Chromonic liquid crystals are formed by the addition of aromatic molecules such as disodium chromoglycate (cromolyn) to water. In this study, we investigate the addition of salts to the lyotropic nematic phase of cromolyn aqueous solutions. The addition of sodium and potassium salts shifts the isotropic-nematic phase boundary upward by more than 10°C, so that samples that were isotropic at room temperature are transformed into nematic phases. Salt effects are predominantly dictated by the cation, not the anion, and appear to differ based on cation size. In contrast to small, hydrated cations like sodium, large, weakly hydrated cations such as tetraethylammonium and tetrabutylammonium shift the phase boundary downward, thus stabilizing the isotropic phase at the expense of the nematic one. The phase behavior results are highly correlated with viscosity measurements, with an upward shift in the phase boundary correlating with an increase in solution viscosity and vice versa. We also probe the microstructure in cromolyn-salt solutions, both indirectly by small-angle neutron scattering (SANS) and directly by cryo-transmission electron microscopy (cryo-TEM). The cryo-TEM images show the presence of rodlike aggregates that possibly undergo a higher order aggregation into bundles in the presence of salt.

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.

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We have studied isotropic-to-nematic pretransitional fluctuations in an aqueous solution of disodium cromoglycate (cromolyn) by static and dynamic light scattering. Cromolyn is a representative of lyotropic chromonic liquid crystals with building units being elongated rods formed by aggregates of disk-like molecules. By combining light-scattering and viscosity measurements we have determined the correlation length and relaxation time of the orientational order-parameter fluctuations and estimated the size of the cromolyn aggregates. The pretransitional behavior of light scattering does not completely follow the classic Landau-de Gennes model. This feature is most probably associated with the variable length of cromolyn aggregates. We have observed a dramatic increase of the shear viscosity near the transition to the nematic phase, the fact which correlates with the idea of growing supramolecular aggregates. The steep temperature dependence of the viscosity is accompanied by a practically temperature-independent translational diffusion coefficient.

We show that the approach to asymptotic fluctuation-induced critical behavior in polymer solutions is governed by a competition between a correlation length diverging at the critical point and an additional mesoscopic length-scale, the radius of gyration. Accurate light-scattering experiments on polystyrene solutions in cyclohexane with polymer molecular weights ranging from 200,000 up to 11.4 million clearly demonstrate a crossover between two universal regimes: a regime with Ising asymptotic critical behavior, where the correlation length prevails, and a regime with tricritical theta-point behavior determined by a mesoscopic polymer-chain length. PACS: 64.75.+g; 61.25.Hg; 05.70.Jk Close enough to the critical point, the correlation length ξ of the fluctuations of the order parameter has grown so large that the microscopic and even the mesoscopic structure of fluids become unimportant: complex fluids become "simple". This feature is known as criticalpoint universality [1]. Within a universality class, determined by the nature of the order parameter, properly chosen physical properties of different systems exhibit the same near-critical behavior. All critical phase-separation transitions in fluids belong to the 3-dimensional Isingmodel universality class, as the order parameter (associated with density or/and concentration) is a scalar. However, in practice, the pure asymptotic regime is often hardly accessible. Even in simple fluids, like xenon and helium, the physical properties in the critical region show a tendency to crossover from Ising asymptotic behavior to mean-field behavior [2,3]. This crossover depends on the microscopic structure of the system, namely, on the range of interaction and on a molecular-size "cutoff". In simple fluids, crossover to mean-field critical behavior is never completed within the critical domain (which can be defined roughly as within 10% of the critical temperature): the "cutoff" length and the range of interactions are too short. In complex fluids, regardless of the range of interaction, the role of the cutoff is played by a mesoscopic characteristic length scale ξ D that is associated with a particular mesoscopic structure [4]. If the cutoff length is mesoscopic, it can compete with the correlation length ξ within the critical domain. The temperature at which the correlation length becomes equal to the structural length can be naturally defined as a crossover temperature between two regimes, namely, an Ising asymptotic critical regime and a regime determined by the nature of the mesoscopic structure of the complex fluid. In some complex fluids, like polymer solutions, it is possible to tune the structural length-scale and make it very large. If both lengths, the correlation length of the critical fluctuations associated with the fluid-fluid separation and the structural correlation length, diverge at the same point, this point will be a multicritical point. A perfect example of such a multicritical phenomenon appears in a polymer solution near the theta point. The theta p...

