Laser Raman spectroscopic study of water in gelatin–surfactant solutions and gels

A Raman spectroscopic study was carried out on water in gelatin at 4% w/v in gel (25 °C) and sol (40–60 °C) states at various concentrations (0.5, 1, 5, 10 and 15 mM) of anionic surfactant, sodium dodecyl sulfate (SDS). The in-phase collective stretching mode vibration of hydrogen-bonded -OH oscillators, centered around 3250 cm⁻¹ in a tetrahedral network of water molecules, was observed to be significantly affected by temperature and the presence of SDS. According to our observation this may be due to the thinning of the hydration water around the gelatin molecules due to strong thermal agitation. The peak center of the collective bands of water decreased linearly with SDS concentration in the gel state which implied that with the increase in concentration of SDS, the -OH oscillators gradually lost their attachment to gelatin chains and were replaced by SDS molecules. Ultimately this resulted in a thinning of the hydration layer around the gelatin and the oscillation frequency of -OH oscillators moved towards 3250 cm⁻¹ at 1 mM SDS concentration resulting in increased coupling of -OH oscillators to form the tetrahedral network at the critical micelle concentration (cmc) of SDS. The variation in the peak amplitudes and the systematic reversal of their trend about the cmc axis was surprising. At 40 °C the amplitude of the peak at 3250 cm⁻¹ increased drastically due to a possible coil expansion by about 7–8% which accommodated more interstitial water into the pseudonetwork leading to an increase in the number of nearest neighbors and for about 6% increase in the C value. However, at the cmc the peak amplitude was observed to be independent of temperature. Continuous shifting of the peak center and full width at half-maxima towards lower values was observed with increasing SDS concentrations in the gel state. Received: 28 September 1998 Accepted in revised form: 8 March 1999     

Association of ionic surfactant in binary water—ethanol media as indicator of changes in structure and properties of solvent

Russian Chemical Bulletin - Association of sodium dodecyl sulfate (SDS) ionic surfactant in concentration range of 1–100 mmol L?1 in binary water—ethanol media with alcohol volume...     

Dilational properties of gemini surfactant/polymer systems at the air–water surface

The surface properties of mixed system containing gemini anionic surfactant 1,2,3,4-butanetetracarboxylic sodium, 2,3-didodecyl ester and partly hydrolyzed polyacrylamide were investigated by surface tension measurements and oscillating bubble methods. The influences of surfactant concentration, dilational frequency, temperature, pH, as well as salts on dilational modulus were explored. Meanwhile, the interfacial tension relaxation method was employed to obtain the characteristic time of surface relaxation process. The polymers play important roles in changing the interfacial properties especially at lower surfactant concentration. The possible mechanism of the polymer in changing the interfacial properties is proposed. Both the hydrophobic and electrostatic interaction among the surfactants and polymers dominate the surface properties of mixed system. These dynamic properties are of fundamental interest in understanding the structure of adsorption layers, dynamics of surfactant molecules, and their interaction with polymers at the surface.     

Self-association and reverse hemimicelle formation at solid–water interfaces in dilute surfactant solutions

Using monodisperse anatase and hematite particles, such classical techniques as colloid stability, ζ potential and wettability measurements along with fluorescence spectroscopy show that anionic alkyl sulfate and sulfonate surfactants first associate into hemimicelles at the interface and cause the particles to begin to coagulate and to become hydrophobic. Maximum coagulation and hydrophobicity occur under conditions where the ζ potential is zero. At higher surfactant concentrations, the particles redisperse and regain their hydrophilicity. Upon reversal of the ζ potential, the adsorbed surfactant ions appear to form hemimicelles with reverse orientation or bilayers, at surfactant concentrations significantly lower than the critical micelle concentration. The results of these experiments correlate well with spectroscopic and adsorption measurements.     

Micellization and surface activity of Triton X-100 in water–orthophosphoric medium

The adsorption isotherms of Triton X-100 for air/water–orthophosphoric acid interfaces were determined by the stripping method. The surface chemical parameters, Γ_max, F and ΔG°_A, and the aggregation ones, CMC and the ΔG_M, are determined in different H₂O/H₃PO₄ mixtures. For concentrations higher than 4 M, the values of the CMC, ΔG_M, Γ_max and ΔG°_A increase with increasing acid concentrations due to the occurring changes in the medium structure. ©2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SASsurface tension / non-ionic surfactant / micellization / orthophosphoric acid     

