Adsorption of randomly annealed polyampholytes at the silica-water interface

We have investigated the adsorption of randomly annealed polyampholytes containing [2-(dimethylamino)ethyl methacrylate)] (DMAEMA), methacrylic acid (MAA), and [3-(2-methylpropionamido)propyl] trimethylammonium chloride (MAPTAC) with various molar compositions. The adsorption was performed from dilute aqueous solutions onto silicon substrates. The adsorbed layers were characterized by reflectivity techniques such as reflectometry, ellipsometry, and neutron specular reflection. As expected for annealed polyampholytes, the adsorption was found to depend strongly on the pH, with a maximum within the isoelectric domain of the polyampholyte. The monomer volume fraction profiles of the adsorbed layers were determined from neutron specular reflection measurements. In the isoelectric domain, the polyampholyte chains adopt a compact conformation, with a layer thickness of about 60 A. The polyampholyte layer is as dense as the adsorbed layer of fully charged polyelectrolyte but much thicker. Finally, we found that changing the ratio of neutral units along the polyampholyte chain in the isoelectric domain had no significant effect on the concentration profile of the adsorbed layer.     

Interfacial pH at an isolated silica-water surface

Tethering a pH-sensitive dye to a silica surface enables the interfacial pH to be measured optically and is found to be 2 pH units lower than that of the bulk. The positive H+ ions are attracted by the surface potential and have an enhanced concentration consistent with the previously determined surface potential of order -120 mV, with respect to zero in the bulk. A stable near-surface charged layer once formed is not disrupted by the bulk pH.     

On the wavelength dependence of picosecond relaxation in crystal violet

The depopulation-repopulation kinetics of ground-state crystal violet in glycerol at room temperature has been investigated. A D₂O generated picosecond continuum served as a probe source and a 530 nm single pulse as an excitation source. The repopulation rate varies with probe wavelength; this dependence is ascribed to the presence of two rotamers of crystal violet.     

Thermal analytic study of the adsorption of crystal violet by laponite

The adsorption of crystal violet on laponite was investigated by X-ray diffraction and thermal analysis. DTA, TG and DTG curves were recorded in air. The evolved H₂O, CO₂, NO₂, H₂ and C₂H₆ were simultaneously determined by mass spectrometry. The thermal analysis curves were compared on one hand with the thermal analysis curves of laponite and on the other hand with thermal analysis curves of non-adsorbed crystal violet and of crystal violet adsorbed on montmorillonite. The thermal analysis curves of crystal violet adsorbed on laponite show similarities to the curves of the non-adsorbed crystal violet, but differ from the curves of crystal violet adsorbed on montmorillonite. The differences in the thermal behaviour were attributed toπ interactions which do not occur between crystal violet and laponite but do occur between this dye and montmorillonite.     

Concomitant adsorption of poly(ethylene oxide)-b-poly(epsilon-caprolactone) copolymers and sodium dodecyl sulfate at the silica-water interface

Upon addition of silica to aqueous solutions of poly(ethylene oxide)-b-poly(epsilon-caprolactone) copolymers (PEO-b-PCL) and sodium dodecyl sulfate (SDS), adsorption of the solutes occurs at the silica-water interface. The amount of the adsorbed constituents has been measured by the total concentration depletion method. Small-angle neutron scattering experiments (SANS) have been carried out to investigate the structure of the adsorbed layer. Although SDS is not spontaneously adsorbed onto hydrophilic silica, adsorption is observed in the presence of PEO-b-PCL diblocks, in relation to the relative concentration of the two compounds. Conversely, SDS has a depressive effect on the adsorption of the copolymer, whose structure at the interface is modified. Copolymer desorption is however never complete at high SDS content. These observations have been rationalized by the associative behavior of PEO-b-PCL and SDS in water.     

Use of the BET adsorption isotherm equation to examine styrene adsolubilization by nonionic surfactants at the water-silica interface

A modified BET adsorption isotherm equation was used to explain adsolubilization at the water-silica interface, producing a method for understanding adsolubilization that is independent of admicelle structure. The two constants k1 and k2 provide a measure of the surfactant-solute and solute-solute interactions, respectively. As predicted, k1 changed with various factors including surfactant structure, the presence and quantity of added lipophilic linker and hexane. On the other hand, k2 was fairly constant for all examined systems having the same solute. Commonly used partition coefficients can be easily related to the model and adsolubilization isotherms may be reproduced by the developed equation. Inferences about solute distribution at the interface and its location within the admicelle may be extracted from the results. The model is useful in explaining trends in adsolubilization and in predicting adsolubilization behavior for similar systems.     

Molecular dynamics simulations of surfactants at the silica-water interface: anionic vs nonionic headgroups

Understanding surfactant adsorption on surfaces at the molecular level will provide us with the ability to design specific surfactants for surface modification. We conducted molecular dynamics simulations for sodium dodecyl sulfate (SDS) and hexaethylene glycol monododecyl ether (C(12)E(6)) adsorbed on silica substrates with varying degree of hydroxylation. Our results shed light on the effects of hydroxylation on the surfactant aggregate morphology. The discrete charge distribution on the substrate surface appears to dictate both surfactant adsorption and aggregate morphology. The differences in aggregate morphology observed for anionic SDS and non-ionic C(12)E(6) on silica substrates are discussed quantitatively and compared to available experimental data.     

