Solvation Shell Water Research Articles

Density functional theory based molecular-dynamics study of aqueous fluoride solvation

We use density functional theory based molecular-dynamics simulations to study the aqueous solvation of the fluoride anion. Our studies are focused on the first solvation shell and have resulted in detailed information on its structural and dynamical properties. The fluoride ion leads to the formation of a rigid solvation shell, qualitatively consistent with simulation and experimental studies, classifying fluoride as a "structure making" particle. However, quantitatively we find the solvation shell to be less structured and more mobile than predicted from empirical force-field simulation. The influence on the intramolecular electronical and structural properties of water is minimal, as observed for other halogens. We propose two distinct mechanisms for the exchange of bulk and first solvation shell water molecules.

Hydration of small peptides

The results for the sequential hydration of small peptides (<15 residues) obtained in our group are reviewed and put in perspective with other work published in the literature where appropriate. Our findings are based on hydration equilibrium measurements in a high-pressure drift cell inserted into an electrospray mass spectrometer and on calculations employing molecular mechanics and density functional theory methods. It is found that the ionic functional groups typically present in peptides, the ammonium, guanidinium, and carboxylate groups, are the primary target of water molecules binding to peptides. Whereas the water–guanidinium binding energy is fairly constant at 9 ± 1 kcal/mol, the water binding energy of an ammonium group ranges from 7 to 15 kcal/mol depending on how exposed the ammonium group is. A five-residue peptide containing an ammonium group is in favorable cases large enough to fully self-solvate the charge, but a pentapeptide containing a guanidinium group is too small to efficiently shield the charge of this much larger ionic group. The water–carboxylate interaction amounts to 13 kcal/mol with smaller values for a shielded carboxylate group. Both water bound to water in a second solvation shell and charge remote water molecules on the surface of the peptide are bound by 7–8 kcal/mol. The presence of several ionic groups in multiply charged peptides increases the number of favorable hydration sites, but does not enhance the water–peptide binding energy significantly. Water binding energies measured for the first four water molecules bound to protonated bradykinin do not show the declining trend typically observed for other peptides but are constant at 10 kcal/mol, a result consistent with a molecule containing a salt bridge with several good hydration sites. Questions regarding peptide structural changes as a function of number of solvating water molecules are discussed. Not much is known at present about the effect of individual water molecules on the conformation of peptides and on the stability of peptide zwitterions.

Vibrational relaxation of liquid water in ionic solvation shells

Femtosecond two-color pump–probe spectroscopy is used to measure the vibrational lifetime of the O–H stretch vibration in solutions of KF, NaCl, NaBr, and NaI in HDO:D 2O. We observe a slow component (roughly 2–4 times slower than in HDO:D 2O) in the decay of the absorption change, which is due to O–H groups that are hydrogen bonded to the dissolved anions. The time constant of this slow component depends on the nature of the anions and is observed to decrease with temperature, in contrast with the temperature dependence of the relaxation of the OH stretch vibration in pure HDO:D 2O.

Calcium hydration on montmorillonite clay surfaces studied by Monte Carlo simulation

Monte Carlo computer simulations were used to investigate the interlayer structure of Ca-montmorillonite for hydration levels ranging from the dry clay to a three-layer system. Our model of montmorillonite consists of negative charge sites in both the octahedral and tetrahedral layers. Our results are compared with X-ray diffraction data from Ca-clays, which have suggested an interlayer structure similar to Mg-clays. However, the Ca 2+ co-ordination number in these systems and in bulk liquid is a subject of ongoing debate. Our results support an eightfold water solvation shell for calcium, even within the confines of a clay pore. The Ca 2+ co-ordination number decreased at lower water content, as the water layer became constrained and less like bulk liquid. Additionally, Ca 2+ solvation shells included surface oxygen atoms at low water content, although this may be a result of the Ca-O interaction potentials.

A first principles molecular dynamics simulation of the hydrated magnesium ion

First principles molecular dynamics has been used to investigate the solvation of Mg 2+ in water. In agreement with experiment, we find that the first solvation shell around Mg 2+ contains six water molecules in an octahedral arrangement. The electronic structure of first solvation shell water molecules has been examined with a localized orbital analysis. We find that water molecules tend to asymmetrically coordinate Mg 2+ through one of the oxygen lone pair orbitals and that the first solvation shell dipole moments increase by 0.2 Debye relative to pure liquid water.

