Monte Carlo modeling of ion adsorption at the energetically heterogeneous metal oxide/electrolyte interface: Micro- and macroscopic correlations between adsorption energies

Grand canonical Monte Carlo simulations are carried out for the basic Stern model of the electrical double layer formed at the energetically heterogeneous metal oxide/electrolyte interface. The effect of the global (macroscopic) and local (microscopic) adsorption energies correlations as well as the influence of the model parameter on the surface charge density curves were investigated. The linear dependence of point of zero charge (PZC) as a function of H+ ion adsorption energy proves that the acidic/basic properties of the system are mainly governed by proton uptake/release. Two kinds of systems were taken into account: one neglecting lateral interactions and the other one including them. The effect of electrolyte concentrations as well as the surface heterogeneity on the surface charge density curves were shown too. The presented simulation algorithm allows to model two experimentally observed instances of the metal oxide/electrolyte interface: one possessing a common intersection point (CIP) at pH = PZC and the other one with CIP not equal PZC.     

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.     

Confinement in nanopores at the oxide/water interface: modification of alumina adsorption properties

There is limited knowledge on the influence of the pore size on surface phenomena (adsorption, dissolution, precipitation, etc.) at the oxide/water interface and a better understanding of the space confinement in nanoscale pores should have practical implications in different areas, such as transport of contaminants in the environment or heterogeneous catalyst preparation, to name a few. To investigate the modifications of the oxide adsorption properties at the oxide/water interface in a confined environment, the surface acidobasic and ion adsorption properties of six different aluminas (5 porous commercial aluminas with pore diameters ranging from 25 to 200 A and 1 non-porous alumina) were determined by means of acid-base titration and Ni(II) adsorption. It is shown that the confinement has a moderate impact on the alumina adsorption capacity because all materials have similar surface charging behaviours and ion saturation coverages. However, a confined geometry has a much larger impact on the ion adsorption constants, which decrease drastically when the average pore diameter decreases below 200 A. These results are discussed in terms of nanoscale pore space confinement.     

Electokinetic and adsorption properties of different titanium dioxides at the solid/solution interface

Six samples of titanium dioxide of different phase compositions and specific surface areas have been characterized by XRD, Raman-and FTIR spectroscopy, adsorption of nitrogen, electrophoresis. Adsorption of Zn(II) ions at the TiO₂/NaCl aqueous solution interface as well as the effect of adsorption on the structure of electrical double layer have been studied. The influence of ionic strength, pH and presence of ions on the adsorption of Zn(II) ions at the TiO₂/NaCl solution interface have also been investigated. The zeta potential, surface charge density, parameters of adsorption edge pH_50% and ΔpH_10–90% for different concentrations of basic electrolyte have been determined. Studied unpurified samples showed lower values of isoelectric point pH_iep compared with literature data due to the presence of anion impurities. The antibate dependence between pH_iep values and particle size has been established. Adsorption of Zn(II) ions using monophase samples is completed at a lower pH than for the biphase TiO₂. Appearance of the point CR3 is associated with the charge turnover from positive to negative at high values of pH and formation of Zn(OH)₂.      

Novel method for linking CdS nanoparticles at liquid-liquid interface

Liquid-liquid interface of water-hexane provides a unique reaction environment in which CdS nanoparticles capped with mercaptoethylamine could be linked together to form a homodimer with a divalent acid chloride, sebacoyl chloride. Prior to the reaction, mercaptoethylamine-capped CdS in aqueous solution was purified by dialysis and freeze-drying. The observation with a transmission electron microscope suggested the formation of a homodimer of CdS nanoparticles.     

