Flow of nonfreezing water interlayers and frost heaving

Using the thermodynamics of irreversible processes as the basis, the problem of the mass exchange between nonfreezing interlayers has been solved for the slit model of a pore filled with ice, as well as between the thawed and the frozen zone of a porous body. There was shown the important role of disjoining pressure in the formation of equilibrium thicknesses of interlayers and the phenomenon of the frost destruction of porous materials and frost heaving. Experimental corroboration of the developed theory is presented. The problems have been formulated involving further investigations in the field of the theory of motion of moisture in frozen porous bodies.     

Disjoining Pressure of Thin Unfreezing Water Layers between the Pore Walls and Ice in Porous Bodies

Thickness h of unfreezing water layers formed between the ice column and the inner surface of thin molecularly smooth capillary (radius 1 m) was measured as a function of temperature (varying from –0.14 to –1°C) and external pressure (up to 8 MPa) by dilatometry. Disjoining pressure isotherms (h) were calculated on the basis of obtained data. The isotherms obtained give the same exponential dependence of onh, as the known isotherms for colloidal systems obtained using surface force apparatus and atomic force microscopy. The disjoining pressure arising in frozen bodies is balanced by the stresses in the skeleton of a porous body and can cause the local fracture of pore walls.     

Effect of pressure on thermal conductivity and pressure collapse of ice in a polymer-hydrogel and kinetic unfreezing at 1 GPa

We report a study of aqueous solutions of poly(vinylalcohol) and its hydrogel by thermal conductivity, κ, and specific heat measurements. In particular, we investigate (i) the changes in the solution and the hydrogel at 0.1 MPa observed in the 350-90 K range and of the frozen hydrogel at 130 K observed in the range from 0.1 MPa to 1.3 GPa, and (ii) the nature of the pressure collapse of ice in the frozen hydrogel and kinetic unfreezing on heating of its high density water at 1 GPa. The water component of the polymer solution on cooling either first phase separates and then freezes to hexagonal ice or freezes without phase separation and the dispersed polymer chains freeze-concentrate in nanoscopic and microscopic regions of the grain boundaries and grain junctions of the ice crystals in the frozen state of water in the hydrogel. The change in κ with temperature at 1 bar is reversible with some hysteresis, but not reversible with pressure after compression to 0.8 GPa at 130 K. At high pressures the crystallized state collapses showing features of κ and specific heat characteristic of formation of high density amorphous solid water. The pressure of structural collapse is 0.08 GPa higher than that of ice at 130 K. The slowly formed collapsed state shows kinetic unfreezing or glass-liquid transition temperature at 140 K for a time scale of 1 s. Comparison with the change in the properties observed for ice shows that κ decreases when the polymer is added.     

Disjoining pressure of thin nonfreezing interlayers

At subzero temperatures, it has been found that nonfreezing water interlayers form between the inner smooth surface of a thin quartz capillary and the ice in the capillary core. The dilatometry method has been used to measure the thicknesses of these nonfreezing interlayers over a range of temperatures between -1 and -0.14 degrees C and for applied pressures up to 8 MPa. The measured data are used to calculate the disjoining pressure isotherms of nonfreezing water interlayers. It is shown that structural forces caused by structural changes of nonfreezing water are the main contribution to the disjoining pressure. The structural changes are also responsible for the phenomenon of the nonfreezing interlayers.     

Flow of nonfreezing water interlayers and frost damage in porous solids

On the basis of the thermodynamics of irreversible processes, a solution has been obtained for the problem of mass transfer between nonfreezing interlayers in a slit model of a pore filled with ice, and also between the thawed and frozen zones of a porous solid. The disjoining pressure plays a role in determining the equilibrium thickneses of the interlayers and in the phenomena of frost damage and heaving of soils. The theory that is developed in this work is supported by experimental findings, and guidelines are formulated for future research in the field of the theory of moisture movement in porous solids during the freezing process.     

Fulfillment of the Conditions of Thermodynamic Equilibrium in an Ice/Electrolyte Solution System with Constrained Geometry

For small volumes of a NaCl solution (10^–6 cm³) with concentrations of 0.1 and 1 M, temperature dependences of the length lof solution columns frozen in thin quartz capillaries (5–10 m in radius) are obtained. At the temperatures t above –4 and –8°C (for 0.1 and 1 M solutions, respectively), the l(t) dependences are reversible, independent of the direction of changes in temperature, and, hence, correspond to the equilibrium conditions of ice/solution system. From the constant mass condition of the solute, an expression for l(t) is derived that includes only one thermodynamic characteristic, namely, the temperature dependence of the solution concentration in equilibrium with ice. Deviations from the calculated l(t) dependences are observed at a temperatures below –2 and –5°C (for 0.1 and 1 M solutions, respectively), which can be explained by the adhesion of frozen solution to the capillary walls. The arising internal stresses lead to the deviations from the thermodynamic equilibrium conditions known for the bulk systems. On approaching the melting zone, the adhesion is failed because of the formation of thin nonfreezing water interlayers on the quartz surface.     

