Thermal and viscoelastic properties of xanthan gum/chitosan complexes in aqueous solutions

The viscoelasticity and thermal properties of aqueous solutions of blended xanthan gum (XA) with chitosan were investigated in order to study the electrostatic interaction established between two polysaccharides. Storage modulus G’, loss modulus G” and zero shear rate viscosity η_O attain a maximum at a chitosan concentration C _max. The above results indicate that the junction between XA and chitosan is formed in a concentration range lower than C _max and the viscoelasticity of systems increases with increasing concentration. In a concentration range higher than C _max, junction formation may not occur effectively since the excess amount of chitosan completely screens anions of XA. The chain rigidity of XA decreases by the screening of the repulsive interaction between anions on XA chains. The ineffective junction formation and the decrease of XA chain rigidity may cause the decrease of viscoelasticity of systems with increasing concentration. The value of C _max decreases with increasing molecular mass of chitosan. From melting enthalpy of the above system measured by DSC, the amount of non-freezing water (W _nf) was evaluated. W _nf shows a minimum at the concentration C _max. This fact suggests that hydrophobic fields increased by junction structure formation through ion-complexation between XA and chitosan molecules.     

Co-gelation mechanism of xanthan and galactomannan

Synergistic gelation of dilute (0.1% total gums) mixed solutions of xanthan and galactomannan isolated from seeds of Delonix regia was investigated. Gelation occurred in a mixed solution of xanthan and galactomannan at 0.1% total gums at room temperature (25 °C). The flow curves of mixed solutions of native xanthan and galactomannan showed plastic behavior. The maximum elastic modulus was obtained when the ratio of the xanthan to galactomannan was 2:1 at room temperature (25 °C). The largest elastic modulus was observed in the mixture solution of deacetylated xanthan. However, a small elastic modulus was obtained in the mixture with depyruvated xanthan. The results obtained supported the interaction mechanism between xanthan and galactomannan (locust bean gum) previously proposed, and the pyruvate methyl groups might also take part in the interaction.     

Electric conductivity of concentrated aqueous solutions of propionic acid, sodium propionate and their mixtures

Specific electric conductivity (EC) of concentrated aqueous solutions of propionic acid (PA), sodium propionate (SP), and water/PA/SP mixtures is measured in the temperature range of 15–90°C. Specific EC passes a maximum at the increase in the electrolyte concentration in the mixtures of water/PA, water/SP, and water/PA/SP containing a similar PA concentration. The maximum EC value of the aqueous PA solution at the given temperature is used as the generalizing term. It is shown that the values of reduced EC (ratio of EC and its maximum value at the given temperature) fall on a single curve in the whole studied range of temperatures and concentrations of the water/PA mixture. The EC activation energy is calculated for all the studied solutions. It is found that the EC activation energy of these solutions decreases at the temperature increase and grows at the increase of the concentration of electrolyte.     

Rheological properties of concentrated aqueous solutions of anionic and cationic polyelectrolyte mixtures

The rheological properties of aqueous solutions of poly(acrylic acid), poly(diallyldimethyldiammonium chloride), and their mixtures at 25°C have been studied. The concentrated solutions of the mixtures contain 18 wt % of both polymers taken at different ratios. The ratio of cationogenic and anionogenic groups φ varied from 0 to 0.4 is taken as a criterion for selection of mixture composition. An increase in φ, reflecting a more intense formation of polyelectrolyte complexes in solution, is accompanied by a significant rise in the low-frequency loss modulus and, especially, in the storage modulus, as well as by an increase in viscosity over the entire studied range of shear rates. This behavior may be explained by the presence of an additional spatial structure with junctions formed by interacting complementary charged groups. In the general case, the formation of poly(acrylic acid)-poly(diallyldimethyldiammonium chloride) polyelectrolyte complexes is said to take place in solution. The excess of rheological characteristics of mixture solutions over the corresponding characteristics of poly(acrylic acid) solutions is found to be the power function of parameter φ. The additional spatial network derived from polyelectrolyte complexes and occurring in solution is destroyed at lower shear stresses than is the network of intermolecular entanglements. At high shear stresses, orientational effects may cause phase separation of the systems owing to a change in the hydrophilic-hydrophobic balance between complexes of poly(acrylic acid) with poly(diallyldimethyldiammonium chloride) and water.     

