Nuclear magnetic resonance relaxation of methane protons in aqueous solutions

Proton nuclear magnetic resonance longitudinal relaxation rates are reported at 100MHz for CH₄ and CHD₃ in deuterium oxide solutions. The results demonstrate that methane reorients with a correlation time on the order of 0.1 psec, considerably faster than molecule correlation times in aqueous solutions.     

Nuclear magnetic relaxation and structure of aqueous solutions

By the interpretation of the intermolecular nuclear magnetic relaxation rate of¹⁹F the orientation of the water molecules in the hydration sphere of F^– can be determined. Similarly, the orientation of the water molecules around the methyl group of propionic acid in aqueous solution has been studied. Experiments are described which give information about the nature of association of solute in aqueous solution of a number of carboxylic acids and of ethanol. The local dynamic details of the I^– ion have been investigated. Some new results are briefly descussed regarding the nuclear magnetic relaxation by quadrupole interaction in electrolyte solutions.This paper was presented at the symposium, The Physical Chemistry of Aqueous Systems, held at the University of Pittsburgh, Pittsburgh, Pennsylvania, June 12–14, 1972, in honor of the 70th birthday of Professor H. S. Frank.     

Nuclear magnetic relaxation in polyolefin resins

Nuclear magnetic resonance (NMR) spin–lattice relaxation times (T₁) in various polyethylene and polypropylene resins were measured at 20 MHz and at temperatures of 130–490 K. At each temperature and for all resins, only a single value of T₁ was found, which was consistent with the occurrence of rapid spin diffusion throughout the protons attached to the polymer chains. The data were analyzed for the estimation of activation energies corresponding to molecular motion causing spin–lattice relaxation. Two well‐defined minima were found for log_e(T₁) plotted as a function of temperature for all of the polypropylene resins. Single very broad minima were found for all of the polyethylene samples. In contrast, the free induction decay signals from all of the resins following single radio‐frequency pulses were observed to contain a rapidly decaying component followed by a much more slowly decaying signal. These components were used to estimate the amount of rigid component present in the solid resins at room temperature. Samples of one high‐density polyethylene and one low‐density polyethylene were irradiated with γ radiation up to a 500‐kGy dose to examine the effects of crosslinking on the NMR relaxation. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 572–584, 2002; DOI 10.1002/polb.10116     

Temperature dependence of volume changes on glycine-PEG and L-alanine-PEG in aqueous solution

The volume changes on mixing aqueous solutions of glycine-PEG and L-alanine-PEG were respectively measured with a vibration densimeter as functions of concentration and temperature. The volume changes ( _m)V for the glucine-PEG-H₂O system were positive, and these _v(x,m) values of different total molality almost linearly decreased with an increase in temperature and converged around 110°C by extrapolation. On the other hand, those for the L-alanine-PEG-H₂O system were almost zero and independent of temperature. It is considered that the influence of hydration cospheres on glycine-PEG interaction for the glycine-PEG-H₂O system disappears around 110°C.     

Nuclear magnetic resonance studies of aqueous solutions of alkylureas: Proton and carbon-13 chemical shifts

Aqueous solutions of methylated and ethylated derivatives of urea have been investigated by proton and carbon-13 magnetic resonance technique. The chemical shifts of water protons in solution of alkylureas, relative to that of pure water, are as a measure of the hydrogen-bonding reinforcing or weakening ability of these substances. The dependence of this effect on the concentration and on the temperature has also been studied. The results obtained indicate that different contrasting effects act in these solutions, among then the water structuring effect of the nonpolar surface of the alkylureas and the effect of the volume of the solute.¹³C chemical shifts of the carbonyl groups of the alkylureas give useful information about the interactions between water and the polar part of the ureas. Carbonyl-water interactions exist but are partially destroyed by increasing concentrations of the solute.     

Titration microcalorimetry of poly(vinylpyrrolidone) and sodium dodecylbenzenesulphonate in aqueous solutions

Microcalorimetric titrations are carried out on solutions containing the anionic surfactant sodium dodecylbenzenesulphonate (SDBS), and mixtures of SDBS and the uncharged polymer poly(vinylpyrrolidone) (PVP). Measurements are taken at different temperatures. Micellisation of SDBS is driven by hydrophobic bonding. The interaction enthalpy of mixed PVP/SDBS systems shows clearly a consecutive endothermic and exothermic region with increasing surfactant concentration. The endothermic part can be looked upon as an incremental binding isotherm and reflects the number of surfactant molecules involved in the association process. The exothermic region features inverse hydrophobic bonding behaviour. This is related to the flexible nature of the adsorbent, i.e. the polymer. Electrostatic repulsion between neighbouring surfactant molecules causes at increased surfactant concentrations structural rearrangements of the polymer-surfactant complexes. This is accompanied by losing inter- and intrachain linking and entropy gain since the expanded complexes can move more freely. Additional surfactants continue to adsorb on the vacant hydrophobic adsorption sites. The influence of the initial amount of polymer and the electrolyte concentration support our proposals.     

