Antonio Sacco

Temperature-dependent self-diffusion coefficients of water and six selected molecular liquids for calibration in accurate 1H NMR PFG measurements

Pulsed magnetic field gradient (PFG) NMR is today a routine method for the determination of self-diffusion coefficients, D. However, a remaining goal is the improvement of the precision of the method. The best procedure for the determination of accurate diffusion coefficients by PFG NMR is a calibration with a sample of precisely known D-value. In continuation of our previous work on calibration at 25°C (M. Holz and H. Weingartner, J. Magn. Reson., 1991, 92, 115) we present reference data as a function of temperature. Since H2O plays an outstanding role as liquid and as primary standard, we carefully measured self-diffusion coefficients of water by 1H PFG NMR in the temperature range from + 5 to + 55°C and added literature data obtained from tracer methods in the range between 0 and + 100°C. This comparatively large collection of data could then be fitted to a Speedy–Angell power law, showing the excellent congruence of the results of two completely different methods and proofing the certainty of the absolute values for water. In this manner reliable primary standard values with error limits of <1% were obtained, allowing us to adapt the temperature of the standard water to the sample temperature of interest. We further give 1H PFG NMR self-diffusion reference data in the range from + 5 to + 55°C for six easily accessible solvents, which we propose as secondary calibration standards, namely cyclohexane, dioxane, dodecane, DMSO, tetradecane and pentanol, covering a large range of absolute D-values and allowing us to match in addition the absolute D-values of calibration sample and sample under investigation. Furthermore, the gained accurate self-diffusion data are suited for an elaborate check of theoretical approaches in the physics of molecular liquids.

Experimental study of dynamic isotope effects in molecular liquids: Detection of translation‐rotation coupling

How and to what extent do molecular motions in a liquid depend upon the molecular mass? Since this fundamental problem could not yet be satisfactorily resolved by theoretical approaches, we present in this paper an experimental approach. In continuation of previous work from our laboratory, the effects of isotopic H–D substitution on viscosity and self‐diffusion have been measured for twelve molecular liquids in a certain temperature range near room temperature. These liquids are the often used solvents: methanol, ethanol, formamide, N,N‐dimethylformamide, dimethylsulfoxide, acetone, acetonitrile, pyridine, nitromethane, tetrahydrofuran, dioxane, and benzene; we have found comparatively high dynamic isotopic effects between 3% and 14%. Beside the NMR self‐diffusion measurements we have, for the first time, performed measurements of the effect of mass change on the molecular rotational diffusion via 17O and 14N intramolecular quadrupole relaxation. In all the given molecular liquids, except benzene, the dy...

Nuclear magnetic resonance study of self-association of small hydrophobic solutes in water: salt effects and the lyotropic series

The self-association behaviour of the relatively small hydrophobic organic components in mixtures of dimethyl sulfoxide (DMSO)–water, of tert-butyl alcohol (ButOH)–water and of tetramethylurea(TMU)–water has been studied by the application of an NMR technique. The so-called association parameter A22, which essentially is obtained from experimental intermolecular dipole–dipole relaxation data of 1H nuclei and from NMR measurements of the self-diffusion coefficients of the organic components, has been determined. The composition dependence of A22 in all the systems under investigation shows that self-association of the organic component occurs. The association parameters Aij and the Kirkwood–Buff integrals Gij are related quantities, which might yield complementary information. Thus a qualitative comparison between A22 and G22 is given for aqueous mixtures of DMSO and of ButOH. The main aim of the present work is the study of the influence of salts on the association behaviour. The A22 parameters obtained for salt-containing mixtures show that the salts (Lil, Csl and CsAc in DMSO–water; LiBr and CsBr in ButOH–water; LiBr, LiClO4 and LiAc in TMU–water) change the attractive interaction between the organic species. For ButOH and TMU the influence of salt essentially follows the lyotropic or Hofmeister series; however, for DMSO the reverse sequence of effectiveness of the ions is found. Our results indicate hydrophobic association for ButOH and TMU, while for DMSO obviously a short-range interaction of another kind is acting. Finally, a new attempt is made to explain the well known reverse order in the Hofmeister series for cations and for anions with respect to their structure-breaking ability.

The effect of site-specific isotopic substitutions on transport coefficients of liquid methanol

A new approach is suggested for studying the nature of molecular transport in simple liquids, which makes use of site‐specific isotopic substitutions. Its application represents the first systematic experimental study of a theoretically predicted correlation between transport coefficients in liquids and molecular moments of inertia. For this purpose, we have determined the viscosities and self‐diffusion coefficients at 25 °C of normal methanol and seven isotopically labeled methanol species: CH3OD, CD3OH, CD3OD, CH2DOH, CHD2OH, CHD2OD, and 13CH3OH. Except for 12C/13C substitution, the observed isotope effects are significantly larger than predicted by a square root of mass dependence, but are well correlated with the square roots of the moments of inertia. The results give strong evidence that translation–rotation coupling influences the transport processes in methanol, thus confirming earlier interpretations of isotope effects upon the transport in water.

