Determination of the quaternary phase diagram of the water-ethylene glycol-sucrose-NaCl system and a comparison between two theoretical methods for synthetic phase diagrams

Characterization of the thermodynamic properties of multi-solute aqueous solutions is of critical importance for biological and biochemical research. For example, the phase diagrams of aqueous systems, containing salts, saccharides, and plasma membrane permeating solutes, are indispensible in the field of cryobiology and pharmacology. However, only a few ternary phase diagrams are currently available for these systems. In this study, an auto-sampler differential scanning calorimeter (DSC) was used to determine the quaternary phase diagram of the water-ethylene glycol-sucrose-NaCl system. To improve the accuracy of melting point measurement, a “mass-redemption” method was also applied for the DSC technique. Base on the analyses of these experimental data, a comparison was made between the two practical approaches to generate phase diagrams of multi-solute solutions from those of single-solute solutions: the summation of cubic polynomial melting point equations versus the use of osmotic virial equations with cross coefficients. The calculated values of the model standard deviations suggested that both methods are satisfactory for characterizing this quaternary system.     

Crystallization and phase behavior of 1,3-propanediol esters II. 1,3-propanediol distearate/1,3-propanediol dipalmitate (SS/PP) and 1,3-propanediol distearate/1,3-propanediol dimyristate (SS/MM) binary systems

Polymorphic influences on the phase behavior of two types of binary mixtures of saturated monoacid 1,3-propanediol esters (PADEs), dipalmitate/distearate (PP/SS) and dimyristate/distearate (MM/SS) were examined by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and by solid fat content (SFC), hardness and microscopy measurements. Three stacking modes have been found in the PP/SS binary system. Mixed SS-PP bilayers were detected in all mixtures, SS-SS bilayers in x(PP)=0.0-0.4 mixtures and PP-PP bilayers in x(PP)=0.6-0.1 mixtures. Two different but close beta polymorphs and one beta' polymorph were detected for this system. beta' was only detected in x(PP)=0.5-0.9 mixtures for the mixed bilayers. For the MM/SS binary system, only MM-MM and SS-SS bilayers were detected and both solid phases crystallized in two different beta forms. XRD data evidenced clearly that the MM and SS components were completely immiscible in the solid state. The phase diagrams constructed using DSC data, exhibited a typical eutectic-type phase boundary. The presence of eutectics, the shape of the solidus lines as well as the analysis of the individual enthalpies of melting indicated typical phase separation for both systems. A thermodynamic study based on the Hildebrand equation and using the Bragg-Williams approximation for non-ideality of mixing confirmed the phase separation in the solid phase and suggested that the PP and SS were miscible in the liquid phase and that SS formed an ideal mixing with MM. Avrami analysis of SFC vs. time curves indicated heterogeneous nucleation and spherulitic crystal development from sporadic nuclei, and suggested that the nucleation rate was higher for the mixture at the eutectic composition. The relative hardness was correlated with the enthalpies, the final SFC and the microscopy measurements.     

Phase diagram of the system sodium alginate/water: a model for biofilms

Sodium alginate is a polyelectrolyte consisting of the monomer units β-d-mannuronate and -l-guluronate. Mainly based on the theory of Khokhlow et al., the state diagram of the binary system alginate/water has been calculated using different sets of parameters like degree of ionization, degree of polymerization and interaction function. The calculations comprise miscibility gaps, liquidus curves, eutectic points and the behaviour at temperatures below the melting point of water. Also gel and swelling curves have been treated, where gels are physically crosslinked. The DSC diagram of a 0.5 by wt.% polymer sol shows a double melting peak, which is explained by a heterogeneity above 0 °C. The crystallization of water seems to concentrate the gelled system irreversibly.     

Phase quilibria in the system dimyristoylphosphatidyl choline/N-hexadecyl-N,N,N,-trimethylammoniumbromide/water at 30°C. Swelling behaviour of the lamellar phase with different electrolyte solutions

The phase equilibria of a swelling amphiphile, dimyristoylphosphatidylcholine (DMPC) and a cationic soap, cetyltrimethylammoniumbromide (CTAB) has been investigated in water at 30°C. The swelling of the lamellar phase of DMPC in water increases dramatically in the presènce of a few mol% of CTAB. Maximum swelling of the lamellar phase in water is obtained when the lamellar phase is in equilibrium with aqueous solution whose concentration of CTAB is less than the critical micell concentration (cmc). Vesicles are easily formed by the lamellar phase in this region. When the cme of CTAB is exceeded, DMPC is solubilized in micelles. The solubilizing capacity corresponds to about 1 mol of DMPC per 3 mol of CTAB. The phase behaviour of DMPC and CTAB in water is very similar to the corresponding equilibria for amphiphite compounds with shorter hydrocarbon chains. The swelling of DMPC and CTAB is influenced by the presence of electrolytes in the water phase. Due to electrostatic effects the swelling of the lamellar phase is lower in electrolyte solutions than in pure water.