Thermal and structural properties of surfactant–picrate compounds

The novel surfactant?Cpicrate compounds were synthesized and characterized by thermal and XRD analysis. The synthesis, based on the electrostatic interactions of components in polar solvents, was carried out using picric acid and cationic surfactants (dodecyltrimethylammonium, didodecyldimethylammonium, and tridodecylmethylammonium halide). The idea was to investigate the dependence of physico-chemical properties and thermal transitions of picrate?Csurfactant compounds with raising number of dodecyl chains sited on the same ammoniumm head group. The equimolar mixed aqueous solutions are characterized as lyotropic, strongly promoted by picrate anion. The main crystal structure feature of investigated picrates is layered structure with stacked aromatic rings of one picrate molecule on the top of the other one via strong ?С??? interactions and connection to the alkylammonium molecules with their nitro groups by C?CH??O hydrogen bonds. Surfactant?Cpicrate bilayer-like structures are interrupted with layers of polar heads and picrate counterions and the observed width of such a bilayers are functions of more complex structural behaviors which ensures alternation in space of equal numbers of positive and negative charges. Although some of the surfactants used posses thermotropic porperties, like examined tridodecylmethylammonium chloride, no thermotropic mesomorphism is detected in solid state of investigated surfactant?Cpicrate compounds. The thermodynamic parameters of solid?Cliquid thermal transitions depend linear on the number of dodecyl chains, and for double- and triple-tailed picrate compounds the marked temporal hysteresis of the melt crystallization is registered.     

Determination of methylmercury fluxes across the air–water and air–soil interfaces by gas chromatography with electron capture detection

A method for the determination of methylmercury (MeHg) fluxes across the air–water and air–soil interfaces was developed using an in situ chamber. The MeHg in the air coming out of the chamber was captured by a column containing sulfhydryl cotton fiber adsorbent. MeHg was then desorbed from the column by using 2 mol L^–1 HCl. The MeHg in the effluent was extracted with benzene, and determined by gas chromatography with electron capture detection. Finally, the MeHg flux was calculated using the chamber. The method was applied to simulated experiments, and the results showed that the MeHg fluxes in the air–water system were higher than those in the air–soil–water system. The method was also successfully applied to the field measurements of an environment polluted by a chemical factory, and the results showed that the MeHg fluxes across the air–soil and air–water interfaces were 0.21–3.09 and 0.14–0.79 ng m^–2 h^–1, respectively. The method will be a useful tool in the environmental study of MeHg.     

Ultrathin dynamic networks formed by the surfactant SPAN 65 at the air–water and oil–water interface

Some surfactants tend to form ultrathin films at the surface of water or at the interface between oil and water. A representative of these surface-active compounds is SPAN 65 (trioctadecanyl ester of sorbic acid). Induced by attractive interactions these molecules can self-associate to form temporary networks. At the planar surface we measured the two-dimensional relaxation modulus, the storage modulus, the loss modulus and the relaxation spectrum. In addition to these measurements, we have also investigated the molecular structure of these networks with “Brewster-angle-microscopy”. The results indicate that temporary cross-linking points, which have lifetimes of the order of a few seconds, tend to stabilize these films. This dynamic network formation is also interesting for numerous technical applications, and it might be used for the preparation of emulsions, foams or microcapsules. Received: 23 October 2000 Accepted: 3 November 2000     

A comparative study of the thermodynamic properties at the air–water interface and in the bulk of structurally related phenothiazine drugs aqueous solutions

Surface tension, conductivity, density, and ultrasound velocity measurements have been performed in order to determine in a systematic manner some of the aggregation properties of the phenothiazine drugs promazine and triflupromazine hydrochlorides. Both drugs are structurally related, differing in an extra CF₃ group in the triflupromazine molecular structure. Surface tension data showed that the presence of an extra CF₃ in the molecular structure of triflupromazine involves a higher hydrophobicity of this drug and a restriction in the number of conformations molecules can adopt due to the presence of this bulkier atomic group. This involves a larger surface area in order to accommodate triflupromazine molecules at the interface. From conductivity measurements at different temperatures, the thermodynamic quantities of the micellization process of these drugs indicate that the aggregation is a spontaneous process, mainly enthalpic, where the London-dispersion forces play an active role. Using density and ultrasound velocity measurements, apparent molar volume and adiabatic compressibility of aqueous solutions of the amphiphilic cationic drugs have been determined. Positive deviations from the Debye–Hückel limiting law of the apparent molar volume were obtained from both drugs over the whole temperature range, which provides evidence of possible pre-association at concentrations below the critical concentration. Apparent molar adiabatic compressibility of the aggregates formed by these drugs was typical of those corresponding for an aggregate formed by a stacking process.     

