The Heavy-Atom Effect on the Photophysics of Aromatic Hydrocarbons in the Presence of Solid Clays

The exchange of the original cation present on a Laponite clay (usually Na⁺) for heavy atoms such as Rb⁺, Cs⁺, and Tl⁺ significantly alters the emission characteristics of some aromatic hydrocarbons (p-terphenyl, naphthalene, pyrene, and biphenyl). The increase of the atomic mass of the cation induces a decrease of the fluorescence emission simultaneous with an increase of the emission in the region of lower energies of the spectra, ascribed to the phosphorescence of those hydrocarbons. Time-resolved experiments for the pyrene–clay system showed a decrease of singlet lifetimes for the heavier atoms. Hydrocarbon aggregates were also detected from both the emission spectra and the time-resolved studies. The “excimer-like” emission showed longer lifetimes (10–25 ns) than the monomolecular hydrocarbons (1–3 ns), as already found for other similar systems. The amount of aggregates increased for the heavier cations due to the smaller surface available on the clay particles. Experiments increasing the amount of Tl⁺ in samples containing a constant concentration of naphthalene allowed evaluation of the distance between the heavy atoms and the probe on the clay surface. The Perrin model treatment was used and resulted in approximately R₀=9.2 Å.     

Effect of pore structure on surface characteristics of zirconium phosphate-modified silica

Three samples of silica of different pore structure-predominantly microporous, S1; mesoporous, S2; and nonporous, S3-were modified with zirconium phosphate and examined. Pore structure analysis showed that modification had taken place in wider pores of S1 leaving a totally microporous sample, and in large pores of S2 giving a mesoporous sample of narrower pore size distribution. The modification of the nonporous sample decreased the surface area and pore volume to a lower extent than in the other two samples, but resulted in a surface of lower energy toward N2. The different distribution of surface silanol groups on the surfaces of different porosity may result in variable pictures on the modified surfaces as reflected in the differences observed in Br?nsted acidity of modified surfaces. The use of these modified silica samples for amino acid adsorption (L-glutamic acid and L-alanine) indicated that both the isoelectric point of the amino acid and the distribution of surface groups on modified solids are controlling the adsorption process.     

Electroosmosis through a Cation-Exchange Membrane: Effect of an ac Perturbation on the Electroosmotic Flow

Electroosmosis experiments through a cation-exchange membrane have been performed using NaCl solutions in different experimental situations. The influence of an alternating (ac) sinusoidal perturbation, of known angular frequency and small amplitude, superimposed to the usual applied continuous (dc) signal on the electroosmotic flow has been studied. The experimental results show that the presence of the ac perturbation affects the electroosmotic flow value, depending on the frequency of the ac signal and on the solution stirring conditions. In the frequency range studied, two regions have been observed where the electroosmotic flow reaches a maximum value: one at low frequencies (Hz); and another at frequencies of the order of kHz. These regions could be related to membrane relaxation phenomena.     

A structural study of amphiphilic PAMAM (poly(amido amine)) dendrimers in Langmuir and Langmuir-Blodgett films

Two amphiphilic PAMAM dendrimers are synthesized by attaching 12-hydroxydodecanoic acid (HA) chains to a poly(amido amine) (PAMAM) dendrimer core (including generation I and generation II). The limiting molecular area obtained from the surface pressure-area isotherm at the air/water interface suggests the edge-on configuration for both dendrimers in Langmuir films. The edge-on arrangement is also supported by the atomic force microscopic (AFM) studies of the Langmuir-Blodgett films.     

Effect of Insoluble Surfactants on Drainage and Rupture of a Film between Drops Interacting under a Constant Force