The approach to asymptotic critical behavior in polymer solutions is governed by a competition between the correlation length of critical fluctuations diverging at the critical point of phase separation and an additional mesoscopic length scale, the radius of gyration. In this paper we present a theory for crossover between two universal regimes: a regime with Ising (fluctuation-induced) asymptotic critical behavior, where the correlation length prevails, and a mean-field tricritical regime with theta-point behavior controlled by the mesoscopic polymer chain. The theory yields a universal scaled description of existing experimental phase-equilibria data and is in excellent agreement with our light-scattering experiments on polystyrene solutions in cyclohexane with polymer molecular weights ranging from 2 x 10(5) up to 11.4 x 10(6). The experiments demonstrate unambiguously that crossover to theta-point tricriticality is controlled by a competition of the two mesoscales. The critical amplitudes deduced from our experiments depend on the polymer molecular weight as predicted by de Gennes [Phys. Lett. 26A, 313 (1968)]. Experimental evidence for the presence of logarithmic corrections to mean-field tricritical theta-point behavior in the molecular-weight dependence of the critical parameters is also presented.

The results from a dynamic light scattering study of concentrated aqueous systems of polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer (PEO-PPO-PEO, Pluronic F108) are reported at pressures to 2500 bar and temperatures from 0 to 110 °C. The pressure-temperature (P-T ) phase behavior for this micellar system exhibits several phase transformations at high concentrations including a transition from a liquid to a gel phase and a subsequent re-entrant transition from the gel phase to a liquid phase with increasing temperature. The gel phase region is bounded at low temperatures by a liquidto-gel phase transition curve that increases in pressure with increasing temperature, eventually exhibits a maximum in pressure, and then at high temperatures rapidly decreases as the temperature is further increased. The high temperature branch of this curve represents a gel-melting transition. Aqueous F108 samples are doped with latex probe nanoparticles to facilitate the characterization of the local viscosity and also to serve as a reference for the measurement of the light intensity scattered by the Pluronic systems. Both the intrinsic collective dynamics of the Pluronic systems and the diffusive dynamics of the nanoparticle probes are characterized. The diffusive dynamics of the nanoparticle probes reveal the nonmonotonic character of viscosity that results from phase transformations dependent on pressure and temperature. The micellar system scattering intensity, obtained from DLS data through comparison with the light intensity scattered by the probe nanoparticles, increases with heating indicating critical pretransitional behavior of concentration fluctuations on approach to the cloud-point boundary over a broad temperature interval that spans the gel phase temperature range.

We have performed accurate dynamic light-scattering measurements near critical demixing points of solutions of polystyrene in cyclohexane with polymer molecular weight ranging from 200,000 to 11.4 million. Two dynamic modes have been observed, "slow" and "fast", that result from a coupling between diffusive relaxation of critical fluctuations of the concentration and visco-elastic relaxation associated with the entanglement network of the polymer chains. The coupling with the visco-elastic mode causes an additional slowdown of the critical mode on top of the uncoupled diffusion mode. By implementing crossover from critical to theta-point tricritical behavior for both static and dynamic properties, we are able to present a quantitative description of the phenomenon and to obtain a scaling of the visco-elastic parameters as a function of the molecular weight. 61.41.+eCritical phenomena in high-molecular-weight polymer solutions differ from critical phenomena in simple fluids. In polymer solutions, the thermodynamic properties and the static correlations near the critical point of mixing are determined by a competition of two mesoscale lengths, namely, the correlation length of critical fluctuations of the concentration (tuned by the distance to the critical point) and the radius of gyration of the polymer molecules (tuned by the molecular weight) [1,2]. In the asymptotic vicinity of the critical point, the correlation length becomes much larger than the radius of gyration and the polymer solution exhibits Ising critical behavior. With increase of the polymer molecular weight and, hence, of the radius of gyration, the range of asymptotic Ising critical behavior shrinks, ultimately yielding to theta-point tricritical behavior [2]. This competition of two mesoscales arises from a coupling between two different order parameters belonging to two different static universality classes, namely, one associated with phase separation and another one with self-avoiding-walk singularities of long polymer chains [3]. Such a competition of two mesoscales and, consequently, crossover from critical to multicritical behavior are expected to be a general feature of phase transitions with coupled order parameters [4]. Hence, it is natural to expect that the dynamic critical behavior near the theta point will be affected by a coupling between two soft dynamic modes associated with the two order parameters [5].In the present communication we report a study of dynamic correlations in near-critical solutions of polystyrene in cyclohexane at the critical volume fraction φ c as a function of temperature and of polymer molecular weight, ranging from 200,000 to 11.4 million. Specifically, we have found that starting with a molecular weight of about one million, two effective dynamic modes emerge, that result from a coupling between two soft critical modes, a diffusion mode (association with the decay of critical fluctuations) and a visco-elastic mode (associated with entanglements of long polymer chains). Very close to the critical point, as...