Uniform magnesium oxide adsorbents

Dash, J. G.; Ecke, Ramona; Stoltenberg, J.; Vilches, O. E.; Whittemore, O. J.

doi:10.1021/j100501a024

Cited by 23 publications

(18 citation statements)

References 2 publications

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“…In this paper we present new results which compare the onset of solidification of 3 He and 4 He on a magnesium oxide substrate, which has cubic symmetry, with that already reported 1 * 2 for solidification on Grafoil, which has a hexagonal symmetry. The Grafoil experiments had shown that hep 4 He undergoes continuous solid nucleation whereas, surprisingly, bcc 3 He did not nucleate similarly.…”

Section: Role Of Substrate Symmetry In Nucleating Solid Heliummentioning

confidence: 65%

“…For superfluid 4 He there is no evidence of continuous nucleation of either the hep or the bcc solid from the substrate. The experiment contrasts the influence of the square lattice of MgO with the triangular lattice of a basal-plane graphite substrate.…”

Section: Role Of Substrate Symmetry In Nucleating Solid Heliummentioning

confidence: 99%

“…The Grafoil experiments had shown that hep 4 He undergoes continuous solid nucleation whereas, surprisingly, bcc 3 He did not nucleate similarly. Since the van der Waals forces between the substrate and either isotope are identical, and this force is considered 1 to be responsible for the superpressurization of the first few helium layers on the substrate, we were led to suggest that the substrate symmetry (hexagonal graphite encouraging the hep 4 He) is the determining factor in the onset of solidification.…”

Section: Role Of Substrate Symmetry In Nucleating Solid Heliummentioning

confidence: 99%

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Role of Substrate Symmetry in Nucleating Solid Helium

1980

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The continuous nucleation of bcc solid 3 He from the liquid phase due to the presence of a MgO substrate is detected. For superfluid 4 He there is no evidence of continuous nucleation of either the hep or the bcc solid from the substrate. The experiment contrasts the influence of the square lattice of MgO with the triangular lattice of a basal-plane graphite substrate. The phenomenon of pore condensation for solid 3 He is discussed.PACS numbers: 67.80. Cx, 68.45.Da In this paper we present new results which compare the onset of solidification of 3 He and 4 He on a magnesium oxide substrate, which has cubic symmetry, with that already reported 1 * 2 for solidification on Grafoil, which has a hexagonal symmetry. The Grafoil experiments had shown that hep 4 He undergoes continuous solid nucleation whereas, surprisingly, bcc 3 He did not nucleate similarly. Since the van der Waals forces between the substrate and either isotope are identical, and this force is considered 1 to be responsible for the superpressurization of the first few helium layers on the substrate, we were led to suggest that the substrate symmetry (hexagonal graphite encouraging the hep 4 He) is the determining factor in the onset of solidification. To test this conclusion we repeated the experiments with a MgO substrate 3 * 4 which has a cubic symmetry. For this substrate we observed uniform nucjeation of the bcc 3 He whereas 4 He does not show any signs of surface solidification in either hep or bcc regions.We should emphasize that the experiment on Grafoil alone did not prove the relevance of substrate symmetry. During the condensation of the first layer of He on Grafoil, it is known from adsorption studies 5 that initially an ordered state is reached in which the helium atoms are registered with the graphite lattice. Subsequently, as the coverage is increased, a close-packed triangular two-dimensional lattice is formed, with a spacing incommensurate with the graphite lattice. It can then be argued that this layer, which is formed by either isotope, is the natural base for the continuing growth of hep 4 He, whereas it would not encourage the further growth of a bcc solid. The present experiments disprove the universality of this model, and demonstrate the importance of a matching substrate symmetry. Further experimental evidence from direct observation of the growth of aligned 4 He crystals on the Grafoil substrate will also be presented to support the conclusion.The experimental evidence for the onset of solidification is deduced from the measurement of equilibrium points along isopycnals 6 in the vicinity of the bulk melting curve. There appears a substantial difference between the isopycnal and the isochore for the bulk liquid when surface solidification occurs, in the presence of a substrate of sufficiently large surface area. The present experiments used a 2.9-g MgO sample prepared from compressed smoke at the University of Washington. 3 Its properties have been measured by Dash etal. 4 who showed that it produces krypton step like adsor...

