Neutron reflectivity studies of the free liquid surface of methylcyclohexane–perfluoromethylcyclohexane near the critical endpoint

Neilson, Martin M.; Bowers, James; Manzanares-Papayanopoulos, Emilio; Howse, Jonathan R.; Vergara-Gutierrez, Marcos C.; Clements, Patricia J.; Burgess, A. N.; McLure, Ian A.

doi:10.1039/a904343h

Cited by 5 publications

(4 citation statements)

References 25 publications

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“…Although we have modelled those limiting values by using monolayer-calculated surface areas, the present thermodynamic formalism does not incorporate such an assumption. In other words, if the surface phase of pure liquid components were constituted by two layers of molecules as, for example, assumed by McLure and co-workers [54] for an organic/organic liquid mixture, then the actual molar surface area would be about one half of the value indicated by a monolayer model.…”

Section: Theoretical Comparison With Previous Approachesmentioning

confidence: 96%

New Thermodynamics for Evaluating the Surface‐phase Enrichment in the Lower Surface Tension Component

Santos¹,

Reis²

2014

ChemPhysChem

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Regarding the surface phase of liquid mixtures as a thermodynamic phase, ideal surface phases are designed so that at fixed bulk-phase composition, real and ideal surface phases have the same chemical composition and identical limiting slopes for the dependence of surface tension on mole fraction. Standard chemical potentials are introduced for surface phase components, and quasi-exact expressions are worked out to compute ideal surface tensions and surface-phase compositions of real liquid mixtures. Guidelines for choosing molecular models to estimate the molar surface area of pure constituents are given. Ideal and excess surface tensions are calculated by using literature data for aqueous ethanol solutions at 298 K. These results show treatment based on Butler's equations grossly overestimate predicted surface tensions, thus leading to lower ethanol content in the surface phase. These inaccuracies are ascribed to the use of molar surface areas in model equations that are too small.

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Section: Theoretical Comparison With Previous Approachesmentioning

confidence: 96%

New Thermodynamics for Evaluating the Surface‐phase Enrichment in the Lower Surface Tension Component

Santos¹,

Reis²

2014

ChemPhysChem

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“…Ever since Cahn has suggested theoretically such a first-order phase transition at an interface and Moldover and Cahn have demonstrated experimentally for the methanol/cyclohexane/air system, this kind of wetting−nonwetting transition for air (or vapor)/liquid/liquid systems has been investigated by many workers. − These are well summarized and reviewed also in some books. − The wetting properties of three liquid-phase systems such as water−surfactant−oil mixtures have been also inspected and examined mainly from the viewpoint of the structure of the middle phase microemulsions. − Furthermore, the wetting and other interfacial properties of alkane molecules in the surfactant adsorbed films at the air/aqueous solution interface have been investigated extensively by the thermodynamic analysis of the surface tension data, − the structure and composition analysis of the neutron reflection, the ellipsometry, , and the viscoelastic properties . Among them, Bain et al have proved, and Aveyard et al and Jayalakshmi et al have referred to, the first-order phase transition of the mixed monolayers of the alkane and surfactant.…”

Section: Introductionmentioning

confidence: 99%

“…Ever since Cahn has suggested theoretically such a firstorder phase transition at an interface 1 and Moldover and Cahn have demonstrated experimentally for the methanol/ cyclohexane/air system, 2 this kind of wetting-nonwetting transition for air (or vapor)/liquid/liquid systems has been investigated by many workers. [3][4][5][6][7][8][9] These are well summarized and reviewed also in some books. [10][11][12] The wetting properties of three liquid-phase systems such as watersurfactant-oil mixtures have been also inspected and examined mainly from the viewpoint of the structure of the middle phase microemulsions.…”

