Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.1 Solvent effects on chemical reactivity 755
attributed to the weak basicity of MeNO2 which poorly solvates cations. As a result, ion
pairing is stronger in MeNO2 in spite of the fact that long-range ion-ion interactions in the
two solvents are equal.
Finally, a potential problem with polarity rests in the fact that this term is typically associated
with enthalpy. But caution is urged in interpreting the like-dissolves-like rule in
terms of enthalpy. It is often stated for example that nonpolar liquids such as octane and carbon
tetrachloride are miscible because the molecules are held together by weak dispersion
forces. However, spontaneous mixing of the two phases is driven not by enthalpy, but by entropy.
Water’s anomalies
The outstanding properties and anomalies of water have fascinated and likewise intrigued
physicists and physical chemists for a long time. During the past decades much effort has
been devoted to finding phenomenological models that explain the (roughly ten) anomalous
thermodynamic and kinetic properties, including the density maximum at 4°C, the expansion
upon freezing, the isothermal compressibility minimum at 46°C, the high heat capacity,
the decrease of viscosity with pressure, and the remarkable variety of crystalline
structures. Furthermore, isotope effects on the densities and transport properties do not possess
the ordinary mass or square-root-mass behavior.
Some of these properties are known from long ago, but their origin has been controversial.
From the increasingly unmanageable number of papers that have been published on
the topic, let us quote only a few that appear to be essential. Above all, it seems to be clear
that the exceptional behavior of water is not simply due to hydrogen bonding, but instead
due to additional “trivial” vdW forces as present in any liquid. A hydrogen bond occurs
when a hydrogen atom is shared between generally two electronegative atoms; vdW attractions
arise from interactions among fixed or induced dipoles. The superimposition and competition
of both is satisfactorily accommodated in the framework of a “mixture model”.
The mixture model for liquid water, promoted in an embryonic form by Röntgen 64
over a century ago, but later discredited by Kauzmann 65 and others, 66 is increasingly gaining
ground. Accordingly there are supposed to be two major types of intermolecular bonding
configurations, an open bonding form, with a low density, such as occurs in ice-Ih, plus a
dense bonding form, such as occurs in the most thermodynamically stable dense forms of
ice, e.g., ice-II, -III, -V, and -VI. 67 In these terms, water has many properties of the glassy
states associated with multiple hydrogen-bond network structures. 68 Clearly, for fluid properties,
discrete units, (H2O) n, which can move independently of each other are required. The
clusters could well be octamers dissociating into tetramers, or decamers dissociating into
pentamers. 69,70 (Note by the way that the unit cell of ice contains eight water molecules.)
However, this mixture is not conceived to be a mixture of ices, but rather is a dynamic (rapidly
fluctuating) mixture of intermolecular bonding types found in the polymorphs of ice. A
theoretical study of the dynamics of liquid water has shown that there exist local collective
motions of water molecules and fluctuation associated with hydrogen bond rearrangement
dynamics. 68 The half-life of a single H bond estimated from transition theory is about
2x10 -10 s at 300K. 71 In view of this tiny lifetime it seems more relevant to identify the two
mixtures not in terms of different cluster sizes, but rather in terms of two different bonding
modes. Thus, there is a competition between dispersion interactions that favor random
dense states and hydrogen bonding that favors ordered open states. Experimental verification
of the two types of bonding has been reviewed by Cho et al. 72