Interfacial dilational properties of partly hydrolyzed polyacrylamide and gemini surfactant at the decane–water interface

The dilational viscoelastic properties of partly hydrolyzed polyacrylamide (HPAM) and surfactant (C₁₂COONa-p-C₉SO₃Na) in the absence or presence of electrolyte were investigated at the decane–water interface by means of longitudinal method and the interfacial tension relaxation method. The polymer plays different roles in influencing the structure of HPAM–surfactant mix-adsorbed layer at different surfactant concentration. At low surfactant concentration, the addition of polymer could sharply decrease the dilational elasticity mainly due to the weakening of the “entanglement” among long alkyl chains in surfactant molecules, while the addition of the polymer may enhance the dilational elasticity due to the slow diffusivity of the polymer chains at higher surfactant concentration. And the added electrolyte, which results in screening of electrostatic interactions between the ionized groups, generally decreases the interfacial dilational elasticity and increases the dilational viscosity. The data obtained on the relaxation processes via interfacial tension relaxation measurement can explain the results from dilational viscoelasticity measurements very well.     

Dilational viscoelasticity of anionic polyelectrolyte/surfactant adsorption films at the water–octane interface

In this article, the interfacial tension and interfacial dilational viscoelasticity of polystyrene sulfonate/surfactant adsorption films at the water–octane interface have been studied by spinning drop method and oscillating barriers method respectively. The experimental results show that different interfacial behaviors can be observed in different type of polyelectrolyte/surfactant systems. Polystyrene sulfonate sodium (PSS)/cationic surfactant hexadecanetrimethyl–ammonium bromide systems show the classical behavior of oppositely charged polyelectrolyte/surfactant systems and can be explained well by electrostatic interaction. In the case of PSS/anionic surfactant sodium dodecyl sulfate (SDS) systems, the coadsorption of PSS at interface through hydrophobic interaction with alkyl chain of SDS leads to the increase of interfacial tension and the decrease of dilational elasticity. For PSS/nonionic surfactant TX100 systems, PSS may form a sub-layer contiguous to the aqueous phase with partly hydrophobic polyoxyethylene chain of TX100, which has little effect on the TX100 adsorption film and interfacial tension.     

Vibrational sum-frequency generation at protein modified air–water interfaces: Effects of molecular structure and surface charging

Protein and surfactant modified air–water interfaces are an important model system for colloid science as many applications for example aqueous foams in food products rely on our knowledge and ability to tune molecular structures at these interfaces. That is because interfaces are a fundamental building block in the hierarchical structure of foam, where in fact the molecular level can determine properties on larger length scales. For that reason it is of great importance to increase our ability to study air–water interfaces with molecular level probes and to obtain not only information on coverage but also direct information on interfacial composition, molecular order, orientations as well as information on the charged state of an interface. Vibrational sum-frequency generation (SFG) is a powerful tool that can help to address these issues and is inherently surface sensitive. In this contribution we will review recent developments in the use of SFG for studies of biomolecules at aqueous interfaces and discuss current issues with the interpretation of SFG spectra from electrified interfaces. In order to guide interpretations from interface spectroscopy we invoke the use of complementary methods such as ellipsometry and zetapotential measurements of bulk molecules.     

Theoretical advances in the dissolution studies of mineral–water interfaces

This article presents theoretical advances in computational modeling of dissolution at mineral–water interfaces with specific emphasis on silicates. Two different Monte Carlo methods have been developed that target equilibrium properties and kinetics in silicate–water dissolution. The equilibrium properties are explored using the combined reactive Monte Carlo and configurational bias Monte Carlo (RxMC-CBMC) method. The new RxMC-CBMC method is designed to affordably simulate the three-dimensional structure of the mineral with explicit water molecules. The kinetics of the overall dissolution process is studied using a stochastic kinetic Monte Carlo method that utilizes rate constants obtained from accurate ab initio calculations. Both these methods provide important complementary perspective of the complex dynamics involving chemical and physical interactions at the mineral–water interface. The results are compared to experimental and previous computational data available in the literature.     

Bulk properties and hydration numbers of components of water–salt,water–urea,and water–urea–salt systems

With an increase in the concentration of additives, the hydration numbers of compounds decrease. Thus, in a saturated 54.6% solution, urea loses approximately 3/4 of the initial amount of water, forming an aquacomplex of the composition (NH₂)₂CO?H₂O. In a supersaturated 44% solution, the sodium chloride aquacomplex is dehydrated by 2/3, and in a supersaturated 67% solution, sodium sulfate is dehydrated by 5/6. The density of these solutions is 1.354÷1.360 g/cm³ (44% NaCl) and 1.800÷1.849 g/cm³ (67% Na₂SO₄). In a saturated urea solution, NaNO₃, NaCl, and Na₂SO₄ complexes lose 53÷55% of hydration water. It is shown that the interactions in the binary water–urea system somewhat increase the hydration number of the salts (structural hydration). The hydration water density, a structurally important characteristic, increases in the series of solutions of urea, NaNO₃, NaCl, and Na₂SO₄. In the same series of additives, the excess volume of binary water–urea and water–salt systems becomes more negative.     