Ab initio molecular dynamics study on the hydrolysis of molecular chlorine

The hydrolysis of Cl 2 in water and its reverse reaction, modeled by a box containing one Cl 2 and 19 H 2O molecules, are studied by ab initio molecular dynamics. It is observed that the hydrolysis reaction involves only Cl 2 and one H 2O molecule directly in bond breaking and formation and that solvated chloride and hydrogen ions are directly produced in the hydrolysis. For the reverse reaction between HCl and HOCl, the approaching HOCl must first penetrate a solvation shell of water around the chloride ion, and the calculated free energy barrier is approximately 4.6 kcal mol −1, much lower than that in the gas phase. The presence of (H 2OCl) +⋯Cl − is identified when the Cl–Cl distance is constrained at 2.2 or 2.4 Å.

Size Dependence of Blackbody Radiation Induced Hydrogen Formation in Al+(H2O)n Hydrated Aluminum Cations and Their Reactivity with Hydrogen Chloride

Trapped hydrated aluminum cluster ions are studied by FT-ICR mass spectrometry on a time scale of several seconds. Blackbody radiation, besides causing the fragmentation of the cluster by stepwise loss of individual water molecules, induces in hydrated aluminum clusters Al+(H2O)n an intracluster reaction yielding hydrated hydroxide and releasing molecular hydrogen. Local deviations of the individual rate constants for the water loss process from a linear size dependence are due to increased rigidity of certain sizes due to the formation of stabilizing hydrogen-bonded bridges. In comparison with previously studied hydrated ions, surface versus internal solvation is discussed. The preferential occurrence of the intracluster reaction in the size region of n = 11−24 is attributed to a concerted proton-transfer mechanism, in which a chain of at least two water molecules is needed to transfer a proton between two first solvation shell water molecules, leading to formation of an Al(OH)2+(H2O)n hydrated aluminum dihydroxide cation and molecular hydrogen. The Al+(H2O)n species with n ≥ 13 and all investigated Al(OH)2+(H2O)m species are able to react with and “dissolve” HCl. The maximum number of HCl molecules in the cluster strongly depends on the number of water molecules available for solvation. The presence of HCl in the cluster removes the upper limit for the intracluster reaction, which leads to the formation of molecular hydrogen, driven by blackbody radiation. This is taken as further evidence for the validity of the proton-transfer mechanism.

Atomic-scale analysis of the solvation thermodynamics of hydrophobic hydration

Molecular dynamics simulations are used to model the transfer thermodynamics of krypton from the gas phase into water. Extra long, nanosecond simulations are required to reduce the statistical uncertainty of the calculated "solvation" enthalpy to an acceptable level. Thermodynamic integration is used to calculate the "solvation" free energy, which together with the enthalpy is used to calculate the "solvation" entropy. A comparison series of simulations are conducted using a single Lennard-Jones sphere model of water to identify the contribution of hydrogen bonding to the thermodynamic quantities. In contrast to the classical "iceberg" model of hydrophobic hydration, the favorable enthalpy change for the transfer process at room temperature is found to be due primarily to the strong van der Waals interaction between the solute and solvent. Although some stabilization of hydrogen bonding does occur in the solvation shell, this is overshadowed by a destabilization due to packing constraints. Similarly, whereas some of the unfavorable change in entropy is attributed to the reduced rotational motion of the solvation shell waters, the major component is due to a decrease in the number of positional arrangements associated with the translational motions.

Prediction of High-Performance Liquid Chromatographic Retention Data of Carboxamides and Oxadiazoles

Abstract Regression models that predict the high-performance liquid chromatographic retention behavior of carboxamides and oxadiazoles were proposed. A new intermolecular interaction parameter was developed that combines dispersion interaction and total water solvation shell surface energy ratio of structural(o, m, p) isomers to form nonpolar bonding constant descriptor. Also resonance effect constant and field effect constant were used as electronic descriptor. A three-variable model indicated high multiple correlation(R>0.996) between the observed and the calculated values.

Solvent isotope effect in ion-pair extraction of aqueous lanthanoid(III) picrates with a crown ether

Lanthanoid-specific, global extractability doubling was achieved without changing the original cation-selectivity sequence by using D2O, instead of water, as an aqueous phase in the solvent extraction of aqueous light lanthanoid(III) picrates (La3+–Gd3+) with 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6) in dichloromethane. Under comparable conditions, on appreciable effects were observed with alkali- and alkaline-earth-metal picrates. Quantitative solvent extraction studies using water, D2O, and their 1:1 mixture revealed that the first extraction step, affording a 1:1 complex between partially hydrated lanthanoid and the ligand, is exclusively responsible for the solvent isotope effect, while the subsequent sandwich complexation of the 1:1 complex with another ligand molecule in the organic phase is not affected by the deuteriated aqueous phase. Thus, the lanthanoid-specific solvent isotope effect is attributed to hydrogen-bond breaking between hydrating water in the first solvation shell and bulk water upon the extraction of incompletely dehydrated lanthanoid ions into the dichloromethane phase.