Isotherms of Capillary Condensation Influenced by Formation of Adsorption Films

Isotherms of capillary condensation are often used to determine the vapor sorption capacity of porous adsorbents as well as the pore size distribution by radii. In this paper, for calculating the volume of capillary condensate and of adsorption films in a porous body, an approach based on the theory of surface forces is used. Adsorption isotherms and disjoining pressure isotherms of wetting films are presented here in an exponential form discussed earlier. The calculations were made for straight cylindrical capillaries of different radii and slit pores of different width. The mechanisms of capillary condensation differ in cylindrical and slit pores. In cylindrical pores capillary condensation occurs due to capillary instability of curved wetting films on a capillary surface, when film thickness grows. In the case of slit pores, coalescence of wetting films formed on opposite slit surfaces proceeds under the action of attractive dispersion forces. Partial volumes of liquid in the state of both capillary condensate and adsorbed films are calculated dependent on the relative vapor pressure in a surrounding media. Copyright 2000 Academic Press.     

On the thermodynamic equilibrium between ice and electrolyte solutions in the conditions of confined geometry

For a small volume (of about 10⁻⁶ cm³) of NaCl and other electrolyte solutions (C = 0.1 and 1 M) in thin (r = 5/10 μm) single quartz capillaries, dependencies of the column length l of frozen solutions on the temperature t were measured using comparator IZA-2 in a thermostated chamber. At temperatures range t > −4 °C (for C = 0.1 M) and t > −8 °C (for C = 1 M) the l(t) dependencies are reversible and therefore correspond to establishment of an equilibrium between ice-1 and the solution.

From the constants mass condition of the dissolved salt in a frozen column, the l(t) expression was derived, which includes thermodynamic relation between solution concentration in an equilibrium with ice, C_s, and the temperature t for bulk systems. Deviations from the data known for bulk solutions were observed in thin capillaries when temperature t decreased to −3 °C (for 0.1 M NaCl) and to −6 °C for 1 M NaCl solution.

This effect may be a result of strong adhesion of the ice column to capillary walls. In this case, some internal stresses arise in frozen solution resulting in a deviation from thermodynamic equilibrium conditions for bulk systems. When approaching the temperature of ice melting, adhesion forces decrease due to formation of a thin non-freezing water interlayer on the capillary wall. In this temperature range the experimental data are in agreement with the predictions for bulk systems. It was supposed that the observed deviation in thin capillaries may be caused by formation of an amorphous ice phase with higher density as compared with the ice-1 during rapid freezing, or by an effect of ice microlenses formation. Both effects will result in a deviation from the phase diagram corresponding to a bulk solution.     

Investigation of dynamic streaming potential by dimensional analysis

The theory of streaming potential at sinusoidal flow of liquid in a porous medium is a convenient and fruitful tool for determination of the interface properties of materials and also for construction of apparatus for zeta potential measurements and electrokinetic transducers. An investigation of the dynamic streaming potential by the method of dimensional analysis is presented. This method provides a wider approach to the problem under consideration. As a result, relationships between streaming potential in a porous medium and mechanical quantities are established. These quantities include pressure gradient in a liquid inside pores and capillaries, acceleration of capillaries, and the solid part of a porous medium, and the viscous friction force the liquid exerts on the solid part. The corresponding formulas for streaming potential are presented. The relationship between the streaming potential and viscous friction force does not depend on the frequency of oscillation and pore size. All these formulas in particular cases are transformed to known formulas for the streaming potential.     

Does water condense in carbon pores?