Self-consistent field modeling of non-ionic surfactants at the silica-water interface: incorporating molecular detail

We have constructed a model to predict the properties of non-ionic (alkyl-ethylene oxide) (C(n)E(m)) surfactants, both in aqueous solutions and near a silica surface, based upon the self-consistent field theory using the Scheutjens-Fleer discretisation scheme. The system has the pH and the ionic strength as additional control parameters. At high ionic strength, the solvent quality for the surfactant head groups is affected, which changes both the bulk and the adsorption behavior of the surfactant. For example, with increasing ionic strength, the CMC drops and the aggregation increases. Surfactants adsorb above the critical surface association concentration (CSAC). The CSAC is a function of the surfactant and the surface properties. Therefore, the CSAC varies with both the ionic strength and the pH. We predict that with increasing ionic strength, the CSAC will first slightly increase but then drop substantially. The charge on the surface is pH dependent, and as the head groups bind through H-bonding to the silanol groups, the CSAC increases with increasing pH. We focus on adsorption/desorption transitions for the surfactants and compare these to the experimental data. Both the equilibrium predictions and the consequences for the kinetics of adsorption follow experimental findings. Our results show that molecularly realistic models can reveal a much richer interfacial behavior than anticipated from more generic models.     

Enthalpies of adsorption of methylene blue and crystal violet to montmorillonite

Calorimetric measurements of the enthalpy of adsorption on montmorillonite indicate different patterns for methylene blue (MB) and crystal violet (CV). The enthalpy of adsorption of MB is endothermic up to 73% of the cation exchange capacity (CEC) (i.e., about 0.6 mmol g^-1 clay), whereas at higher adsorption ratios the adsorption reaction becomes exothermic. The enthalpy of adsorption of CV is exothermic for all amounts adsorbed. These results were confirmed with adsorption experiments that prove that adsorption of MB increase with temperature, whereas CV adsorption decreases. This behavior indicates changes in the equilibrium coefficient according to Van't Hoff's equation. This revised version was published online in July 2006 with corrections to the Cover Date.     

The adsorption behavior of crystal violet in functionalized sporopollenin-mediated column arrangements

The adsorption behavior of Crystal Violet (CV) on a sporopollenin-based solid phase, carboxylated diaminoethane sporopollenin (CDAE-S), was investigated under column conditions, and the obtained breakthrough profiles were used in evaluations and quantifications. The adsorption capacity of the CDAE-S was observed to be considerably higher than that of diaminoethane sporopollenin (DAE-S), revealing the importance of electrostatic interactions and carboxyl groups in the adsorption of CV on the CDAE-S. The binding of CV on the DAE-S was found to be a typical nonspecific adsorption, whilst cation-exchange was proposed as the main mechanism for monolayer adsorption of CV on the CDAE-S. Hence in the present study, the cation-exchange is suggested as an effective process for removal and recovery of CV from aqueous effluents, and in view of the pH point of zero charge matter, multifunctionality of the CDAE-S is discussed in detail, and various application possibilities based on "aminocarboxylic acid" functionality are also drawn.     

Molar volume and adsorption isotherm dependence of capillary forces in nanoasperity contacts

The magnitude of the capillary force at any given temperature and adsorbate partial pressure depends primarily on four factors: the surface tension of the adsorbate, its liquid molar volume, its isothermal behavior, and the contact geometry. At large contacting radii, the adsorbate surface tension and the contact geometry are dominating. This is the case of surface force apparatus measurements and atomic force microscopy (AFM) experiments with micrometer-size spheres. However, as the size of contacting asperities decreases to the nanoscale as in AFM experiments with sharp tips, the molar volume and isotherm of the adsorbate become very important to capillary formation as well as capillary adhesion. This effect is experimentally and theoretically explored with simple alcohol molecules (ethanol, 1-butanol, and 1-pentanol) which have comparable surface tensions but differing liquid molar volumes. Adsorption isotherms for these alcohols on silicon oxide are also reported.     

pH dependence of adsorption of n-dodecyl-beta-D-maltoside on solids

Adsorption of surfactants on solids plays an important role in industrial operations such as separation, lubrication, flotation, dispersion, chemical mechanical polishing, and enhanced oil recovery. In this work, adsorption of a typical biodegradable nonionic surfactant, n-dodecyl-beta-d-maltoside, on solids was studied to explore its potential applications. Even though it is a nonionic surfactant, significant pH-dependence was revealed for the adsorption on alumina in the range from pH 4 to 7. The adsorption density was found to be proportional to the concentration of surface AlOH group among Al(OH(2))(+) and AlO(-) groups. The equilibriums among the surface species are governed by pH through surface ionization reactions. The surface AlOH group evidently determines the formation of hydrogen bonding between the surfactant molecules and the solid surface and thus the adsorption. Similar correlation was also found in the case of hematite. The results help to understand the mechanism of adsorption of sugar-based surfactant on solids.     

