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.

Figure 1

Proposed phase diagram of aqueous solutions of 3-methylpyridine and sodium bromide inferred from the experimental data observed by Jacob et al. [17, 21, 29] and reported in Ref. 20. The circles represent the experimental values of $T_c$ for various mass fractions of NaBr; the solid curve represents the critical locus implied by these values. The dashed curve indicates the boundary of the temperatures and salt concentrations where the presence of a possible microheterogeneous phase was assumed. The crosses indicate the observed appearance of a large background scattering and the horizontal bar the appearance of clustering inferred from SAXS measurements [21]. The critical locus over a wide range of salt concentrations is shown as an inset.

Figure 2

Locus of lower critical temperatures of aqueous solutions of 3-methylpyridine and sodium bromide: (◇): this study; (엯): Ref. 18; (×): Ref. 25; and (▵): Ref. 28.

Figure 3

Observed light-scattering intensity as a function of time at $21.83°\mathrm{C}$ (lower curve) and at $26.88°\mathrm{C}$ (upper curve) for an aqueous solution of 3-methylpyridine and sodium bromide with 16.7% mass fraction of NaBr.

Figure 4

Dynamic correlation function at $21.83°\mathrm{C}$ (dashed curve) and at $26.88°\mathrm{C}$ (solid curve) for an aqueous solution of 3-methylpyridine and sodium bromide with 16.7% mass fraction of NaBr.

Figure 5

Experimental light-scattering intensities, measured at $\theta =90°$ and corrected for multiple scattering and turbidity losses, as a function of $\epsilon$ for all samples of aqueous solutions of 3-methylpyridine and NaBr investigated in the present study. Data symbols: ▷ 14% NaBr (Bangalore sample), ▵ 16% NaBr, ◇ 16% NaBr (Madrid sample), ⊕ 16.7% NaBr run 1, 엯 16.7% NaBr run 2, ▿ 17% NaBr, ◻ 17% NaBr (Bangalore sample), and ⊠ 24% NaBr.

Figure 6

Experimental light-scattering intensities, corrected for multiple scattering and turbidity losses, obtained for the aqueous solution of 3-methylpyridine with 17% mass fraction of NaBr from our sample (▿) and from the old sample prepared in Bangalore (⊕). The solid curve represents a fit to the crossover theory. The differences, $\delta$, between experimental and calculated intensities are shown at the bottom. The temperature dependence of the effective susceptibility exponent ${\gamma }_{\mathrm{eff}}$ deduced from the fit to the data for our sample (solid curve) and for the fit to the data for the old Bangalore sample is shown in the inset.

Figure 7

Experimental light-scattering intensities, corrected for multiple scattering and turbidity losses, obtained for the aqueous solution of 3-methylpyridine with 16% mass fraction of NaBr from our sample (▵) and from the sample prepared in Madrid (엯). The solid curve represents a fit to the crossover theory. The differences, $\delta$, between experimental and calculated intensities are shown at the bottom. The temperature dependence of the effective susceptibility exponent ${\gamma }_{\mathrm{eff}}$ deduced from the fit to the data for our sample (solid curve) and for the fit to the data for the Madrid sample is shown in the inset.

Figure 8

Reduced crossover temperature ${\epsilon }_{\times }$ for different salt concentrations in the aqueous solutions of 3-methylpyridine and sodium bromide obtained in this study as compared with ${\epsilon }_{\times }$ from Ref. 18. 엯 from our samples, ⊕ from our measurements for samples prepared in Bangalore, ⊠ from our measurements for a sample prepared in Madrid [28], + Ref. 18.

Figure 9

(a) Dynamic correlation functions $g_2\left(\tau \right)$ for an aqueous solution of 3-methylpyridine at $20°\mathrm{C}$ (circles) and at $60°\mathrm{C}$ (squares); (b) corresponding probability distributions $H\left(\tau \right)$ of the decay time $\tau$ at $20°\mathrm{C}$ (solid curve) and at $60°\mathrm{C}$ (dashed curve).

Figure 10

Dynamic correlation functions $g_2\left(\tau \right)$ and corresponding probability distributions $H\left(\tau \right)$ of the decay time $\tau$ for two aqueous solutions of 3-methylpyridine at $20°\mathrm{C}$. The circles and the solid curve are for the same transparent sample (sample No. 1) shown in Fig. 9; the triangles and dashed curve are for a turbid sample (sample No. 2).

Figure 11

Result of an experimental study of long-living nonequilibrium states in aqueous solutions of 3-methylpyridine. We show the scattering intensity for four scattering angles as a function of time while the temperature is cycled between 25 and $80°\mathrm{C}$.

Figure 12

Experimental light-scattering intensities for an initially turbid aqueous solution of 3-methylpyridine with 17% mass fraction of NaBr after the addition of salt. Data obtained with the freshly prepared sample are indicated by +; data from Ref. 20 for a sample with the same concentration are indicated by $\Delta$. Data obtained the next day at room temperature are indicated by ×, and subsequent runs by, ◻, 엯, and ⊕.