Consolute Temperature Research Articles

Molecular dynamics of interfaces in opposing fields

Equilibrium binary liquid mixture below its upper critical consolute temperature, confined between opposing walls, is investigated by molecular dynamics simulations. The wall potentials are chosen strong enough to ensure that both walls are wet and therefore the interface is delocalized. The interfacial widths (W) are determined from density profiles and from order parameter profiles for a series of wall to wall distances (D). Early saturation of the width with increasing D is found already for D∼20σ and data at the other end are consistent with the theoretical prediction of linear dependence of W2 on D. Also, the error function complement is found slightly superior to the hyperbolic tangent, on the basis of a new test of consistency proposed.

Quenching of capillary waves in composite wetting films from a binary vapor: An x-ray reflectivity study

We report x-ray reflectivity measurements of the internal structure of thin wetting films formed on a silicon substrate from the vapor of a binary mixture of methylcyclohexane (MC) and perfluoromethylcyclohexane (PF). At $T=30\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ (below the bulk consolute temperature of ${T}_{c}\ensuremath{\approx}46\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$ variation in the difference in temperature between the substrate and the vapor induces changes in film thickness (25 to $135\AA{})$ that are consistent with complete wetting of both surfaces, with a MC-rich phase wetting the substrate and a PF-rich phase wetting the free surface. The width of the internal liquid/liquid interface, however, is found to be noticeably smaller than values predicted from the convolution of the intrinsic interfacial profile between bulk liquids with the capillary wave roughness as modified by dispersion interactions with the substrate. The difference is attributed to the effects of confinement. Although the width of the liquid/liquid interface is less well defined above the consolute point $(T=60\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}),$ we still find PF enrichment at the free surface and MC enrichment at the substrate.

Shear viscosity studies above and below the critical consolute point in a nitrobenzene-decane mixture

The shear viscosity has been studied in a nitrobenzene-decane critical mixture above the critical consolute temperature T(C), in the homogeneous phase, and below T(C), in coexisting phases. The form of background viscosity for coexisting phases has been postulated. The same value of the critical exponent straight phi has been obtained in the lower (L), upper (U), and homogeneous (H) phases. The pretransitional amplitudes (A(L,U)) in coexisting phases are approximately the same, whereas A(L,U)/A(H) approximately 0.965. In the homogeneous phase the possibility of the appearance of the quasinematic, field-induced structure of critical fluctuations has been discussed.

Nonionic Cellulose Ethers as Potential Drug Delivery Systems for Periodontal Anesthesia

Nonionic cellulose ethers displaying a lower consolute temperature, or cloud-point, close to body temperature were investigated as potential carrier systems for the delivery of local anesthetic agents to the periodontal pocket. The interaction between the polymers, i.e., ethyl(hydroxyethyl)cellulose (EHEC) and hydrophobically modified EHEC (HM-EHEC), and ionic surfactants was determined in the absence and in the presence of the local anesthetic agents lidocaine and prilocaine. The cloud-point and rheology data indicate interactions between the polymer and both anionic and cationic surfactants. More precisely, a number of ionic surfactants were found to result in an increase in cloud-point at higher surfactant concentrations, a surfactant-concentration-dependent thickening, and a temperature-induced gelation upon heating. Upon addition of the local anesthetic agents lidocaine and prilocaine in their uncharged form to EHEC and HM-EHEC, in the absence of surfactants, only minor interaction with the polymer could be inferred. However, these substances were found to affect the polymer–surfactant interaction. In particular, the drug release rate in vitro as well as the stability and temperature-dependent viscosity were followed for an EHEC/SDS system and EHEC/myristoylcholine bromide system upon addition of lidocaine and prilocaine. The data indicate a possibility of formulating a local anesthetic drug delivery system suitable for administration into the periodontal pocket where at least small amounts of active ingredients can be incorporated into the system without severely affecting the gelation behavior. The results found for the cationic myristoylcholine bromide system are particularly interesting for the application in focus here since this surfactant is antibacterial and readily biodegradable.

