Thermoreversible gelation and phase separation in aqueous methyl cellulose solutions

We derived typical phase diagrams for aqueous solutions of methyl cellulose (MC) of different molecular weights via micro‐differential scanning calorimetry, small‐angle X‐ray scattering, and visual inspection. The phase diagrams showed the cooccurrence of gelation and phase separation and qualitatively agreed with the theoretically calculated diagrams. The sol–gel transition line and phase separation line of a lower critical solution point type shifted toward lower temperatures and lower concentrations with an increase in the MC molecular weight. The sol–gel transition line intersected at a temperature higher than the critical point of the phase separation; therefore, both sol–gel phase separation and gel–gel phase separation were possible, depending on the temperature. Specifically, through visual inspection of a high molecular weight MC sample in the critical temperature region, we observed phase separation into two coexisting gels with different polymer concentrations. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 91–100, 2001     

Interplay between gelation and phase separation in aqueous solutions of methylcellulose and hydroxypropylmethylcellulose

Thermally induced gelation in aqueous solutions of methylcellulose (MC) and hydroxypropylmethylcellulose (HPMC) has been studied by rheological, optical microscopy, and turbidimetry measurements. The structural and mechanical properties of these hydrogels are dominated by the interplay between phase separation and gelation. In MC solutions, phase separation takes place almost simultaneously with gelation. An increase in the storage modulus is coupled to the appearance of a bicontinuous structure upon heating. However, a thermal gap exists between phase separation and gelation in the case of HPMC solutions. The storage modulus shows a dramatic decrease during phase separation and then rises in the subsequent gelation. A macroporous structure forms in the gels via "viscoelastic phase separation" linked to "double phase separation".     

Clouding: origin of phase separation in oppositely charged polyelectrolyte/surfactant mixed solutions

The phase behavior in the system of cationic modified poly(vinyl alcohol) (CPVA)-sodium dodecylsulfate (SDS)-water has been investigated. Samples were found phase separated near electroneutral mixing at CPVA concentrations < or =6%, while in a medium CPVA concentration of 7-12%, the phase separation disappeared and the system transformed into bluish homogeneous solution. At > or =13% CPVA concentrations, the mixed systems became colorless homogeneous. Preclouding phenomenon was observed in 5-8% CPVA-SDS mixed systems at an electroneutral mixing ratio. The addition of inorganic salts, such as Na2SO4, NaCl, NaBr, and NaSCN, could exclude the bluish and phase separation phenomenon that was found to be caused by the increase of clouding point in these systems. The clouding phenomenon was proven to be the origin of the phase separation in the CPVA-SDS mixed system. The ability for the inorganic salts to increase the clouding point follows the order of the Hofmeister series.     

Mutual and self-diffusion of charged porphyrines in aqueous solutions

We have investigated the diffusion properties for an ionic porphyrin in water. Specifically, for the {tetrasodium tetraphenylporphyrintetrasulfonate (Na₄TPPS) + water} binary system, the self-diffusion coefficients of TPPS⁴⁻ and Na⁺, and the mutual diffusion coefficients were experimentally determined as a function of Na₄TPPS concentration from (0 to 4) · 10⁻³ mol · dm⁻³ at T = 298.15 K. Absorption spectra for this system were obtained over the same concentration range. Molecular mechanics were used to compute size and shape of the TPPS⁴⁻ porphyrin. We have found that, at low solute concentrations (<0.5 · 10⁻³ mol · dm⁻³), the mutual diffusion coefficient sharply decreases as the concentration increases. This can be related to both the ionic nature of the porphyrin and complex associative processes in solution. Our experimental results are discussed on the basis of the Nernst equation, Onsager–Fuoss theory and porphyrin metal ion association. In addition, self-diffusion of TPPS⁴⁻ was used, together with the Stokes–Einstein equation, to determine the equivalent hydrodynamic radius of TPPS⁴⁻. By approximating this porphyrin to a disk, we have estimated structural parameters of TPPS⁴⁻. These were found to be in good agreement with those obtained using molecular mechanics. Our work shows how the self-diffusion coefficient of an ionic porphyrin in water is substantially different from the corresponding mutual-diffusion coefficient in both magnitude and concentration dependence. This aspect should be taken into account when diffusion-based transport is modelled for in vitro and in vivo applications of pharmaceutical relevance.     