Physical properties of nanobubbles on hydrophobic surfaces in water and aqueous solutions

In recent years there has been an accumulation of evidence for the existence of nanobubbles on hydrophobic surfaces in water, despite predictions that such small bubbles should rapidly dissolve because of the high internal pressure associated with the interfacial curvature and the resulting increase in gas solubility. Nanobubbles are of interest among surface scientists because of their potential importance in the long-range hydrophobic attraction, microfluidics, and adsorption at hydrophobic surfaces. Here we employ recently developed techniques designed to induce nanobubbles, coupled with high-resolution tapping-mode atomic force microscopy (TM-AFM) to measure some of the physical properties of nanobubbles in a reliable and repeatable manner. We have reproduced the earlier findings reported by Hu and co-workers. We have also studied the effect of a wide range of solutes on the stability and morphology of these deliberately formed nanobubbles, including monovalent and multivalent salts, cationic, anionic, and nonionic surfactants, as well as solution pH. The measured physical properties of these nanobubbles are in broad agreement with those of macroscopic bubbles, with one notable exception: the contact angle. The nanobubble contact angle (measured through the denser aqueous phase) was found to be much larger than the macroscopic contact angle on the same substrate. The larger contact angle results in a larger radius of curvature and a commensurate decrease in the Laplace pressure. These findings provide further evidence that nanobubbles can be formed in water under some conditions. Once formed, these nanobubbles remain on hydrophobic surfaces for hours, and this apparent stability still remains a well-recognized mystery. The implications for sample preparation in surface science and in surface chemistry are discussed.     

Temperature depending light scattering measurements of aqueous gelatin and alginate solutions and their mixtures

Highly diluted solutions of an alkaline treated pig bone gelatin and a seaweed alginate of Macrocystis pyrifera have been investigated by means of static light scattering at various temperatures. With these results the thermodynamic properties found by other authors and us before can be verified. Furthermore, diluted mixtures of both polymers with different ratios of composition have been investigated. These measurements show that both polymers in all mixtures behave quasi-binarily in the investigated temperature range. They maintain their association phenomena of the pure polymer solutions.     

Preparation of iron-aluminum powders from aqueous solutions and their physicochemical properties

Iron and iron-aluminum powders were obtained by the reduction of Fe(III) to α-Fe, occurring on a dispersed aluminum support. Their physical and mechanical properties were studied, and the possibility of obtaining compact samples with a lower bulk density as compared to known analogs, an elevated strength, and a high porosity was examined.     

Surface properties of aqueous amino acid solutions II. Leucine-leucine hydrochloride and leucine-sodium leucinate mixtures

To understand the distinction between the effects of zwitterionic, anionic, and cationic l-leucine upon adsorption and lateral interactions at air/water surface, the surface tensions of aqueous solutions of l-leucine-l-leucine hydrochloride and l-leucine-sodium l-leucinate mixtures were measured as a function of concentration and composition at 25 degrees C. The surface activity decreases in the order l-leucine >l-leucine hydrochloride > sodium l-leucinate. Both l-leucine hydrochloride and sodium l-leucinate form gaseous adsorbed films through the experimentally accessible concentration range, while the adsorbed film of zwitterionic l-leucine shows a transition between gaseous and expanded film.     

Micellization and surface properties of aqueous solutions of binary mixtures of tween-80 and bromide cetyltrimethylammonium

The physicochemical properties of solutions of mixtures of cationic (cetiltrimethylammonum bromide) and nonionic (polyoxyethylene (20) sorbitan monooleate) surfactant, i.e., Tween-80, have been studied. The critical concentration of micellization, adsorption at the interface of a mixture of surfactants solution/air, and the minimum area occupied by a surfactant molecule have been determined. It was shown that, in the whole region of ratios between surfactants, sinergic effects are observed for both micellization and for the process of adsorption at the interface of a mixture of surfactants solution/air. The results obtained have been analyzed according to the regular solution theory (RST).     