Interfacial and Monolayer Properties of Poly(vinylcaprolactam)

The surface properties of a polydisperse poly(vinycaprolactam) (48 kDa) have been studied at the air/water and hexane/water interfaces. Adsorption of the polymer at both interfaces shows substantial time-dependent surface tension lowering at remarkably low concentrations in the aqueous phase. This adsorption is increased by addition of electrolyte, which also lowers the lower consolute temperature (LCT) of the polymer in solution. Adsorption is endothermic, corresponding with the approach to the LCT as the temperature is raised. Spreading of the polymer at the air/water interface from aqueous solutions is near quantitative. The surface pressure−area isotherms indicate extensive coiling in the monolayer at low pressures and looping of the polymer monolayer into the aqueous phase in the higher pressure range.

Study of Phase Behavior of Poly(ethylene glycol)–Polysorbate 80 and Poly(ethylene glycol)–Polysorbate 80–Water Mixtures

Mixtures of poly(ethylene glycols) (PEGs) with polysorbate 80 are often used to dissolve poorly water‐soluble drugs in dosage forms, where polysorbate 80 helps either in enhancing dispersion or in inhibiting precipitation of drugs once the solution is mixed with water. Binary phase diagrams of polysorbate 80 with several low molecular weight PEGs and a ternary phase diagram of polysorbate 80 with PEG 400 and water are presented. Two phases were observed in the binary mixtures when the concentration of PEG 200, PEG 300, PEG 400, or PEG 600 was >55%(w/w). The miscibility of the binary mixtures increases with an increase in temperature; the upper consolute temperatures of PEG 200–polysorbate 80, PEG 300–polysorbate 80, PEG 400–polysorbate 80, and PEG 600–polysorbate 80 mixtures were 100, 85, 75, and 40 °C, respectively. The upper consolute temperature of PEG 1000–polysorbate 80 could not be determined because the melting temperature of the mixtures is ∼40 °C and the consolute temperature appeared to be less than this temperature. The decrease in upper consolute temperature with an increase in PEG molecular weight indicated a greater miscibility of the two components. In the ternary system, phase separation of polysorbate 80 was observed when the concentration of PEG 400 was >50–60 % (w/w), possibly because of the high exclusion volume of PEG 400. © 2000 Wiley‐Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 89:946–950, 2000

Isotope effect on the coexistence curve and crossover behavior of water+tetrahydrofuran

Liquid–liquid coexistence curves of the system (tetrahydrofuran+water) and the pseudo-binary system (tetrahydrofuran+deuterated tetrahydrofuran+heavy water) were measured in the reduced temperature range 3×10 −5< τ<7×10 −2 from their lower critical points. The composition of the deuterated system was adjusted to get the critical temperature close to that of the normal binary mixtures. This ensures that the effect of the upper consolute temperatures is the same in both cases, thus facilitating data interpretation. The refractive index and the Lorentz–Lorenz function were chosen as composition variables for analyzing the order parameter. For τ<10 −3 both mixtures exhibit pure Ising behavior. The leading critical amplitude increases in the deuterated mixture. The crossover behavior is found to be sensitive to changes in deuteration.

The solubility loop of nicotine:water

The lower consolute temperature in the nicotine:water system arises from the equilibrium formation of a covalent hydrate, revealed by its unique circular dichroism.

Calculation of metastable immiscibility region in the Al2O3–SiO2system using molecular dynamics simulation

The metastable immiscibility region in the Al2O3–SiO2system was calculated by conventional thermodynamic equations using thermodynamic parameters obtained from molecular dynamics simulation. The calculated miscibility gap has a consolute temperature of around 1500 °C at the critical composition of about 20 mol% Al2O3and spreads more widely towards the Al2O3-rich composition side than the SiO2-rich side. The calculated miscibility gap in this study showed a fair agreement with that reported by Banet al.[T. Ban, S. Hayashi, A. Yasumori, and K. Okada, J. Mater. Res.11,1421 (1996)] calculated by a regular solution model, but the present calculated region is somewhat narrower in the Al2O3-rich composition side than that reported by Banet al.