Polymerized rodlike micelle adsorption at the solid-liquid interface

The adsorption of rodlike polymer-micelle aggregates of cetyltrimethylammonium 4-vinylbenzoate (p-C16TVB) at the silica-water interface has been characterized using a combination of quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM) studies. Adsorption isotherm data, recorded by QCM-D, indicate a two-stage mechanism: an adsorbed film of free CTA+ ions is initially produced at low concentrations until the surface is charge reversed, whereupon the weakly anionic aggregates can adsorb and the adsorbed mass is seen to increase dramatically. The adsorbed rodlike micelle aggregates are seen to form a close-packed monolayer from AFM images with a high degree of order over micrometer length scales. AFM force-distance data indicate that the adsorbed aggregates retain their cylindrical structure and little or no flattening is seen. Rinsing of the film did not result in removal of the adsorbed layer, and the persistence of these nanoscale ordered films at the solid-liquid interface suggests many possible applications.     

On the adsorption properties of surface chemically pure CHAPS at the air/water interface

CHAPS, a surface-active derivative of the steroids' basic structure of the cholic acid [3-[(3-cholamidopropyl)dimethyl-ammonio]-1-propanesulfonate] has become a very important material in biological and pharmaceutical application. Investigations of the adsorption properties of aqueous, surface-chemically pure CHAPS solutions at the air/water interface were performed using surface tension and surface potential measurements. Unlike ordinary extended-chain surfactants, the amphiphilic structure of CHAPS is prone to adopt different concentration-dependent surface states of the adsorption layer. These are well reflected in the adsorption isotherm and in the electric surface properties. They are explained by changes in the adsorbate molecule's orientation and/or conformation as a result of the latter's different surface area demand. The versatile favorable application properties of the CHAPS molecule are obviously due to its complicated molecular structure, which enables it to comply with rather different interfacial and colloidal challenges.     

Effect of sucrose on the properties of caffeine adsorption layers at the air/solution interface

Sweet and bitter tastes are known to be mediated by G-protein-coupled receptors. The relationship between the chemical structure of gustable molecules and their molecular organization in saliva (aqueous solution) near the surface of the tongue provides a useful tool for elucidating the mechanism of chemoreception. The interactions between stimulus and membrane receptors occur in an anisotropic system. They might be influenced by the molecular packing of gustable molecules within an aqueous solvent (saliva) close to the receptor protein. To investigate the molecular organization of a sweet molecule (sucrose), a bitter molecule (caffeine), and their mixture in an aqueous phase near a "wall", a hydrophobic phase, we modeled this using an air/liquid interface as an anisotropic system. The experimental (tensiometry and ellipsometry) data unambiguously show that caffeine molecules form an adsorption layer, whereas sucrose induces a desorption layer at the air/water interface. The adsorption of caffeine molecules at the air/water interface gradually increases with the volume concentration and is delayed when sucrose is added to the solution. Spectroscopic ellipsometry data show that caffeine in the adsorption layer has optical properties practically identical to those of the molecule in solution. The results are interpreted in terms of molecular association of caffeine with itself at the interface with and without sucrose in the subphase, using the theory of ideal gases.     

Cross linking and rheological characterization of adsorbed protein layers at the oil-water interface

The dilatational rheological properties of cross-linked protein layers adsorbed at the oil-water interface were investigated with help of a modified drop tensiometer allowing successive replacements of the external phase. This setup enables one to perform cross-linking reactions at the interface only, that is, without any contact between the cross-linking agent and protein molecules in solution, under continuous monitoring of the interfacial tension. The mechanical properties of the resulting interface were investigated with dilatational large strain experiments. Measured rheological properties were related to the expected stability of an emulsion against disproportionation by considering the ratio of the interfacial elasticity to the interfacial tension. In an attempt to increase this ratio to improve the resistance against disproportionation, experiments were performed with densified protein layers obtained via reduction of the droplet area prior to cross linking. To highlight the influence of the protein morphology on the dilatational rheological properties of the cross-linked adsorbed layers, experiments were performed with random coil (beta-casein) as well as globular (beta-lactoglobulin) proteins. Glutaraldehyde was used as a cross-linking agent. Experiments were performed at 55 degrees C and pH 7.0 in 20 mM imidazole buffer for later comparison with enzymatically cross-linked adsorbed protein layers. The present work demonstrated substantial qualitative and quantitative differences in the interfacial rheological properties of cross-linked random coil and globular proteins.     