Isotopic substitution effects on the volumetric and viscosimetric properties of water-dimethylsulfoxide mixtures at 25°C

Densities and viscosities of binary mixtures (H2O or D2O) (1) + (DMSO or DMSO-D6)(2) have been measured over the entire mole fraction range; and the excess volumes, excess viscosities, and excess partial molar volumes Vf of the components have been obtained. All systems show negative excess volume Ve at all compositions, values for mixtures containing D2O being more negative than those with H2O byca. 0.03 cm3-mol-1 at x1, = 0.6, where a minimum is observed. The difference between DMSO and DMSO-D6 containing mixtures is negligible. The excess viscosity ηe is always positive and shows a maximum at x1 = 0.65; at this composition, the substitution of H2O with D2O causes an excess viscosity increment ofca. 0.35 mPa-s, while deuteration of DMSO brings about a smaller increase,ca. 0.1 mPa-s. The trend of V2E with concentration shows the characteristic features of moderately hydrophobic solutes in water (negative values and a minimum in the water-rich region), features that are slightly but significantly more marked in D2O than in H2O. The V2E values in the water-diluted region and at x1, =0 are more negative for D2O than for H2O.

Excess volumes and viscosity of water—sulfolane mixtures at 30, 40 and 50°C

Abstract Densities and viscosities of water—sulfolane mixtures have been measured at 30, 40 and 50°C over the whole mole fraction range. From density data apparent molar volumes of both components and deviations from ideal volumes of mixing have been evaluated at the three temperatures. From viscosity data activation parameters of viscous flow have been computed. Data obtained seem to confirm that sulfolane acts as a structure-breaker to water even at low concentrations.

NMR Studies on hydrophobic interactions in solution Part 3 Salt effects on the self-association of ethanol in water at two different temperatures

The salt effect on the self-association of ethanol has been investigated in water-rich aqueous mixtures at 25 and 40 °C by the NMR A-parameter method. In order to investigate the anion dependence we used salts with the common sodium cation, namely Na2SO4, CF3COONa, NaBr, NaI, NaClO4 and NaSCN at fixed concentration. The association parameter A22 was obtained by means of the measurement of intermolecular 1H–1H dipole–dipole relaxation rates and self-diffusion coefficients of the ethanol molecules in D2O. The influence of the anions on the alcohol self-association at 25 °C follows the sequence: SO42− > CF3COO− > Br− > ClO4− ≈ I− > SCN−, which corresponds exactly to the well known Hofmeister or lyotropic series determining e.g. the stabilisation and destabilisation of proteins in aqueous solutions. The two salts Na2SO4 and CF3COONa promote the alcohol self-association, whereas the other salts reduce the association tendency. Thus, small organic molecules can be used for basic investigations with respect to salt effects on biologically interesting molecules. At 40 °C the relative promotingeffect of the two salts is weakened but the opposite effect of the other salts is strengthened. At this temperature the sequence ofthe anion influence is slightly different in the sense that Br− and ClO4− exchange position. Finally, we propose consideration of a special anion–apolar group interaction in water in the explanation of the anion sequence in the Hofmeister series.

NMR STUDIES ON HYDROPHOBIC INTERACTIONS IN SOLUTION PART 2.-TEMPERATURE AND UREA EFFECT ON THE SELF-ASSOCIATION OF ETHANOL IN WATER

The self-association of ethanol has been investigated in binary aqueous mixtures and in mixtures containing urea as a third component by the application of NMR. We obtain the so-called association parameter A 22 by means of the measurement of intermolecular 1 H– 1 H dipole–dipole relaxation rates and of self-diffusion coefficients of the ethanol molecules. The composition dependence of A 22 shows a surprisingly high self-association of ethanol. This result is in agreement with that found recently from other authors using different experimental methods. Addition of urea further increases the self-association of ethanol. For the first time we measure the A 22 parameter as a function of temperature, ranging from 10 to 40°C. We observe that the association of ethanol increases with increasing temperature in agreement with theoretical predictions. The structure breaking effect of urea obviously acts on the hydrophobic association in the same way as an increase of temperature.

Volumes and heat capacities of binary liquid mixtures of water—sulfolane and water—hexamethylphosphotriamide

Abstract Density and heat capacity measurements of water—sulfolane mixtures at 303.15 K and water—hexamethylphosphotriamide mixtures at 298.15 K have been performed over the whole composition range. Molar, excess, apparent molar volumes and heat capacities were calculated for the two systems. The trends of these functions are discussed in terms of specific interactions between the components of the solvent mixtures and the changes caused by the organic solvents on the water structure.

Solute—Solvent interactions in water-rich mixtures. I. Ionic conductances in water—Acetonitrile mixtures at 25°C

Conductance measurements oni-Am3BuNI (TABI), NaBPh4, NaI, NaBr, NaCl, and KCl are reported for aqueous mixtures containing up to 20 mole% acetonitrile at 25°C. Experimental data were analyzed by the 1965 Fuoss-Onsager-Skinner equation. Limiting ionic equivalent conductances in water-acetonitrile mixtures were calculated assuming that the contribution to the limiting conductance foriAm3BuNBPh4 is the same for both ions involved. The trends observed for the limiting ionic conductance-viscosity products for several ions in these water-rich solvents are discussed in terms of structural effects and ion-solvent interaction.