Gary S. Whiting

Hydrogen bonding: XVI. A new solute salvation parameter, π2H, from gas chromatographic data

Abstract The general salvation equation, log VG0 (or log L) = c + rR2 + sπ2H + aα2H + bβ2H + l log L16 has been used to set up a new π2H parameter of solute dipolarity-polarisability, mainly through the extensive data of McReynolds and Patte et al. Values of π2H are tabulated for several hundred solutes, and two simple rules have been formulated to enable π2H to be estimated for many types of aliphatic functionally substituted compounds. A coherent set of effective solvation parameters, Σπ2H, Σα2H, Σβ2H, and also R2 and log L16, allows the application of the general solvation equation to the characterisation of any gas-liquid chromatographic stationary phase.

Hydrogen bonding. Part 34. The factors that influence the solubility of gases and vapours in water at 298 K, and a new method for its determination

The solubility of 408 gaseous compounds in water at 298 K has been correlated through eqn. (i), where the solubility is expressed as the Ostwald solubility coefficient, Lw, and the solute explanatory variables are R2 an excess molar refraction, π2H the dipolarity/polarizability, Σα2H and Σβ2H the effective hydrogen-bond acidity and basicity, and Vx the McGowan characteristic volume. A similar equation using the log L16 parameter instead of Vx can also be used; L16 is the Ostwald solubility coefficient on hexadecane at 298 K. log Lw=–0.994 + 0.577R2+ 2.549 π2H+ 3.813Σα2H+ 4.841Σβ2H– 0.869 Vx(i), n= 408 ρ= 0.9976 sd = 0.151 F= 16810 The main factors leading to increased solubility are solute π2H, Σα2H and Σβ2H values; conversely, the corresponding properties of water are dipolarity/polarizability, hydrogen-bond basicity and hydrogen-bond acidity. Solute size plays a minor role, and slightly decreases solubility, contrary to observations on all non-aqueous solvents. It is shown that this peculiar behaviour of water is due to (a) a greater increase in the unfavourable cavity effect with increase in solute size, for solvent water, and (b) a smaller increase in the favourable general dispersion interaction with size, for solvent water.A new method for the determination of log Lw values is put forward, using the relationship Lw=L16/P where L16 is as above, and P is either the water–hexadecane partition coefficient or the water–alkane partition coefficient. For 14 solutes using the former P-value, agreement with values calculated through eqn. (i) is 0.08 log units on average and for 45 solutes using the latter P-value, the corresponding agreement is 0.15 log units, with log Lw values ranging up to 8 log units.

Hydrogen bonding. XV, A new characterisation of the McReynolds 77-stationary phase set

Abstract The following equation has been applied to all the phases in the McReynolds 77-stationary phase set. In this equation, V0G is the specific retention volume for a series of solutes on a given stationary phase, and the explanatory variables are R2 a modified solute molar refraction, π*2 the solute dipolarity, αH2 the solute hydrogen-bond acidity, βH2 the solute hydrogen-bond basicity, and log L16 where L16 is the solute Ostwald absorption coefficient on hexadecane at 25°c. The constants in the equation are obtained by multiple linear regression analysis, using about 150 data points in eacy regression, and values of r, s, a, b and l are regarded as characteristic constants of the phases that serve to classify the 77-phase set. It is shown that the classification of the phases into clusters is in accord with chemical principles, and is in excellent agreement with previous work using hierarchical clustering, minimum spanning tree techniques, and pattern cognition methods. The above equation allows the factors that lead to gas-liquid chromatographic separations to be identified, and provides quantitative information on the various solute-solvent interactions that give rise to these factors.