Physical Chemistry of Organic Eutectic and Monotectic: Hexamethylbenzene−Succinonitrile System

A phase diagram of hexamethylbenzene−succinonitrile system, determined by the thaw−melt method, shows the formation of a eutectic and a monotectic with large miscibility gap in the system. The eutectic and the monotectic contain 0.150 and 0.996 mol fraction of succinonitrile, respectively, and the consolute temperature is 111.5 °C above the monotectic horizontal. Growth behavior of the eutectic, the monotectic, and the pure components, studied at different undercooling temperatures by measuring the rate of movement of solid−liquid interface in a capillary, suggests that the data obey the square relationship between growth velocity and the undercooling. Using enthalpy of fusion data, determined by the DSC method, entropy of fusion, enthalpy of mixing, size of the critical nucleus, interfacial energy, and excess thermodynamic functions were calculated. While the microstructures of the pure components, namely, hexamethylbenzene and succinonitrile show faceted and nonfaceted morphology, respectively, those of the eutectic and the monotectic give lamellar structures.

Liquid–liquid equilibria for the ternary system water+ n-dodecane+2-(2- n-hexyloxyethoxy)ethanol

Experiments of the fish-shaped phase diagram for the ternary system water+ n-dodecane+2-(2- n-hexyloxyethoxy)ethanol (abbreviated by C 6E 2 hereafter) under atmospheric pressure were performed at constant water/ n-dodecane weight ratio (1/1) to locate the critical end points. The upper and lower critical consolute temperatures for the system of interest are 307.80 K and 282.30 K, respectively. Compositions of two- and three-phase liquid–liquid equilibrium for the ternary system water+ n-dodecane+C 6E 2 at 293.15 K and 303.15 K under atmospheric pressure are presented in this paper.

Solidification behaviour of binary organic eutectics and monotectics; 1,2,4,5-tetrachlorobenzene– m-aminophenol system

The phase-diagram of the 1,2,4,5-tetrachlorobenzene– m-aminophenol system studied by the thaw–melt method shows the formation of a eutectic at 120.0°C (0.946 mole fraction of m-aminophenol) and a monotectic at 136.0°C (0.094 mole fraction of m-aminophenol) with a consolute temperature at 177.0°C. The growth kinetic studies at different undercoolings (Δ T) by the capillary method give straight lines for the plots between log v and log Δ T where v is the rate of movement of solid–liquid interface. From the enthalpy of fusion of the pure components, the eutectic and the monotectic determined by the DSC method, different thermodynamic parameters such as enthalpy of mixing, excess thermodynamic functions and entropy of fusion were calculated. While the microstructure of the pure components give their faceted growth, the eutectic and the monotectic show their characteristic features.

Suppression of the rate of hydrolysis of t-amylchloride at the consolute composition of isobutyric acid+water

We have measured the rate of hydrolysis of t-amylchloride in the solvent, isobutyric acid+water, prepared at its consolute composition. The consolute temperature of the mixture depends upon the initial amount of t-amylchloride, being 32.3°C when 100 μl are added to 25 ml of the solvent and 36.46°C when 150 μl are added. In both cases, however, we found that the specific rate of reaction is suppressed over a temperature range of about 0.2°C on either side of the local consolute temperature, T c. These observations serve to demonstrate the concept of `critical slowing down' in chemical kinetics. The decrease in specific rate with temperature, T, could be fitted by | T− T c| − x with x=0.63±0.28. Within the experimental error, this value of the exponent is consistent with the value ( x=0.80) predicted theoretically on the basis of the principle of universality.

Critical Slowing Down of Chemical Reactions in Liquid Mixtures

We have observed suppression in the specific rate of hydrolysis of five organic halides in three water-based liquid mixtures near their respective equilibrium consolute points. The systems examined were tert-amyl chloride in isobutyric acid + water, tert-butyl chloride in isobutyric acid + water, tert-butyl bromide in triethylamine + water, 3-chloro-3-methylpentane in 2-butoxyethanol + water, and 4-methylbenzylbromide in 2-butoxyethanol + water. The first two have upper consolute temperatures, while the second three have lower consolute temperatures. The slowing down effect occurred within a few tenths °C on either side of the consolute temperature. In the case of tert-amyl chloride in isobutyric acid + water, the effect followed a shift in consolute temperature produced by a change in the initial concentration of tert-amyl chloride. This indicates that all points along the critical line are equivalent. These observations demonstrate the existence of critical slowing down of chemical reaction rates in liquid mixtures.