Inclusion complexation of novocaine by beta-cyclodextrin in aqueous solutions

The formation of inclusion complexes between beta-cyclodextrin (beta-CD) and the local anesthetic 2-(diethylamino)ethyl-p-amino-benzoate (novocaine) in aqueous solutions under different acidity conditions, using steady-state fluorescence or UV-vis spectroscopies, electrical conductivity, or the kinetic study of both the nitrosation reaction of the primary amine group in a mild acid medium and the hydrolysis of the ester function under an alkaline medium, has been studied. The inclusion complex formation between neutral or protonated novocaine and beta-CD of 1:1 stoichiometry was observed; however, the magnitude of the binding constants depends on the nature of both the guest and the host, and the higher-affinity guest-host was found under conditions when both the novocaine and the beta-CD were neutral molecules.     

Kinetics of thermally induced phase separation in ternary polymer solutions. I. Modeling of phase separation dynamics

Simulations based on Cahn–Hilliard spinodal decomposition theory for phase separation in thermally quenched polymer/solvent/nonsolvent systems are presented. Two common membrane‐forming systems are studied, cellulose acetate [CA]/acetone/water, and poly(ethersulfone) [PES]/dimethylsulfoxide [DMSO]/water. The effects of initial polymer and nonsolvent composition on the structure‐formation dynamics are elucidated, and growth rates at specific points within the ternary phase diagram are quantified. Predicted pore growth rate curves exhibit a relative maximum with nonsolvent composition. For shallow quenches (lower nonsolvent content) near a phase boundary, the pore growth rate increases with increasing quench depth, whereas for deep quenches, where the composition of the polymer‐rich phase approaches that of a glass, the pore growth rate decreases with increasing quench depth. With increasing initial polymer concentration, the overall rate of structure growth is lowered and the growth rate maximum shifts to higher nonsolvent compositions. This behavior appears to be a universal phenomenon in quenched polymer solutions which can undergo a glass transition, and is a result of an interplay between thermodynamic and kinetic driving forces. These results suggest a mechanism for the locking‐in of the two‐phase structure that occurs during nonsolvent‐induced phase inversion. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1449–1460, 1999     

Thermodynamic characteristics of and intermolecular interactions in aqueous solutions of N-methylformamide

The thermodynamic characteristics of aqueous solutions of N-methylformamide were calculated over the whole range of mixture compositions. Intermolecular interaction parameters were determined, and the boundaries of concentration regions with different types of intercomponent association and solution structural organization were established.     

Thermodynamic characteristics and intermolecular interactions in aqueous solutions of oxyethylated glycols

The thermodynamic characteristics of aqueous solutions of mono-, di-, tri-, and tetraethylene glycols were calculated in the entire range of compositions of the mixtures for various temperatures. The specific and nonspecific terms of the total energy of intermolecular interaction were determined within the framework of a model approach using the internal pressure as a measure of nonspecific interactions in a liquid. The concentration ranges with different types of intercomponent association and of structural organization of solutions depend on the temperature and number of ether groups in the glycol molecules.     

Structural thermodynamic characteristics and intermolecular interactions in aqueous solutions of hexamethylphosphorotriamide

Structural thermodynamic parameters of aqueous solutions of hexamethylphosphorotriamide are calculated. They are discussed together with previously obtained data on aqueous solutions of disubstituted amides of carboxylic acids. The specific and non-specific components of the total energy of intermolecular interactions are determined. The boundaries of concentration regions with different structural organizations of solutions are found, and the preferential solvation parameters of solution components are estimated.     