Mutual influence of cetyltrimethylammonium bromide and Triton X-100 on their adsorption at the water–air interface

On the basis of surface tension values of the aqueous solution of cetyltrimethylammonium bromide (CTAB) and Triton X-100 (TX-100) mixtures measured at 293 K as a function of CTAB or TX-100 concentration at constant TX-100 or CTAB concentration, respectively, the real surface area occupied by these surfactants at the water–air interface was established which is inaccessible in the literature. It appeared that at the concentration of the CTAB and TX-100 mixture in the bulk phase corresponding to the unsaturated monolayer at the water air-interface this area is the same as in the monolayer formed by the single surfactant at the same concentration as in the mixture. In the saturated mixed monolayer at this interface the area occupied by both surfactants is lower than that in the single surfactant monolayer corresponding to the same concentration in the aqueous solution. However, the decrease of the CTAB adsorption is lower than that of TX-100 and the total area occupied by the mixture of surfactants is also lower than that of the single one. The area of particular surfactants in the mixed saturated monolayer changes as a function of TX-100 and CTAB mixture concentration and at the concentrations close to CMC or higher the area occupied by both surfactants is the same. The changes of the composition of the mixed surface monolayer are connected with the synergetic effect in the reduction of the water surface tension by the adsorption of CTAB and TX-100 at the water–air interface. This effect was confirmed by the values of the standard Gibbs free energy of adsorption of both individual surfactants and their mixtures with different compositions in the bulk phase determined by using the Langmuir equation if RT instead of nRT was applied in this equation.     

Dissolution of zinc oxide in a protic ionic liquid with the 1-methylimidazolium cation and electrodeposition of zinc from ZnO/ionic liquid and ZnO/ionic liquid–water mixtures

We show that zinc oxide can be dissolved in the protic ionic liquid 1-methylimidazolium trifluoromethylsulfonate, [MIm]TfO at quite a high concentration (~ 2.5 mol/L). FTIR and Raman spectra revealed the association of zinc ions with 1-methylimidazole. The ZnO/[MIm]TfO solutions and their mixtures with water were employed as electrolytes for the electrodeposition of zinc. High current density electrodeposition of zinc can be achieved in the employed electrolytes. Spongy-like zinc structures with a high porosity were obtained in ZnO/[MIm]TfO and the formation of Au_1.2Zn_8.8 alloy was observed. Compact and hexagonal zinc deposits were found in the presence of water. The present results show the potential of ionic liquids as electrolytes for rechargeable zinc–air batteries.     

Synthesis of amphiphilic V-type silica nanogels and study of their self-assembling at the air–water interface

The methods for the synthesis of silica nanogels and a modifying agent, as well as related amphiphilic V-type silica nanogels are presented. Self-assembly of the synthesized amphi philic nanogels during the formation of Langmuir monolayers at the air–water interface is considered. Aggregate structures were revealed to form during ordering of the silica V-type nanogels at the air–water interface after collapse of the Langmuir monolayer. For the low-molecular-weight fraction of the silica nanogels, the aggregates do not completely decompose upon the expansion of the Langmuir layer since are formed by mutually penetrating macromolecules. For the highmolecular- weight fraction, they are reversibly formed and decompose in consecutive compression– expansion cycles, which is characteristic of Langmuir layers of nanoparticles.     

The shear hysteresis in lamellar structure of surfactant–water binary system

We study the shear effect on the lamellar structure of surfactants in water using dissipative particle dynamics simulations. Starting from a lamellar structure without shear flow, we increase the shear rate and then decrease it stepwisely. A weak shear changes the lamellar plane to be parallel to the shear direction though the lamellar normal has no specific direction on the plane normal to the shear direction. By increasing the shear rate, the lamellar normal eventually flips to the vorticity direction regardless of the initial configuration. Lamellar normal would stay along the vorticity direction on decreasing the shear rate. The hysteresis is also found in shear-stress. By varying the shear rate, the time needed to reach the final unique state is significantly shortened compared with that observed with a constant shear rate. We find a correlation between the excess shear-stress and the tilt angle of surfactant in lamellar.