Effect of nanostructure on the properties of water at the water-hydrophobic interface: a molecular dynamics simulation

The local structure of water near hydrophobic surfaces of different surface topographies has been analyzed by molecular dynamics simulation. An alkane crystal has been taken as the parent model for a hydrophobic surface. Surface structures were created by placing pits into it, which were half a nanometer deep and several nanometers wide. Around all structures, the water has a lower density, less orientational ordering, fewer water-water hydrogen bonds, and fewer surface contacts than for a flat unstructured surface. This indicates that the structured surfaces are more hydrophobic than the flat surface. Of the structures investigated, pits with a diameter of approximately 2.5 nm were effective in increasing the hydrophobic character of the surface.     

Diffusion-controlled adsorption kinetics at the air/solution interface

 To describe diffusion-controlled adsorption, the diffusion equation is solved under different initial and boundary conditions by means of a Laplace transformation. By solving this equation, it has been found that the solution, which Ward and Tordai used, is only applicable for x>0; therefore, it is incorrect if the derivation is made at x = 0. Ward and Tordai did not notice this and the first derivation was made at x = 0 in order to get the dynamic surface adsorption, Γ(t). In this paper, an accurate solution, which is applicable for x≥ 0, is given and the expression for Γ(t) is obtained. Furthermore the relationship between the dynamic surface tension and Γ(t) is derived. As an example, the dynamic surface tensions of an aqueous octyl-β-d-glucopyranosid solution were measured by means of the maximum bubble pressure method. By using the derived theory it has been proved that the controlling mechanism of the adsorption process of this surfactant at the long-time-adsorption limits changes as a function of the bulk concentration; only at dilute concentration is it controlled by diffusion. Received: 26 July 1999/Accepted in revised form: 16 September 1999     

Polystyrene latex adsorption at the gold/electrolyte interface

Irreversible deposition of polystyrene latex particles (average diameter, 1.5 microm) on various solid/electrolyte interfaces was studied experimentally by using the direct microscope observation method. The substrate surfaces included bare mica (reference interface), gold covered mica (layer thickness of 50 nm), and solid gold plate. The morphology and thickness of the gold layer on mica was determined by atomic force microscopy. Well-defined transport conditions of particles were created by using the new impinging-jet cell. A characteristic feature of the cell was that the suspension stream was directed obliquely to the interface. This unique characteristic was advantageous allowing one for direct, in situ, observation of particle deposition at metals and other nontransparent interfaces. Experiments performed for various flow intensities indicated that the initial deposition kinetics at all interfaces was identical within the error bounds, in accordance with the model based on the convective-diffusion theory. It was concluded that the limiting flux was governed by the bulk transport rather than by the specific surface interactions.     

Protein adsorption at the electrified air-water interface: implications on foam stability

The surface chemistry of ions, water molecules, and proteins as well as their ability to form stable networks in foams can influence and control macroscopic properties such as taste and texture of dairy products considerably. Despite the significant relevance of protein adsorption at liquid interfaces, a molecular level understanding on the arrangement of proteins at interfaces and their interactions has been elusive. Therefore, we have addressed the adsorption of the model protein bovine serum albumin (BSA) at the air-water interface with vibrational sum-frequency generation (SFG) and ellipsometry. SFG provides specific information on the composition and average orientation of molecules at interfaces, while complementary information on the thickness of the adsorbed layer can be obtained with ellipsometry. Adsorption of charged BSA proteins at the water surface leads to an electrified interface, pH dependent charging, and electric field-induced polar ordering of interfacial H(2)O and BSA. Varying the bulk pH of protein solutions changes the intensities of the protein related vibrational bands substantially, while dramatic changes in vibrational bands of interfacial H(2)O are simultaneously observed. These observations have allowed us to determine the isoelectric point of BSA directly at the electrolyte-air interface for the first time. BSA covered air-water interfaces with a pH near the isoelectric point form an amorphous network of possibly agglomerated BSA proteins. Finally, we provide a direct correlation of the molecular structure of BSA interfaces with foam stability and new information on the link between microscopic properties of BSA at water surfaces and macroscopic properties such as the stability of protein foams.     