Using grand canonical Monte Carlo (GCMC) simulations of molecular models, we investigate the nature of water adsorption and desorption in slit pores with graphitelike surfaces. Special emphasis is placed on the question of whether water exhibits capillary condensation (i.e., condensation when the external pressure is below the bulk vapor pressure). Three models of water have been considered. These are the SPC and SPC/E models and a model where the hydrogen bonding is described by tetrahedrally coordinated square-well association sites. The water-carbon interaction was described by the Steele 10-4-3 potential. In addition to determining adsorption/desorption isotherms, we also locate the states where vapor-liquid equilibrium occurs for both the bulk and confined states of the models. We find that for wider pores (widths >1 nm), condensation does not occur in the GCMC simulations until the pressure is higher than the bulk vapor pressure, P0. This is consistent with a physical picture where a lack of hydrogen bonding with the graphite surface destabilizes dense water phases relative to the bulk. For narrow pores where the slit width is comparable to the molecular diameter, strong dispersion interactions with both carbon surfaces can stabilize dense water phases relative to the bulk so that pore condensation can occur for P < P0 in some cases. For the narrowest pores studied--a pore width of 0.6 nm--pore condensation is again shifted to P > P0. The phase-equilibrium calculations indicate vapor-liquid coexistence in the slit pores for P < P0 for all but the narrowest pores. We discuss the implications of our results for interpreting water adsorption/desorption isotherms in porous carbons.     

1H NMR cryoporosimetry of swollen hypercrosslinked polymers

The porous structure of two laboratory samples of hypercrosslinked polystyrene networks with a crosslinking degree of 200%, the commercial hypercrosslinked sorbent MN-270 (Purolite Int.), and macroporous polystyrene sorbents Amberlite XAD-4 and Amberlite XAD-1600 were investigated by means of ¹H NMR cryoporosimetry. It was determined that the signal intensity of benzol, dioxane, dichloroethane and water protons previously frozen in polymer pores increases during solvent melt. Dependencies of the integral proton signal intensities on the sample temperature obtained in various solvents are discussed in terms of sample micropore accessibility and solvent—polystyrene interaction energy. Pore dimensions are estimated using the melting temperature value of ice according to the Gibbs—Thomson equation. We conclude that the obtained distribution curves reveal all hypercrosslinked polymers to be mainly microporous with diameter distribution maxima around 10 Å. It is shown that along with pores 120 Å in diameter, XAD-4 has a considerable amount of micropores. In contrast, XAD-1600 is ranged as a mesoporous sorbent.     

Alkane films on water: stability and wetting transitions

Types of surface forces determining the disjoining pressure isotherms of wetting films of low-molecular-weight alkanes on water surface are discussed. The van der Waals forces in alkane interlayers at different temperatures were calculated using a combination of exact equations of the Dzyaloshinsky—Lifshitz—Pitaevsky macroscopic theory and the multi-oscillator model for representation of the dielectric permittivity spectra of contacting bodies. Taking account of competitive action of the van der Waals and image forces allows one not only to reproduce specific features of wetting in the systems studied at different temperatures, but also to describe quantitatively the contact angles and the experimentally observed isotherms of polymolecular adsorption. The experimentally detected wetting transition in the water—pentane—vapor system was rationalized using the results of calculations mentioned above and the Derjaguin—Frumkin theory of wetting. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 256–266, February, 2008.     

Confined water inside single-walled carbon nanotubes: global phase diagram and effect of finite length

Studies on confined water are important not only from the viewpoint of scientific interest but also for the development of new nanoscale devices. In this work, we aimed to clarify the properties of confined water in the cylindrical pores of single-walled carbon nanotubes (SWCNTs) that had diameters in the range of 1.46 to 2.40 nm. A combination of x-ray diffraction (XRD), nuclear magnetic resonance, and electrical resistance measurements revealed that water inside SWCNTs with diameters between 1.68 and 2.40 nm undergoes a wet-dry type transition with the lowering of temperature; below the transition temperature T(wd), water was ejected from the SWCNTs. T(wd) increased with increasing SWCNT diameter D. For the SWCNTs with D = 1.68, 2.00, 2.18, and 2.40 nm, T(wd) obtained by the XRD measurements were 218, 225, 236, and 237 K, respectively. We performed a systematic study on finite length SWCNT systems using classical molecular dynamics calculations to clarify the effect of open ends of the SWCNTs and water content on the water structure. It was found that ice structures that were formed at low temperatures were strongly affected by the bore diameter, a = D - σ(OC), where σ(OC) is gap distance between the SWCNT and oxygen atom in water, and the number of water molecules in the system. In small pores (a < 1.02 nm), tubule ices or the so-called ice nanotubes (ice NTs) were formed irrespective of the water content. On the other hand, in larger pores (a > 1.10 nm) with small water content, filled water clusters were formed leaving some empty space in the SWCNT pore, which grew to fill the pore with increasing water content. For pores with sizes in between these two regimes (1.02 < a < 1.10 nm), tubule ice also appeared with small water content and grew with increasing water content. However, once the tubule ice filled the entire SWCNT pore, further increase in the water content resulted in encapsulation of the additional water molecules inside the tubule ice. Corresponding XRD measurements on SWCNTs with a mean diameter of 1.46 nm strongly suggested the presence of such a filled structure.     