Thermodynamics of saturated aqueous solutions including mixtures of NaCl,KCl, and CsCl

The activity coefficients for saturated aqueous KCl, CsCl and mixtures of NaCl with each of these electrolytes are calculated from solution properties using the ion interaction model as well as from the solubility. The agreement between the two sets of results for both single and mixed electrolytes is, in general, good when it is considered that the saturated solution molatities are often much higher than those whose properties were used in the evaluation of the ion interaction parameters. Also, for pure KCl (aq) the agreement is good up to 300°C, an extrapolation 50°C above the range of data on which the equations were based.     

Drying dissipative patterns of aqueous solutions of simple electrolytes and their binary mixtures on a cover glass

The drying dissipative patterns of aqueous solutions of simple electrolytes, KCl, NaCl, CaCl₂, and LaCl₃, were observed on a cover glass. The macroscopic broad rings were formed at the outside edge of the drying film area, which shrunk from the initial solution area especially at low salt concentrations. The drying area and the broad ring size decreased as the salt concentration decreased. The microscopic block-like and dendritic cross-like patterns were observed for all the salts. Size of single crystals dried on a cover glass increased as salt concentration increased. The drying patterns of the binary mixtures of the salts were also observed. Size of the broad ring increased sharply by mixing. The microscopic patterns were, on the other hand, insensitive to the mixing.     

Phase behavior in mixtures of cationic and anionic surfactants in aqueous solutions

Phase behavior of cationic/anionic surfactant mixtures of the same chain length (n=10, 12 or 14) strongly depends on the molar ratio and actual concentration of the surfactants. Precipitation of catanionic surfactant and mixed micelles formation are observed over the concentration range investigated. Coacervate and liquid crystals are found to coexist in the transition region from crystalline catanionic surfactant to mixed micelles.The addition of oppositely charged surfactant diminishes the surface charge density at the mixed micelle/solution interface and enhances the apparent degree of counterion dissociation from mixed micelles. Cationic surfactants have a greater tendency to be incorporated in mixed micelles than anionic ones.     

Emission properties of dihydropterins in aqueous solutions

Pterins belong to a class of heterocyclic compounds present in a wide range of living systems and accumulate in the skin of patients affected by vitiligo, a depigmentation disorder. The study of the emission of 7,8-dihydropterins is difficult because these compounds are more or less unstable in the presence of O(2) and their solutions are contaminated with oxidized pterins which have much higher fluorescence quantum yields (Φ(F)). In this work, the emission properties of six compounds of the dihydropterin family (6-formyl-7,8-dihydropterin (H(2)Fop), sepiapterin (Sep), 7,8-dihydrobiopterin (H(2)Bip), 7,8-dihydroneopterin (H(2)Nep), 6-hydroxymethyl-7,8-dihydropterin (H(2)Hmp), and 6-methyl-7,8-dihydropterin (H(2)Mep)) have been studied in aqueous solution. The fluorescence characteristics (spectra, Φ(F), lifetimes (τ(F))) of the neutral form of these compounds have been investigated using the single-photon-counting technique. Φ(F) and τ(F) values obtained lie in the ranges 3-9 × 10(-3) and 0.18-0.34 ns, respectively. The results are compared to those previously reported for oxidized pterins.     

Short-lived metal clusters in aqueous solutions: formation, identification, and properties

The formation and properties of short-lived homo- and heteronuclear metal clusters in aqueous solutions were investigated by pulse radiolysis. These species arise in early stages of the aggregation of metal atoms and ions in unusual oxidation states. The conditions favoring the formation of positively charged “magic” clusters and intermediate heterometallic clusters were elucidated. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 1–15, January, 1999.     

