Interface Of Cyclohexane Research Articles

The monotonicity behavior of density profiles at vapor-liquid interfaces of mixtures

In their seminal monograph ’Molecular Theory of Capillarity’, Rowlinson and Widom describe different possible shapes of density profiles at the vapor-liquid interface of mixtures. They postulated that in some instances, density profiles could possibly be non-monotonic, exhibiting either a maximum and/ or a minimum. This contribution revisits this statement in the light of four decades of posterior research. We summarize the distinct morphologies at the vapor-liquid interface suggested in the literature recognizing that the condition of a single minimum in the profile has not yet been reported. Interfacial density profiles with a single maximum as well as fully monotonic density profiles have been observed and reported extensively. The case of a simultaneous maximum and minimum is more controversial, as it has only been predicted using theoretical approaches such as density gradient theory (DGT). This ambiguity is further investigated in this work using the example of the vapor-liquid interface of cyclohexane + butanol. Both DGT in combination with several distinct equations of state and molecular dynamics simulations are used. The results from the two methods are found to be contradictory: while the DGT results predict a maximum/minimum structure, the computer experiment results indicate only a single maximum in the density profiles. This work thereby emphasizes that results from DGT for highly non-ideal mixtures should not be taken for granted.

Water to cyclohexane transfer free energy calculations for a carbon nanotube

We calculate the difference in the solvation free energy of a carbon nanotube in water and cyclohexane and introduce two methods to perform these calculations. One is to use thermodynamic integration and gradually decouple the interactions of the nanotube with these solvents. The other is to build a water–cyclohexane system and create a transition path for the nanotube on a direction normal to the water–cyclohexane interface, and employ umbrella sampling simulations on the path and calculate the free energy difference between the states. By combining the solvation free energies of the nanotube computed by thermodynamic integration, we infer correlations with the free energies of transfering the nanotube directly from the one solvent to the other, computed by umbrella simulations. The correlations depend on how many molecules of the one solvent partition into the other together with the nanotube. We measure this amount by averaging the water accessible surface area of the nanotube over the umbrella sampling simulations. When the direction is from cyclohexane to water, cyclohexane molecules are coextracted with the nanotube into the water phase. These molecules remain adsorbed inside the pore, even when the nanotube enters adequately the aqueous solution. The adsorbed phase of cyclohexane configures an almost complete monolayer over the internal surface of the nanotube making it inaccessible to water.

Controllable self-assembly of NaREF4 upconversion nanoparticles and their distinctive fluorescence properties

The paper presents the growth of hexagonal NaYF4:Yb3+, Tm3+ nanocrystals with tunable sizes induced by different contents of doped Yb3+ ions (10%–99.5%) using the thermal decomposition method. These nanoparticles, which have different sizes, are then self-assembled at the interface of cyclohexane and ethylene and transferred onto a normal glass slide. It is found that the size of nanoparticles directs their self-assembly. Due to the appropriate size of 40.5 nm, 15% Yb3+ ions doped nanoparticles are able to be self-assembled into an ordered inorganic monolayer membrane with a large area of about 10 × 10 μm2. More importantly, the obvious short-wave (300–500 nm) fluorescence improvement of the ordered 2D self-assembly structure is observed to be relative to disordered nanoparticles, which is because intrinsic absorption and scattering of upconversion nanoparticles leads to the self-loss of fluorescence, especially the short-wave fluorescence inside the disordered structure, and the relative emission of short-wave fluorescence is reduced. The construction of a 2D self-assembly structure can effectively avoid this and improve the radiated short-wave fluorescence, especially UV photons, and is able to direct the design of new types of solid-state optical materials in many fields.

Tunable Pickering emulsions with polymer-grafted lignin nanoparticles (PGLNs)

Lignin is an abundant biopolymer that has native interfacial functions but aggregates strongly in aqueous media. Polyacrylamide was grafted onto kraft lignin nanoparticles using reversible addition–fragmentation chain transfer (RAFT) chemistry to form polymer-grafted lignin nanoparticles (PGLNs) that tune aggregation strength while retaining interfacial activities in forming Pickering emulsions. Polymer graft density on the particle surface, ionic strength, and initial water and cyclohexane volume fractions were varied and found to have profound effects on emulsion characteristics, including emulsion volume fraction, droplet size, and particle interfacial concentration that were attributed to changes in lignin aggregation and hydrophobic interactions. In particular, salt concentration was found to have a significant effect on aggregation, zeta potential, and interfacial tension, which was attributed to changes in solubility of both the kraft lignin and the polyacrylamide grafts. Dynamic light scattering, UV–vis spectroscopy, optical microscopy, and tensiometry were used to quantify emulsion properties and nanoparticle behavior. Under all conditions, the emulsions exhibited relatively fast creaming but were stable against coalescence and Ostwald ripening for a period of months. All emulsions were also oil-in-water (o/w) emulsions, as predicted by the Bancroft rule, and no catastrophic phase inversions were observed for any nanoparticle compositions. We conclude that lower grafting density of polyacrylamide on a lignin core resulted in high levels of interfacial activity, as characterized by higher concentration at the water–cyclohexane interface with a corresponding decrease in interfacial tension. These results indicate that the interfacial properties of polymer-grafted lignin nanoparticles are primarily due to the native hydrophobic interactions of the lignin core. These results suggest that the forces that drive aggregation are also correlated with interfacial activities, and polymer–nanoparticle interactions are critical for optimizing interfacial activities. Controlled radical polymerization is a powerful tool for polymer grafting that can leverage the intrinsic interfacial functions of lignin for the formation of Pickering emulsions.

