An explanation based on kinetic considerations is developed for the failure of some pure liquids to crystallize. The important parameters are (1) α, the ratio of the molar liquid-solid surface tension to the heat of fusion ΔHf, (2) β, the ratio of the entropy of fusion to R, (3) ΔG′, the kinetic barrier to nucleation, and (4) ΔG″, the kinetic barrier to the growth of the finite crystal. In the absence of foreign nucleating agents sensible crystallization will not occur if αβ1/30.9 and ΔG′ 0, or if ΔG′ or ΔG″ 30(ΔHf/β) = 30 RTm. In nonreconstructive crystallization the molecules incorporated into the crystal are the same as those existing in the liquid. In this case AG' and AG" are likely to be the same and of the order of magnitude of the free energy of activation for fluid flow, ΔGf. Usually, αβ 1/3∼1/2 and ΔG/f≪30 RTm, in nonreconstructive crystallization and the nucleation of crystals in clean liquids, is sensible at some undercooling. There are some organic liquids in which crystallization is nonreconstructive and that do not crystallize when clean. We have found that the ratio Tb/Tm of the normal boiling point to the melting point is abnormally large in these cases and near two or more for those compounds that clearly form glasses. Since α and ΔG′/RTm are both likely to increase with T b/Tm, the difficulty of crystallization would increase with this ratio. It is probable that this correlation also holds for inorganic molecular liquids. In reconstructive crystallization the molecular entities in the crystal differ from those which predominate in the liquid and bonds between atoms must be broken in order for nucleation to occur. Thus ΔG′ can be of the order of the bond energy, which often exceeds 30 RTm, and, excepting in network liquids, much greater than ΔGf. Bonds between atoms also must be broken for the reconstructive growth of finite crystals. However, this process can be promoted by impurities and molecular fragments in ways not possible in homogeneous nucleation so that ΔG″ as well as ΔGf can be much less than ΔG′;. The reconstructive growth of finite crystals at the expense of a different crystal phase can be enhanced similarly so that ΔG″ may be much less than ΔG′ in this case also.
All Science Journal Classification (ASJC) codes
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry