Evidence of clustering in an aqueous electrolyte solution: a small-angle X-ray scattering study

Confined Water Molecules in Binary Mixtures of Water and 2,6-Lutidine Near Lower Solution Critical Temperature

We study the concentration and temperature dependence of the reorientation dynamics of water molecules in binary mixtures of water and 2,6-lutidine below the lower solution critical temperature (LSCT) with femtosecond mid-infrared pump–probe spectroscopy. The measurements reveal the presence of water molecules interacting with both the hydrophobic groups of lutidine and forming a hydrogen bond with the nitrogen atom of lutidine. Both types of molecules show a strongly decreased rotational mobility in comparison to bulk water. From the temperature dependence of the slow water fraction, we conclude that the lutidine molecules form clusters that decrease in size when the temperature is decreased further below the LSCT.

Salt-induced microheterogeneities in binary liquid mixtures

The salt-induced microheterogeneity (MH) formation in binary liquid mixtures is studied by small-angle x-ray scattering (SAXS) and liquid state theory. Previous experiments have shown that this phenomenon occurs for antagonistic salts, whose cations and anions prefer different components of the solvent mixture. However, so far the precise mechanism leading to the characteristic length scale of MHs has remained unclear. Here, it is shown that MHs can be generated by the competition of short-ranged interactions and long-ranged monopole-dipole interactions. The experimental SAXS patterns can be reproduced quantitatively by fitting to the derived correlation functions without assuming any specific model. The dependency of the MH structure with respect to ionic strength and temperature is analyzed. Close to the demixing phase transition, critical-like behavior occurs with respect to the spinodal line in the phase diagram.

Chemical reactivity and solution structure: on the way to a paradigm shift?

Reagent molecules inside solution domains {R1} and {R2} cannot contact hence react. For this reason solution structure may influence chemical reactivity.

Precipitation in aqueous mixtures with addition of a strongly hydrophilic or hydrophobic solute

We examine phase separation in aqueous mixtures due to preferential solvation with a low-density solute (hydrophilic ions or hydrophobic particles). For hydrophilic ions, preferential solvation can stabilize water domains enriched with ions. This precipitation occurs above a critical solute density n(p) in wide ranges of the temperature and the average composition, where the mixture solvent would be in a one-phase state without solute. The volume fraction of precipitated domains tends to zero as the average solute density n is decreased to np or as the interaction parameter χ is decreased to a critical value χ(p). If we start with one-phase states with n>n(p) or χ>χ(p), precipitation proceeds via homogeneous nucleation or via heterogeneous nucleation, for example, around suspended colloids. In the latter case, colloid particles are wrapped by thick wetting layers. We also predict a first-order prewetting transition for n or χ slightly below np or χ(p) for neutral colloids.

Ion-induced multiply reentrant liquid-liquid transitions and the nature of criticality in ethanol-water mixture

We report a quite unusual feature of four liquid-liquid reentrant transitions in ethanol (E)+water (W)+ammonium sulfate mixture by meticulous tuning of the ammonium sulfate concentration in a narrow range, as a function of temperature, at atmospheric pressure. Detailed exploration of the intricate phase behavior in terms of E/W sections shows that the range of triple reentrance shrinks with increasing E/W. The behavior of osmotic susceptibility is investigated by light scattering, near the critical point, in the one-phase region by varying the temperature at fixed concentration of the components, in a particular E/W section. The critical exponent of susceptibility (gamma) and correlation length (nu) are observed to have Fisher renormalized Ising values [Phys. Rev. 176, 237 (1968)], with gamma(r)=1.41 and nu(r)=0.718. The effective susceptibility exponent, gamma(eff), exhibits a sharp, nonmonotonic crossover from Ising to mean-field critical behavior, which is completed outside the critical regime. The amplitude of the correlation length, xi(o)(=21.2+/-0.4 A), deduced from light scattering experiment, is an order of magnitude larger than the typical values in usual aqueous electrolyte systems. This value of xi(o) is further verified from small-angle x-ray scattering (SAXS) experiments and found to be consistent. SAXS experiments on the critical sample reveal the presence of long-ranged intermolecular correlations, leading to supramolecular structuring, at a temperature far away from the critical point. These results convincingly demonstrate that the finite length scale arising due to the structuring competes with the diverging correlation length of critical concentration fluctuations, which influences the nonasymptotic critical behavior in this aqueous electrolyte system. The sulphate ions play a dominant role in both structuring and the complex phase behavior.

