Isotope Effects
Another area of compatibility of our mixed bonding model with the real liquid is the "thermal lag" effect described in Refs. [3], [20] and [33]. The thermal lag is a structural effect caused by the difference in zero-point energies of the heavier isotopic forms compared with that of H2O. This purely liquid state effect is directly responsible for the increased temperatures of maximum density for D2O (11.185°C), and T2O (~13.4°C) as compared with H2O (3.984°C). When applied to the viscosities , a vibrational [45] (MD2O/MH2O)1/2 , not a rotational, mass effect has been seen to emerge over a wide temperature range. This result removes past questions [40] arising from the supposedly non-typical isotopic mass dependence of transport properties in water. For example, the huge D2O/H2O viscosity ratios at very low temperatures, which have been a concern in the past [41], become modified to a normal square-root-mass ratio when the thermal lag effect is applied. This thermal lag concept may be used to obtain temperature-pressure dependent structural properties, such as the density or the viscosity, for D2O and T2O when such data are not available (as is the case for T2O viscosities), or serve as a check by using the higher precision data available for H2O. Also, we have shown [46] that it is possible to convert any of the existing empirical viscosity relationships for H2O to obtain D2O viscosities within a 1% uncertainty over a wide temperature range without additional parameter adjustment.
The thermal lag concept, which was an outgrowth of our type-I/type-II outer structure mixture model [20] depends on achieving a structural equivalence of these isotopic forms for analysis of the densities and the dynamics. This means that a higher temperature is required in these isotopic modifications of water to reach the same degree of mixed bonding as in H2O. Since isotopic substitution experiments on water are so important in biology and chemistry, it is important that isotope effects in this solvent are understood. Very recently, our thermal lag effect has been used in other laboratories to help understand H2O/D2O isotope effects on dielectric relaxation in water [42], and to explain lysozyme solubilities [43] and second virial coefficients of lysozyme-lysozyme interactions [44] in D2O and H2O.
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