Freezing bulk water

Cooling to temperatures below ice crystallisation results in dehydration of membranes as the bulk water freezes and solutes are zone-refined to the regions of unfreezable water at the membrane aqueous interface. Saturated solutions of electrolytes and solutes affect membrane phase behaviour by screening charges on acidic lipids which are known to be important for the overall phase behaviour of the membrane. 

In Chapter 4 we considered gases, in which intermolecular forces play only a minor role. Here, we deal with liquids and solids, in which the forces that hold molecules together are of crucial importance for determining the physical properties of bulk samples. Individual water molecules, for instance, are not wet, but bulk water is wet because water molecules are attracted to other substances and spread over their surfaces. Individual water molecules neither freeze nor boil, but bulk water does, because in the process of freezing molecules stick together and form a rigid array and in boiling they separate from one another and form a gas. 

Careful cooling of pure water at atmospheric pressure can result in water that is able to remain liquid to at least 38 °C below its normal freezing point (0 °C) without crystallizing. This supercooled water is metastable and will crystallize rapidly upon being disturbed. The lower the temperature of the supercooled water, the more likely that ice will nucleate. Bulk water can be supercooled to about — 38 °C (Ball, 2001 Chaplin, 2004). By increasing the pressure to about 210 MPa, liquid water may be supercooled to — 92 °C (Chaplin, 2004). A second critical point (C ) has been hypothesized (Tc = 220 K and Pc = 100 MPa), below which the supercooled liquid phase separates into two distinct liquid phases a low-density liquid (LDL) phase and a high-density liquid (HDL) phase (Mishima and Stanley, 1998 Poole et al., 1992 Stanley et al., 2000). Water near the hypothesized second critical point is a fluctuating mixture of LDL and HDL phases. 

Water which is bound to cellulose (or any other natural polymer) has properties different from those of unbound (bulk) water. For example, it has a higher density and a lower freezing point. The... 

ESR spectroscopy can be used with adsorbed paramagnetic ions to study the liquid associated with mineral surfaces. Cu(II) and Mn(II) have been used in his type of investigation, although difficulties are encountered with observing a resonance from Mn(II) in distorted environments. Measurements of Cu(II) on silica at room temperature and above have shown that adsorbed water behaves in the same manner as bulk water, but at lower temperatures it experiences a decreased mobility (61). On freezing two types of water are found one which is freezable and undergoes crystallization and the other which is unfreezable, in which the ice structure cannot be formed because of the surface interaction. NMR, IR and differential thermal... 

Another concern with freeze-drying LEH is the instability of liposome structure upon lyophilization. Vesicle formation occurs in the presence of bulk water and when water is removed, loss of structural integrity is inevitable. Fusion, crystal formation, and phase transition are observed, resulting... 

From experiments, Lehmann and Siegenthaler (1991) determined the equilibrium H-isotope fractionation between ice and water to be +21.2%o. Under natural conditions, however, ice will not necessarily be formed in isotopic equilibrium with the bulk water, depending mainly on the freezing rate. 

The DNA solvation shell consists of about 20-22 water molecules per nucleotide of these, — 15-17 waters associate with the nucleoside and —5 waters associate with the phosphate group [13,14]. Water outside the solvation layer is termed bulk water. Upon freezing, the DNA solvation water forms two primary phases the ice phase, consisting of one or more of the crystalline forms of ice, and a DNA-associated phase, consisting of ordered water which comes in direct contact with the DNA (primary layer) and disordered water in the secondary layer. DNA hydration is expressed in terms of F, the number of water molecules per nucleotide. 

In an aqueous solution of DNA, the water outside of the solvation shell is referred to as bulk water. When DNA solutions are frozen, the bulk water crystallizes as a separate phase—ice. Ice does not form if the concentration of DNA is brought to a level where only the solvation shell remains, about 20-22 waters/nucleotide. If brought to this concentration slowly, a film is formed. Freezing a film does not create ice. Another type of sample is prepared by first lyophilizing DNA and then letting it sit at a preselected humidity that determines the level of hydration, typically 2.5 < F < 22. Subsequent freezing of these cotton-like samples does not yield ice. 

Water in food products can be described as being free or bound. The definition of what consitiutes bound water is far from clear (see Fennema, 1985) but it can be considered as that part of the water in a food which does not freeze at — 40°C and exists in the vicinity of solutes and other non-aqueous constituents, has reduced molecular mobility and other significantly altered properties compared with the bulk water of the same system (Fennema, 1985). The actual amount of bound water varies in different products and the amount measured is often a function of the assay technique. Bound water is not permanently immobilized since interchange of bound water molecules occurs frequently. 

