Thermal Expansion and Structure of 1,3-Dimethylurea, Tetramethylurea, and Tetrabutylammonium Bromide Aqueous Solutions Derived from Density Measurements

Abstract

On the basis of density measurements, the determination of apparent and partial molar volumes of the solutes in the title solutions have been carried out previously at and around room temperatures both in H 2 O and D 2 O solutions as a function of composition. They showed minima at intermediate compositions, and a number of other properties of these solutions also indicated radical structural changes and differences versus composition. On this basis, we have chosen these solutions to analyze also the solvent partial molar volume and its thermal expansibility, which were not involved in the earlier studies, to these structural changes and differences. To obtain reliable thermal expansibility results, the density measurements were extended down to low temperatures. High precision density measurements of the normal water solutions were carried out in wide temperature [(274.15 to 308.15) K] and molality, m [(0.05 to 3 < 6) mol • kg -1 ], ranges at closely spaced temperature intervals. From the measured densities, both the solute and the solvent partial molar volumes and their thermal expansibilities were calculated. The results are analyzed in terms of the cage model of the hydrophobic effect, and they are correlated to the solvent excess enthalpy and entropy and to their H 2 O + D 2 O isotope effects. Although all the title solutions show dominant "interstitial" solution character, their structure is determined by a variety of structural equilibriums, depending mainly on the polar/apolar balance of the "mixed" solute molecules, on their H-bond donor/acceptor ability, and on the solute/solvent ratio.

Counteraction ability of TMAO toward different denaturing agents

“…Experimental density values of all the considered solutions have been determined at 25°C and 1 atm. Present density values are in agreement with literature data for the already measured aqueous solutions, and have been used to perform classic SPT calculations. Density values play a fundamental role in classic SPT calculations, because they: (a) provide information on the actual interactions existing among the molecules making up the solution; (b) determine the number density of the solution and its volume packing density, the fraction of the liquid volume really occupied by particles, ξ 3 = (π/6)·∑ρ j ·σ j 3 , where ρ j is the number density, in molecules per Å, of the species j and σ j is the corresponding hard sphere diameter.…”

Thermal expansion and structure of tetrapropylammonium bromide aqueous solutions derived from density measurements

“…The initial negative slopes and the compositions of the V 2 -minima mðV 2;min: Þ, (except for Me 4 N + ) were rationalized by Wen and Saito [7] in terms of an ''interstitial" solution process, in which the alkyl chains are hidden in a complete or partial solvent cage. The lished earlier with the Bu 4 N-Br solution [5]. The derivatives have maxima and their magnitude increases with the hydrophobic character of the solutes and with decreasing temperature, i.e., with the stability of the hydrophobic cage.…”

“…The combined expanded uncertainty for the property: 2d(q) = 0.00004. Our earlier density measurements, carried out over wide concentration and temperature ranges for Me 4 N-Cl [12], Et 4 N-Cl [13] and Bu 4 N-Br [3,5] aqueous solutions, make it possible to compare the concentration and temperature dependences of the solute and solvent partial molar volumes on their alkyl chain lengths. Because the ''unique" thermodynamic (including volume) behaviour of the R 4 N-halide series are determined by the organic cations [14][15][16], the Cl À vs. Br À influence on extremes must be negligible.…”

“…From density measurements carried out over wide temperature and concentration ranges, for a series of solutes with increasingly apolar character (1,3-dimethylurea, DMU, tetramethylurea, TMU, and tetrabutylammonium bromide, Bu 4 NBr), we recently determined the solute (2) and solvent (1) partial molar volumes ðV 2 ; V 1 Þ and their derivatives with respect to temperature [5]. These properties, representing the second and third derivatives of the Gibbs free energy [6], proved to be suitable for detecting a variety of structural equilibria in these solutions including both solvent structure promotion and disruption, depending on the concentration and on the ratio of polar to apolar components of the solutes.…”

Densities and Volumetric Properties of Aqueous Solutions of {Water (1) + N-Methylurea (2)} Mixtures at Temperatures of 274.15–333.15 K and at Pressures up to 100 MPa

“…Volume properties of urea aqueous solutions and its alkyl derivatives were studied earlier − ,,,,− and for single atmospheric pressure only. All these studies were conducted in conditions, where concentration, temperature, alkyl group length, and the number of these groups were the variables.…”