Thermodynamic activation parameters for viscous flow of dilute aqueous solutions of ethylenediamine,trimethylenediamine and N,N-dimethyltrimethylenediamine

The density and the viscosity data have been used to determine the thermodynamic activation parameters, free energies (ΔG ^?), enthalpies (ΔH ^?) and entropies (ΔS ^?), for viscous flow of the systems; water (W) + ethylenediamine (ED), W + trimethylenediamine (TMD) and W + N,N-dimethyltrimethylenediamine (DMTMD) in the temperature range of 303.15–323.15 K over the composition range of 0 ≤ X ₂ ≤ 0.45, where X ₂ is the mole fraction of diamines. On addition of diamines to water, ΔG ^?, ΔH ^? and ΔS ^? values increase sharply, pass through a maximum and then decline. The heights of maximum in the ΔG ^? versus X ₂ curve vary as, W + DMTMD > W + TMD > W + ED. For all systems, the excess properties, ΔG ^{? E} , ΔH ^{? E} and ΔS ^{? E} are positive. The observed increase in thermodynamic values may be due to combined effect of hydrophobic hydration of diamines and water–diamine interaction as a result of hydrophilic effect.     

Molecular motion and interactions in aqueous carbohydrate solutions. III. A combined nuclear magnetic and dielectric-relaxation strategy

A strategy is developed for the complementary use of dielectric and nuclear magnetic relaxation methods to elucidate the molecular dynamics in aqueous solutions of small hydrophilic molecules, and hence determine extents of hydration. The nuclear magnetic relaxation data, as well as characterizing the motional properties of various carbohydrate solutes, is used here to test alternative models for the resolution of the dielectric spectra into their component relaxation processes. This approach results in a much more confident analysis of solvent relaxation properties than has in the past been usual, to yield information relating to the extents of hydration of small sugars and the possible orientation-specific nature of this hydration. It is demonstrated that the dielectric relaxation of the sugar molecules themselves is unequivocally not due to the reorientation of the whole molecule and most likely is dominated by the rotation of hydrate side chain groups (hydroxyls and hydroxymethyl). In proton magnetic relaxation studies of glucose in D₂O it is observed that one particular proton (H-1 in the -form only) is extremely susceptible to inter-molecular proton-proton interactions while the remaining protons are very effectively shielded. This observation is shown to be fully consistent with the conformational and hydration properties of glucose.     

Enthalpic relaxation in frozen aqueous trehalose solutions

Our object was to investigate the effect of annealing on the glass transition temperatures and enthalpic recovery of frozen aqueous solutions of trehalose. Trehalose solutions were subjected to differential scanning calorimetry wherein they were first cooled from room temperature to −60 °C, and heated to the annealing temperature, which ranged between −34 and −48 °C. Following isothermal annealing for the desired time period, the glass transition temperatures and the enthalpic recovery were determined in the final heating scan. Frozen unannealed trehalose solutions were characterized by two glass transition events. At a heating rate of 2 °C/min, the observed Tg₁′ and Tg₂′ were ∼−45 and −31 °C, respectively. Annealing resulted in an increase in the lower transition temperature, Tg₁′, while the higher transition temperature, Tg₂′, was unaffected. Enthalpic recovery due to annealing was associated only with Tg₂′. Annealing at −36 °C resulted in the highest value of Tg₁′ and the maximum enthalpic recovery. Irrespective of the heating rates, the magnitude of enthalpic recovery and Tg₁′ showed a similar trend (first an increase, followed by a decrease) as a function of annealing temperature. This suggests that annealing led to crystallization of ice and subsequently the system became maximally freeze-concentrated. Annealing, at temperatures higher than −36 °C, led to a reduction in enthalpic recovery associated with Tg₂′ and a lowering of Tg₁′. These observations are consistent with the hypothesis that the higher transition temperature coincides with the onset of ice melting. We have attempted to bridge two conflicting schools of thought regarding the origin of multiple glass transitions in frozen aqueous sugar solutions. The two glass transitions are attributed to the formation of two “populations” in the freeze-concentrated phase during “non-equilibrium” or rapid cooling—one, that is maximally freeze-concentrated and the other that contains a higher amount of water. The higher transition temperature also overlaps with the onset of ice melting.     