Hydration of the DNA has also a strong influence on the radical yield at 77K (Wang et al. 1993). The G values increase by over fourfold upon addition of the primary hydration layer, that is about 20 water molecules per nucleotide. Upon further water addition, the excess water freezes into an apparently independent bulk ice phase which steals about five water molecules from the hydration layer and thus reduces the DNA radical yield. It has been concluded that efficient hole... 

Abstract A simplified quintuple model for the description of freezing and thawing processes in gas and liquid saturated porous materials is investigated by using a continuum mechanical approach based on the Theory of Porous Media (TPM). The porous solid consists of two phases, namely a granular or structured porous matrix and an ice phase. The liquid phase is divided in bulk water in the macro pores and gel water in the micro pores. In contrast to the bulk water the gel water is substantially affected by the surface of the solid. This phenomenon is already apparent by the fact that this water is frozen by homogeneous nucleation. 

Bulk or free water and gas are in the macroscopic pores with a hydraulic diameter greater than 0.1 fim. The gel pores are filled with pore solution (gel water). Their diameter is much smaller (1 - 30 nm). During cooling below the freezing point of bulk water ice is formed in the larger pores with sufficient su-... 

Taking into account the aforementioned effects of ice formation in porous materials, a macroscopic quintuple model within the framework of the Theory of Porous Media (TPM) for the numerical simulation of initial and boundary value problems of freezing and thawing processes in saturated porous materials will be investigated. The porous solid is made up of a granular or structured porous matrix (a = S) and ice (a = I), where it will be assumed that both phases have the same motion. Due to the different freezing points of water in the macro and micro pores, the liquid will be distinguished into bulk water ( a = L) in the macro pores and gel water (a = P, pore solution) in the micro pores. With exception of the gas phase (a = G), all constituents will be considered as incompressible. 

Using ESR spin probe technique one can allow for the contribution of bulk water due to its preferential freezing as compared to the fraction of water modified by the dispersed particles surface [7,8], Surface water molecules and spin probe localized in that water retains its mobility at lower temperatures than bulk water molecules do and EPR signal from these spin probes dominates in spectrum. Specifically the sequence of freezing of water attached to different chemical groups at the surface is as follows water at nonpolar, polar and, finally, at charges surface sites [9,10],... 

The primary hydration shell is different from bulk water. Of the 20 water molecules per nucleotide, only 11 to 12 are directly bound to DNA. They form a shell which is impermeable to cations [861] and does not freeze into an ice-like state [862], It is these water molecules which are observed in crystal structure analyses and are hydrogen-bonded to DNA oxygen and nitrogen atoms. 

Besides direct damage of the DNA strand, it is also possible for the surrounding water molecules to be involved in radiation damage mechanisms. The hydration layer of DNA consists of a primary layer (approximately 20 or 21 water molecules per nucleotide), which possesses properties different from crystalline ice upon freezing, and a secondary layer, which cannot be distinguished from bulk water upon crystallization. Upon irradiation of water, many different products can be formed ... 

If the sponge is left to dry in the sun, this adsorbed water will evaporate, leaving only a small proportion of water bound chemically to the salts and to the cellulose of the sponge fibers. Like water in sponge, water is held in food by various physical and chemical mechanisms (Table 3.1). It is a convenient oversimplification to distinguish between free and bound water. The definition of bound water in such a classification poses problems. Fennema (1985) reports seven different definitions of bound water. Some of these definitions are based on the freezability of the bound component, and others rely on its availability as a solvent. He prefers a definition in which bound water is that which exists in the vicinity of solutes and other non-aqueous constituents, exhibits reduced molecular activity and other significantly altered properties as compared with bulk water in the same system, and does not freeze at -40"C."... 

One might anticipate that if the water in the cell wall were to be frozen and then the ice were to be subliminated off there should be no liquid capillary tension, no cell wall shrinkage and it should be possible to create a porous cell wall. However, sublimating the water molecules from the cell wall at -20°C does not prevent collapse of the internal pore structure (Merchant, 1957). This implies that the cell wall water is not actually frozen at this temperature the cell wall still shrinks and very little internal surface is created. Indeed there is evidence (Tarkow, 1971) that at least some adsorbed water does not lose the mobility characteristic of the liquid phase until very low temperatures (<-80°C). Of course water in the lumens behaves like bulk water and freezes at a temperature between -0.1°C and -2.0°C, depending on the concentration of dissolved sugars in the sap. 

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