The deformation, drainage, and rupture of an axisymmetrical film between colliding drops in the presence of insoluble surfactants under the influence of van der Waals forces is studied numerically at small capillary and Reynolds numbers and small surfactant concentrations. Constant-force collisions of Newtonian drops in another Newtonian fluid are considered. The mathematical model is based on the lubrication equations in the gap between drops and the creeping flow approximation of Navier–Stokes equations in the drops, coupled with velocity and stress boundary conditions at the interfaces. A nonuniform surfactant concentration on the interfaces, governed by a convection–diffusion equation, leads to a gradient of the interfacial tension which in turn leads to additional tangential stress on the interfaces (Marangoni effects). The mathematical problem is solved by a finite-difference method on a nonuniform mesh at the interfaces and a boundary-integral method in the drops. The whole range of the dispersed to continuous-phase viscosity ratios is investigated for a range of values of the dimensionless surfactant concentration, Peclét number, and dimensionless Hamaker constant (covering both “nose” and “rim” rupture). In the limit of the large Peclét number and the small dimensionless Hamaker constant (characteristic of drops in the millimeter size range) a fair approximation to the results is provided by a simple expression for the critical surfactant concentration, drainage being virtually uninfluenced by the surfactant for concentrations below the critical surfactant concentration and corresponding to that for immobile interfaces for concentrations above it.     

A Phenomenon of the Change in Particle Drift Velocity Direction in High-Field Electrophoresis

The sign–alternating electric field in rectangular impulses has been used to eliminate linear electrophoresis and to study nonlinear electrophoresis in water at strong fields. We found out that the particle drift velocity could change its direction with the growth of the strength of the field. This new phenomenon has obtained its explanation in the framework of the Debye–Hückel theory of strong electrolyte where we consider a particle as a “heavy” effective ion and take into account all relevant nonlinear effects, grounding ourselves on basic physics. With the help of our theory we have succeeded in good fitting of our experimental data on black oil and Al₂O₃ particles in distilled water using reasonable values for the basic parameters such as the Debye screening length.     

Effect of β-Cyclodextrin on the Photoinduced Charge Transfer in Sodium 1-Anilino-8-naphthalene Sulfate (ANS)/CdS Colloidal System

The S-center radical (ANS^·) of sodium 1-anilino-8-naphthalene sulfate (ANS) generated by photoinduced charge transfer in ANS/CdS and ANS/CdS/β-cyclodextrin(β-CD) systems has been studied by using spin trapping electron spin resonance techniques, UV-visible spectroscopic methods, and fluorescence spectroscopic methods. It was found that the S-centered radical (ANS^·) was produced by the charge transfer reaction between the ground state ANS and the positive hole h⁺(CdS) from the valence band of CdS colloids, by the charge transfer from the excited singlet state ¹ANS* to the conduction band of CdS colloids, or by both in the ANS/CdS and ANS/CdS/β-CD systems. The ESR signal intensity of the spin adduct (5,5′-dimethyl-1-pyrroline-N-oxide (DMPO)–ANS)^·, which is formed from ANS^· trapped by DMPO, in the latter system is 15 times stronger than that in the former system. The apparent association constants between ANS and CdS colloids in the absence and presence of β-CD determined from fluorescence quenching experiments are 1097 and 1606 M⁻¹, respectively. From ESR and fluorescence results, it is estimated that the efficiency of photoinduced charge transfer from ANS to CdS colloids in the ANS/CdS/β-CD system is 12.5 times that in the ANS/CdS system.     

Electroacoustic and Potentiometric Studies of the Hematite/Water Interface

The basic charging properties of nearly spherical hematite particles were studied by using potentiometric titration and the electroacoustic technique. Both the pH and the ionic strength dependence of the surface charge and the ζ-potential were studied in detail. For calculating the ζ-potential from mobility data a few different theories were used and obtained differences are discussed. At pH values higher than 7 and at high electrolyte concentrations (50 mM and 100 mM NaNO₃), it was difficult to fit the mobility data by using the full mobility spectra including both magnitude and phase angle at several frequencies. In this regime the best fits were obtained by using a theory for aggregated complexes (porous particles). From potentiometric titrations in 0.01, 0.1, and 1.0 M NaNO₃, parameters for a 1-pK Basic Stern Model were determined. The model was used to examine the possibility of correlating the experimentally determined ζ-potentials to the model-calculated potentials at the Stern plane. Qualitatively, the model predicted the correct ionic strength dependence of the ζ-potentials, and there was also a rather good quantitative agreement at high ionic strengths (50 and 100 mM NaNO₃). However, at lower ionic strengths the model predicted values up to 40% higher than those found from the electroacoustic study. Surface conduction behind the slip plane was discussed as a possible cause for this discrepancy.     