Surfactant adsorption at the metal-oil interface

The structure of the adsorbed palmitic acid at the iron oxide/oil interface has been investigated using polarized neutron reflectometry. The palmitic acid was found to be strongly adsorbed at the oxide/oil interface resulting in a monolayer of thickness 16 ± 4 ? for 150 and 500 ppm palmitic acid concentrations (16 ± 5 ? for the 1000 ppm solution). These layer thicknesses suggest tilt for the palmitic acid molecules with respect to the interface. The model also requires a second diffuse layer extending in the bulk oil. The thickness of this diffuse layer was 35 ± 17 ? for the 150 ppm solution and 45 ± 22 ? for 500 and 1000 ppm solution. The composition profiles at the interface suggest a depletion of the oil in the vicinity of the interface as the concentration of palmitic acid increases.     

Adsorption of poly(ethylene oxide)-b-poly(epsilon-caprolactone) copolymers at the silica-water interface

The adsorption of amphiphilic poly(ethylene oxide)-b-poly(epsilon-caprolactone) and poly(ethylene oxide)-b-poly(gamma-methyl-epsilon-caprolactone) copolymers in aqueous solution on silica and glass surfaces has been investigated by flow microcalorimetry, small-angle neutron scattering (SANS), surface forces, and complementary techniques. The studied copolymers consist of a poly(ethylene oxide) (PEO) block of M(n) = 5000 and a hydrophobic polyester block of poly(epsilon-caprolactone) (PCL) or poly(gamma-methyl-epsilon-caprolactone) (PMCL) of M(n) in the 950-2200 range. Compared to homoPEO, the adsorption of the copolymers is significantly increased by the connection of PEO to an aliphatic polyester block. According to calorimetric experiments, the copolymers interact with the surface mainly through the hydrophilic block. At low surface coverage, the PEO block interacts with the surface such that both PEO and PCL chains are exposed to the aqueous solution. At high surface coverage, a dense copolymer layer is observed with the PEO blocks oriented toward the solution. The structure of the copolymer layer has been analyzed by neutron scattering using the contrast matching technique and by tapping mode atomic force microscopy. The experimental observations agree with the coadsorption of micelles and free copolymer chains at the interface.     

Effects of pH and ionic strength on the adsorption of phosphate and arsenate at the goethite-water interface

The surface properties of a well-crystallized synthetic goethite have been studied by acid-base potentiometric titrations, electrophoresis, and phosphate and arsenate adsorption isotherms at different pH and electrolyte concentrations. The PZC and IEP of the studied goethite were 9.3+/-0.1 and 9.3+/-0.2, respectively. Phosphate and arsenate adsorption decrease as the pH increases in either 0.1 or 0.01 M KNO(3) solutions. Phosphate adsorption is more sensitive to changes in pH and ionic strength than that of arsenate. The combined effects of pH and ionic strength result in higher phosphate adsorption in acidic media at most ionic strengths, but result in lower phosphate adsorption in basic media and low ionic strengths. The CD-MUSIC model yields rather good fit of the experimental data. For phosphate it was necessary to postulate the presence of three inner-sphere surface complexes (monodentate nonprotonated, bidentate nonprotonated, and bidentate protonated). In contrast, arsenate could be well described by postulating only the presence of the two bidenate species. A small improvement of the arsenate adsorption data could be achieved by assuming the presence of a monodentate protonated species. Model predictions are in agreement with spectroscopic evidence, which suggest, especially for the case of arsenate, that mainly bidentate inner-sphere complexes are formed at the goethite-water interface.     

Numerical analysis of surfactant dynamics at air-water interface using the Henry isotherm

This paper focuses on the surfactant behavior at air-water interface, taking into account the diffusion-controlled model together with the Henry isotherm to model the relation between the surface and the subsurface concentrations. The existence and uniqueness of a weak solution is stated. Fully discrete approximations are obtained by using a finite element method and the backward Euler scheme. Error estimates are then proved from which, under adequate additional regularity conditions, the linear convergence of the algorithm is derived. Finally, some numerical simulations are presented in order to demonstrate the accuracy of the algorithm and the behavior of the solution.     

Estimation of the average heat of gas adsorption from the adsorption isotherm measured at supercritical temperatures and pressures

Using a high-precision volumetric—gravimetric experimental unit, the adsorption iso-therms of methane, argon, and nitrogen were measured at pressures of 0.1—40 MPa and at temperatures of 303—373 K on four carbon adsorbents with different porosities. To calculate the characteristic energy and average heat of adsorption of gases in the supercritical region, two parameters, which characterize an adsorptive were introduced: critical temperature T _c of the gas and the analog of the saturated vapor pressure “P _s”, which is defined assuming that densities of an adsorbate and an adsorptive are equal. The average heats of gas adsorption well agree with the average isosteric heats for the adsorption systems over the entire pressure and temperature intervals under study.