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.     

Characterization of Titania/Silica Gel by Means of Low-Pressure Nitrogen Adsorption

Adsorbents synthesized by grafting of titania onto mesoporous silica gel surfaces at different temperatures were studied by means of nitrogen adsorption–desorption and water desorption. The pore size distribution f(R_p) of titania/silica gel depends on the titania concentration (C_TiO2) and the temperature of titania synthesis. Nonuniformity of TiO₂ phase is maximal at a low C_TiO2 value (3.2 wt.% anatase deposited at 473 K), and two peaks of the fractal dimension distribution f(D) are observed at such a concentration of titania, but at larger C_TiO2 values, only one f(D) peak is seen. More ordered filling of pores and adsorption sites by nitrogen, reflecting in the shape of adsorption energy distributions f(E) at different pressures of adsorbate, is observed for adsorbent with titania (rutile+anatase) grafted on silica gel at a higher temperature (673 K).     

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.     

Interactions between Poly(ethylene oxide) and Sodium Alkylcarboxylates As Studied by Conductivity and Gel Permeation Chromatography

The interactions between PEO and sodium alkylcarboxylates (octyl, decyl, and dodecyl) have been investigated by conductivity measurements and gel permeation chromatography (GPC). Also included in the study was sodium dodecyl sulfate. From the conductivity measurements the critical aggregation concentration, ionization degree, and binding ratios were determined; the binding ratio was also determined from GPC. PEO–surfactant interactions were observed for all the studied surfactants, except sodium octanoate. For the polymer–surfactant complexes the ionization degree was in all cases observed to be about 0.2 higher than the ionization degree for the corresponding aqueous micelles. Further, the binding ratio decreased somewhat with decreasing chain length of the alkylcarboxylate. The Gibbs free energy showed that the polymer–surfactant interaction decreases with decreasing chain length of the alkylcarboxylates and is weaker for alkylcarboxylate compared to alkylsulfate of similar chain length.     

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.     

Analysis of the Silica Surface Free Energy by the Imbibition Technique

The surface free energy of silica and its components have been evaluated from imbibition experiments performed with liquids of differing surface properties by the distance–time method. Data were analyzed by a parabolic fit to Washburn's equation, because of the uncertainty in the exact position and time at which penetration begins in these kinds of experiments. In addition to the mathematical treatment of the experimental results, the influence of the components and parameters of the surface tension of the liquids used on the values of the solid surface free-energy components has been analyzed.     

Dynamic surface tension and its diffusional decay of dodecyl ethoxylates with different homologue distribution

Dynamic surface tension and its diffusional decay have been studied with four different polydisperse C12E7 at different temperatures and different concentrations. The CMC and the headgroup area from equilibrium surface tension were shown with polydispersity and temperature. The chain length of oxyethylene on the surface was derived from comparison between the headgroup area of monodisperse dodecyl ethoxylates and that of polydisperse C12E7. The values for (Deff/D) were deduced with a diffusion-controlled adsorption model using parameters obtained from equilibrium surface tension. It was shown at short adsorption time that molecules were really adsorbed onto the surface in a diffusion-controlled manner. At a comparably long adsorption time, the ratios (Deff/D) were calculated by assuming the selective adsorption onto the surface. The modified Arrhenius-type equation was proposed by putting a concentration term in front of the exponential terms. The modified Arrhenius-type equation gave Ea=30 kJ/mole for this system. Ea directly derived without an Arrhenius plot was between 9 to 11 kJ/mole. It was an indication that the activation energy alone was not enough to explain the decay of dynamic surface tensions.     

Contribution to the Determination of Kinetic Parameters of the Sol-Gel Transformation by Rheological Measurements

The system tetraethoxysilane(TEOS)–water–ethanol has been studied by rheological measurements. Different molar ratios of TEOS : water (1 : 4, 1 : 10, and 1 : 20) are studied at different temperatures (30, 40, and 50°C). The dynamic viscosity (rotating mode) at a constant shear rate (100 s⁻¹) and the elastic and viscous moduli (oscillating mode) at a constant frequency (1 Hz) are determined. The viscosity–time curves are evaluated by application of a nucleation and particle growth model. Good agreement between experiments and theory is observed. The model allows the determination of the complex rate constant of silica precipitation. The temperature-dependent measurements gave the possibility to determine the apparent energy of activation by common methods. The results are in agreement with data from the literature. The gel time defined as intersection point of elastic and plastic moduli and its dependence on temperature are evaluated by the Smoluchowski model. The energy of activation for the coagulation was determined and found to be in the correct order of magnitude.     

Wetting Study of Hydrophobic Membranes via Liquid Entry Pressure Measurements with Aqueous Alcohol Solutions

A new concept of liquid entry pressure measurements is applied to study the hydrophobicity of microporous membranes for aqueous alcohol solutions. The effects of alcohol concentration, type of alcohol, and temperature on liquid entry pressure of the membrane have been studied. Two theoretical equations for the determination of membrane pore size have been proposed. The former equation was developed taking into account the deviation from the Laplace–Young equation due to the membrane structure by means of the structure angle. The latter equation was established considering only the range of alcohol concentration in which the dispersion component of liquid surface tension remains practically constant. Hydrophobicity has been expressed in terms of wetting surface tension, γ_L^w. Based on these measurements, the maximum concentration before the spontaneous wetting occurs would be predicted.     

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.     

