Polarized Raman spectra of aqueous solutions and band component analysis

Polarized Raman spectra of water in ν (OH) and δ (OH₂) regions has been recorded for various aqueous solutions of electrolytes of types AB and AB₂ (A=Li, Na, K, Ca; B=F, Cl, Br, I, NO₃) over a wide concentration range. The isotropic and anisotropic components of the intensity of ν (OH) band has been analysed as a function of concentration and as function of the nature of the ions by means of the Fourier transform analysis.     

Counterion binding in the aqueous solutions of bile acid salts, as studied by computer simulation methods

We investigate the structural and dynamical properties of counterion binding in sodium cholate and sodium deoxycholate micelles at three different concentration, namely, 30, 90, and 300 mM, by means of molecular dynamics simulations at the atomistic level. The obtained results can resolve a long-standing, apparent contradiction between different experiments that reported discordant values for the degree of counterion binding. Namely, our results suggest that certain experimental techniques, such as freezing point depression, are only sensitive to the contact counterions, and hence, the degree of contact binding of the counterions is measured. On the other hand, in experiments employing, e.g., electrode potential or nuclear magnetic resonance measurements, the solvent-separated counterions also contribute to the signal detected, and hence, the counterions that are measured as bound ones do include the solvent-separated counterions as well.     

Ion association studies in aqueous NiSO4 solutions by positron lifetime spectroscopy

Ion association has been studied by positron lifetime spectroscopy in aqueous solutions containing the Ni²⁺ and SO ₄ ^2– ions at 294 K with the double aim of assessing the reliability of the method for quantitative determination of complex formation constants and of probing the validity of various expressions to calculate single-ion activity coefficients at high ionic strength. The existence of two complexes, identified as NiSO₄ and Ni₂SO ₄ ²⁺ , is shown by the data analysis. Considering the formation constant of the former, K_I=(196±10)M^–1, determined in previous works leads to discarding several of the expressions commonly used for activity corrections. Two possible values are retained for K_I, (193±20)M^–1 and (179±20)M^–1, while K_II related to Ni₂SO ₄ ²⁺ is better defined, as (2.57±0.14)M^–1.     

Determination of ratios of sulfate to bisulfate ions in aqueous solutions by Raman spectroscopy

Raman spectra are presented for aqueous ammonium bisulfate and zinc sulfate-sulfuric acid solutions over a wide range of concentration and from 20 to 85°C. The heights of the 980 cm ^–1 (primarily SO ₄ ^–2 ) and 1053 cm ^–1 (primarily HSO ₄ ^– ) bands are correlated with the pH of the solutions. The ratio of the species SO ₄ ^–2 and HSO ₄ ^– can be obtained from the ratio of scattering intensities at 980 and 1053 cm ^–1 . The H obtained from plots of the log of the concentration products versus 1/T is –37±5 kJ-mol ^–1 for ammonium bisulfate solutions and –48±4kJ-mol ^–1 for the ZnSO ₄ -H ₂ SO ₄ solutions. This compares to –22 kJ-mol ^–1 for the H of bisulfate ion dissociation as obtained from thermodynamic equilibrium constants.     

Raman spectroscopic study of the conformation of dicarboxylic acid salts in aqueous solutions

It is already known that the molecules of long chain monocarboxylic acid salts have a tendency to form micelles in aqueous solutions, the molecular chain taking the all-trans zigzag structure. However it is considered difficult for dicarboxylic acid salts to adopt the same structure as the monocarboxylic acid salts as they have two carboxyl groups, one on each end of the molecular chain. Therefore, a special structure is expected to exist for dicarboxylic acid salts in aqueous solution. In order to examine this, Raman spectra of suberic acid salt and azelaic acid salt in aqueous solution were measured and the normal vibrational calculation carried out, showing that dicarboxylic acid salts have a helical structure in aqueous solution.     