Assembly of Alternated Multivalent Ion/Polyelectrolyte Layers on Colloidal Particles. Stability of the Multilayers and Encapsulation of Macromolecules into Polyelectrolyte Capsules

Alternating adsorption of multivalent ions and oppositely charged polyelectrolytes on colloid particles has been investigated. Multilayer films composed of Tb³⁺/polysterene sulfonate (PSS) and 4-pyrene sulfate/polyallylamine (PAH) were successfully assembled on polysterene sulfonate (PS) and melamine formaldehyde (MF) latex particles. The amount of assembled material was estimated by fluorescence and the linear growth of the film versus the number of layers was demonstrated. These multilayers are not stable and can be decomposed by salt and temperature. Dissolution of MF particles leads to formation of hollow capsules consisting of multivalent ion/polyelectrolyte multilayers. Comparative analysis of the capsules was done by confocal and scanning force microscopy. Complex hollow spheres consisting of Tb³⁺/PSS or 4-PS/PAH as an inner shell and stable PSS/PAH as an outer shell were produced. Due to selective permeability of the outer shell after degradation of the inner shell the multivalent ions are released out of the capsule while the polyelectrolytes fill the capsule interior. This is indicative of swelling of the capsule by osmotic pressure. The filled capsules were studied by confocal and scanning electron microscopy. Possibilities of encapsulating macromolecules in defined amounts per capsule are discussed.     

Effects of Nonionic and Mixed Cationic-Nonionic Micelles on the Rate of Alkaline Hydrolysis of 4-Nitrophthalimide

Pseudo-first-order rate constants (k_obs) for alkaline hydrolysis of 4-nitrophthalimide show a monotonic decrease with increase in [C₁₂E₂₃]_T (total concentration of Brij 35) at constant [CH₃CN] and [NaOH]. This micellar effect is explained in terms of a pseudophase micelle model. The rate of hydrolysis becomes too slow to monitor at [C₁₂E₂₃]_T≥0.03 M in the absence of cetyltrimethylammonium bromide (CTABr) and at [C₁₂E₂₃]_T≥0.04 M in the presence of 0.006–0.02 M CTABr at 0.01 M NaOH. The plots of k_obs versus [C₁₂E₂₃]_T show minima at 0.006 and 0.01 M CTABr, while such a minimum is not visible at 0.02 M CTABr.     

Adsorption of 1,4-Benzenedithiol on Gold and Silver Surfaces: Surface-Enhanced Raman Scattering Study

The adsorption behavior of 1,4-benzenedithiol (1,4-BDT) on colloidal gold and silver surfaces has been investigated by means of surface-enhanced Raman scattering (SERS). 1,4-BDT chemisorbed dissociatively on both gold and silver surfaces but as mono- and dithiolate, respectively. Regardless of the bulk concentration of 1,4-BDT, only a monolayer was assembled on the silver surface with a flat orientation by forming two Ag–S bonds. On the gold surface, the monothiolate species,1,4-BDT⁻¹, appeared to assume a rather flat orientation at a very low surface coverage, but as the surface coverage was increased, the adsorbate took a perpendicular orientation. Furthermore, when the bulk concentration of 1,4-BDT was close to that required for a full-monolayer coverage limit, a band assignable to the S–S stretching vibration appeared at 536 cm⁻¹ in the gold sol SERS spectra. A separate ellipsometry measurement performed with vacuum-evaporated gold substrates revealed that up to tetralayers could be assembled on gold in 1 mM n-hexane solution of 1,4-BDT while at best a bilayer formed in either methanol or ethanol solution. The different adsorbate structure of 1,4-BDT on gold and silver was overall quite comparable to that of p-xylene-α,α′-dithiol.     

Surface layers of 6-thioguanine on the mercury electrode

The adsorption behavior of 6-thioguanine (6TG) on a hanging mercury drop electrode has been studied with ac and cyclic voltammetry in 0.1 M Na2SO4 and 0.01 M sodium acetate solutions at pH 4.3. Several condensed phases of chemically adsorbed 6TG as well as one phase of physically adsorbed 6TG have been characterized. Under total coverage conditions, the films of chemiadsorbed molecules inhibit rather efficiently the electrode reaction of mercury oxide formation.     