Study of intermolecular interactions in aqueous solutions by millimeter spectroscopy

The absorption of millimeter electromagnetic radiation (v=1.4, 1.71, and 5 cm⁻¹) by aqueous solutions of glycine (pH 6.1–6.2) in the concentration range of 0.5–2.5 mol L⁻¹ was measured. It was found that the absorbing ability of the water present in the solutions, is higher than that of pure water. This phenomenon is explained by the presence of a center of negative hydration in the structure of the glycine zwitterion, which results in an increase in the rotational mobility of water molecules immobilized in the hydrate shell of the glycine zwitterion. For Part 5, see Ref. 1. Deceased. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1305–1307, July, 1997.     

Structral and thermodynamic characteristics and intermolecular interactions in aqueous solutions of diols

The structural and thermodynamic characteristics of aqueous solutions of ethanediol, 1,2-and 1,3-propanediols, and 1,2-and 1,4-butanediols were calculated over the whole range of the compositions of the mixtures. The specific and nonspecific components of the total energy of intermolecular interactions were determined. The boundaries of the concentration regions with different structural organizations of solutions were established, and the parameters of preferable solvation of the solution components were evaluated.     

Laser speckle analysis on correlation between gelation and phase separation in aqueous methyl cellulose solutions

Structure formation by coupling between formation of crosslinking points and liquid–liquid phase separation was investigated for aqueous methyl cellulose solution by small‐angle X‐ray scattering (SAXS) and light scattering (LS) techniques. The sol–gel phase diagram and the SAXS results suggested that the liquid–liquid phase separation occurred before gelation. By LS measurements, the structure due to the liquid–liquid phase separation was directly observed. By applying speckle analysis on the LS profiles, it was suggested that the gelation and the phase separation strongly coupled each other: the increase in the apparent molecular weight by crosslinking induced the liquid–liquid phase separation. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 168–174, 2010     

Electrochemical and complexation behavior of neptunium in aqueous perchlorate and nitrate solutions

Electrochemical and complexation properties of neptunium (Np) are investigated in aqueous perchlorate and nitrate solutions by means of cyclic voltammetry, bulk electrolysis, UV-visible absorption, and Np L(III)-edge X-ray absorption spectroscopies. The redox reactions of Np(III)/Np(IV) and Np(V)/Np(VI) couples are reversible or quasi-reversible, while the electrochemical reaction between Np(III/IV) and Np(V/VI) is irreversible because they undergo structural rearrangement from spherical coordinating ions (Np(3+) and Np(4+)) to transdioxoneptunyl ions (NpO2(n+), n = 1 for Np(V) and 2 for Np(VI)). The redox reaction of the Np(V)/Np(VI) couple involves no structural rearrangement on their equatorial planes in acidic perchlorate and nitrate solutions. A detailed analysis on extended X-ray absorption fine structure (EXAFS) spectra suggests that Np(IV) forms a decaaquo complex of [Np(H2O)10](4+) in 1.0 M HClO4, while Np(V) and Np(VI) exist dominantly as pentaaquoneptunyl complexes, [NpO2(H2O)5](n+) (n = 1 for Np(V) and 2 for Np(VI)). A systematic change is observed on the Fourier transforms of the EXAFS spectra for all of the Np oxidation states as the nitrate concentration is increased in the sample, revealing that the hydrate water molecules are replaced by bidentate-coordinating nitrate ions on the primary coordination sphere of Np.     

Structural thermodynamic parameters and intermolecular interactions in aqueous solutions of secondary amides

Structural thermodynamic parameters are calculated for aqueous solutions of secondary amides of carboxylic acids. Specific and nonspecific contributions to the total energy of intermolecular interactions are determined and the boundaries of concentration regions for a various structural organization of solutions are found.     