Liquid organization and solvation properties at polar solid/liquid interfaces

Second order nonlinear optical spectroscopy has been employed to examine the organization of four different liquids at the hydrophilic silica/liquid interface. The liquids - cyclohexane, methylcyclohexane, 1-propanol, and 2-propanol - were chosen to isolate how intermolecular forces between the liquid and the substrate competed with steric effects to control liquid structure and solvating properties across the interfacial region. Vibrational sum frequency generation (VSFG) data showed that cyclohexane structure at the silica/liquid cyclohexane interface closely resembled the structure of a cyclohexane monolayer adsorbed to the silica/vapor interface. Methylcyclohexane, however, showed evidence of large structural reorganization between the silica/liquid and silica/monolayer/vapor interfaces. 1-Propanol at a silica/vapor interface formed a well-ordered, Langmuir-like monolayer due to strong hydrogen bonding with the surface silanols and cohesive van der Waals interactions between carbon chains. 1-Propanol at the silica/liquid interface retained the same ordered structure. In contrast, 2-propanol adopted different structures adsorbed to the solid/vapor and at the solid/ liquid interfaces. Specifically, the plane defined by 2-propanol's three carbon atoms changed orientation from being perpendicular to the surface (silica/vapor) to parallel to the surface (silica/liquid). Surface mediated liquid structure affected the solvation of adsorbed solutes. Resonance enhanced second harmonic generation (SHG) data showed that silica/alkane interfaces were significantly more polar than would be expected based on a solute's bulk solution solvatochromic behavior. Both silica/alcohol interfaces exhibited alkane-like polarity, a result that was interpreted in terms of a reduction in hydrogen bonding opportunities for adsorbed solutes.

Methyl Trichlorosilane Prepared Methyl Sodium Silicate

This paper studied how to prepare Methyl Sodium Silicate from Methyl Trichlorosilane. Methyl trichlorosilane hydrolyzed on the interface of cyclohexane and water, then hydrolysate and sodium hydroxide prepared methyl sodium silicate at molten state. Manufactrue was characterised by XRD, IR, and differential thermal analysis. The experimental results indicate that the best hydrolyze condition was the proportion of methyl trichlorosilane and water was 10~12:100 , at 5°C, lasting for 45min; the best condition of prepared methyl sodium silicate was the proportion of sodium hydroxide and hydrolysate was 2.1~2.3:1, at 300 ~450°C, lasting for 50min. The manufacture was white powder, water-solubility and well stabilization bellow 450°C.

Adsorption and coalescence in mixed surfactant systems: Water−hydrocarbon interface

Major commercial uses of surfactants involve a mixture of these compounds. The properties of the monolayer formed at the water−hydrocarbon interface depend upon the interaction between the surfactant molecules. The presence of salt influences the interactions between the surfactants in the monolayer and in the micelle. In this work, these interactions in presence and in absence of a 1:1 salt were studied and the results were analyzed using the nonideal solution theory. An anionic, a cationic and a nonionic surfactant were used. The stability of emulsions is significantly affected by the interaction between the surfactants. To study this effect, coalescence of cyclohexane drops at a flat water−cyclohexane interface was studied in the mixed surfactant systems, and the results were compared with single surfactant systems. The effect of salt on coalescence time was studied. The results were analyzed using the film drainage and stochastic theories of coalescence. Seven film drainage models were used to predict the coalescence time, and the values predicted by these models were compared with the experimental data. The parameters of the stochastic model were analyzed based on the properties of the systems.

Effect of Organized Assemblies, Part VII: Adsorption Behavior of Polyoxyethylated Nonyl Phenol at Silica−Cyclohexane Interface and Its Efficiency in Stabilizing the Silica−Cyclohexane Dispersion

The adsorption behavior of a series of nonionic polyoxyethylated nonyl phenol surfactants having same hydrocarbon chain length but varying oxyethylene chain has been studied spectrophotometrically at the silica−cyclohexane interface. The adsorption density is found to be independent of the number of oxyethylene units throughout the adsorption isotherms, except at higher surface coverage, where the adsorption density decreases with the increase in the number of oxyethylene units. The settling rate of the silica particles in cyclohexane decreases with the increase in adsorption density as well as oxyethylene chain length. On the other hand, the sediment volume decreases with the increase in the adsorption density, with a minimum for the surfactant having the least number of oxyethylene units. Surfactants with a longer oxyethylene chain partitions preferentially to the bulk solvent at higher concentrations, as a result of the change in the polarity of the medium. On the basis of adsorption density, settling ...

Properties of a two-dimensional asphaltene network at the water-cyclohexane interface deduced from dynamic tensiometry.