Thermocapillary Alignment of Gas Bubbles Induced by Convective Transport

The effect of a weak convective heat transfer on the thermocapillary interaction of two bubbles with an arbitrary orientation relative to an externally imposed temperature gradient is examined. Asymptotic analysis of the case of large separation distances, Z, suggests that the corrections to the bubbles' velocities are of (Pe/Z²), rather than (Pe²) previously found for an isolated bubble. Equal-sized bubbles are known to move with the same velocities, as if they were isolated, when heat conduction is the only transport mechanism. However, the convective transport results in a relative motion of the bubbles. The tendency of equal bubbles to line up in a plane perpendicular to the applied thermal gradient is shown analytically in the weakly nonlinear limit of small Pe numbers, and an interesting interaction behavior in the case of unequal bubbles is discussed.     

Application of the JKR Method to the Measurement of Adhesion to Langmuir-Blodgett Cellulose Surfaces

The JKR method has been applied for studying adhesion between poly(dimethylsiloxane) (PDMS) caps and Langmuir–Blodgett cellulose surfaces including the substrate, hydrophobized mica, and two flat mineral surfaces, bare mica and glass. The self-adhesion of PDMS caps and oxidized PDMS caps are included as a reference to compare with literature data. The results of the measurements have been compared with previous studies using the surface force apparatus and similar systems. A satisfactory agreement is obtained for simple systems showing no, or very limited, hysteresis between loading and unloading curves. In several cases, however, a large hysteresis is found between loading and unloading curves, with a larger adhesion measured from the pull-off force than from the JKR-curve determined on loading. This is, for instance, the case for PDMS against cellulose. The situation is analogous to that found in wetting studies showing a large hysteresis between advancing and receding contact angles.     

Substituent Effects on Binding Constants of Carotenoids to n-Heptane/AOT Reverse Micelles

The absorption spectra of 6′-apo-β-caroten-6′-ol (1), 6′-apo-β-caroten-6′-oic acid (2), and ethyl 6′-apo-β-caroten-6′-oate (3) were analyzed in homogeneous media and in reversed micelles of AOT (sodium 1,4-bis(2-ethylhexyl) sulfosuccinate) in n-heptane. The possible solute–solvent interactions of these compounds were analyzed in pure solvents by Taft and Kamlet's solvatochromic comparison method. These carotenoids show sensitivity similar to that of medium polarity-polarizability as measured by π*. Moreover, the absorption spectra of carotenoid 3 and to much less extent carotenoid 2 display broadening of the visible bands induced by polar solvents characteristic of carotenoids that contain a carbonyl functional group in conjugation with the carbon–carbon π-electron system. They are also sensitive to the ability of the solvent to accept protons in a hydrogen bond interaction measured by β. This sensitivity follows the expected order: 2>1>3. In the reverse micellar system, while the spectra for 3 remain unchanged, the intensity of the absorption band characteristic of n-heptane for 1 and 2 decreases as the AOT concentration increases, and a new band develops. This new band is attributed to the solute bound to the micelle interface. These changes allowed us to determine the binding constant (K_b) between these compounds and AOT. At W₀=[H₂O]/[AOT]=0 the values of K_b of 326±5 and 6.2±0.3 were found for the acid 2 and the alcohol 1, respectively. The strength of binding is interpreted considering their hydrogen-bond donor ability and the solubility in the organic pseudophase. For 1K_bdecreases as W₀ is increased, while for 2 no variation was observed. These effects are discussed in terms of carotenoid–water competition for interfacial binding sites.     

