© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

10.5.3 Solubility and Solvents

Gas solubility always declines, while the solubility of most solids rises, with higher temperature (Section 9.2.6). Liquid solutes behave as solids in this regard, but note that unit volumes of miscible fluids are not precisely additive. For example, if 1 liter of ethanol is added to 1 liter of water, the result is only 1.93 liters of solution.²⁰³⁶

A solution of a nonvolatile nonelectrolyte solute (e.g., glucose) has a boiling point T_boil that is elevated above the boiling point of pure solvent by DT_boil = n k_b Ml, where Ml is the molality of the solution (moles solute per kilogram of solvent), k_b is the boiling point constant for the solvent (Table 10.7), and n = 1. The presence of solute also slightly lowers solvent freezing point T_freeze by DT_freeze = n k_f Ml, where k_f is the freezing point constant for the solvent (Table 10.7). Both effects are a direct consequence of the lowering of solvent vapor pressure by a solute. The constants may be estimated as:³⁹⁰

{Eqn. 10.22}

{Eqn. 10.23}

where R = 8.31 J/mole-K (universal gas constant), MW is molecular weight in gm/mole or daltons, T_freeze and T_boil in K, and DH_fus and DH_vap are the heats of fusion and vaporization, respectively (Table 10.8).

In the case of electrolyte solutes (e.g., KCl) in dilute solutions (e.g., Ml <~ 0.01 molal), n is approximately the moles of ions per mole of solute (e.g., n ~ 2 for KCl). In more concentrated solutions, experimental values of n are slightly lower, as explained by Debye-Huckel theory due to ion-ion and ion-solvent interactions. For example, with KCl in water, n = 1.94 at Ml = 0.01 molal but n = 1.80 at Ml = 0.50 molal.²⁰⁵⁰ The most concentrated salt solution (e.g., 6.2 molal NaCl) depresses the freezing point of water by ~20°C.

At still lower temperatures, solvents with much lower melting points may be employed to avoid solvent freezing. In general, substances dissolve in liquids that are chemically similar. Water (itself a polar solvent) and many organic compounds including glucose are soluble in ethanol, which remains a liquid to -117°C (156 K).⁷⁶³ Sodium chloride is slightly soluble in ethanol, and also in liquid ammonia which melts at -78°C (195 K).⁷⁶³ Some natural enzymes are known to retain their function in liquid ammonia, and others in supercritical carbon dioxide; artificial enzyme systems based on natural peptides, or other polymers with protein-like conformational properties, could in principle operate in cryogenic solvents such as tetrafluoromethane,²⁶¹ carbon monoxide (a polar solvent, liquid from -191.5°C to -199°C) or liquid nitrogen (a nonpolar solvent, liquid -195.8°C to -209.9°C). Lactate dehydrogenase enzyme found in cold water Antarctic fishes operates as fast as the related enzyme in animals with higher body temperatures, even though enzyme action normally halves for every 10°C temperature drop (Section 4.6.4). The Antarctic fish enzyme compensates for the cold with modifications near the enzyme's active site that increase flexibility and mobility, in effect "greasing the hinges so that the enzyme can move more quickly."²¹⁵²

Last updated on 24 February 2003