Density and Viscosity Study of Interactions of Some Amino Acids in Aqueous Solutions of Sodium Benzoate

The density (ρ) and viscosity (η) of three amino acids, glycine, l-alanine, and l-valine, have been determined as a function of amino acid concentration in aqueous solutions of 0.1011, 0.3088, and 0.5245 mol·kg^–1 sodium benzoate at temperatures 303.15, 308.15, and 313.15 K. Apparent molar volumes (<i>V</i>_ϕ), limiting values of apparent molar volumes (<i>V</i>⁰_ϕ), and transfer volumes (Δ_t<i>V</i>⁰_ϕ) have been calculated from the density data. The viscosity data could be fit to the Jones–Dole equation, and <i>B</i> coefficients and variation of <i>B</i> with temperature d<i>B</i>/d<i>T</i> were obtained at different concentrations and temperatures. Free energies of activation of the solvent (Δμ₁^0≠) and solute (Δμ₂^0≠) were also calculated by application of the Eyring transition-state theory. The obtained thermophysical data have been interpreted in terms of the structure of the amino acids and their interactions with the sodium benzoate solution

Transport Properties of the 1‑Hexyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)amide–Trichloromethane Binary System: Indication of Trichloromethane Segregation

Self-diffusion coefficients and electrical conductivity were studied for the binary system 1-hexyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­amide–trichloromethane ([C₆mim]­[NTf₂]–CHCl₃) as a function of composition and temperature. Self-diffusion coefficients of cation and anion are identical for ionic liquid mole fractions <i>x</i>_IL < 0.95. The self-diffusion coefficient of CHCl₃ is consistently larger than that of the ions by a factor of 4. A double logarithmic plot for the ratio of self-diffusion coefficient and temperature versus viscosity is linear for ionic liquid mole fractions 0.1 < <i>x</i>_IL < 0.9 indicating (a) a fractional Stokes–Einstein applies where self-diffusion is inverse proportional to some power <i>b</i> of viscosity (<i>D</i> ∼ η^–<i>b</i>) and (b) single average length scales are associated with the mass transport of [C₆mim]­[NTf₂] and CHCl₃. However, the obtained length scale for CHCl₃ is unreasonably small, which is indicative of CHCl₃ segregation. The molar conductivity displays a maximum near <i>x</i>_IL = 0.2. Evaluation of the ionicity from molar conductivity and self-diffusion coefficients indicates a gradual speciation change from charged species to neutral species for <i>x</i>_IL < 0.5. The temperature dependencies of self-diffusion and molar conductivity follow Arrhenius behavior. The obtained <i>x</i>_IL-dependent activation energies are found to be linear for molar conductivity and largest for the cation and anion self-diffusion coefficients. The activation energies for the self-diffusion of CHCl₃ appear to be identical with those obtained from fluidity data

Comparing Composition- and Temperature-Dependent Excess Molar Volumes of Binary Systems Involving Ionic Liquids

A total of 167 data sets of composition- and temperature-dependent excess molar volumes, <i>V</i>^E, which are derived from density measurements, for ionic liquid–molecular solvent binary systems are inspected concerning their temperature dependence at fixed compositions. It is found that the <i>V</i>^E temperature dependence is generally linear regardless of the sign and shape of the <i>V</i>^E composition dependence. Plotting the slopes and intercepts of the linear <i>V</i>^E temperature dependence as a function of composition allows for convenient comparisons of existing data sets. In doing so, <i>V</i>^E data sets are critically examined with respect to data quality as well as obtaining insight into structure–property relationships. This study also includes new density measurements for the binary systems 1-hexyl-3-methylimidazolium bis­(trifluoro­methyl­sulfonyl)­amide–chloroform and 1-butyl-3-methylimidazolium methylsulfonate–water, measured in the respective temperature ranges of (288.15 and 318.15) K and (298.15 and 358.15) K

Combining Freezing Point Depression and Self-Diffusion Data for Characterizing Aggregation

The colligative property freezing point depression is evaluated as a means for estimating the extent of aggregation for solutions of poly­(ethylene oxide) alcohol (C₁₀E₆) nonionic surfactant in cyclohexane. Combined with additional measurements of self-diffusion coefficients, it is shown that both unaggregated C₁₀E₆ as well as reverse micelles are significantly present for the entire range of measured C₁₀E₆ concentration (0.048–2.35 mol kg^–1). A change in speciation near 0.2 mol kg^–1 is indicated by the results from both freezing point depression and self-diffusion coefficient measurements. It is shown that average reverse micelle radii and aggregation numbers obtained from the ratio of solvent and C₁₀E₆ self-diffusion coefficients are consistent with prior reported results. However, unreasonably small radii for the reverse micelles as well as for the cyclohexane were obtained from analysis of the results by the Stokes–Einstein equation using additional measured solution viscosities. The concentration of reverse micelles and unaggregated C₁₀E₆ was calculated from the freezing point depression results using the aggregation numbers obtained from ratio of self-diffusion coefficients. These concentrations indicate that the reverse micelles become smaller in average size and increase in number with increasing temperature without an increase in unaggregated C₁₀E₆

Comparative Study of the Magnetic Field Dependent Signal Enhancement in Solid-State Dynamic Nuclear Polarization Experiments