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Section: Role Of Substrate Symmetry In Nucleating Solid Heliummentioning

confidence: 65%

Section: Role Of Substrate Symmetry In Nucleating Solid Heliummentioning

confidence: 99%

Section: Role Of Substrate Symmetry In Nucleating Solid Heliummentioning

confidence: 99%

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Role of Substrate Symmetry in Nucleating Solid Helium

1980

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“…If the water molecule rotates such that one of these hydrogen bonds cannot be made, due to the presence of the solute boundary, then we can describe the difference in the enthalpy of these two states by AH. If there are N water molecules in the solvent shell and each molecule exists in two thermodynamic states, separated by an enthalpy difference of AH, the expression for the excess heat capacity, assuming the N molecules behave independently, is25 ( A m 2 e -( u / R ) (~-~d C, -C," = N-RF [1 + e-(w/R)(T-Tm)]2 (1) Here Cp is the heat capacity of water in the solvent shell. The heat capacity of the ground state, Cpo, is where these solvent Among other features the completely cooperative model predicts a nonlinear dependence of heat capacity with N o r surface area.…”

Section: Theorymentioning

confidence: 99%

Anomalous heat capacity of hydrophobic solvation

Gill¹,

Dec²,

Olofsson³

et al. 1985

J. Phys. Chem.

216

103

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The large heat capacity difference between gaseous and dissolved nonpolar molecules in water is commonly attributed to "iceberg" formation around the molecule. Recent experimental measurements show that the heat capacity difference is correlated with the number of water molecules in the first solvation shell. To a first approximation a two-state model in which each water molecule in the solvation shell behaves independently provides a satisfactory basis to quantitatively describe the heat capacity properties of the solvation shell. This noncooperative model explains the observed dependence of thermodynamic parameters upon molecular surface area, the magnitude of the heat capacity in water in terms of reasonable hydrogen-bond energies, and the temperature dependence of the excess heat capacity of apolar molecules in water. IntroductionThe unusual thermodynamic properties of nonpolar solutes in water were interpreted by Frank and Evans' in terms of ordering of the water molecules around the solute. They described the process as "iceberg" formation. A critical question has largely remained unanswered: what is the nature of the "iceberg"? The perturbed water surrounding the solute has been described as a "flickering cluster" by Frank and Wen2 Miller and Hildebrand3 suggested that the ability of the perturbed water molecules to form hydrogen bonds depends upon the total surface of the solute. The nature of the perturbed water surrounding the solute is revealed by detailed theoretical and computer simulation studies" where it has been shown that water significantly altered resides in the first solvation shell.The "iceberg" concept has provided a convenient qualitative explanation for the unusually large entropy losses and large heat capacity changes observed upon dissolution of a nonpolar solute in water. Without describing the detailed structural features of the "iceberg", Nemethy and Scheraga7 developed a quantitative description of the perturbed water in terms of modified energy levels of the solvent surrounding the solute. This enabled them to describe many thermodynamic properties of hydrocarbons in water, including the large heat capacities of these materials in water. An alternate approach, based more on the cooperative "iceberg" nature of the water surrounding an apolar molecule, has been described by Hvidt to explain the temperature dependence of the solubility of hydrocarbons in water.s She assumes that N water molecules in the solvation shell form a cooperative unit with different thermodynamic properties than normal water. Shinodag has argued from a more general thermodynamic view that "iceberg" formation will be eliminated at temperatures above 160 " C where a regular solution is presumed. Thus, the unusual entropy and heat capacity values of nonpolar solutes in water should disappear at high temperatures. Examination of solubility (free energy) and entropy properties in terms of molecular models is complicated by standard-state Heat capacity

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“…Following Dash et al, 9 we take these as indicators of the accumulation of the first and second layers on the surface, qualitatively similar to what is observed on highly uniform substrates. 1 The detailed behavior, however, is different.…”

Section: Isothermsmentioning

confidence: 94%

3He on MgO: Nuclear magnetic relaxation properties

Buzerak

Rollefson

1980

J Low Temp Phys

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Using MgO powder as a substrate, the magnetic relaxation of 3He and adsorption isotherms of 3He and Kr have been measured. Relaxation data as a function of coverage were obtained at temperatures of 2.0, 2. 7, 3.5, and 4.2 K, and as a function of temperature at coverages of x = 0.84 and 1.0 monolayer. The Kr isotherms show steplike features whose size increases following an extended bakeout at 950~ under vacuum. This is interpreted as indicating that the MgO surface consists of a mixture of uniform and nonuniform regions. Extended heating increases the fraction of the total surface area that is uniform. The values of Te vs. coverage at T= 2 and 2.7K are found to change from being roughly coverage independent to decreasing as x -3 at a coverage of x = O. 9, indicating a decrease in film mobility. At 3.5 and 4.2 K an approximately constant T2 is found at all coverages with no evidence of a sudden change of mobility. The low-temperature T 2 data continue to decrease as the second layer forms, implying a low mobility for small second-layer coverages. On the basis of the relatively short values obtained for T1 it is concluded that a relaxation mechanism in addition to dipolar coupling is presen t.

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Uniform magnesium oxide adsorbents

Cited by 23 publications

References 2 publications

Role of Substrate Symmetry in Nucleating Solid Helium

Role of Substrate Symmetry in Nucleating Solid Helium

Anomalous heat capacity of hydrophobic solvation

3He on MgO: Nuclear magnetic relaxation properties

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