Molecular dynamics simulation of solvated protein at high pressure.

We have completed a molecular dynamics simulation of protein (bovine pancreatic trypsin inhibitor, BPTI) in solution at high pressure (10 kbar). The structural and energetic effects of the application of high pressure to solvated protein are analyzed by comparing the results of the high-pressure simulation with a corresponding simulation at low pressure. The volume of the simulation cell containing one protein molecule plus 2943 water molecules decreases by 24.7% at high pressure. This corresponds to a compressibility for the protein solution of beta = 1.8 x 10(-2) kbar-1. The compressibility of the protein is estimated to be about one-tenth that of bulk water, while the protein hydration layer water is found to have a greater compressibility as compared to the bulk, especially for water associated with hydrophobic groups. The radius of gyration of BPTI decreases by 2% and there is a one third decrease in the protein backbone atomic fluctuations at high pressure. We have analyzed pressure effects on the hydration energy of the protein. The total hydration energy is slightly (4%) more favorable at high pressure even though the surface accessibility of the protein has decreased by a corresponding amount. Large pressure-induced changes in the structure of the hydration shell are observed. Overall, the solvation shell waters appear more ordered at high pressure; the pressure-induced ordering is greatest for nonpolar surface groups. We do not observe evidence of pressure-induced unfolding of the protein over the 100-ps duration of the high-pressure simulation. This is consistent with the results of high-pressure optical experiments on BPTI.(ABSTRACT TRUNCATED AT 250 WORDS)

Theories of cloud-curve phase separation in non-ionic alkyl polyoxyethylene micellar solutions

Two contrasting theories have been used to investigate the low critical volume fractions observed for the cloud curves of a range of alkyl polyoxyethylene surfactant solutions. These are a thermodynamic perturbation theory and Flory–Huggins solution theory. The former theory uses a model based on spherical micelles with a surrounding water solvation shell, and the mechanism of phase separation is considered to be the reduction of this solvation shell with temperature. The latter theory utilises the idea that micellar growth is largely responsible for cloud-curve behaviour and assumes the micelles are elongated. Both theories meet with limited success when compared with recent experimental data. Although it is possible to fit the cloud curves with both models the physical assumptions in the model are often unreasonable. The model of solvated spherical micelles can account for the cloud-curve behaviour of C8E4 with a critical volume fraction of 7% with acceptable parameters; however, fitting the lower critical volume fractions observed for C12E4, C12E6, C12E8 or C10E4 results in extensive solvation shells. Flory–Huggins theory on the other hand predicts substantial elongation of the micelles of all the above five systems, with aggregation numbers far in excess of those given by experiment. We conclude that micellar growth alone cannot account for the low critical volume fractions observed for many of these systems. A combination of the two effects is likely for those surfactant solutions with low critical volume fractions: a solvation shell combined with a degree of elongation of the micelle.

Enthalpies of aqueous solution of noble gases at 25�C

The standard enthalpies of solution of rare gases (helium, neon, argon, krypton, and xenon) in water at 25°C have been measured by a high precision steady-state calorimetric method. The aqueous solvation process is energetically favorable at 25°C for the gases studied. Values of the standard free energy, enthalpy, and entropy changes are found to be well correlated with cavity surface areas and the number of water molecules in the first solvation shell. Also, the values of the standard enthalpy and entropy of solution for the rare gases are found to have the same dependence on the number of solvation shell water molecules as inorganic and hydrocarbon gases. These results imply that the dominant source of enthalpy and entropy change resides in the first solvation shell.

Interactions Between Micelles in Nematic Lyomesophases

Abstract Interactions between micelles are analyzed, taking into account Coulombian repulsion, van der Waals attraction and excluded volume, including the water solvation shell bound to the micelles. Curves of interaction energy and force are obtained as a function of the distance between micelles for planar and cylindrical micelles for specific nematic lyomesophases, having the fraction α of ionized polar groups as parameter. It is verified that for both geometries the attractive interaction is always larger than Coulombian repulsion for α ≤ 0.1; the high ionic strength of the medium leads to screening of the micelles since α values are smaller than this limit. Therefore, the systems are in flocculation conditions, with behavior determined by the interplay of the net attractive interaction, the amount of bound water giving the length of the excluded volume interaction and thermal agitation. Possible formation of aggregates of micelles is discussed in view of the results obtained.

