Freezing-point depression Reduction

Many chemicals when added to water cause a freezing point depression, as shown in Table 1, and thus are termed antifreezes. The antifreeze properties of these chemicals vary widely as a function of their coUigative, or concentrative, properties. The reduction in freeze point depends both on the chemical itself and the concentration of the chemical in water. The freeze point depression increases as the antifreeze chemical is added to the water, until a characteristic concentration is achieved. Further addition of the antifreeze chemical to water will either result in insolubility or serve to increase the freezing point of the mixture, as illustrated in Figure 1. 

The depression of freezing point or reduction of the vapour pressure of a solvent is proportional to the molar concentration of the solute. 

The phenomena of freezing-point depression, boiling-point elevation, and osmotic pressure all result from the reduction in the escaping tendency of solvent in a solution due to the addition of solute. They can all be used to determine the activity of the solvent. For example, as long as pure solvent freezes out of the... 

There are four main colligative properties, or properties of a solvent that are affected by the presence of a solute vapor-pressure reduction, boiling-point elevation, freezing-point depression, and osmotic pressure. 

If a 20% fructose solution is selected. Young [17] shows in diagram (Fig. 2.63), that pure ice freezes out from this mixture at appr. -2.5°C this is a result of the known phenomenon of freezing point reduction in solutions. The freezing-out of pure ice, however, causes an increase in the sugar concentration and thus a further freezing point depression. This continues until a mixture of ice and fmctose dihydrate is present at -10°C. This lowest common solidification point is called eutectic point . 

Increases in pressure increase the solubility of gaseous solutes, but have little effect on solid solutes. Similarly, decreases in pressure decrease the solubility of gases in liquids and have little effect on solid solutes. There are four main coUigative properties, or properties of a solvent that are affected by the presence of a solute vapor-pressure reduction, boiling-point elevation, freezing-point depression, and osmotic pressure. 

The colligative properties of solutions, e.g. osmotic pressure, boiling point elevations, freezing point depression and vapour pressure reduction, depend on the effect of solute concentration on the solvent aetivity. The ehemieal potential, yt, of a non-electrolyte in dilute solution may be expressed by... 

According to this, the freezing-point depression is proportional to the relative reduction of vapor pressure or to the molar loading of the solute. For dissociation of the solnte, the number of emerging ions has to be considered again. Also, here it is reqnired to assume that the solute has a neghgible vapor pressure and that the solntion is dilute. 

Data reduction of cryoscopic measurements is made by applying the relation for the freezing point depression of a binary mixture to obtain solvent activities ... 

Lambert C, Montreuil C, Vanderslice J (2003) Application of Organic Freeze-Point Depressants in Aqueous Urea Solutions Effect on NO Reduction. SAE Technical Paper... 

The results stated so far has been with saturated vapor or liquid as the equilibrium bulk phase. Liquid-like state in pore, however, can hold with reduced vapor pressure in bulk the well-known capillary condensed state. One of the most important feature of the capillary condensation is the liquid s pressure Young-Laplace effect of the curved surface of the capillary-condensed liquid will pull up the liquid and reduce its pressure, which can easily reach down to a negative value. In the section 2 we modeled the elevated freezing point as a result of increased pressure caused by the compression by the excess potential. An extension of this concept will lead to an expectation that the capillary-condensed liquid, or liquid under tensile condition, must be accompanied with depressed freezing temperature compared with that under saturated vapor. Then, even at a constant temperature, a reduction in equilibrium vapor pressure would cause phase transition. In the following another simulation study will show this behavior. 

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