Freezing phase transition lowering

With the suggestion that the last common genetic ancestor is a hyperthermophile, the role of temperature on the origins of life is important. The lower temperature limit in water is limited by the phase transition from liquid to ice. This is a problem because the density of ice is lower than that of water and the increase in volume on freezing will cause the cell structure to become disrupted in the same way that pipes burst in the winter. The lower limit for bacterial growth reported so far is -20°C, which is the temperature at which intracellular ice is formed. Adaptation to the cold requires a considerable salt content to depress the melting point of water the Don Juan Pond in Antarctica, which has a saturated CaCE solution, preserves the liquid phase at temperatures as low as —53°C. 

As we saw in Section 5.1, a single substance can exist in a variety of different phases, or different physical forms. The phases of matter include the solid, liquid, and gaseous forms and the different solid forms, such as the diamond and graphite forms of carbon. In one unique case— helium—there are two liquid forms of the same substance. There are several different forms of ice, which differ in the way the water molecules pack together when high pressures are applied. The conversion of a substance from one phase to another, such as the melting of ice, the vaporization of water, or the conversion of graphite into diamond, is called a phase transition. Phase transitions take place at specific temperatures and pressures that depend on the purity of the substance. Seawater, for instance, freezes at a lower temperature than pure water does. 

We saw in Section 10.5 that the vapor pressure of a liquid rises with increasing temperature and that the liquid boils when its vapor pressure equals atmospheric pressure. Because a solution of a nonvolatile solute has a lower vapor pressure than a pure solvent has at a given temperature, the solution must be heated to a higher temperature to cause it to boil. Furthermore, the lower vapor pressure of the solution means that the liquid /vapor phase transition line on a phase diagram is always lower for the solution than for the pure solvent. As a result, the triplepoint temperature Tt is lower for the solution, the solid/liquid phase transition line is shifted to a lower temperature for the solution, and the solution must be cooled to a lower temperature to freeze. Figure 11.12 shows the situation. 

One type of extrinsic deviation is found in the lowering of the freezing point or the raising of the boiling point for small liquid droplets from that for the bulk state. Such effects are usually attributed to the absence of phase transition nuclei. The absence of such nuclei stems from the fact that the bulk material from which the aerosol particles are formed probably contains only minute traces of foreign material (nuclei) per unit volume, so that there is only a very small probability that any small aerosol particle will contain even one nucleus. This circumstance results in the situation that nearly all aerosol particles formed by vapor condensation and subsequent cooling well below the melting point of the parent material are likely to be in a... 

The same techniques developed for bulk fats can be equally employed to measure the crystallization of emulsified oils. In these cases, the experiments tend to be easier as the contraction of the fat on freezing does not lead to the formation of air spaces in the sample, but, on the other hand, because a smaller volume of material is undergoing a phase transition, the magnitude of the signal change, and thus the precision, is lower. 

Moving from structure to dynamics, we recall the NMR results, [Ty91, Ya91a] which indicate that the molecules in solid Ceo tumble rapidly at room temperature but freeze at lower temperatures. Considering the structural data reviewed above, one would expect a more-or-less continuous rotational diffusion in the room temperature fee phase, and either small oscillations or molecular jumps between favorable orientations in the low temperature phase. This difference was very clearly seen in the overall spin-lattice relaxation time near the orientational phase transition.[Ty91b] It was also found that the... 

The preferred nucleation in the subsurface can be due to several factors. First, the volume change upon the phase transition (change in the molar volume upon freezing is positive for water) will direct the process to the region of lower density, i.e., towards the interface. However, the water molecules at the very interface are very disordered and undercoordinated and cannot freeze. 

Figure 41.1 shows the gel-to-liquid crystalline phase transition temperatures (Tm) of DPPC-cholesterol mixtures as a function of the cholesterol-lipid molar ratio. The Tm of fully hydrated DPPC is 42°C (Crowe and Crowe, 1988 Vist and Davis, 1990 McMullen et al., 1993 Ohtake et al., 2004). Upon the addition of cholesterol, the transition enthalpy decreases continuously imtil it is no longer observable at 50 mol% cholesterol. The disappearance of the melting transition has been attributed to strong interactions between cholesterol and DPPC (McCoimell, 2003). Upon dehydration, the Tm for DPPC increases from 42 to 105°C (Crowe and Crowe, 1988 Ohtake et al., 2004). This Tm increase is caused by the reduction in the spacing between the phospholipids, which allows for increased van der Waals interactions between the lipid hydrocarbon chains (Koster et al., 1994). Between 10 and 70 mol% cholesterol, two endothermic transitions are observed, both lower than the Tm of the pure phospholipid (Figure 41.1). High-sensitivity DSC studies on fully hydrated DPPC-cholesterol systems reported endotherms consisting of two components, suggesting the existence of domains enriched/depleted in cholesterol (Vist and Davis, 1990 McMullen et al., 1993). The two peaks present in our freeze-dried systems also suggest the...

The difference between a freezing point determined in the presence of air at a pressure of one atmosphere and the triple-point temperature will depend on the effect of dissolved air and on the pressure difference. The presence of dissolved gas in the liquid phase always lowers the transition temperature. The amount by which it does so can be calculated from the freezing point depression equations if the solubility is known and ideal behaviour is assumed. The effect of pressure can be calculated from the Clapeyron equation... 

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