Gus Somsen

Some properties of binary aqueous liquid mixtures. Apparent molar volumes and heat capacities at 298.15 K over the whole mole fraction range

Densities and molar heat capacities have been measured for the following binary systems at 298.15 K: water + formamide, water +N,N-dimethylacetamide, water + dimethylsulphoxide and water + acetonitrile over the whole mole fraction range. From these data apparent molar volumes and heat capacities have been calculated for both water and the organic components. The volumes have been discussed in terms of enhanced water–water interactions and as being the result of inter-component interaction. The volume of water infinitely diluted in the various solvents might be well understood from the packing of hard sphere molecules. Similar reasoning cannot be applied to the heat capacities. Most of the curves are typical for weakly hydrophobic solutes, while the apparent molar heat capacity curve of formamide in water + formamide resembles that of hydrophilic solutes. Finally, the apparent molar heat capacities of water at infinite dilution in the various solvents are unexpectedly high.

Thermochemical behavior of mixtures ofN,N-dimethylformamide with dimethylsulfoxide, acetonitrile, andN-methylformamide: Volumes and heat capacities

Densities and molar heat capacities have been measured for mixtures ofN,N-dimethylformamide with dimethylsulfoxide, acetonitrile, andN-methylformamide at 25°C over the complete mole fraction range. From these data the apparent molar volumes and heat capacities have been calculated for both components. These quantities, as a function of the mole fraction, deviate very little from their molar values, indicating that the mixtures can be regarded as almost ideal.

Interactions between terminally substituted amino acids in an aqueous and a non-aqueous environment. Enthalpic interaction coefficients in water and in N,N-dimethylformamide at 25°C

Enthalpies of dilution of the N-acetyl amides of glycine, L-alanine, L-valine, L-leucine, and L-phenylalanine, dissolved in N,N-dimethylformamide (DMF) as a solvent have been measured at 25°C. The results obtained have been analyzed to give the enthalpic interaction (or virial) coefficients of the solutes and these are compared with information previously obtained in aqueous systems. There are marked differences in the interaction properties in the two solvents and, while the additivity approach of Savage and Wood is applicable to the solutes in water it is not suitable for representing the interactions in DMF. A correlation is presented between the enthalpic second virial coefficients in DMF and the propensity of side-chains to be in proximity in globular proteins.

Solute-solute interactions in non-aqueous solvents. Enthalpic interaction coefficients of substituted acetamides dissolved in N,N-dimethylformamide

Enthalpies of dilution of acetamide, N-methylacetamide, N-ethylacetamide, N-propylacetamide, N-butylacetamide, N,N-dimethylacetamide, N,N-diethylacetamide, N,N-dipropylacetamide and N,N-dibutylacetamide dissolved in N,N-dimethylformamide as solvent have been measured calorimetrically at 25°C. The results are interpreted in terms of the McMillan-Mayer theory. The enthalpic interaction parameters are obtained for pairs, triplets and some quadruplets of solute molecules. All enthalpic pair interaction coefficients but one in this non-aqueous solvent are negative, whereas the triplet coefficients are positive. The concept of ‘solvophobic interaction’ can be used to explain these results in connection with the assumption of the formation of solute-solvent associates. The enthalpic pair interaction coefficients can be described by the additivity approach of Savage and Wood.

Solvation and hydrophobic hydration of alkyl-substituted ureas and amides in NN-dimethylformamide + water mixtures

Enthalpies of solution of five alkyl-substituted ureas and seven different amides have been determined at 298.15 K in mixtures of NN-dimethylformamide (DMF) and H2O. Methyl substitution of the ureas causes changes in the enthalpies of transfer from H2O to DMF which show that either side of these molecules is solvated independently. From the measurements on the amides it is concluded that methyl substitution at the N atom gives changes in the enthalpies of transfer from H2O to DMF which are different from those caused by methyl substitution at the C atom. Analysis of the data in the mixed solvent shows that introduction of more or longer alkyl groups into the molecules makes both ureas and amides considerably more hydrophobic. After accounting for the influence of the NH protons, in both ureas and amides, the enthalpic effect of hydrophobic hydration of the solutes was calculated by application of a clathrate-like hydration model. Enthalpic effect of N-substituted methyl groups in ureas and amides prove to be virtually equal. The variation in the enthalpic effects of hydrophobic hydration with the number of C atoms in the n-alkyl group is comparable to that found for alcohols and amines.