A detailed study of the magnetic field dependent signal enhancement in solid-state dynamic nuclear polarization (DNP) experiments is presented for a specific sample consisting of AMUPol dissolved in the nonionic surfactant C₁₀E₆. C₁₀E₆ displays a superposition of “direct” and “indirect channel” resonances in ¹³C MAS DNP NMR spectra. The shapes of the DNP enhancement profiles are essentially identical for the ¹H MAS, ¹H → ¹³C CP MAS, and ¹³C MAS indirect channel signals, which confirms that the same polarization transfer process from electron to proton is responsible for the obtained enhancements of these experiments. The shape of the DNP enhancement profiles of ¹H and of ¹³C direct channel resonances reveals that the cross effect is the dominant polarization transfer mechanism for the studied sample. The magnitudes of the ¹³C MAS DNP enhancement profiles for ¹H → ¹³C CP MAS, direct and indirect channel signals were found to be not uniform. For ¹H → ¹³C CP MAS and the indirect channel signals, this observation is related to relaxation effects of the methyl group carbon. For the ¹³C MAS direct channel resonances, differences in magnitudes are discussed in terms of preferential structural orientation of the polar ethylene oxide headgroup of C₁₀E₆ toward the AMUPol radical

Comparing Composition- and Temperature-Dependent Viscosities of Binary Systems Involving Ionic Liquids

More than 200 composition- and temperature-dependent viscosity datasets for binary systems involving ionic liquids are analyzed with the Arrhenius model to inspect the composition dependence of the Arrhenius fit parameters activation energy, <i>E</i>_a, and <i>y</i>-intercept, ln <i>A</i>. The analysis also includes a new viscosity dataset for the binary system 1-hexyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­amide–trichloromethane. The majority of the binary systems show linear dependence of <i>E</i>_a and ln <i>A</i> with mole fraction, either over the entire range of composition or over a wide range of compositions, typically between 0.2 < <i>x</i>_IL < 1.0. These findings are useful for estimating unknown viscosities for binary systems involving ILs. As a side-outcome from the Arrhenius analysis and careful comparisons between datasets, a number of datasets are identified that are suspect of experimental inaccuracies

Composition Dependent Physicochemical Property Data for the Binary System Water and the Ionic Liquid 1‑Butyl-3-methylimidazolium Methanesulfonate ([C₄mim][MeSO₃])

A number of composition dependent physicochemical properties were measured for the water–[C₄mim]­[MeSO₃] binary system at 85 °C. Specifically, we provide data for density, heat capacity, heats of dissolution, conductivity, viscosity, as well as the self-diffusion coefficients of the cation, anion, and water. Ionic liquids based on the MeSO₃ anion constitute a suitable medium for various chemistries. This study addresses the need to provide physicochemical data on [C₄mim]­[MeSO₃] and the water–[C₄mim]­[MeSO₃] binary systems that are currently unavailable. Additional analysis of the combined data is also presented to provide further insight into the molecular level behavior of the water–[C₄mim]­[MeSO₃] binary system

Directly vs Indirectly Enhanced ¹³C in Dynamic Nuclear Polarization Magic Angle Spinning NMR Experiments of Nonionic Surfactant Systems

A study of dynamic nuclear polarization (DNP) in polyethylene glycol and related nonionic surfactants is presented. In these experiments, we found the surprising result that DNP enhanced ¹³C magic angle spinning (MAS) spectra display two sets of resonances, one with broad and one with sharp spectral features that are 180° opposite in phase. These two sets indicate the presence of a direct polarization transfer channel as expected for ¹³C MAS experiments, and a second unexpected indirect polarization transfer channel. Plots of DNP enhancements as a function of applied magnetic field for the two resonances show a superposition of two DNP enhancement profiles for AMUpol in the nonionic surfactant C₁₀E₆. The indirect polarization channel can be suppressed by application of a string of ¹H 180° pulses during ¹³C DNP buildup. The presence of direct and indirect polarization channels is observed in a total of four different nonionic surfactants and with three different radicals, showing that these concurring polarization mechanisms are of general nature. Therefore, the presented findings, including the demonstration of how the indirect polarization channel can be suppressed, are of high importance for all future applications of direct ¹³C MAS DNP

Influence of APTES-Decorated Mesoporous Silica on the Dynamics of Ethylene Glycol MoleculesInsights from Variable Temperature ²H Solid-State NMR

The physicochemical effects of decorating pore walls of high surface area materials with functional groups are not sufficiently understood, despite the use of these materials in a multitude of applications such as catalysis, separations, or drug delivery. In this study, the influence of 3-amino-propyl triethoxysilane (APTES)-modified SBA-15 on the dynamics of deuterated ethylene glycol (EG-d4) is inspected by comparing three systems: EG-d4 in the bulk phase (sample 1), EG-d4 confined in SBA-15 (sample 2), and EG-d4 confined in SBA-15 modified with APTES (sample 3). The phase behavior (i.e., melting, crystallization, glass formation, etc.) of EG-d4 in these three systems is studied by differential scanning calorimetry. Through line shape analysis of the 2H solid-state NMR (2H ssNMR) spectra of the three systems recorded at different temperatures, two signal patterns, (i) a Lorentzian (liquid-like) and (ii) a Pake pattern (solid-like), are identified from which the distribution of activation energies for the dynamic processes is calculated employing a two-phase model