Lattice model of equilibrium polymerization. V. Scattering properties and the width of the critical regime for phase separation

Dynamic clustering associated with self-assembly in many complex fluids can qualitatively alter the shape of phase boundaries and produce large changes in the scale of critical fluctuations that are difficult to comprehend within the existing framework of theories of critical phenomena for nonassociating fluids. In order to elucidate the scattering and critical properties of associating fluids, we consider several models of equilibrium polymerization that describe widely occurring types of associating fluids at equilibrium and that exhibit the well defined cluster geometry of linear polymer chains. Specifically, a Flory-Huggins-type lattice theory is used, in conjunction with the random phase approximation, to compute the correlation length amplitude xi(o) and the Ginzburg number Gi corresponding, respectively, to the scale of composition fluctuations and to a parameter characterizing the temperature range over which Ising critical behavior is exhibited. Our calculations indicate that upon increasing the interparticle association energy, the polymer chains become increasingly long in the vicinity of the critical point, leading naturally to a more asymmetric phase boundary. This increase in the average degree of polymerization implies, in turn, a larger xi(o) and a drastically reduced width of the critical region (as measured by Gi). We thus obtain insight into the common appearance of asymmetric phase boundaries in a wide range of "complex" fluids and into the observation of apparent mean field critical behavior even rather close to the critical point.

Critical behavior of the aqueous electrolytic system 3-methylpyridine+D2O+NaBr

The system 3-methylpyridine(3MP) + water(H2O)+NaBr has been the subject of an intense scientific debate since the work of Jacob et al. [Phys. Rev. E. 58, 2188 (1988)] and Anisimov et al. [Phys. Rev. Lett. 85, 2336 (2000)]. The crossover critical behavior of this system seemed to show remarkable sensitivity to the weight fraction (X) of the ionic impurity NaBr. In the range X< or =0.10 the system displayed Ising behavior and a pronounced crossover to mean-field behavior in the range 0.10< or =X< or =0.16. A complete mean-field behavior was observed at X=0.17, a result that was later attributed to the existence of long-living nonequilibrium states in this system [Kostko et al., Phys. Rev. E. 70, 026118 (2004)]. In this paper, we report the near-critical behavior of osmotic susceptibility in the isotopically related ternary system, 3MP+heavy water(D2O)+NaBr. Detailed light-scattering experiments performed at exactly the same NaBr concentrations as investigated by Jacob et al. reveal that the system 3MP + D2O + NaBr shows a simple Ising-type critical behavior with gamma approximately 1.24 and nu approximately 0.63 over the entire NaBr concentration range 0< or =X< or =0.1900. The crossover behavior is predominantly nonmonotonic and is completed well outside the critical domain. An analysis in terms of the effective susceptibility exponent (gammaeff) reveals that the crossover behavior is nonmonotonic for 0< or =X< or =0.1793 and tends to become monotonic for X>0.1793. The correlation length amplitude xio, has a value of approximately 2 A for 0.0250< or =X< or =0.1900, whereas for X=0, xio approximately 3.179 A. Since isotopic H-->D substitution is not expected to change the critical behavior of the system, our results support the recent results obtained by Kostko et al. [Phys. Rev. E. 70, 026118 (2004)] that 3MP + H2O + NaBr exhibits universal Ising-type critical behavior typical for other aqueous solutions.

NMR probing of structural peculiarities in ionic solutions close to critical point

(1)H, (23)Na, (35)Cl, (79)Br, and (81)Br NMR chemical shifts (delta) and signal half widths (Delta(12)) have been measured in aqueous electrolyte mixtures [tetrahydrofuran/H(2)ONaCl and 3-methylpyridine (3MP)H(2)ONaBr] at different mass fractions of salt (X) in the one-phase region, close to their lower critical solution points (T(CL)). Discontinuous changes in slope of delta=f(X) and Delta(12)=f(X) have been found in (23)Na and (81)Br NMR spectra of 3MP/water/NaBr solution at X approximately 0.1 and T=301 K. The dependency of (1)H NMR signals of 3MP is continuous over the whole investigated range of X=0.002-0.2, whereas changes in the slope of H(2)O chemical shifts are hardly noticeable. In the two-phase region, i.e., at T>T(CL), a doubling of all NMR signals has been observed. The sensitivity of NMR parameters depends more on composition of solution for anions (Cl(-) and Br(-)) than for cations (Na(+)). A very strong relaxation effect for (81)Br nuclei with relaxation rates reaching 14 000 s(-1) was observed. The results are interpreted in terms of ion-molecular clustering and changes in coherency of dipole configurations of water molecules during supramolecular restructuring of solutions.