Temperature induced phase separation of luminescent silica nanoparticles in Triton X-100 solutions

The aggregation and cloud point behavior of Tb(III)-doped silica nanoparticles has been studied in Triton X-100 (TX-100) solutions at various concentration conditions by fluorimetry, dynamic light scattering, electrophoresis and transmission electron microscopy methods. The temperature responsive behavior of nanoparticles is observed at definite concentration of TX-100, where the aggregation of TX-100 at the silica/water interface is evident from the increased size of the silica nanoparticles. The reversible dehydration of TX-100 aggregates at the silica/water interface should be assumed as the main reason of the temperature induced phase separation of silica nanoparticles. The distribution of nanoparticles between aqueous and surfactant rich phases at the phase separation conditions can be modified by the effect of additives.     

Liquid-liquid phase transition of protein aqueous solutions isothermally induced by protein cross-linking

We experimentally demonstrated that liquid-liquid phase separation (LLPS) of protein aqueous solutions can be induced by isothermal protein oligomerization. This phenomenon is analogous to LLPS induced by the polymerization of small organic molecules in solution. Specifically, using glutaraldehyde for protein cross-linking, we observed the formation of protein-rich liquid droplets for bovine serum albumin and chicken egg lysozyme at 25 degrees C. These droplets evolved into cross-linked protein microspheres. If the aqueous solutions of the protein monomer do not show LLPS at temperatures lower than the oligomerization temperature, protein-rich droplets are not observed. We experimentally linked the formation of these droplets to the increase of LLPS temperature during protein oligomerization. When macroscopic aggregation competes with LLPS, a rationale choice of pH, polyethylene glycol, and salt concentrations can be used to favor LLPS relative to aggregation. Although glutaraldehyde has been extensively used to cross-link protein molecules, to our knowledge, its use in homogeneous aqueous solutions to induce LLPS has not been previously described. This work contributes to the fundamental understanding of both phase transitions of protein solutions and the morphology of protein condensed phases. It also provides guidance for the development of new methods based on mild experimental conditions for the preparation of protein-based materials.     

Nonresonant and resonant mode-specific intermolecular vibrational energy transfers in electrolyte aqueous solutions

The donor/acceptor energy mismatch and vibrational coupling strength dependences of interionic vibrational energy transfer kinetics in electrolyte aqueous solutions were investigated with ultrafast multiple-dimensional vibrational spectroscopy. An analytical equation derived from the Fermi's Golden rule that correlates molecular structural parameters and vibrational energy transfer kinetics was found to be able to describe the intermolecular mode specific vibrational energy transfer. Under the assumption of the dipole-dipole approximation, the distance between anions in the aqueous solutions was obtained from the vibrational energy transfer measurements, confirmed with measurements on the corresponding crystalline samples. The result demonstrates that the mode-specific vibrational energy transfer method holds promise as an angstrom molecular ruler.     

Cloud points and phase separation of aqueous poly(N-vinylacetamide) solutions in the presence of salts

Aqueous poly(N-vinylacetamide) (PNVA) solution was found to exhibit the cloud point in the presence of salt. This cloud point was shown to correspond to a liquid-liquid phase separation, as confirmed when the PNVA-salt solutions were maintained at a temperature above the cloud point. The upper layer had a higher polymer concentration and a lower salt concentration than those in the lower layer. Thus interaction between PNVA and salts are repulsive. The lower critical solution temperatures were estimated to be 18±1°C for 1.25 molal (NH₄)₂SO₄ and 25±1°C for 0.76 molal Na₂SO₄. Divalent anions such as SO _2– ⁴ , SO _2– ³ , HPO _2– ⁴ and CO _2– ³ were effective in causing turbidity when examined at 25°C. Dependence of the effect on the cationic species was similar to but significantly different from that for acetyltetraglycine ethylester. The cloud points of PNVA decreased linearly with the increase of the polymer concentration at a fixed salt concentration or with the increase of the salt concentration at a fixed polymer concentration. A parameter analogous to the salting-out constant was empirically derived from the dependencies of the cloud points on the concentrations of polymer and salt.