Solubility of pure 1,3 butadiene and methyl propene and their mixtures in pure n-methyl-2-pyrrolidone and in its aqueous solutions

The solubility of pure 1,3 butadiene and methyl propene and their mixtures in pure n-methyl-2-pyrrolidone and in its aqueous solutions was measured at several temperatures and pressures. For this purpose, experimental methodologies were established in order to evaluate thermodynamic equilibrium variables. A simple weighing method was used to obtain the experimental data. From the experimental results, the parameters of conventional excess Gibbs free energy models, Wilson, NRTL and UNIQUAC, were determined through the Maximum Likelihood method.     

Photochemical and photophysical properties of lumazine in aqueous solutions

Lumazine (pteridine-2,4(1H,3H)-dione, LU) was investigated for its efficiency of singlet oxygen (¹O₂) production and quenching in aqueous solution. The quantum yield of ¹O₂ production (Φ_Δ) was determined by measurements of the ¹O₂ luminescence in the near-infrared upon continuous excitation of the sensitizer. Values of Φ_Δ are sensitive to the pH and were found to be 0.44 ± 0.01 and 0.080 ± 0.004 in acidic and alkaline media, respectively. The photochemical stability of LU was investigated under different pH conditions, in the presence and in the absence of O₂. The photochemical consumption of LU in aqueous solution at room temperature under irradiation at 350 nm was followed by UV–vis spectrophotometry and HPLC. Values of the quantum yields of LU disappearance are low, indicating that LU is rather photostable under physiological conditions.     

Molecular weight effects on gelation and rheological properties of konjac glucomannan–xanthan mixtures

Fractions of konjac glucomannan (KGM) with various viscosity‐average molecular weights (M_v) ranging from 4.00 × 10⁵ to 2.50 × 10⁶ were prepared by hydrolysis degradation in hydrochloride acid/ethanol. Effect of M_v of KGM on the critical gelation temperature (T_gel) determined by Winter–Chambon criterion and the elasticity of KGM/xanthan mixed gels, a kind of binary gel formed by synergistic gelation, were investigated by dynamic viscoelastic measurements. It was shown that the value of T_gel of the gel was shifted to a higher temperature and the gel strength was enhanced when M_v of KGM was increased. The critical M_v (1.91 × 10⁶) was observed, above which the T_gel and elasticity of the mixed gels showed no or slight increase. It was suggested that T_gel and elasticity of KGM/xanthan mixed gels mainly depend on the structure of junction zones driven by the strong interaction between KGM and xanthan, which was gradually improved with increasing M_v of KGM. It was found that the critical strain and yield stress of the mixed gels increased monotonically with the increasing M_v of KGM. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 313–321, 2010     

Controlled synthesis of hydrophilic concentrated polymer brushes and their friction/lubrication properties in aqueous solutions

Polymer brushes of water‐soluble polymers, poly(2‐hydroxyethyl acrylate) (PHEA) and poly(poly(oxyethyleneglycol)methylether acrylate) (PPEGA), were synthesized on a silicon wafer and a silica particle by applying photo‐induced organotellurium‐mediated radical polymerization to surface‐initiated graft polymerization. High graft densities were obtained, corresponding to reduced graft densities of about 0.32 and 0.42 for the PHEA and PPEGA brushes, respectively. These values were high enough to be categorized in the regime of “concentrated” polymer brushes (CPBs). Atomic force microscopic (AFM) study revealed that the CPB of PPEGA was allowed to be highly swollen in water but the CPB of PHEA did not. This means that water is reasonably good for PPEGA but not for PHEA. The AFM microtribological study between swollen brushes revealed two lubrication regimes, namely, boundary‐ and hydrodynamic‐lubrication regimes, with different shear‐velocity dependencies. Reflecting insufficient quality of water as a solvent, the CPB of PHEA showed adhesive interaction and thereby a higher frictional coefficient μ in the boundary lubrication. More interestingly, super lubrication was achieved for the CPB of PPEGA with a μ value in the order of 10^?4 in water and in 0.1 M aqueous NaCl solution (without the help of electrostatic repulsion). Super lubrication was concluded to be a characteristic feature of the CPB, even in an aqueous system. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011