and TX-100, at the prewetted silica gel/n-heptane and dried silica gel/n-heptane interfaces has been compared by the determination of the average adsorption isotherms of the polydisperse surfactants and of displacement enthalpies. From HPLC experiments, we could also separately quantify the adsorption of each ethyleneoxide (EO) fractions for silica gel from the polydisperse surfactant solution. The adsorption isotherms clearly indicate an incomplete preferential adsorption of the large (EO) chains over the small ones, as well on dried silica gel as on a prehydrated sample. This preferential adsorption and its driving force follow the solubility rules of the poly(oxyethylene) glycol alkylphenyl ether in an apolar solvent and support the idea of a solubility-limited adsorption: solubility in organic solvents of the smaller (EO) chains is much more significant than that of the longer ones and hence prevents adsorption of the smaller species. Consequently, it is observed that the presence of interfacial water decreases the affinity of TX-35 molecules for the hydrophilic silica surface due to the hydration of (EO) chains. In contrast, for TX-100 adsorption after the prewetting treatment the clearest trend is a drastic increase of the adsorption ascribed to the additional solubilization (and micellization) of the TX-100 molecules in the interfacial aqueous phase. The differential molar enthalpies of displacement show a change in the adsorption mechanism, depending on the presence of molecular water on the surface. In the initial part of the adsorption isotherm, a prevailing exothermic process is obtained with prehydrated silica and suggests that hydration of the polar heads of TX-35 and the solubilization of the TX-35 in interfacial water are occurring. For higher equilibrium concentrations, the enthalpies of displacement observed with the prehydrated adsorbent become slightly lower than those obtained with dry silica gel. It may be that this difference is due to the micellization phenomenon of the surfactant species with longer EO chains in interfacial water. These features emphasize the influence of interfacial water on the adsorption of EO fractions from organic solvent. Copyright 2000 Academic Press.

Journal Article↗