Physical chemistry of the organic analog of metal–metal eutectic and monotectic alloys

The phase diagram of an organic analog of a metal–metal system involving succinonitrile (SCN) and carbontetrabromide (CTB) shows the formation of a eutectic (0.996 mole fraction of SCN) and a monotectic (0.040 mole fraction of SCN) with a wide range of miscibility gap in the system. The consolute temperature is 111.5°C above the monotectic horizontal. The growth data, determined by measuring the rate of movement of solid–liquid interface in a capillary, at different undercooling (Δ T) suggest that they obey the parabolic law, v= u(Δ T) n , where v is the linear velocity of crystallization and u and n are constants depending on the nature of materials involved. Using enthalpy of fusion of the pure components, the eutectic and the monotectic, entropy of fusion, Jackson's roughness parameter, interfacial energy, size of the critical nucleus and excess thermodynamic functions were calculated. While microstructural investigations of pure components give dendritic microstructures, those of eutectic and monotectic give characteristic lamellar structures.

Phase diagram and thermochemical properties of organic eutectic in a monotectic system

The phase diagram of a binary organic system involving diphenyl and succinonitrile shows the formation of a eutectic (0·968 mole fraction of succinonitrile) and a monotectic (0·074 mole fraction of succinonitrile) with a large miscibility gap in the system, the upper consolute temperature being 53·5°C above the monotectic horizontal. From the enthalpy of fusion of the pure components, the eutectic and the monotectic, determined by the DSC method, the enthalpy of mixing, Jackson’s roughness parameter, interfacial energy, size of the critical nucleus and excess thermodynamic functions were calculated.

Experimental study of stability in a layer of a model alloy with a liquid miscibility gap

The stability of Rayleigh-Benard convection in a transparent model alloy was investigated with holographic interferometry. The dispersed minority phase is the heavier component. The majority component is lighter. A vertical temperature difference was applied to the liquid layer keeping the hot temperature always above phase separation. The upper cold side temperature was kept above or below the phase separation temperature. Fluid separation occurs when the cold temperature drops below consolute temperature. When the liquid is kept at higher than consolute temperature, it is noticed that the stability of the layer is increased when the average temperature approaches the consolute temperature.

Surface and colloid properties of cyclic amides II. phase separation, surface activity and micellization in mixtures of N-hexyl-2-pyrrolidone and water

N-hexyl-2-pyrrolidone (HP) and water are partially miscible above a lower consolute temperature (LCT) of 19.1°C. The interfacial tension of the two phase system was measured as a function of temperature. HP is surface active at the air/solution interface and forms micelles in solution at all temperatures from the freezing point up to the LCT and up to 25°C in the water-rich phase above the LCT. At low water contents, water is positively adsorbed at the air/solution interface.

Structural changes of the molecular complexes and consolute phenomena in binary liquid mixtures

AbstractConsolute phenomena in the aqueous solutions of the polymers are considered In view of the temperature induced structural changes of the hydrogen bonds between water and functional groups of polymer. The lower and upper critical consolute points are attributed to the appearance of the “critical” concentration of the complexes with one hydrogen bond between single water molecule and functional group of polymer. Namely such kind of the hydrogen bonds are responsible for the formation of the strongly associated water clusters, that may be followed by phase separation. Experimentally observed dependences of the critical consolute temperatures for the aqueous solutions of polyethylene glycol on the molecular weight of polymer and adding of salts are well reproduced in the framework of the proposed model.

Wetting observed by evanescent-wave-generated fluorescence spectroscopy III: Fluorescence lifetimes: n-Heptane+perfluorohexane at the upper critical endpoint

The technique of evanescent-wave-generated-fluorescence spectroscopy EWGFS has been applied here to the investigation of the critical-point wetting in the phase-separated mixture (n-heptane+perfluoro-n-hexane). The discrimination between the two coexisting liquid phases was achieved by exploiting the fluorescence lifetime difference of the flurophor 1,6-diphenylhexatriene in each of the phases. The wetting transition temperature T W is clearly identified as being (31.5 ± 0.2) °C, in relation to the upper consolute temperature of ca. 42°C. This version of the EWGFS technique for studying critical-point wetting may be useful for the future determination of composition and relative thickness of the wetting layer, in addition to the elucidation of the order of the transition. Other related application of the technique include the study of critical-point wetting and critical adsorption at modified solid surfaces.