Static and dynamic tensiometries show that a newly prepared water/asphaltenated cyclohexane interface behaves as expected: the mean area occupied per asphaltene molecule is 2 nm2, and variations of interfacial tension and dilatational elastic modulus with time indicate that equilibrium is reached more slowly than that for usual surfactants. The use of the time/temperature superposition principle allows a detailed rheological study of a 2 day old interface of the same type which has reached equilibrium. It is found that the two-dimensional asphaltene network exhibits a glass transition zone, behaves as a gel near its gelation point, and is built by a universal process of aggregation.

Effect of Temperature on the Mobility of Nitroxide Probes in Cyclohexane and at the Alumina–Cyclohexane Interface

Nitroxide spin probes have been widely used to study the behavior of surfactants in solution as well at solid–liquid interfaces. In this study the effect of temperature on the mobility of nitroxide spin probes in cyclohexane and at the alumina/cyclohexane interface is investigated in the presence of anionic aerosol-OT (AOT). The probe mobility in AOT micelles in solution is markedly dependent on the amount of solubilized water. The variation with temperature of the mobilities of adsorbed doxyl stearic acid with the nitroxide probe at the 5th, 10th, and 16th positions from the –COOH group has shed some light on the mechanisms of interaction of the probe with the solid surface as well as the molecular structure of the adsorbed layer. The results indicate that the adsorption of the probe occurs mainly through interactions of the –COOH group with the surface while the –NO group interacts only weakly with the surface. The adsorption of AOT causes changes in probe mobility with the changes being most significant when the nitroxide group on the stearic acid chain is farthest from the anchoring group. It is suggested that the adsorption of the surfactant causes an orientational rearrangement of the probe molecules. The relatively low-temperature effect on the probe mobility observed in the presence of surfactant is attributed to the compactness of the adsorbed layer which is found to be stable even at elevated temperatures.

Wetting and layering in critical binary fluid mixtures

We present an ellipsometric study of wetting and adsorption at the liquid–vapor interface of cyclohexane–methanol and cyclohexane–deuterated methanol systems. Due to the first-order character of the wetting transition and the hysteresis associated with it, either a very thin or a thick film of the heavy phase intrudes between the lighter phase and the vapor. In the cyclohexane–methanol mixture, the thin film grows in layers with a thickness on the order of the molecular diameter rather than continuously; a series of first-order layering transitions takes place at the interface. In the deuterated system, no clear layering effects are observed. As the hydrogen bonds in the latter system are somewhat weaker, this suggests that hydrogen bonding plays a role in the observed layering effects. Measurements above the critical temperature TC allow for the conclusion that the thin film corresponds to the critical adsorption state. Residual layering effects may persist above TC for the cyclohexane–methanol system.

Transient effects in liquid—liquid interface motions through glass capillaries

We present an experimental study of the nonstationary motion of a water—cyclohexane interface in millimiter—size glass capillary tubes initially saturated with cyclohexane. Starting from an initially high value, the interface velocity is observed to relax exponentially toward an asymptotic steady motion. At a given applied counterpressure, the time variation of the interface velocity follows a law independent of the experimental procedure. These effects are particularly strong at very low capillary numbers between 3 × 10 −9 and 5 × 10 −7; the relaxation time constant increases roughly as the inverse of the final interface velocity in this range and reaches values of several thousand seconds at the lowest velocities. We suggest that these slow relaxations are due to adsorption sites with a wide range of characteristic times: decreasing the velocity increases the number of sites active for adsorption and enhances the wetting of the high-energy glass surface by water.

Light scattering at the solid-liquid interface of cyclohexane

Anomalous Rayleigh scattering in the interface layer of cyclohexane crystals has been studied during crystal growth. For the first time we have observed anomalous Rayleigh scattering in the interface layer during melting. Cyclohexane has been chosen because the molecule has no permanent dipole moment. Thus all models based on the existence of long range dipolar interaction to describe these light scattering phenomena can be eliminated. A new crystal growth geometry has been developed to eliminate any possibility of contamination. A liquid sphere inside of a multiple zone refined single crystal is melted. The same light scattering phenomena are observed during inward growth, which we know from experiments during zone refining. Light scattering is also observed during melting of the solid. The solid is not superheated and thus no instabilities can occur at the interface. Thus it is concluded that light is not scattered by surface corrugations, small scale convections or by segregated impurities. Quasielastic light scattering techniques have been combined with elastic light scattering measurements in our studies of the freezing process. Model calculations for the angular dependence of the intensities of the scattered light have been compared with the experimental data. Calculations have been performed on the basis of 3 models: Mie theory for spherical particles, continuous fluctuations with an Ornstein-Zernike type pair correlation function and continuous fluctuations with a Debye-Bueche type pair correlation function. Only continuous fluctuations are compatible with the experiment. Thus models based on the assumption of gas bubbles or crystallites in front of the crystal-liquid interface can also be eliminated. The experimental results with cyclohexane are compatible with earlier experiments with H 2O and salol, indicating that the interface layer is a general phenomenon of growing dielectric crystals.