Influence of Surface Hydrophobic Groups on the Adsorption of Proteins onto Nonporous Polymeric Particles with Immobilized Metal Ions

Iminodiacetic acid (IDA) and octyl moieties were covalently bound on nonporous particles, which were prepared from dispersion polymerization of methyl methacrylate and glycidyl methacrylate. After being charged with copper ions, the IDA-bound particles could specifically adsorb deoxyribonuclease I (DNase I) through the affinity interaction between protein and immobilized metal ion. A mixed-ligand (metal–chelate and octyl–bound) support was obtained after hydrophobic (octyl) groups were also introduced to the particle surface. The affinity adsorption of DNase I on the copper–IDA chelate was influenced by interaction between the protein and the bound octyl group. Both the affinity and the hydrophobic interactions could be well described by the Langmuir isotherms. The equilibrium adsorption constants were estimated separately to be 0.96 and 0.50 liter g⁻¹ for affinity and hydrophobic bindings, respectively. For binding on mixed-ligand support, the adsorption constant was 0.45 liter g⁻¹. It was evident that both affinity and hydrophobic interactions are involved in the adsorption of proteins onto mixed-ligand particles. Desorption of the inactive proteins from the support was possible by increasing the hydrophobicity of the solution.     

Influence of Sodium Periodate and Tyrosinase on Binding of Alginate to Adlayers of Mytilus edulis Foot Protein 1

Mytilus edulis foot protein 1 (Mefp-1) is the most well-characterized component of this sea mussel's adhesive plaque. The plaque is a condensed, heterogeneous mixture consisting of a large proportion of cross-linked biopolymers that bonds the mussel to a chosen mooring. Mefp-1 is densely populated with lysine and -3,4-dihyroxyphenylalanine ( -dopa) residues incorporated into a repeating amino acid sequence motif. It has been proposed that one plaque cross-linking reaction is the nucleophilic addition of the ε-amino groups of the lysine residues into the oxidized catechol (o-diphenol) functionality (quinone) of the -dopa residues. In order to determine if this reaction occurs in adlayers of Mefp-1, a previously developed assay for ε-amino groups was applied. Adlayers of Mefp-1 were exposed to an oxidant, either the enzyme, mushroom tyrosinase, or sodium periodate. Binding of alginate to adlayers was used to probe for accessibility of ε-amino groups. It was found that lysine residues lose the ability to bind alginate after exposure to sodium periodate, but that this loss is not clearly due to a reaction with -dopa residues. There is a slight decrease of binding of alginate to adlayers of Mefp-1 exposed to either active or thermally deactivated mushroom tyrosinase, probably due to the obstruction of binding sites by bound enzyme. Adsorption kinetics of mushroom tyrosinase onto adlayers of Mefp-1 for active and thermally inactivated enzyme were nearly identical. Attenuated total reflection Fourier transform infrared spectroscopy was used to characterize these interactions at a germanium (Ge) interface.     

Use of the Gaussian Distribution Function as a Tool to Estimate Continuous Heterogeneity in Adsorbing Systems

In environmental engineering, adsorption and desorption are phenomena commonly referred to as responsible for pollution dispersion, retention, or retardation in soils, aquifers, and hydrologic systems. They are also used to remove organic pollutants from water or odorous compounds in gas deodorization. Most often, the characterization of the aqueous adsorption systems that are of engineering interest involves a narrow adsorbate concentration range and low values of the adsorbate concentration. The practice is to use the Freundlich equation that best fits most data and is considered sufficient to design adsorption contactors. However, no physical or chemical meaning can be associated with the values taken by the parameters. The present paper gives a new way of analyzing adsorption data, using an extension of the Freundlich equation and the Gaussian distribution function that makes it possible to associate parameter values of this extension with the adsorbate–adsorbent normal interaction energy, its heterogeneity, and to some extent the adsorbate–adsorbate lateral interaction energy.     

Self-Assembled Monolayers of Organoselenium Compounds on Gold: Surface-Enhanced Raman Scattering Study

To understand the binding nature of organoselenium compounds on gold, we have examined the adsorption behavior of several representative organoselenium compounds, i.e., benzeneselenol (BSe), diphenyl diselenide, dibenzyl diselenide, dioctyl diselenide, and benzyl phenyl selenide (BPSe) on the Au surface by virtue of surface-enhanced Raman spectroscopy (SERS). BSe chemisorbs on gold as selenolate with a tilted orientation. Upon adsorption, the Se–Se bonds of diselenides are cleaved to form selenolates, analogous to the formation of thiolate monolayers from disulfides. BPSe adsorbs on gold without any C–Se bond scission. The benzyl moiety of BPSe assumes a rather vertical stance while the phenyl moiety is more tilted to the gold surface.