Selective Water Sorbents for Multiple Applications. 11. CaCl2 Confined to Expanded Vermiculite

This paper presents the water sorption properties of a new selective water sorbent based on expanded vermiculite as a host matrix and calcium chloride as a hygroscopic salt. Sorption isobars, isosters and isotherms at T = 30–150°C and vapor partial pressure 8.2–42.0 mbar clearly show that at low water contents crystalline hydrates with 0.33, 1 and 2 molecules of water per 1 molecule of CaCl₂ are formed in the pores. These hydrates are stable over a temperature change of 20–30°C and exhibit kinetically slow transformations. At higher water uptake, the vapor absorption leads to the formation of a CaCl₂ aqueous solution inside the pores, which properties are close to those in the bulk. Isosteric sorption heat was found to depend on water sorption and change from 76.3 kJ/mol for solid hydrates to 39.1–46.6 kJ/mol.     

Thermodynamic properties of methane hydrate in quartz powder

Using the experimental method of precision adiabatic calorimetry, the thermodynamic (equilibrium) properties of methane hydrate in quartz sand with a grain size of 90-100 microm have been studied in the temperature range of 260-290 K and at pressures up to 10 MPa. The equilibrium curves for the water-methane hydrate-gas and ice-methane hydrate-gas transitions, hydration number, latent heat of hydrate decomposition along the equilibrium three-phase curves, and the specific heat capacity of the hydrate have been obtained. It has been experimentally shown that the equilibrium three-phase curves of the methane hydrate in porous media are shifted to the lower temperature and high pressure with respect to the equilibrium curves of the bulk hydrate. In these experiments, we have found that the specific heat capacity of the hydrate, within the accuracy of our measurements, coincides with the heat capacity of ice. The latent heat of the hydrate dissociation for the ice-hydrate-gas transition is equal to 143 +/- 10 J/g, whereas, for the transition from hydrate to water and gas, the latent heat is 415 +/- 15 J/g. The hydration number has been evaluated in the different hydrate conditions and has been found to be equal to n = 6.16 +/- 0.06. In addition, the influence of the water saturation of the porous media and its distribution over the porous space on the measured parameters has been experimentally studied.     

多孔介质中甲烷水合物的分解特性

利用定容降压方法测定了在不同多孔介质中甲烷水合物的分解实验数据, 所使用的多孔介质平均孔径分别为9.03, 12.95, 17.96和33.20 nm, 其中孔径为12.95 nm的多孔介质采用了3个粒径范围, 分别为0.105～0.150, 0.150～0.200和0.300～0.450 mm; 其它孔径的多孔介质的粒径范围为0.105～0.150 mm. 在封闭的条件下测定了不同温度与不同初始生成压力下甲烷水合物的分解实验数据(实验温度范围为269.15～278.15 K, 初始生成压力范围为4.1～11.0 MPa), 结果表明, 水合物的分解速度随着初始生成压力的增加和水浴温度的降低而升高, 也随孔径的增加而升高, 但随多孔介质粒径的增大而降低. 在孔径较大和分解温度较低时, 多孔介质中水合物分解引起的温度降低会使水结冰, 从而减缓水合物的分解速度.     

Preparation method of porous polymer materials by radiation technique and its application

The radiation polymerization of hydrophilic hydroxyethyl methacrylate monomer solution at temperatures below 0 °C leads to the formation of a porous structure in the polymers. The melting peak of the eutectic water–monomer composition at the eutectic point (above ?24 °C) could be distingushed and a glass transition temperature was observed at ?96 °C. The porous structure was developed after melting small ice pieces in the polymers after polymerization. The porous structure formed above 0 °C contained discontinuous pores and that formed below 0 °C had continuous pores leading to reticular structure. In a mixture of water –dioxane –monomer, the pore diameter decreased with increasing monomer concentration. Replacing dioxane with decane led to a maximum pore diameter at 70% monomer concentration. The pore diameter in 70% monomer concentration using water and dioxane was 1～4 µm, maximum activity in immobilized enzyme tablets was observed at this diameter. The porous structure was also varied by controlling the polymerization temperature. The durability of the immobilized enzyme tablets was demonstrated by the retention of high enzyme activities after repeated batch enzyme reactions. Copyright © 2001 John Wiley & Sons, Ltd.