Capillary condensation of hexafluoroethane in an ordered mesoporous silica

Machin, William D.

doi:10.1039/b207515f

Cited by 13 publications

(9 citation statements)

References 21 publications

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“…Here, in all systems desorption isotherm runs on the adsorption isotherm and does not show hysteresis loop. This fact signifies that observed capillary condensation is an second-order process which occurs above the capillary critical temperature. , However, as discussed below, adsorbed phase can be identified as a liquid acetonitrile. Adsorption mechanism awaits the further adsorption experiments at wide temperatures range and will be reported in the future.…”

Section: Resultsmentioning

confidence: 95%

Low Temperature Properties of Acetonitrile Confined in MCM-41

Kittaka¹,

Iwashita²,

Serizawa³

et al. 2005

J. Phys. Chem. B

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The effect of confinement on the phase changes and dynamics of acetonitrile in mesoporous MCM-41 was studied by use of adsorption, FT-IR, DSC, and quasi-elastic neutron scattering (QENS) measurements. Acetonitrile molecules in a monolayer interact strongly with surface hydroxyls to be registered and perturb the triple bond in the C[triple bond]N group. Adsorbed molecules above the monolayer through to the central part of the cylindrical pores are capillary condensed molecules (cc-acetonitrile), but they do not show the hysteresis loop in adsorption-desorption isotherms, i.e., second order capillary condensation. FT-IR measurements indicated that the condensed phase is very similar to the bulk liquid. The cc-acetonitrile freezes at temperatures that depend on the pore size of the MCM-41 down to 29.1 A (C14), below which it is not frozen. In addition, phase changes between alpha-type and beta-type acetonitriles were observed below the melting points. Application of the Gibbs-Thomson equation, assuming the unfrozen layer thickness to be 0.7 nm, gave the interface free energy differences between the interfaces, i.e., Deltagamma(l/alpha) = 22.4 mJ m(-2) for the liquid/pore surface (ps) and alpha-type/ps, and Deltagamma(alpha/beta) = 3.17 mJ m(-2) for alpha-type/ps and beta-type/ps, respectively. QENS experiments substantiate the differing behaviors of monolayer acetonitrile and cc-acetonitrile. The monolayer acetonitrile molecules are anchored so as not to translate. The two Lorentzian analysis of QENS spectra for cc-acetonitriles showed translational motion but markedly slowed. However, the activation energy for cc-acetonitrile in MCM-41 (C18) is 7.0 kJ mol(-1) compared to the bulk value of 12.7 kJ mol(-1). The relaxation times for tumbling rotational diffusion of cc-acetonitrile are similar to bulk values.

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Section: Resultsmentioning

confidence: 95%

Low Temperature Properties of Acetonitrile Confined in MCM-41

Kittaka¹,

Iwashita²,

Serizawa³

et al. 2005

J. Phys. Chem. B

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“…In these constricted regions, condensation transpires at higher temperatures and lower humidities than equilibrium values. Confined regions in the context of capillary condensation can refer, however not limited, to the sharp cracks, mesoporous media, , and contacts between planar and spherical surfaces. , In the case of water drops on cold hydrophobic surfaces, trijunction adjacent between a water drop and the hydrophobic solid substrate is considered to be convexly confined, except for Teflon with a contact angle of 108°, on which spreading does not occur (Figure ). At this region, condensation transpires at higher temperatures and lower RH values compared to equilibrium.…”

Section: Resultsmentioning

confidence: 99%

Cold-Induced Spreading of Water Drops on Hydrophobic Surfaces

Tavakoli

Kavehpour

2015

Langmuir

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Superhydrophobic surfaces are characterized by their peculiarities, such as water-repellent, anti-icing, and freezing-delay properties. Wetting dynamics of deposited water drops on cooling hydrophobic surfaces, which directly affects the aforementioned properties, has not been studied thoroughly. Here, water drops are cooled on different hydrophobic surfaces in a controlled environment. During the cooling process, a significant increase in the drop footprint and decrease in the apparent contact angle are observed because of premature and capillary condensation, followed by thin water film formation adjacent to the solid-liquid-gas line. The water thin film propagates on the hydrophobic substrates radially away from the trijunction, followed by spreading of the drop on the film, which was experimentally validated through high-speed visualization. In addition, the roles of physical variables, such as the substrate temperature, humidity of surrounding air, types of hydrophobic surfaces, surface roughness, and drop volume, on post-spreading shape are investigated experimentally.

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“…The resulting pseudo pore critical temperature, T ‘ CP , is always larger than the hysteresis critical temperature T CH . Our results indicate that both T CH and T ‘ CP for confined water decrease as the degree of confinement increases, in qualitative agreement with: (a) simulations for simple fluids; (b) theoretical arguments; ,, (c) simulation results for confined water; , and (d) experimental data for simple fluids in ordered pores. ,,, To validate our approach for the estimation of T ‘ CP , we calculate adsorption isotherms for water in all the pores at temperatures above the predicted T ‘ CP . The simulated adsorption isotherms exhibit a gradual increase in density, pointing to the absence of a phase transition for confined water (see discussion of Figures and ).…”

Section: Simulation Resultsmentioning

confidence: 99%

“…Experimental evidence , and theoretical arguments 14 suggest that T CH is lower than the pore critical temperature, T CP . However, while the existence of the critical point is accepted for fluids confined in slit-shaped pores, no critical point can be defined for fluids in narrow cylindrical pores ,, In contrast, T CH is observed for pores of all geometries.…”

Section: Introductionmentioning

confidence: 99%

Effect of Temperature on the Adsorption of Water in Porous Carbons

et al. 2005

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We report experimental and simulation studies to investigate the effect of temperature on the adsorption isotherms for water in carbons. Adsorption isotherms are measured by a gravimetric technique in carbon-fiber monoliths at 378 and 423 K and studied by molecular simulation in ideal carbon pores in the temperature range 298-600 K. Experimental adsorption isotherms show a gradual water uptake, as the pressure increases, and narrow adsorption-desorption hysteresis loops. In contrast, simulated adsorption isotherms at room temperature are characterized by negligible uptake at low pressures, sudden and complete pore filling once a threshold pressure is reached, and wide adsorption-desorption hysteresis loops. As the temperature increases, the relative pressure at which pore filling occurs increases and the size of the hysteresis loop decreases. Experimental adsorption-desorption hysteresis loops are narrower than those from simulation. Discrepancies between simulation and experimental results are attributed to heterogeneities in chemical composition, pore connectivity, and nonuniform pore-size distribution, which are not accounted for in the simulation model. The hysteresis phase diagram for confined water is obtained by recording the pressure-density conditions that bound the simulated hysteresis loop at each temperature. We find that the hysteresis critical temperature, i.e., the lowest temperature at which no hysteresis is detected, can be hundreds of degrees lower than the vapor-liquid critical temperature for bulk model water. The properties of confined water are discussed with the aid of simulation snapshots and by analyzing the structure of the confined fluid.

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Capillary condensation of hexafluoroethane in an ordered mesoporous silica

Cited by 13 publications

References 21 publications

Low Temperature Properties of Acetonitrile Confined in MCM-41

Low Temperature Properties of Acetonitrile Confined in MCM-41

Cold-Induced Spreading of Water Drops on Hydrophobic Surfaces

Effect of Temperature on the Adsorption of Water in Porous Carbons

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