Section: Introductionmentioning

confidence: 99%

Interfacial Films and Wetting Behavior of the Air/Hexadecane/Aqueous Solution of a Surfactant System

et al. 2001

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The tension of the three interfaces and the dihedral angle of the lens of the air/hexadecane lens/aqueous solution of tetramethylammonium dodecyl sulfate (TMADS) system were measured as a function of the surfactant concentration. The air/water surface tension vs concentration curves both with and without an oil lens exhibited a break point corresponding to the phase transition of the adsorbed films. From the thermodynamic analysis, it was shown that the phase transition drives the spreading of hexadecane molecules into the adsorbed film. The dihedral angle decreased very rapidly from about 50°at zero concentration to about 6°at the phase transition point and then increased slowly to about 20°at the critical micelle concentration (cmc), suggesting the very drastic change in wetting properties at the phase transition point. The plausible gap was observed between the dihedral angles measured and those calculated from the interfacial tension values at the concentration range from just above the phase transition to near the cmc. This suggests a case where the Neumann relation does not hold even when a lens exists stably. One of the possibilities may be that the wetting film is rather thick just above the phase transition point.

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“…Although methylcyclohexane + perfluoromethylcyclohexane (MCH + PFMCH) has been greatly studied in the critical mixing region, [11][12][13][14][15][16][17][18][19][20][21] its transport properties, like those of many other mixtures studied in this region, have been much less thoroughly explored than the equilibrium properties. Partly in an effort to redress this imbalance, we have determined the near-critical shear viscosity and the thermal conductivity in the upper critical mixing region.…”

Section: Introductionmentioning

confidence: 99%

Shear Viscosities of Methycyclohexane, Perfluoromethylcyclohexane, and Their Mixtures in the Vicinity of the Upper Critical Mixing Temperature. 1. Critical Isopleth and Coexistence Curve

et al. 2006

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Shear viscosities η are reported for pure liquid methylcyclohexane (MCH) from 298.610 K to 333.694 K, for perfluoromethylcyclohexane (PFMCH) from 319.196 K to 333.114 K and for a MCH + PFMCH mixture of overall PFMCH near-critical mole fraction, x c ) 0.3640, from (T UCS /K -7) in the region of biphase liquid coexistence to (T UCS /K + 20) in the uniphase region, where T UCS ) 320.13 K is the air-saturated upper liquidliquid critical solution temperature. The measurements were made using a capillary rheometer that permits the measurement of the viscosity of thermally equilibrated coexisting-liquid phases. The results confirm that the near-critical viscosity exhibits a weak enhancement that strictly speaking becomes a divergence when account is taken of the finite shear gradients in the capillary during measurements. The viscosity of the uniphase mixture of critical composition is well-described after shear gradient correction by a multiplicative combination of an Arrhenius background and a critical power expression with an index close to the now-accepted universal value y ) 0.0435. The chief objective of the work, in addition to contributing to knowledge of this aspect of near-critical rheology, is the development for the biphase of a simple expression for the temperature dependence of the viscosities of the coexisting phases, η + and η -, that combines (a) an expression for the viscosity diameter <η> ) 1/2(η + + η -) similar to that for the viscosity of the critical mixture in the uniphase region, with a similar best critical index y′ between 0.041 and 0.0435, and (b) an expression for ∆η ) (η + -η -) that behaves like an order parameter, with an index very close to the normal value ) 0.325 and as many Wegner correction terms as the data require. The best two-phase fit emerges from a freely fitted exponent y′ ≈ 0.037 with one Wegner-extended scaling term, but we believe that were shear gradient correction to be applied, the best y′ would be the consensus value y′ ) 0.0435. The magnitude of y′ notwithstanding, we believe that our primary objective has been satisfied, namely, the formulation of an expression that affords a good description of the shear and background viscosities of nearcritical mixtures in the one-and two-liquid phases in relation to our estimates of the nano-or molecular-viscosity derived from measurements of fluorescence polarization decay rates.

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Neutron reflectivity studies of the free liquid surface of methylcyclohexane–perfluoromethylcyclohexane near the critical endpoint

Cited by 5 publications

References 25 publications

New Thermodynamics for Evaluating the Surface‐phase Enrichment in the Lower Surface Tension Component

New Thermodynamics for Evaluating the Surface‐phase Enrichment in the Lower Surface Tension Component

Interfacial Films and Wetting Behavior of the Air/Hexadecane/Aqueous Solution of a Surfactant System

Shear Viscosities of Methycyclohexane, Perfluoromethylcyclohexane, and Their Mixtures in the Vicinity of the Upper Critical Mixing Temperature. 1. Critical Isopleth and Coexistence Curve

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