Heterogeneous freezing

Elias, P. M., Friend, D. S. and Goerke, J. 1979. Membrane sterol heterogeneity. Freeze-fracture detection with saponins and filipin. J. Histochem. Cytochem. 27, 1247-1260. 

It has been pointed out recently, that the question of homogenous and heterogeneous freezing of stratospheric aerosols is one of the key questions in the understanding of the physico-chemical behavior of the PSCs. According to the phase diagram of liquid sulfuric acid solutions, PSCs were initially believed to consist solely of solid particles, while it now seems clear from experimental observations and theoretical considerations that most of the PSCs stay liquid above the ice frost point.It is still not clear, however, why sometimes solid PSCs are found above the ice frost point, and how they are formed. 

Prom these results we conclude that homogeneous freezing of stratospheric aerosols is only of importance at very low temperatures which might be reached in arctic regions only for short periods and in limited regions of space. The observed solid PSC particles are therefore most likely due to heterogeneous freezing. 

Urey, H. C., L. H. Dawsey and F. O. Rice (1929) The absorption spectrum and decomposition of hydrogen peroxide by light. Journal of the American Chemical Society 51, 1371-83 Uri, N. (1952) Inorganic free radicals in solution. Chemical Reviews 50, 375-454 Vali, G. (2008) Repeatability and randomness in heterogeneous freezing nucleation. Atmospheric Chemistry and Physics 8, 5017-5031... 

Cantrell W, Robinson C (2006) Heterogeneous freezing of ammonium sulfate and sodium chloride solutions by long chain alcohols. Geophys Res Lett 33 L07802. doi 10.1029/ 2005GL024945... 

As air parcels rise through the 0°C isotherm the droplets remaining become supercooled and ice begins to form at slightly lower temperatures. Laboratory and field studies show that at temperatures higher than w -35°C most atmospheric ice crystals form primarily via heterogeneous freezing of supercooled... 

P. J. DeMott, D. C. Rogers, S. M. Kreidenweis and Y. Chen. The role of heterogeneous freezing nucleation in upper tropospheric clouds inferences from SUCCESS. Submitted, Geophys. Res. Lett. 1997. 

Dispersion Characteristics The chief characteristics of gas-in-liquid dispersions, like those of hquid-in-gas suspensions, are heterogeneity and instabihty. The composition and structure of an unstable dispersion must be obsei ved in the dynamic situation by looking at the mixture, with or without the aid of optical devices, or by photographing it, preferably in nominal steady state photographs usually are required for quantitative treatment. Stable foams may be examined after the fact of their creation if they are sufficiently robust or if an immobilizing technique such as freezing is employed [Chang et al., Ind. Eng Chem., 48, 2035 (1956)]. 

On the other hand, operations such as distilling or freezing usually tend to separate solutions into the pure substances that were the components of the solution. The nearer alike the components are, the harder it is to separate them from the solution, but even in difficult cases, a variety of methods in succession usually brings about a separation. In nature, solutions are much more common than pure substances, and heterogeneous systems are more common than solutions. When we want pure substances, we often must prepare them from solutions through successive phase changes. 

Reid et al. [ 1.12] described the effect of 1 % addition certain polymers on the heterogeneous nucleation rate at-18 °C the rate was 30 times greater than in distilled, microfiltered water and at -15 °C, the factor was still 10 fold hogher. All added polymers (1 %) influenced the nucleation rate in a more or less temperature-dependent manner. However, the authors could not identify a connection between the polymer structure and nucleation rate. None the less it became clear that the growth of dendritic ice crystals depended on to factors (i) the concentration of the solution (5 % to 30 % sucrose) and (ii) the rate at which the phase boundary water - ice crystals moved. However, the growth was found to be independent of the freezing rate. (Note of the author the freezing rate influences the boundary rate). 

Talsma et al. [1.34] described the freezing behavior of certain liposomes by DSC measurements. Besides the expected influences of freezing and rewarming speeds, and of the CPAs (mannitol and mannitol in Tris-buffer solutions) it was shown, that the heterogeneous and homogeneous crystallization in mannitol solutions exists and the nucleation of ice depends also on the liposome size In small liposomes (e. g. 0.14 pm) mannitol suppressed the heterogeneous crystallization more effectively than in large (0.87 pm) liposomes. If in certain substances no crystallization or eutectic mixtures can be found by DSC (cephalosporin, Williams [1.35]) with the used experimental conditions, one has to seek different conditions [1.32]. 

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