Ion association and hydration in aqueous solutions of copper(II) sulfate from 5 to 65 degrees C by dielectric spectroscopy

Aqueous solutions of copper(II) sulfate have been studied by dielectric relaxation spectroscopy (DRS) over a wide range of frequencies (0.2 less, similar nu/GHz < or = 89), concentrations (0.02 < or = m/mol kg(-1) less, similar 1.4), and temperatures (5 < or = t/ degrees C < or = 65). The spectra show clear evidence for the simultaneous existence of double-solvent-separated, solvent-shared, and contact ion pairs at all temperatures, with increasing formation especially of contact ion pairs with increasing temperature. The overall ion association constant corresponding to the equilibrium: Cu2+(aq) + SO4(2-)(aq) right harpoon over left harpoon CuSO4(0)(aq) was found to be in excellent agreement with literature data over the investigated temperature range. However, the precision of the spectra and other difficulties did not allow a thermodynamic analysis of the formation of the individual ion-pair types. Effective hydration numbers derived from the DRS spectra were high but consistent with simulation and diffraction data from the literature. They indicate that both ions influence solvent water molecules beyond the first hydration sphere. The implications of the present findings for previous observations on copper sulfate solutions are briefly discussed.     

The association in mixtures of dimethylsulphoxide and water as studied by Raman spectroscopy

In mixtures of dimethylsulphoxide (DMSO) and water, formation of a complex (DMSO) · (H₂O)₂ is indicated by wavenumber shifts and linewidth variations of the v SO Raman band. Higher temperatures interfere with this complex formation.     

Dynamic and molecular association in premicellar aqueous solutions of dicarboxylate amino acid-based surfactant as studied by 1H NMR

We examined a series of amino acid-based surfactants with two carboxylic groups separated by a spacer of one, two, or three carbon atoms with sodium and calcium counterions in the premicellar concentration region near the CMC. ¹H nuclear magnetic resonance (NMR) spectroscopy and NMR diffusometry techniques were used to study the local environment, association, and translational dynamics of the surfactant's molecules. We measured the self-diffusion coefficients of the micelles, calculated the effective hydrodynamic radii, and determined the temperature region in which the premicelles exist. With an increase in temperature from 295 to 335 K, the premicellar state of the surfactant is replaced by the monomeric state.     

Diffusion with hydrolysis and ion association in aqueous solutions of beryllium sulfate

Taylor dispersion is used to measure mutual diffusion coefficients for aqueous solutions of beryllium sulfate at concentrations from 0.005 to 1 mol-L^–1 at 25°C. Least-squares analysis of the dispersion profiles shows that diffusion of the partially hydrolyzed salt produces a small additional flow of sulfuric acid, about 0.04 mol sulfuric acid per mole of total beryllium sulfate. Ternary diffusion coefficients measured for the aqueous BeSO₄–H₂SO₄ system are qualitatively consistent with Nernst-Planck predictions based on the formation of beryllium sulfate ion pairs, bisulfate ions, and the hydrolysis equilibria 2Be²⁺+H₂O= Be₂OH³⁺+H⁺, 3Be²⁺+2H₂O=Be₃(OH) ₂ ⁴⁺ +2H⁺. Except for very dilute solutions, the predicted flow of sulfuric acid is small compared to the flow of beryllium sulfate because most of the beryllium ions are protected from hydrolysis by the formation of BeSO₄ ion pairs, and most of the hydrogen ions produced by hydrolysis are converted to less-mobile bisulfate ions.     

Vibrational dynamics of some anions in aqueous solutions from isotropic Raman scattering

From the I_VV and I_VH Raman spectra of a totally symmetric vibration, isotropic scattering profiles have been obtained for several anions in dilute aqueous solutions at various temperatures. The Raman spectrometer was connected to an A/D converter and a small computer, which allowed multiple scanning, data accumulation, and numerical processing of these profiles with good precision over a frequency range of about 20 times their full width at half peak height.The theoretical analysis of the isotropic Raman profiles was accomplished by assuming vibrational phase relaxation based on a MARKOV-GAUSS mechanism. Fitting procedures to the experimental data permitted the determination of the vibrational frequency distribution and its modulation time. Temperature effects are discussed in relation to the distribution and motional characteristics of the immediate environment of water molecules of the anions.     

