Freezing eutectic temperature

Salt Brines The typical curve of freezing point is shown in Fig. II-IIO. Brine of concentration x (water concentration is I-x) will not solidify at 0°C (freezing temperature for water, point A). When the temperature drops to B, the first ciystal of ice is formed. As the temperature decreases to C, ice ciystals continue to form and their mixture with the brine solution forms the slush. At the point C there will be part ice in the mixture /(/i+L), and liquid (brine) /i/(/i-t-L). At point D there is mixture of mi parts eutectic brine solution Di [concentration mi/(mi-t-mg)], and mo parts of ice [concentration mol m -t- mo)]. Coohng the mixture below D solidifies the entire solution at the eutectic temperature. Eutectic temperature is the lowest temperature that can be reached with no solidification. 

The term eutectic temperature is often misused in reference to freeze-drying. A eutectic mixture—an... 

Dried product resistance normally increases with increasing solute concentration and frequently decreases as the temperature of the frozen product approaches the eutectic temperature or T., . Production of larger ice crystals by a lower degree of water supercooling and/or an annealing process during freezing may also decrease the resistance. 

To freeze a substance, it must be cooled to such a temperature at which the water and the solids are fully crystallized, or at which areas of crystallized ice and solids are enclosed in zones in which amorphous concentrated solids and water remain in mechanically solid state (see Section 1.1.2). In the zone of freezing, the ice crystals are growing first, thus concentrating the remaining solution, which can vary the pH value. In many substances a eutectic temperature can be determined, but in many others this value does not exist. The crystallization depends on several factors which influence each other cooling velocity, initial concentration, end temperature of cooling, and the time at this temperature. In several products no crystallization takes place and the product remains in an amorphous, glasslike phase, or a mixture of both occurs. 

As an example, liquefied mixtures are easily formed by using some amino acids protected with acetyl, Boc-, Fmoc- and Z-groups. Some authors have called the resulting systems eutectic mixtures , but this term should be reserved for the textbook definition as the composition of minimum melting point that freezes isothermally at the eutectic temperature. Hence we refer to them as eutectic melts . 

Terentier and Kadeter [3.28] described the freeze-drying of the vaccine Yersinia pestis EV 76 in a solution containing 10% sucrose, 1% gelatin and 0.5% thiourea. The product was frozen on the shelves of a freeze-drying plant at -8 °C/min to -40 °C. From ER measurements it was concluded that below -24.4 °C a glass phase started and the eutectic temperature, Te was -17.1 °C. The drying time was determined as 9 h. If Tsh was controlled in such a way that Te was exceeded after 4.5 h, the survival rate fell to -50% if Te was reached after 6 h, the survival rate was -80%. One can assume that M D should only be terminated after 5 h or more, at which time the temperature could be raised. 

Fig. 5.2. Salt precipitation during seawater freezing to the eutectic temperatures along the Ringer-Nelson-Thompson and Gitterman pathways Reprinted from Marion et al. (1999) with permission...

If Soln. E is allowed to dry by evaporation at 0°C (Fig. 5.14c) or freezing to the eutectic (Fig. 5.14d), then the distribution of precipitated salts is similar to Soln. C except for the large increase in MgSC>4 salts (cf. Figs. 5.14a,b with 5.14c,d). Exactly the same suite of salts precipitate for Solns. C and E. Also, the eutectic temperature is the same (—35.4°C). For Soln. E, the salt quantities fall in the order (MgNa)SC>4 > NaCl > (MgCa)CC>3, in agreement with estimates of salt distribution on the Martian surface (Clark and Van Hart 1981). 

A somewhat surprising result was that 1460 bars of pressure had little effect on the eutectic temperature (238.65 K vs. 237.45 K) (Fig. 5.22). A pressure of 1460 bars, per se, would decrease the freezing point of pure water by about —14.8K (Fig. 3.3). Dropping the temperature at which ice first formed by 12K had only a minor effect on the eutectic (AT = 1.2K) (Fig. 5.22). This is not, however, always the case. For example, for the simpler NaCl-H20 system, the calculated eutectic temperature at lbar is —21.3°C at 1460bars of pressure, the calculated eutectic temperature is —31.3°C (AT = 10.OK). As we point out repeatedly, chemical systems and their response to temperature and pressure depend, ultimately, on thermal and volumetric properties of individual constituents, which makes every system response highly individualistic. 

On further cooling to just below the eutectic temperature, the remaining liquid which has the eutectic composition will freeze immediately according to the eutectic reaction. The solid structure will thus be the mixture of the primary phase of A and the eutectic structure which is the fine mixture of A and B. 

Most inorganic salts, when they melt, are found to flow and conduct electricity according to a simple Arrhenius law at all temperatures down to their melting points. For instance, unless measurements of high precision are used, the alkali halides appear to remain obedient to the Arrhenius equation even down to the deep eutectic temperatures of their mixtures with other salts. LiCl and KCl form a eutectic mixture with a freezing point of 351°C, some 300 K below either pure salt freezing point, yet the viscosity of the melt barely departs from Arrhenius behavior before freezing. 

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