Binary Liquid Phase Separation Research Articles

Experimental Investigation of Phase Equilibria in the Cu–Mo–Ti Ternary System

Phase equilibria of the Cu–Mo–Ti ternary system at 800 and 900 °C have been experimentally investigated using electron probe microanalysis and x-ray diffraction analysis. A ternary intermetallic compound (τ phase) was newly found, exhibiting a homogeneity range at both 800 and 900 °C. Furthermore, the nominal as-cast alloys Cu50Mo10Ti40, Cu30Mo30Ti40, Cu15Mo50Ti35, Cu40Mo30Ti30, Cu15Mo20Ti65, Cu15Mo30Ti55, Cu15Mo25Ti60, Cu40Mo10Ti50, and Cu60Mo5Ti35 presented a common separated microscopic morphology because of the ternary extent of the Cu–Mo binary liquid phase separation. The newly determined phase equilibria may provide essential information to support thermodynamic assessment and development of Cu–Mo-based alloys.

Phase field simulation of liquid phase separation with fluid flow

A phase-field theory of binary liquid phase separation coupled to fluid flow is presented. The respective Cahn–Hilliard-type and Navier–Stokes equations are solved numerically. We incorporate composition and temperature dependent capillary forces. The free energies of the bulk liquid phases are taken from the regular solution model. In the simulations, we observe Marangoni motion, and direct and indirect hydrodynamic interactions between the droplets. We find that coagulation is dramatically accelerated by flow effects. Possible extension of the model to solidification is discussed.

Light Scattering in the Eye Lens Near the Spinodal

Scattered light intensities measurements of the nuclear part of adult bovine lenses are reported. In the investigated samples the existence of a binary liquid phase separation from a metastable state is demonstrated by the phenomenon of hysteresis. Fluctuations near spinodal are studied by means of a tentative theoretical approach. Accordingly the scattered light intensities were analyzed reduced by the intensity obtained for the stable state of the system. Fluctuations in the stable state are ascribed to protein aggregates and are analyzed by means of random density fluctuation theory. To evaluate the correlation length of the fluctuations near spinodal the Ornstein-Zernike theory is adopted. Temperature dependence of the correlation length xi of the fluctuations near spinodal can be described by equation for critical fluctuations [formula: see text]. For the investigated lenses the exponent nu varies from 0.65 to 0.74 and the parameter xi 0 varies from 1.6 nm to 3.6 nm. The spinodal temperature Ts for the investigated samples is evaluated.

Towards a Molecular Understanding of Phase Separation in the Lens: a Comparison of the X-ray Structures of Two HighTcγ-Crystallins, γE and γF, with Two LowTcγ-Crystallins, γB and γD

γ-Crystallins, although closely related in sequence, show intriguing differences in their temperature-dependent interactions: those that have a high or intermediateTcfor phase separation are cryoproteins whereas lowTcγ-crystallins are not. To address the molecular basis of phase separation, X-ray crystallography has been used to define the structural differences between high and lowTcγ-crystallins. A pre-requisite for this study was to clarify the assignment of bovine gene sequences to bovine γ-crystallin proteins used for biophysical measurements. Based on nucleotide sequence analyses of γE and γF bovine crystallin genes, γF corresponds to the previously crystallised highTcprotein bovine γIVa and γE corresponds to the highTcbovine protein fraction previously known as γIIIa. The γF sequence has enabled the completion of the refinement of the bovine γF crystal structure which shows that the molecule has an additional surface tryptophan explaining why γF has different spectroscopic properties from γB. A highTcprotein from rat lens, γE crystallin, has been crystallised and the X-ray structure solved at 2.3 Å resolution. Comparison of the X-ray structures of two highTcproteins, rat γE and bovine γF, with the structures of two lowTcproteins, bovine γB and bovine γD, shows that the main conformational change between high and lowTcproteins is in thecdsurface loop of motif 3. All four structures have numerous ion pairs on their surfaces leading to a high surface charge density, yet with low overall charge. Comparison of the lattice contacts of the two highTcproteins with the two lowTcγ-crystallins indicates that these highTcproteins utilise more amino-aromatic interactions such as between histidine and arginine. Comparison of the sequences of all the γ-crystallins which have been characterised for phase separation temperature indicates that only residue Arg/Lys 163 uniquely distinguishes cryo from non-cryo γ-crystallins and it is close to the altered surface loop. Although this region probably contributes to phase separation,Tcis likely to be a function of an overall global property that is responsive to overall charge distribution. Calculated dipole moments of native γ-crystallins, lowTcγ-crystallin sequences threaded into highTcγ-crystallin structures, andvice versa, show how both sequence and 3D structure contribute to this overall property. HighTcγ-crystallins have on average higher Arg/Lys ratios and higher histidine content. It is hypothesised that this increases the proportion of surface static paired charged networks which thus reduces the repulsive hydration force and so increases the attractive interactions of the protein-rich phase in binary liquid phase separation.