Thermody namic functions of 1,2-alkanediols in dilute aqueous solutions

The heat capacities of binary aqueous solutions of 1,2-ethanediol, 1,2-propanediol and 1,2-butanediol were measured at temperatures ranging from 283.15 to 338.15 K by differential scanning calorimetry. The partial molar heat capacities at the infinite dilution were then calculated for the respective alkanediols. For 1,2-ethanediol or 1,2-propanediol, the partial molar heat capacities at the infinite dilution of increased with increasing temperature. In contrast, the partial molar heat capacities of 1,2-butanediol at the infinite dilution decreased with increasing temperature. Heat capacity changes by dissolution of the alkanediols were also determined. Heat capacity changes caused by the dissolution of 1,2-ethanediol or 1,2-propanediol were increase with increasing temperature. On the other hand, heat capacity changes caused by the dissolution of 1,2-butanediol are decrease with increasing temperature. Thus our results indicated that the structural changes of water caused by the dissolution of 1,2-butanediol differed from that of the two other alkanediols. This revised version was published online in July 2006 with corrections to the Cover Date.     

The viscosity of aqueous solutions of alkali and tetraalkylammonium halides at 25°C

The viscosities of most alkali and tetraalkylammonium halides have been measured in water at 25°C. The relative viscosities can be fitted, up to 1M, with the relation _r =1+A c^1/2+B c+D ². TheA term depends on long-range coulombic forces, andB is a function of the size and hydration of the solute. When combined with partial-molal-volume data, the difference B –0.0025V° is mostly a measure of the solute-solvent interactions. IonicB are obtained if the tetraethylammonium ion is assumed to obey Einstein's law. TheD parameter depends on higher terms of the long-range coulombic forces, on higher terms of the hydrodynamic effect, and on structural solute-solute interactions. As such, it cannot be interpreted unambiguously.     

Solubility of amino acids in aqueous poly (ethylene glycol) solutions

The solubilities of amino acids have been measured in water and aqueous poly(ethylene glycol) (PEG) solutions as a function of temperature and PEG concentration. The free energies of transfer from water to aqueous PEG solutions forl-alanine,l-valine,l-isoleucine andl-leucine were positive, while those forl-phenylalanine andl-tryptophan were negative. The corresponding enthalpies of transfer were almost zero for all amino acids. The equilibrium constants of the binding of amino acids to PEG chain were estimated from the solubility data. Amino acids with larger hydrophobicity are bound more strongly to the PEG chain due to the hydrophobic interaction between the methylene groups of PEG and the side chain of amino acid. The equilibrium constants showed a correlation with the dynamic hydration number (n _DHN) which expresses the hydration properties of amino acids in aqueous solution.     

Dielectric spectroscopy on aqueous solutions of pyridine and its derivatives

The complex (dielectric) permittivity has been measured as a function of frequency between 1 MHz and 40 GHz for aqueous solutions of pyridine, 2- and 3-methylpyridine, as well as 2,4- and 2,6-dimethylpyridine at various temperatures and solute concentrations. Different relaxation spectral functions are used to analytically represent the data, in particular the Cole-Cole function. The solute contribution to the extrapolated static permittivity has been calculated to show that, in correspondence with other aqueous solutions of organic molecules and ions, the permittivity of the solvent seems to be enhanced with respect to the pure water value. Also in accordance with other aqueous systems it is found that the principal dielectric relaxation time for equimolar solutions of stereo isomers at the same temperature may significantly differ from one another. A further result is the finding of an unusually strong temperature dependence in the relaxation time of the 1 molar solution of 2,6-dimethylpyridine.     