Solvation and hydrophobic hydration of different types of alkylamines inN,N-dimethylformamide and water mixtures

Enthalpies of solution of twelve amines of different type have been determined at 25°C in mixtures of N,N-dimethylformamide and water over the whole composition range. The enthalpies of transfer from water to the mixtures deviate substantially from a linear dependence on the mole fraction of water. These deviations appear to contain additive contributions of the different alkyl groups. By application of a simple hydration model the enthalpic effect of hydrophobic hydration has been calculated for each amine. For alkylamines this is determined by the number and size of the alkyl groups present in the molecule. The contribution of each alkyl group is the same in primary, secondary and tertiary amines. Results for the different alkyl groups show a close relationship with values for alcohols obtained previously. Differences between alcohols and amines can be attributed to differences in the hydrophobic hydration of the parts of the solute molecules which are adjacent to the polar group. The influence of the polar group does not seem to extend beyond the second carbon atom.

The pairwise interaction concept of Savage and Wood applied to electrolytes

The pairwise interaction concept used by Savage and Wood to predict enthalpies of interaction of nonelectrolytes has been applied to several tetraalkylammonium bromides, triethylamine, and some trialkylphosphates in binary aqueous amide mixtures. It is possible to use the same model for electrolytes as well. Values of the functional group interaction parameters for both nonelectrolytes and electrolytes are in reasonable agreement.

Volumes and heat capacities of water andN-methylformamide in mixtures of these solvents

The densities of mixtures ofN-methylformamide (NMF) and water (W) have been measured at 5, 15, 25, 35, and 45°C, and the heat capacities of the same system at 25°C, both over the whole mole-fraction range. From the experimental data the apparent molar volumes (Φv) and heat capacities (Φc) of NMF and of water are evaluated. The relatively small difference between the partial molar volumes or heat capacities at infinite dilution and the corresponding molar volumes or heat capacities of the pure liquids for both NMF and water suggests that with regard to these quantities replacement of a NMF molecule by a water molecule or vice versa produces no drastic changes. The partial molar volume of water at infinite dilution in NMF is smaller than the molar volume of pure water, but the corresponding partial molar heat capacity is unexpectedly high.

Enthalpies of interaction of some N-acetyl amino acid amides in aqueous urea solutions at 298.15 K

Enthalpies of dilution of the N-acetyl amino acid amides of glycine, L-alanine, L-valine, L-leucine, L-proline, L-phenylalanine, sarcosine and N-methyl-L-alanine, and the N-acetyl-N′-methyl amino acid amides of glycine, L-alanine, L-leucine, L-proline, sarcosine, and N-methyl-L-alanine dissolved in aqueous 8 mol dm–3 urea have been measured calorimetrically at 298.15 K.The results were used to calculate enthalpic interaction coefficients employing a concentration expansion of the enthalpies of dilution. The enthalpic pairwise interaction coefficients obtained are more positive in 8 mol dm–3 urea than in water in most cases. It is proposed that aqueous urea influences the interactions between peptides in two ways. First, urea solvates the amide groups of the peptides and, secondly, urea reduces the hydrophobic interaction of the apolar side chains of the peptides. The consequences for the ability of aqueous urea to denature globular proteins are discussed.

Enthalpic interaction coefficients of amides dissolved in N,N-dimethylformamide

Enthalpies of dilution of propionamide, butyramide, pentanamide, hexanamide, N-pentylacetamide, N,N-dipentylacetamide, N-ethylhexanamide and N,N-diethylhexanamide dissolved in N,N-dimethylformamide as solvent have been measured calorimetrically at 25°C. The results are interpreted in terms of the McMillan-Mayer theory. Enthalpic interaction parameters are obtained for pairs, triplets and some quadruplets of solute molecules. All enthalpic pair interaction coefficients are negative, whereas those for triplets are positive. For unsubstituted amides the change of the enthalpic coefficients with the number of C-atoms differs considerably from that of the substituted compounds. The concept of polarophobic interaction is used for the interpretation of the results in connection with the assumption of formation of solute-solvent associates. For solutes with longer alkyl chains the results cannot be described satisfactorily in terms of the additivity approach of Savage and Wood. Probably the pair interactions of these compounds are not the result of interaction in a random way. Also the linear dependence of the pair interaction coefficients of the larger molecules with the number of C-atoms and the results for the unsubstituted amides support the occurrence of preferential orientations for these compounds.