Hydrogen bonding: XVII. The characterisation of 24 gas-liquid chromatographic stationary phases studied by Poole and co-workers. including molten salts, and evaluation of solute-stationary phase interactions

Abstract The general solvation equation log K = c + rR 2 + s π 2 H + a α 2 H + b β 2 H + l log L 16 has been used to characterise 24 gas-liquid chromatographic stationary phases for which Poole and co-workers have determined log K values for a series of solutes at 121.4°C. The explanatory variables are R 2 , asolute excess molar refraction, π 2 H , the solute dipolarity, α 2 H and β 2 H , the solute hydrogen-bond acidity and basicity, and log L 16 , where L 16 is the solute gas-liquid partition coefficient on hexadecane at 25°C. It is shown that the b β 2 H term is not significant for any phase, and that the molten salts are all strongly dipolar and basic, with large s and a constants. A term-by-term analysis of the solvation equation yields a quantitative measure of the contribution to log K of various solute-stationary phase interactions, and leads to an understanding of how these interactions affect solute retention. The use of the characteristic constants c, r, s, a, b and l in the selection of stationary phases for particular separations is described.

Hydrogen bonding: XXI. Solvation parameters for alkylaromatic hydrocarbons from gas-liquid chromatographic data

Abstract The truncated solvation equation log SP = c + rR2 + llog L16 has been applied to numerous sets of gas-liquid chromatographic (GLC) data for alkylaromatic hydrocarbons on non-polar stationary phases. Here SP can be VG or can be the relative retention time, and the retention index I can in this context be used in place of log SP. R2 is the solute excess molar refraction, easily obtained from refractive index. A set of solutes of known log L16 is used to set up the equation and then values of log L16 can be back-calculated for other solutes: L16 is originally defined as the solute gas-liquid partition coefficient on hexadecane at 25°C. Through the above equation log L16 values were calculated for 190 solutes. Once log L16 is known, the reduced equation log SP = c + sπH2 + llogL16 can be applied to GLC data on polar stationary phases, and the dipolarity/polarizability parameter πH2 obtained by back-calculation in a similar way. Values of πH2 for 120 solutes are listed. It is shown that n-alkyl substituents affect the πH2 value only slightly, but ortho substituents considerably increase πH2, e.g., benzene (0.52), toluene (0.52), o-xylene (0.56), 1,2,3-trimethylbenzene (0.61), 1,2,3,4-tetra-methylbenzene (0.65), pentamethylbenzene (0.66), hexamethylbenzene (0.72).

Thermodynamics of solute transfer from water to hexadecane

New measurements of enthalpies of solution in hexadecane and in water (ΔH°s), and gas-hexadecane Ostwald solubility coefficients (LH) of neutral monomeric organic solutes are reported. These values, together with literature values of ΔHs°, LH, and gas-water Ostwald solubility coefficients (Lw), have been used to derive the Gibbs energies, enthalpies, and entropies of solute transfer from water to hexadecane (ΔG°tr′, ΔHtr′°, and ΔStr°), as well as water–hexadecane partition coefficients (as log PH). Results have been examined by the method of multiple linear regression analysis, using the equation, SP =c+dδ2+sπ2*+aα2+bβ2+vV2The sπ2* term is difficult to interpret, but the aα2 and bβ2 terms can be shown to arise through hydrogen bonding of solute molecules to the bulk water that is exothermic but rather disfavoured entropically. It is shown also that thevV2 term arises due to a combination of cavity effects and general dispersion interactions in bulk water and bulk hexadecane.

Comparison of two free energy of solvation models for characterizing selectivity of stationary phases used in gas-liquid chromatography

Two solvation energy models, developed independently for the characterization of the solvent properties of gas-liquid chromatographic stationary phases, are compared using accurately determined gas-liquid partition coefficients for 30 test solutes on 25 stationary phases at a common reference temperature. Remarkably good agreement between the regression model of Abraham and the free energy model proposed by Poole is demonstrated for the contributions to retention characterized by cavity formation, nonpolar interactions and polar interactions. Exceptional behavior for the liquid organic salts is explained by the differences between the experimental and predicted gas-liquid partition coefficients for the n-alkanes used in the two models. In addition, both models conclusively demonstrate that at the measurement temperature, 121.4°C, none of the stationary phases behave as significant hydrogen-bond acid solvents, contrary to commonly held beliefs.