Molecular motion and interactions in aqueous carbohydrate solutions. II. Nuclear-magnetic-relaxation studies

Nuclear-magnetic-relaxation studies of a range of aqueous mono- and disaccharide solutions are reported. These include¹⁷O relaxation of solvent and¹H,²H,¹³C, and¹⁷O relaxation of various solutes. The limitations of nuclear-magnetic relaxation for providing direct indications of solvent motions or extents of hydration of these sugars are outlined. In contrast to the solvent studies, the motional properties of the solutes themselves have been reasonably well defined, with¹H,²H, and¹³C studies of the sugar ring C–H groups all indicating very similar rotational correlation times. Shorter correlation times estimated for the –CH₂OH and –OH side chains, implying that internal motions make a significant contribution to the relaxation of these groups. Differences in reorientation rates of pentose monosaccharides, hexose monosaccharides, and disaccharides are discussed in terms of molecular size and solvent interactions. In every case examined, the solute NMR rotational correlation time is in serious disagreement with that expected from previous dielectric-relaxation studies. Some of the implications of this discrepancy are considered.     

The hydrophobic effect in aqueous solutions of nonelectrolytes. I. Self-interactions of alkylureas

In order to clarify some aspects of the hydrophobic interactions, the enthalpies of dilution of monoethylurea, 1,3-dimethylurea, and 1,3-diethylurea have been determined calorimetrically at 25°C. The calorimetric data, expressed in terms of excess enthalpy, permit the evaluation of the pair and triplet interaction coefficients. The analyses of these and of the analogous coefficientsg _xx andg _xxx, derived from osmotic data, indicate a driving force favorable to the interactions among the hydrated solute molecules. Nevertheless, the positive values of theh _xx andh _xxx coefficients seem to suggest that the source of the effect is a rearrangement of the water molecules rather than a direct association of the solute molecules. There are evidences of a strict correlation between the enthalpic and the entropic effects. Preliminary data were presented at the International Conferences on Chemical Thermodynamics at Baden (1973) and Montpellier (1975). The experimental part was carried out at the Istituto Chimico of the University of Trieste. To whom correspondence should be addressed.     

Molecular motion and interactions in aqueous carbohydrate solutions. I. Dielectric-relaxation studies

Dielectric-relaxation studies in the frequency range 200 kHz to 35 GHz are reported for a range of sugars (from mono- to trisaccharides) in aqueous solution. The complex dielectric spectra were analyzed using a weighted least-squares minimization method to resolve the various component relaxations, and the implications of the analyses in terms of the molecular dynamics of solute and solvent and the interactions between solute and solvent are discussed. For the highest concentration studied (ca. 2M), it was found that the most significant analysis required three discrete relaxation processes, whereas lower concentration samples could usually be satisfactorily fitted with two. Irrespective of any uncertainty in model selection, a number of conclusions regarding the solute-solvent interactions can be made, and it is shown how final quantification of the extents of hydration can be made using the input of information from other techniques.     

Effects of inorganic salts on the properties of aqueous poly(vinylpyrrolidone) solutions

Cloud point curves and temperatures have been determined for aqueous solutions of poly(vinylpyrrolidone) at several concentrations for a variety of inorganic salts (phosphates, monohydrogen phosphates, sulfates, carbonates, dihydrogen phosphates and fluorides). The resulting dependency of the critical temperatures (mostly between 289 and 350 K) on the molar concentration can be expressed as sequences showing the decreasing effect of anion species or cation species in salting out the polymer. The decreasing order of effectiveness of the anions in reducing the temperatures is PO ₄ ^3– >HPO ₄ ^2– >SO ₄ ^2– CO ₃ ^2– >H₂PO ₄ ^– >F^–. The order for cation is Na⁺>K⁺. The changes brought about in temperatures by the salts were found to be the results of the changes taking place in the hydrophilic and hydrophobic interactions among polymer, solvent and additive salts and of the change of water structure by structure making or structure breaking ions, and of the influence of salts on the hydration sheath of the polymer.Deceased     

交联PVP的制备与应用研究进展

综述用乙烯基吡咯烷酮（NVP）制备交联PVP的各种方法,概述交联PVP的应用发展状况。利用紫外光或γ－射线照射PVP水溶液,或用过硫酸盐、肼和过氧化氢或在过氧化物存在下用α、ω－二烯烃处理线型PVP的水溶液均可生成交联PVP水凝胶。NVP单体和交联剂、自由基引发剂有机溶剂或电解质水溶液中进行沉淀聚合能够制备交联程度较高的交联PVP。采用不加入自由基引发剂的米花状聚合则可制备高度交联的不溶性PVP。用不同制备方法制得的交联PVP在水中具有不同的溶胀能力,呈现出的形态多种多样,前软凝胶、白色粉末、或多孔粒子,可满足不同的要求。交联PVP以其优良的络合能力、胶体性质和生理惰性,已广泛地应用于医药、食品和化学工业等领域。