Destructive adsorption of CCl4 over lanthanum-based solids: linking activity to acid-base properties

The relative activities of a low-surface crystalline and high-surface amorphous LaOCl, further denoted as S1 and S2, have been compared for the destructive adsorption of CCl4. It was found that the intrinsic activity of S2 is higher than that of S1. Both samples were characterized with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), N2-physisorption, and Raman and infrared (IR) spectroscopy. IR was used in combination with CO2, CO, and methanol as probe molecules. The CO2 experiments showed that different carbonate species are formed on both materials. For S1, a high surface concentration of bidentate carbonate species and a lower concentration of monodentate carbonate were observed. In the case of S2, bulk carbonates were present together with bridged carbonates. CO adsorption shows that S2 and S1 have very similar Lewis acid sites. However, methanol adsorption experiments showed that S2 had a higher number of stronger Lewis acid sites than S1 and that twofold coordinated methoxy species were more strongly bound than threefold coordinated methoxy species. Because of the analogy between methanol dissociation and the removal of the first chlorine atom in the destructive adsorption of CCl4, the sites enabling twofold coordination were likely to be the same Lewis acid sites actively involved in the destructive adsorption of CCl4. La2O3 was less active than the two LaOCl materials, and therefore, the intrinsic activity of the catalyst increases as the strength of the Lewis acid sites increases. S2 contains more chlorine at the surface than S1, which is expressed by the higher number of sites enabling twofold coordination. Moreover, this explains the difference in destructive adsorption capacity for CCl4 that was observed for the samples S1 and S2. Since LaCl3, being the most acidic phase, is not active for the destructive adsorption of CCl4, basic oxygen atoms, however, remain needed to stabilize the reaction intermediate CCl3 as La-O-CCl3.     

Interaction between poly(vinylimidazole) and sodium dodecyl sulfate: binding and adsorption properties at the silica/water interface

The adsorption properties (adsorbed amount, kinetics, and reversibility) of poly(vinylimidazole) (PVI) and sodium dodecyl sulfate from PVI/SDS mixed solutions on negatively charged silica substrates were studied at pH 9 using reflectometry and compared to that measured on colloidal silica by the solution depletion method. In this paper, we will try to gain insight into the effect of PVI/SDS complex composition on the adsorption characteristics of the complex and particularly on the kinetics of the complex adsorption and its consequence on the adsorption reversibility. The properties of the complex in solution were characterized by means of potentiometric titration at a constant pH, binding isotherm, and surface tension measurements. On the basis of the experimental results the prevailing mechanism of the SDS/PVI interaction and the properties of the PVI/SDS complex were evaluated. Both the PVI/SDS complex uptake and the kinetics of the adsorption decreased with the amount of SDS bound to PVI. At low PVI/SDS binding ([SDS](0)CAC) the incoming complex experiences a blocking barrier of an electrostatic nature. This barrier has been confirmed by reversibility measurement, and the respective roles of the complex structure and charge were assessed.     

From molecular level to macroscopic properties: A solid-state NMR biomineralization and biomimetic exploration

Through biomineralization, calcareous composites are produced with exceptional properties, evolution-optimized for specific function. The bioinspired quest to understand how properties are controlled and enhanced is motivated by their fundamental and technological significance. The incorporation of small molecules and/or biopolymers as inter- and intra-crystalline additives in the CaCO₃ matrix, is widely employed by organisms to achieve diverse functions. The interactions between the components during the early events within the precipitation medium, and when entrapped through precipitation-crystallization, are key players of process–property regulation. In addition to identifying the bulk matrices and the incorporated molecules, we show how solid-state NMR methods are tailored to directly report the chemical-structural details of the inorganic interface that surrounds an occlusion. Solid-state NMR is uniquely suited for that and is applicable to stable or spontaneously transforming lattices, crystalline or amorphous. Our findings are grouped to highlight the connection between the molecular level and tunability of macroscopic properties.     

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.