Hydrogen bonding. Part 14. The characterisation of some N-substituted amides as solvents: comparison with gas–liquid chromatography stationary phases

Equations previously used for the characterisation of GLC stationary phases have been found to be equally suitable for the characterisation of common solvents. Thus equation (a) has been applied to solubility data for series of solutes on N-formylmorpholine (NFM), N-methylpyrrolidinone (NMP), N,N-dimethylformamide (DMF), and N,N-dimethylace amide (DMA).SP =c+r·R2+s·π2*+a·α2H+b·β2H+I· log L16(a)In equation (a), SP can be log V°G or log L for a series of solutes on a given solvent where V°G is the specific retention volume and L is the Ostwald solubility coefficient. The solte parameters are R2, a polarisability parameter; π2*, the solute dipolarity; α2H, the solute hydrogen-bond acidity; β2H, the solute hydrogen-bond basicity; and log L16 where L16 is the solute Ostwald solubility coefficient on n-hexadecane at 298 K.It is shown that at 298 K all four amides have about the same dipolarity, as judged by the s-constant, and have nearly the same hydrogen-bond basicity, as judged by the a·α2H term: all have zero hydrogen-bond acidity so that b= 0 in equation (a). Comparison can be made between results for NFM and NMP at 393 K and results for some GLC stationary phases. The two amides are less dipolar than tricyano(ethoxy) propane and diethyleneglycol succinate, about the same as Zonyl E-7®and Carbowax®, and more dipolar than poly(phenyl ether). The amides, however, have rather more hydrogen-bond basicity than any of the above five GLC phases. It is suggeted that equation (a) can be used as the basis of method for characterising condensed phases, such that common solvents as well as GLC stationary phases can be included within the scope of the method.

Solvation of gaseous non-electrolytes

Linear free energy equations, log L=c+sπ*2+aαH2+bβH2+l log L16 log L=c+sµ22+aαH2+bβH2+l log L16 have been used to analyse the solvation of a series of gaseous non-electrolytes in a given bulk solvent as log L values where L is the Ostwald solubility coefficient. The parameters π*2, αH2, βH2, log L16 and µ2 characterise the solutes and the constants c, s, a, b and l are obtained by multiple linear-regression analysis. It is shown that for solvation in the bulk solvents ethyl acetate, acetonitrile, ethanol and methanol, the contribution of hydrogen-bonding terms to solvation is quite small, the main contributing terms being an endoergic cavity term and an exoergic solute–solvent dispersion interaction term. Even with bulk water as the solvent, hydrogen-bonding interactions of the type solute (base)–water (acid) and solute (acid)–water (base) are not more than ca. -15 or -11 kJ mol–1, respectively, for DMSO (base) and ethanol (acid). It is shown also that linear free energy equations can be used for the correlation and prediction of the solubility of gaseous solutes in a given liquid phase, even when the latter is polymeric in nature.

Hydrogen bonding: XIX. The characterisation of two poly(methylphenylsiloxane)s

Abstract Two commercial samples of poly(methylphenylsiloxane) were characterised using our salvation equation, log L = c + rR 2 + s π 2 H + a α 2 H + b β 2 H + l log L 16 where L is the gas-liquid partition coefficient for a series of solutes on a given stationary phase, and the explanatory variables are R 2 an excess molar refraction, π 2 H the solute dipolarity/polarisability, α 2 H and β 2 H the solute hydrogen-bond acidity and basicity, and log L 16 where L 16 is the solute gas-liquid partition coefficient on hexadecane at 25°C. For both samples, a substantial b - constant was found, viz . 1.22 ± 0.07 and 0.49 ± 0.08 at 25°C, suggesting that they can act as hydrogen-bond acid (contrary to their chemical formulation). Examination of the bulk liquid stationary phases by IR showed the presence of OH groups and confirmed our analysis by the solvation equation. It is suggested that workers using the OV or SE series of siloxanes routinely check the bulk stationary phases by IR in order to assess the presence or absence of OH groups.