Generation of Cd +1 in x-irradiated frozen aqueous solutions: Similar hydration shells for Cd +2 and Cd +1 species

Cd +1 has been generated in frozen aqueous solution by X-irradiation at 4.2 and 77 K. No difference in the g-factors or isotropic hyperfine constant has been detected for the two irradiation temperatures in contrast to Ag° species. Although the isotropic coupling constant indicates that 30% of the unpaired electron spin density of Cd +1 is delocalized onto first solvation shell waters it appears that the equilibrium solvation environments for Cd +2 and Cd +1 are essentially the same.

Temperature dependence of the electron spin resonance linewidth of solvated electrons in aqueous alkaline glass: Determination of vibrational and torsional frequencies associated with the solvated electron complex

The electron spin resonance linewidth of solvated electrons produced by γ-irradiation of 10 M NaOH alkaline aqueous glass has been measured from 4.2 to 115 K at 9 GHz and from 12 to 115 K at 35 GHz. At both frequencies the linewidth reversibly decreases with increasing temperature. This is attributed to interaction with vibrational and rotational frequencies of the solvated electron complex. Analysis on this basis yields two frequencies which are associated with the solvated electron including its first solvation shell water molecules. The lower frequency, 1.5 cm −1, is tentatively assigned to torsional or twisting motions of the first solvation shell of water molecules and the higher frequency, 37 cm −1, is assigned to a solvated electron—nearest proton stretching motion. Model theoretical calculations partially support the latter assignment.

Effect of irradiation temperature from 77 to 1.6 K on the optical absorption and scavenging efficiency of localized electrons in aqueous 10 M lithium chloride glasses

Irradiation of the 10 M LiCl/D2O matrix at 77 K stabilizes the solvated electron species, evis−(λmax∼575 nm) while irradiation at 4.2 or 1.6 K stabilizes two localized electron species evis−(λmax∼680 nm) attributed largely to solvated electrons and eIR−(λmax≳2400 nm) attributed largely to presolvated electrons. Between 4.2 and 1.6 K the evis− spectrum is essentially unaffected but the eIR spectrum is shifted to lower energy at 1.6 K. The electron scavenging efficiencies of bromate, nitrate, and chromate for evis− at 77, 4.2, and 1.6 K and for eIR− at 4.2 and 1.6 K were studied. The total scavenging efficiency for all localized electrons in the glass increases as the temperature is lowered over this range. An apparent low scavenging efficiency for evis− at 4.2 K seems due to competitive tunneling to the scavenger by evis− and eIR−. This interpretation is supported by observations of the scavenging efficiency of nitrate in 10 M NaOH aqueous glass over this same temperature range in which only one main type of localized electron species is observed. The scavenging results support the assignments of e−IR to a presolvated electron with a nonequilibrium orientation of first solvation shell waters and of e−vis to a solvated electron with an equilibrium orientation of first solvation shell waters.

Electron spin echo envelope modulation of trapped radicals in disordered glassy systems: Application to the molecular structure around excess electrons in γ-irradiated 10M sodium hydroxide alkaline ice glass

Electron spin echo signals associated with trapped electrons in aqueous matrices show a modulation due to hyperfine interaction with the surrounding magnetic nuclei. The three pulse electron spin echo modulation gives information about the next nearest neighbor deuterons surrounding the electron. Analysis of the phase change in the three pulse modulation gives an effective deuteron interaction distance of 3.6 Å. The two pulse spin echo modulation depends on both the nearest and next nearest neighbor deuterons. Simulations of this modulation support a structural model in which the number of equivalent nearest neighbor deuterons to the electron is 6 at a distance of 2.1 Å and with an isotropic coupling constant of 0.9 MHz (5.8 MHz for protons). The orientation of the water molecules in the first solvation shell is not uniquely determined and two possible models are proposed. In model I there are three first solvation shell water molecules with their molecular dipoles oriented toward the electron so that there are two nearest neighbor deuterons from each molecule. In model II there are six first solvation shell water molecules with one OD bond oriented toward the electron. Model I is approximately consistent with the semicontinuum model for solvated electrons although the experimental distances are 0.2–0.3 Å larger than the predicted point dipole distances.