Light Scattering Study of Turbid Heat-Set Globular Protein Gels Using Cross-Correlation Dynamic Light Scattering

The structure factor of aqueous solutions of the globular protein β-lactoglobulin was determined as a function of heating time at 76°C. We show how the effect of multiple scattering on the scattered light intensity can be effectively corrected using cross-correlation dynamic light scattering even if the transmission is only 1%. The structure factor of aggregated and gelled proteins can be described by the Ornstein–Zernike equation. The system is characterized by a correlation length that increases with heating time and stabilizes some time after the gel is formed. The correlation length of the protein gels decreases with decreasing concentration. Measurements after progressive dilution of a sample close to the gel point showed that the protein aggregates are initially interpenetrated and disinterpenetrate upon dilution.     

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.     

Silylating Agents Grafted onto Silica Derived from Leached Chrysotile

Silica from leached chrysotile fibers (SILO) was silanized with trialkoxyaminosilanes to yield inorganic–organic hybrids designated SILx (x=1–3). The greatest amounts of the immobilized agents were quantified as 2.14, 1.90, and 2.18 mmol g⁻¹ on SIL1, SIL2, and SIL3 for –(CH₂)₃NH₂,–(CH₂)₃NH(CH₂)₂NH₂, and –(CH₂)₃NH(CH₂)₂NH(CH₂)₂NH₂ groups attached to the inorganic support. The infrared spectra for all modified silicas showed the absence of the Si–OH deformation mode, originally found at 950 cm⁻¹, and the appearance of asymmetric and symmetric C–H stretching bands at 2950 and 2840 cm⁻¹. Other important bands associated with the organic moieties were assigned to ν_as(NH) at 3478 and ν_sym(NH) at 3418 cm⁻¹. The NMR spectrum of the solid precursor material suggested two different kinds of silicon atoms: silanol and siloxane groups, between −90 and 110 ppm; however, additional species of silicon that contain the organic moieties bonded to silicon at −58 and −66 ppm appeared after chemical modification. These modified silicas showed a high adsorption capacity for cobalt and copper cations in aqueous solution, in contrast to the original SILO matrix, confirming the unequivocal anchoring of silylating agents on the silica surface.     

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.     

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.     

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.     

Aggregation and Polymerization of PEG-Based Macromonomers with Methacryloyl Group as the Only Hydrophobic Segment

Aggregation behavior in aqueous solution of a series of poly (ethylene glycol) (PEG)-based macromonomers with methacryloyl group as the only hydrophobic segment has been investigated using surface tension, steady-state and time-resolved fluorescence spectroscopy using pyrene as a probe, and small-angle neutron scattering techniques. The general formula of these macromonomers is CH₂=C(CH₃)–CO–O–E_m–CH₃, where E is the ethylene glycol unit and m=8 (ME₈), 18 (ME₁₈), 49 (ME₄₉), and 120 (ME₁₂₀). The results indicate that a macromonomer with 8 ethylene glycol units forms as an aggregate above a certain critical concentration, which can be defined as critical aggregation concentration. The observed high value of I₁/I₃ in pyrene emission spectra at the interface of these aggregates and the inability to scatter a neutron beam by these aggregates indicate that the hydrophobic cluster formed by this macromonomer is remarkably solvated. ME₁₈ has a tendency to aggregate but others do not form any hydrophobic cluster. The homopolymerization behaviors of these macromonomers in an aqueous medium at 70°C are consistent with these possibi- lities.     

Adsorption of Strontium on Illite

Adsorption of strontium on illite type clay has been studied as a function of shaking time, the ratio of solution volume to weight of clay and the concentration of adsorbate, using ⁹⁰Sr as a tracer. The adsorption experiments were carried out using the batch method and initial Sr²⁺ ion concentrations ranged from 10^–6 to 10^–1 M. The influence of Ca²⁺ and Ba²⁺ cations on Sr adsorption were also studied. These effects are correlated with the ionic radii of alkaline earth ions present in the solution. The Freundlich and Dubinin Radushkevich (D-R) isotherm have been applied to the data and the parameters of the isotherm equations were calculated. The mean energy of adsorption, E was also calculated from the adsorption energy constant, K and maximum capacity X _m values were determined from linearized D-R equation. From empirical Freundlich parameters a site distribution function was calculated.     

Adsorption and direct probing of fibrinogen and sodium myristate at the air/water interface

Fibrinogen (FB), a serum protein, is considered a major inhibitor of lung surfactant function at the lining layer of the alveoli. In this study, the adsorption of aqueous bovine FB at the air/water interface was investigated with tensiometry and directly probed for the first time with ellipsometry and infrared reflection adsorption spectroscopy (IRRAS). The tension results show that FB has moderate surface activity. The surface densities of FB were calculated by using two different ellipsometry models to range from 3±0.2 to 17±2 mg/m², for 7.5 to 750 ppm of FB in water at 25°C. Although FB at concentrations from 75 to 750 ppm reached about the same steady surface tension value, the surface densities at 750 ppm FB were substantially larger. The same techniques were used for studying aqueous mixtures of 7.5 to 750 ppm FB with 2 mM of sodium myristate (SM) to investigate a possible interaction of the SM with the protein. The behavior of the FB/SM mixtures was found to be close to that of SM alone. The surface tension of the FB/SM mixtures reached values less than 10 mN/m under surface area oscillation at 20 or 80 rpm. These results and the ellipsometry and the IRRAS results indicate that at a concentration of 2 mM SM, FB, up to 750 ppm, does not inhibit the surfactant surface-tension-lowering function. In certain cases the results demonstrate that FB and SM may act cooperatively in lowering the surface tension.