Freezing point suppression

When a freeze point suppressant is added to water, it loses its heat transfer characteristics to some extent, which depends on the concentration of the suppressant. However, water-based fluids still exhibit a very high heat transfer coefficient compared to nonaqueous fluids. Nonaqueous fluids such as aromatics, aliphatics, and silicones are used in such applications when an aqueous fluid exhibits very high viscosity (>50cP) at the lowest operating temperature or its freezing point is very close to the refrigerant temperature in the evaporator. 

Primary or polyfunctional alcohols such as methanol and ethylene glycol have been used as freezing point suppressants. This could be seen routinely in the antifreezing windshield wash solution used for automobiles. Typically, a 25% methanol solution decreases the freezing point of water by about 36°F to about —4°F, which is called the depressed ice point. Similarly, a typical natural gas forms hydrates in the presence of free water at about 63 °F, whereas it is depressed to 39°F (24°F hydrate depression) in the presence of a 25% methanol solution. Normal and depressed hydrate points are functions of... 

The confinement of water in nanometer-size pores between the walls of the polymer material affects the structure of water. The observed freezing-point suppression [49,50] and reduced dielectric constant of water [115-117], are macroscopic manifestations of this effect. The interfacial area between polymer and water provides a complex environment for proton transport. The complications for the theoretical description are caused by the flexibiHty of the sidechains, their random distributions and their partial penetration into the bulk of water-filled pores. The charged polymer sidechains contribute elastic ( entropic ) and electrostatic terms to the free energy [54,55]. Distribution and mobilities of protons depend strongly on the resulting sidechain-water interactions [54,56,118]. 

At the macroscopic level, proton transport can be studied with electrochemical impedance spectroscopy (EIS). Cappadonia et al. (1994,1995) performed EIS studies to explore variations of proton conductivity with water content and temperature for Nafion 117. The Arrhenius representation of conductivity data revealed activation energies between 0.36 eV at lowest hydration and 0.11 eV at highest hydration, as shown in Figure 2.6. The transition occurs at a critical water content of A-crit 3. At fixed X, the transition between low and high activation energies was observed at 260 K for well-hydrated membranes. This finding was interpreted as a freezing point suppression due to confinement of water in small pores. 

A freezing point suppression has also been found by Cappadonia et al. in Arrhenius plots of conductivity data (Cappadonia et al., 1994,1995). Free water in PEMs possesses the same melting point as bulk water, and it sustains high bulk-like mobility of protons and water. In comparison of the different classification schemes, there seems to be a correlation between surface water and nonfreezable/freezable bound water, but the correspondence is not unique. The distinction of freezable-bound and free water is vague. 

Freezing point. Solvents that are liquids at all anticipated ambient temperatures are desirable since they avoid the need for freeze protection and/or thawing of frozen solvent prior to use. Sometimes an antifreeze compound such as water or an aliphatic hydrocarbon can be added to the solvent, or the solvent is supplied as a mixture of related compounds instead of a single pure component—to suppress the freezing point. 

Thus, if the nucleation explanation is correct, it would only be too cold to snow when T decreases well below T /3. This would imply temperatures below 91 K, or -182 °C This is far colder than any recorded temperature on Earth, indicating that the nucleation factor is not an adequate explanation for snowfall suppression at frigid temperatures. The better explanation considers the water vapor capacity of the atmosphere, which decreases exponentially with decreasing temperature. Cold air holds exponentially less water vapor than warm air, and so the precipitation potential decreases dramatically as temperatures become more frigid. This is the main reason why warm snowstorms, where the air temperature is just slightly below the freezing point, typically produce the greatest snowfall rates. 

Water-based PEMs exhibit proton transport mechanisms and mobilities similar to those in liquid electrolytes like hydrochloric acid proton conductivities could reach up to 0.1 S cm in the case of PFSA-type ionomers, and up to 0.5 S cm in the case of block copolymer systems. The temperature range of operation of PEMs stretches from —30°C to 90°C, the lower bound being determined by the freezing point of water, which is suppressed because of the high surface energy of water in nanopores. The upper limit is determined by evaporation of water only a few water-based PEMs have been demonstrated that could maintain a sufficient conductivity above lOO C. 

In multi-component liquids, stabilization of the liquid is revealed by the formation of eutectics where the freezing temperature is suppressed. In such liquids, the atomic species (say A and B) are not distributed at random. There are more associated AB pairs (or other clusters) than expected for a random distribution. As a result in binary metal-metalloid alloys, such as Fe-B, the low melting-point eutectics occur at preferential compositions. The most common of these is at about 17 at. % B, or an atom ratio of one B for five Fe atoms (Gilman, 1978). This suggests that clusters of metal atoms surrounding metalloid atoms form (trigonal bipyramids). These probably share corners, edges, and faces. 

---