Collective and self-diffusion coefficients in an ionic critical mixture: 3-methylpyridine+water+NaBr

The dynamics of concentration fluctuations of three critical samples of the 3-methylpyridine (3MP)+water+NaBr system have been measured by photon correlation spectroscopy. The collective-diffusion coefficient D shows the usual Ising behavior near the critical temperature T(c). However, as |T-T(c)| increases, the dynamic correlation length calculated from D, xi, takes values higher than the correlation length of the critical fluctuations calculated from static light scattering, xi(s). At the largest |T-T(c)| measured, xi approaches the value, xi(0,d) approximately equal to 1.13 nm, while the amplitude of xi(s) is xi(0,s)=0.38 nm. Pulsed-gradient NMR spectroscopy points out the existence of two dynamic contributions. One of them is consistent with the existence of molecular entities of hydrodynamic radius 0.31 nm, while the other one indicates the existence of aggregates rich in 3MP of radius 1.16 nm. The existence of the aggregates may explain the apparent anomalous behavior of the dynamic light scattering experiments for this system far from the critical point.

Dynamical scaling of the structure factor of some non-Euclidean systems

Predictions of nonlinear theories on dynamics of new phase formation have been examined for the hydration of calcium silicates with light water and heavy water. In the case of hydration with light water, reasonable agreement has been observed with dynamical scaling hypothesis with a new measure of the characteristic length. The characteristic length does not follow a power law relation with time. Hydrating mass is found to be mass fractal throughout hydration, with mass fractal dimension increasing with time. But, in the case of hydration with heavy water, no agreement has been observed with the scaling hypothesis. Hydrating mass undergoes transition from mass fractal to surface fractal and finally again to mass fractal. The qualitative features of the kinetics of hydration, as measured in small-angle scattering experiments, are strikingly different for hydration with light water and heavy water.

Unconventional density dependence of the stochastic dynamics in an organic liquid

The density dependence of the diffusive rotational and center-of-mass dynamics of 2-methyl-pyridine is investigated by means of the concurrent use of quasielastic neutron scattering and molecular dynamics simulations. The dependence of both translation and rotational diffusion coefficients shows a distinctive change of slope with increasing density taking place about rho=0.975 g/cm3. Such a change in the dynamics can be related to observations made in other liquids composed of oblate-spheroidal particles.

Solvation effects in near-critical binary mixtures

A Ginzburg-Landau theory is presented to investigate solvation effects in near-critical polar fluid binary mixtures. Concentration dependence of the dielectric constant gives rise to a shell region around a charged particle within which solvation occurs preferentially. As the critical point is approached, the concentration has a long-range Ornstein-Zernike tail representing strong critical electrostriction. If salt is added, strong coupling arises among the critical fluctuations and the ions. The structure factors of the critical fluctuations and the charge density are calculated and the phase transition behavior is discussed.

Criticality in aqueous solutions of 3-methylpyridine and sodium bromide

We address a controversial issue regarding the nature of critical behavior in ternary electrolyte solutions of water, 3-methylpyridine, and sodium bromide. Earlier light-scattering studies showed an anomalous critical behavior in this system that was attributed to the formation of a microheterogeneous phase associated with ion-molecule clustering [M.A. Anisimov, J. Jacob, A. Kumar, V.A. Agayan, and J. V. Sengers, Phys. Rev. Lett. 85, 2336 (2000)]], while some other investigators subsequently found this system to exhibit ordinary Ising-like critical behavior. This contradiction forced us to revisit the problem and perform an accurate and comprehensive study of light scattering in this system paying attention to the achievement of thermodynamic equilibrium, hysteresis effects, aging, and prehistory of the samples, and a possible role of impurities. We show that properly aged, equilibrium samples of aqueous solutions of 3-methylpyridine with NaBr exhibit universal Ising-like critical behavior, typical for other aqueous solutions. No evidence for an equilibrium microheterogeneous phase was found. We have been able to reproduce anomalous behavior (similar to that reported initially) in a fast run on a freshly prepared sample. We attribute the observed anomalies to mesoscopic nonequilibrium aggregates, possibly associated with supramolecular restructuring in aqueous solutions. To support this conclusion we performed a study of aqueous solutions of 3-methylpyridine without NaBr and have found long-living nonequilibrium states in aqueous solutions of 3-methylpyridine.

Experimental evidence for crossover to mean-field tricritical behavior in a concentrated salt solution

We have discovered a mean-field multicritical point on the critical locus in an aqueous solution of 3-methylpyridine and sodium bromide. Light-scattering measurements indicate Ising-like asymptotic critical behavior at the lower salt concentrations. However, the temperature range of Ising critical behavior shrinks with increasing salt concentration and the critical behavior becomes mean-field-like at a concentration of about 17% mass fraction of NaBr. Emergence of a new characteristic length scale diverging at this point and a simultaneous pronounced increase in the background scattering suggests mean-field tricritical behavior associated with the formation of a microheterogeneous phase due to clustering of ions and molecules.