6.0 Introduction
The Abraham general solvation equation (6.1) is of great interest in understanding physicochemical and biochemical phenomena in which solutes distribute between the gas phase and a condensed phase. ^ As explained in chapter 3, equation (6.1) consists on a linear combination of five solvation descriptors which represent the solute physicochemical properties. E is an excess molar refraction, S is the dipolarity / polarizability, A and B are the overall hydrogen-bond acidity and basicity. Finally, L is defined through logL^^, where is the solute Ostwald solubility coefficient on n- hexadecane at 298K.*
SP = c + g.E + iS.S + <2.A + 6 B + /.L (6.1)
The dependent variable, SP, is the logarithmic value of ‘some property’ of a series of solute in a given phase system. The regression coefficients, c, e, s, a, 6, and I are found by multiple linear regression analysis, MLR A, and reflect the complementary properties of the solvent phase or biophase. Equation (6.1) has been used to analyse and predict numerous gas / solvent partitions, gas / biophase systems, and to a large number of gas chromatographic systems. It is then a well-trained and tested equation. However, the use of equation (6.1) depends on determination of the solute descriptors that are mostly derived from experimental gas liquid chromatographic data, see chapter 3. They have been obtained for more than 3000 solutes and compiled in an in-house database. Out of this database, L is the descriptor for which least values are available. Therefore, in order to make a wider use of equation (6.1), L values need to be assigned for greater number of solutes.^
The solute descriptor, L, initially formulated by Abraham et al., characterises the solute size and its tendency to participate in solute / solvent interactions of the general London dispersion type.^ L is now a well-established descriptor in linear free energy relationship. Hence, it is not surprising that several studies on L determination
have been carried out. L values were originally measured on n-hexadecane stationary phase at 298K.^ However, this technique is limited to volatile and semi-volatile solutes and is often replaced by a number of alternative methods.^'^ These approaches based either on experimental or empirical data were presented in detail in chapter 3.
Other hydrocarbon-like stationary phases, such as squalane, apiezon and apolane, have been proposed as substitutes for n-hexadecane.^ The main advantage of these hydrocarbons is that a larger spread of solutes can be analysed; experiments with such solvent phases are not limited to volatile or semi-volatile compounds but also include non-volatile one^ Two works in this field are presented here. First, Weckwerth and co-workers^ have recently shown that gas / apolane partition coefficient values, correlate with the descriptor L. Apolane, a highly branched nonpolar hydrocarbon synthesised by Ko vats is a stable non-volatile stationary phase that can be used over the temperature range of 300-553K. The authors measured and values for 139 solutes by open-tubular capillary GLC at 313K and 298K respectively, and put forward a strong relationship between logL^^ and log or L, see equation (6.2). Then, knowing the values, it is easy to calculate the corresponding L values. However, it is important to note that only a few L values have been derived from gas / apolane partition coefficient values, so far.
LogL‘® = L = 0.175 (0.024) + 1.1004 (0.0082) log (6.2)
n = 139 y =0.992 ,sd = 0.093
Here and elsewhere, n is the number of data points, r is the overall correlation coefficient, and sd is the standard deviation in the dependent variable. The sd values for the coefficients are given in parentheses.
This recent approach is closely related to the Abraham alternative method for estimation of L / Equation (6.1) provides actually a number of options for the direct determination of L. Values of L can be obtained through GLC measurements on non polar solvent phases in which the /-coefficient is large. GLC retention data can then be fitted to an equation of the form,
Here, SP can be the logarithmic value of retention volume, or relative retention index, or gas / solvent partition coefficients, or SP can be the retention index, I. Since E can be calculated with sufficient accuracy from structure or calculated directly from the refractive index of liquids, this method is an easy way to obtain L value for any given solutes. This method based on the correlation of retention properties on low polarity phases other than n-hexadecane, is a well-established method that has led to a considerable amount of L values.^
In this work, the Abraham method for estimation of L is favoured. Equations similar to equation (6.3) have been developed for gas / squalane partitions, at 298K and gas / apolane partitions, at 298K and 313K. From these equations, some 146 new L values were determined. An equation for water / squalane partitions at 298K was also developed according to the Abraham solvation equation for processes within condensed phases\ see equation (6.4).
SP = c + e.E + 5.S 4- a.A + b.B +v. V (6.4)
This equation for water / squalane partitions together with the equation for gas / squalane systems were compared with similar equations for water / alkanes and gas / alkanes previously developed by Abraham and co-workers.^
6.1. Construction of Solvation Equations for Gas / Alkane and
Water / Alkane Partition Process
6.1.1. Construction o f an Equation f o r log LP9
Squalane (2,6,10,15,19,23-hexamethyltetracosane) is a stable non-polar and non-volatile solvent that can be used over the temperature range of 293-423 K. Thanks to these properties, squalane has been extensively used, and considerable quantities of thermodynamic properties for a wide range of solutes have been accumulated. A survey of the literature showed that there were enough data on squalane at 298K to set up a statistically significant regression equation similar to equation (6.3) where the dependent variable, SP, is the gas / squalane partition coefficient, Next, are presented the various methods in use for the calculation of values from literature data.
6.1.1.1 Calculation of Gas / Squalane Partition Coefficient.
Gas / squalane partition coefficient, at 298K for 396 varied solutes have been obtained from the literature. Some of these values were available as such,^'^"^ others were calculated from the reported activity coefficient^^’^^, y, and Henry’s law coefficient^^’^^, at 298K. A large number of values were derived from gas chromatographic data, such as the solute specific retention volume^’^^’^^’^®, Vg, and the unified retention index at 298K^^'^, UI2 9 8- values for five solids were calculated from their solubility in squalane. The transformation of these several constants into gas / squalane partition coefficients is now covered.
First, activity coefficient and Henry's law coefficient values were transformed into values by means of the following equations.
l