Monte Carlo study of phase separation in aqueous protein solutions

The binary liquid phase separation of aqueous solutions of γ-crystallins is utilized to gain insight into the microscopic interactions between these proteins. The interactions are modeled by a square-well potential with reduced range λ and depth ε. A comparison is made between the experimentally determined phase diagram and the results of a modified Monte Carlo procedure which combines simulations with analytic techniques. The simplicity and economy of the procedure make it practical to investigate the effect on the phase diagram of an essentially continuous variation of λ in the domain 1.05≤λ≤2.40. The coexistence curves are calculated and are in good agreement with the information available from previous standard Monte Carlo simulations conducted at a few specific values of λ. Analysis of the experimental data for the critical volume fractions of the γ-crystallins permits the determination of the actual range of interaction appropriate for these proteins. A comparison of the experimental and calculated widths of the coexistence curves suggests a significant contribution from anisotropy in the real interaction potential of the γ-crystallins. The dependence of the critical volume fraction φc and the reduced critical energy ε̂c upon the reduced range λ is also analyzed in the context of two ‘‘limiting’’ cases; mean field theory (λ→∞) and the Baxter adhesive sphere model (λ→1). Mean field theory fails to describe both the value of φc and the width of the coexistence curve of the γ-crystallins. This is consistent with the observation that mean field is no longer applicable when λ≤1.65. In the opposite case, λ→1, the critical parameters are obtained for ranges much shorter than those investigated in the literature. This allows a reliable determination of the critical volume fraction in the adhesive sphere limit, φc(λ=1)=0.266±0.009.

The effect of salt identity and concentration on liquid–liquid phase separation in aqueous micellar solutions of C8-lecithin

We report measurements of the location and shape of the coexistence curve for binary liquid–phase separation in aqueous micellar solutions of C8 -lecithin, as a function of the concentrations of KI, NaI, LiI, KCl, NaCl, and LiCl. The salts containing iodide dramatically lower the critical temperature for phase separation Tc as the salt concentration is increased. The chloride salts, however, first lower and then raise Tc as the salt concentration is increased. The identity of the added cations used has a comparatively minor effect on Tc . Potassium ions are more effective in changing Tc than sodium ions, which in turn are more effective than lithium ions. We have used our data to deduce the dependence on salt identity and concentration of the parameters C and Δμ, which reflect the strength of intermicellar interactions and the extent of micellar growth, respectively, within a Gibbs free energy model for micellar solutions. The theoretical coexistence curves calculated using the deduced parameters agree well with our experimental findings.

Binary liquid phase separation and critical phenomena in a protein/water solution.

We have investigated the phase diagram of aqueous solutions of the bovine lens protein gamma II-crystallin. For temperatures T less than Tc = 278.5 K, we find that these solutions exhibit a reversible coexistence between two isotropic liquid phases differing in protein concentration. The dilute and concentrated branches of the coexistence curve were characterized, consistently, both by measurements of the two coexisting concentrations, c(T), and by measuring the cloud temperatures for various initial concentrations. We estimate that the critical concentration, cc, is 244 mg of protein per ml solution. The coexistence curve is well represented by the absolute value of (c - cc)/cc = 5.2 square root (Tc - T)/Tc. Using the temperature dependence of the scattered light intensity along isochores parallel to the critical isochore, we estimated the location of the spinodal line and found it to have the form (c - cc)/cc = 3.0 square root (Tc - T)/Tc. The ratio of the widths of the coexistence curve and the spinodal line, (5.2/3.0), is close to the mean-field value square root 3. We have also observed the growth of large crystals of gamma II-crystallin in some of these aqueous solutions and have made preliminary observations as to the factors that promote or delay the onset of crystallization. These findings suggest that selected protein/water systems can serve as excellent model systems for the study of phase transitions and critical phenomena.

Thermal diffusion measurements near a critical end point in a microemulsion system

A new optical technique has been developed for the study of thermal conductivity Λth and thermal diffusivity Dth in critical fluids. The new method combines the advantages of two traditional methods (induced thermal lens and thermal dynamic grating) while eliminating difficulties associated with them. This method has been applied to the investigation of dynamic behavior in a microemulsion composed of n-dodecane, water, n-pentanol, and sodium dodecyl sulfate. This system exhibits critical phase separation (into two microemulsions with different dispersed phases concentrations) that can be considered as analogous to binary liquid phase separation near a consolute point. Over a range of microemulsion compositions there exists a line of such critical points terminating at a critical end point. Near this critical end point both Λth and Dth exhibit an anomalous increase as the temperature approaches the critical temperature. The ratio Λth/Dth is found to be essentially constant near Tc, in agreement with the behavior of independently measured heat capacity values. The results are discussed in the context of current theoretical predictions for the critical transport properties of binary liquids.