Vibrational band shape analysis for dimethylsulfoxide and hexadeuterodimethylsulfoxide in solutions

The influence of the deuterium substitution on the vibrational spectra of liquid and dimethylsulfoxide (DMSO) solutions is analysed from the point of view of the frequency shift and of the molecular dynamics behaviour as well.     

Phase separation dynamics of aqueous solutions of thermoresponsive polymers studied by a laser T-jump technique

Poly(N-isopropylacrylamide) and poly(vinyl methyl ether) are well-known thermoresponsive polymers. The aqueous solutions of these polymers exhibit a phase transition followed by phase separation with LCST approximately 305-310 K. In the present study, the dynamic behavior of the phase separation was analyzed by a laser T-jump method. Two different T-jump methodologies were employed: the first was a dye-photosensitized T-jump technique (indirect heating) using 532 nm laser pulses, while the other was a direct heating T-jump technique using 1.2 mum laser pulses. Both methods gave similar results. The time constants (tau) of the phase separation were systematically determined for 1-10 wt % aqueous solutions of the polymers, and a hydrodynamic radius (R) dependence for tau was clearly observed. The values of tau increased linearly with increasing square of R. The present behavior is interpretable in the framework of Tanaka's model for the volume phase transition of a gel, since each of the polymer chains are entangled in the present sample solutions, which can be regarded as approximating to a gel in solution.     

Molecular aggregates in aqueous solutions of bile acid salts. Molecular dynamics simulation study

The aggregation behavior of two bile acid salts (i.e., sodium cholate and sodium deoxycholate) has been studied in their aqueous solutions of three different concentrations (i.e., 30, 90,and 300 mM) by means of molecular dynamics computer simulations. To let the systems reach thermodynamic equilibrium, rather long simulations have been performed: the equilibration period, lasting for 20-50 ns, has been followed by a 20 ns long production phase, during which the average size of the bile aggregates (regarded to be the slowest varying observable) has already fluctuated around a constant value. The production phase of the runs has been about an order of magnitude longer than the average lifetime of both the monomeric bile ions and the bonds that link two neighboring bile ions together to be part of the same aggregate. This has allowed the bile ions belonging to various aggregates to be in a dynamic equilibrium with the isolated monomers. The observed aggregation behavior of the studied bile ions has been found to be in good qualitative agreement with experimental findings. The analysis of the results has revealed that, due to their molecular structure, which is markedly different from that of the ordinary aliphatic surfactants, the bile ions form rather different aggregates than the usual spherical micelles. In the lowest concentration solution studied, the bile ions only form small oligomers. In the case of deoxycholate, these oligomers, such as the ordinary micelles, are kept together by hydrophobic interactions, whereas in the sodium cholate system, small hydrogen-bonded aggregates (mostly dimers) are also present. In the highest concentration systems, the bile ions form large secondary micelles, which are kept together both by hydrophobic interactions and by hydrogen bonds. Namely, in these secondary micelles, small hydrophobic primary micelles are linked together via the formation of hydrogen bonds between their hydrophilic outer surfaces.     

Raman and infrared spectroscopic investigation of contact ion pair formation in aqueous cadmium sulfate solutions

Raman and IR data for aqueous CdSO₄ and (NH₄)₂SO₄ solutions have been recorded over broad concentration and temperature ranges. Whereas the v₁-SO ₄ ^2– band profile is symmetrical in (NH₄)₂SO₄ solutions, in CdSO₄ solutions a shoulder appears on the high frequency side which increases in intensity with increasing concentration and temperature. The molar scattering coefficient of the v₁-SO ₄ ^2– band is the same for all forms of sulfate in (NH₄)₂SO₄ and CdSO₄ solutions and is independent of temperature up to 99°C. The high frequency shoulder is attributed to the formation of a contact ion pair [Cd²⁺OSO ₃ ^2– ] (11 associate). Also the v₃-SO ₄ ^2– antisymmetric stretching mode shows a splitting in the CdSO₄ solution. Further spectroscopic evidence for contact ion pair formation is provided by IR spectroscopy. No higher associates or anionic complexes are required to interpret the spectroscopic data. The degree of association has been measured as a function of concentration and temperature. The thermodynamic association constant, K_A=0.15±0.05 kg-mol^–1 at 25°C is estimated from the Raman data by an extrapolation procedure by taking account of the activity coefficients. Values are reported for the activity coefficient of the ion pair. From the Raman temperature dependence studies, the enthalpy of formation for the contact ion pair is estimated to be 10±1 kJ-mol^–1.     

Structure and conformation analysis on perfluorodecanoic acid and its lithium,sodium and ammonium salts as studied by Raman spectroscopy

Raman spectroscopy is shown to be a useful method for structure and conformation analysis in fluorinated surfactants (perfluoro decanoic acid and its lithium, sodium and ammonium salts). The most sensitive part of the spectra in this respect is formed by the frequency changes occurring in the 1400–1700 cm^?1 region and the changes in the relative intensity of some bands associated either with C-F or C-C stretching vibrations. The successive increase in the degree of disorder at transitions into liquid crystalline and micellar phases can be seen from a decrease in the 1370/1300 cm^?1 intensity ratio.     

Surface hydration of aqueous calcium minerals as studied by Fourier transform Raman and infrared spectroscopy

Rapid hydration reactions of several calcium minerals are studied using Fourier transform (FT) Raman and infrared (IR) spectroscopy. Oleate adsorption over aqueous synthetic fluorite, calcite and gypsum (pH 9) is investigated and adsorption mechanisms are discussed.     

Ion association and hydration in aqueous solutions of LiCl and Li2SO4 by dielectric spectroscopy

A systematic study of the dielectric relaxation spectra of aqueous solutions of LiCl and Li2SO4 has been made at solute concentrations of 0.05 < or = c/M < or = 1.0 and 2.0, respectively, and over a wide range of frequencies (0.2 < or = nu/GHz < or = 89) at 25 degrees C. The spectra were best described by a superposition of four Debye processes, consisting of the two well-known water relaxations at ca. 8 and 0.5 ps and two ion-pair contributions at ca. 200 and 20 ps, corresponding to the presence of double-solvent-separated (2SIP) and solvent-shared (SIP) ion pairs, respectively. Consistent with spectroscopic studies, no contact ion pairs were detected over the studied concentration range. The overall ion association constants K(o)(A) obtained were in good agreement with literature data for both salts. Detailed analysis of the solvent relaxations indicated that Li+ has a significant second solvation sheath although there were differences between the effective hydration numbers obtained from LiCl and Li2SO4, which might arise from competition for the solvent from the anions.     

Water structure in aqueous solutions of alkali halide salts: FTIR spectroscopy of the OD stretching band

Water structure making/breaking studies in solutions of five alkali halide salts (KF, KI, NaI, CsF and CsCl) in 4 wt% D(2)O in H(2)O mixtures have been performed by FTIR analysis of the OD stretching band in the full solubility range. The proposed method gives a microscopic picture of the water structure making/breaking character of the salts in terms of the hydrogen bonding between the water molecules in the solution. With the exception of CsCl, there is a very good correlation of the structure making/breaking character of the salts determined by FTIR analysis, and the viscosity coefficients of the solutions. The results fully support and explain previous studies of bubble attachment to microscopic salt particles of the above salts. The investigations support the primary importance of interfacial water structure in the explanation of the flotation of alkali halide salts in their brines.