Aggregation and gelation in hydroxypropylmethyl cellulose aqueous solutions

In this work we present an analysis of the thermal behavior of hydroxypropylmethyl cellulose aqueous solutions, from room temperature to higher temperatures, above gelation. We focus on significant aspects, essentially overlooked in previous work, such as the correlation between polymer hydrophobicity and rheological behavior, and the shear effect on thermal gelation. Micropolarity and aggregation of the polymer chains were monitored by both UV/vis and fluorescence spectroscopic techniques, along with polarized light microscopy. Gel formation upon heating was investigated using rheological experiments, with both large strain (rotational) tests at different shear rates and small strain (oscillatory) tests. The present observations allow us to compose a picture of the evolution of the system upon heating: firstly, polymer reptation increases due to thermal motion, which leads to a weaker network. Secondly, above 55 degrees C, the polymer chains become more hydrophobic and polymer clusters start to form. Finally, the number of physical crosslinks between polymer clusters and the respective lifetimes increase and a three-dimensional network is formed. This network is drastically affected if higher shear rates, at non-Newtonian regimes, are applied to the system.     

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".     

Concentration dependence of critical exponents for gelation in gellan gum aqueous solutions upon cooling

The sol-gel transition in aqueous gellan gum solutions induced upon cooling was investigated by rheology measurements. The gelation temperature was determined from the crossover point of storage and loss moduli, i.e., G′ = G′′ (T_c) and from the Winter’s criterion (T_gel), respectively, which increased with gellan concentration. T_gel was higher than T_c and the difference became larger as the gellan concentration got higher. The relaxation critical exponent n was estimated with the Winter’s method and the self-similarity was observed from the critical gel. The scaling for the zero-shear viscosity η₀ before the gel point and the equilibrium modulus G_e after the gel point was established against the relative distance ε from the gel point over the gellan concentration C_g of 1.0-2.5 wt%, giving the critical exponents k and z. The critical exponent n calculated from k and z agrees well with n from the Winter’s criterion. However, no universal n was found for the gelation in aqueous gellan gum solutions, indicating that this gelation should be classified into the cross-linking category for the physical gelation. The critical exponent n decreased with increasing C_g for the gellan gum solution. The fractal dimension d_f calculated from n with the screened hydrodynamic interaction and the excluded volume effect suggested a denser structure in the critical gel with higher C_g.     

Organosilica composite for preconcentration of phenolic compounds from aqueous solutions

A new adsorbent is proposed for the solid-phase extraction of phenol and 1-naphthol from polluted water. The adsorbent (TX-SiO₂) is an organosilica composite made from a bifunctional immobilized layer comprising a major fraction (91%) of hydrophilic diol groups and minor fraction (9%) of the amphiphilic long-chain nonionic surfactant Triton X-100 (polyoxyethylated isooctylphenol) (TX). Under static conditions phenol was quantitatively extracted onto TX-SiO₂ in the form of a 4-nitrophenylazophenolate ion associate with cetyltrimethylammonium bromide. The capacity of TX-SiO₂ for phenol is 2.4 mg g⁻¹ with distribution coefficients up to 3.4 × 10⁴ mL g⁻¹; corresponding data for 1-naphthol are 1.5 mg g⁻¹ and 3 × 10³ mL g⁻¹. The distribution coefficient does not change significantly for solution volumes of 0.025–0.5 L and adsorbent mass less than 0.03 g; 1–90 μg analyte can be easily eluted by 1–3 mL acetonitrile with an overall recovery of 98.2% and 78.3% for phenol and 1-naphthol, respectively. Linear correlation between acetonitrile solution absorbance (A ₅₄₀) and phenol concentration (C) in water was found according to the equation A ₅₄₀ = (6 ± 1) × 10⁻² + (0.9 ± 0.1)C (μmol L⁻¹) with a detection range from 1 × 10⁻⁸ mol L⁻¹ (0.9 μL g⁻¹) to 2 × 10⁻⁷ mol L⁻¹ (19 μL g⁻¹), a limit of quantification of 1 μL g⁻¹ (preconcentration factor 125), correlation coefficient of 0.936, and relative standard deviation of 2.5%. A solid-phase colorimetric method was developed for quantitative determination of 1-naphthol on adsorbent phase using scanner technology and RGB numerical analysis. The detection limit of 1-naphthol with this method is 6 μL g⁻¹ while the quantification limit is 20 μL g⁻¹. A test system was developed for naked eye monitoring of 1-naphthol impurities in water. The proposed test kit allows one to observe changes in the adsorbent color when 1-naphthol concentration in water is 0.08–3.2 mL g⁻¹.     

A new apparatus for the extraction of organic compounds from aqueous solutions

A new apparatus for the extraction of organic compounds from sea water is described. With this apparatus it is possible to extract 54 standard compounds with high recovery percentages from 9 1 of sea water with 3 ml ofn-hexane. The analysis time (about 1 h) is appreciably lower than those of the extraction methods based on RP-18 and Carbopack-B adsorption (about 15 h). Furthermore, it is possible to analyze samples without filtration. An application of this method to the analysis of Tirreno sea water is reported.     

Room temperature sphere-to-rod growth and gelation of PEO-PPO-PEO triblock copolymers in aqueous salt solutions

The effects of NaCl and KF on the sphere-to-rod micellar growth behavior of triblock copolymers having two different compositions, (EO)20(PO)70(EO)20 (P123) and (EO)26(PO)40(EO)26 (P85), have been studied by dynamic light scattering (DLS), small angle neutron scattering (SANS) and dilute solution viscometry. NaCl can effectively tune the sphere-to-rod growth temperature of the micelles of both these copolymers and induce micellar growth down to the room temperature and below. The growth behavior is found to be dependent on the composition of the copolymer as P123 being more hydrophobic shows the room temperature growth in the presence of ethanol at significantly lesser NaCl concentration than the less hydrophobic copolymer P85. DLS studies depict for the first time the growth driven transition of the copolymer solutions from dilute to semi-dilute regime as a function of copolymer and salt concentrations. KF can also induce room temperature growth of the P123 micelles at lesser salt concentration than NaCl but it fails to induce any such growth of the P85 micelles. A pseudo-binary temperature-concentration phase diagram on 15% copolymer solutions shows the variation of the sphere-to-rod transition temperature and the cloud point of the copolymer solutions as a function of salt concentration.     

Radiation-induced nitration of organic compounds in aqueous solutions

Radiation-induced nitration of organic compounds in aqueous solutions was studied. It was found that γ-irradiation of solutions containing acetic and nitric acid and/or their salts gives nitromethane. Dependences of the product yield on the absorbed dose and the contents of components were established. The mechanism of radiation nitration involving radicals is discussed.     

Bound water measurements for aqueous protein solutions and food gels

The role of aqueous media in the stabilization of globular proteins and formation of gels was studied by absorption millimeter spectroscopy. This method allowed to measure bound water, the fraction of water which had decreased rotational mobility owing to the presence of solute. Hydration data for globular proteins were compared with data obtained previously for low-weight molecules and groups. It was found that rotational mobility of water molecules in the hydration shells of various kinds of solutes (groups) decreased in the following order: water structure breaking compounds>polar groups>unfolded proteins>globular proteins>non-polar groups. Time courses of the storage modulus were determined for the chemical acidification by glucono-δ-lactone (GDL) of milk samples prepared from skimmed milk powder (SMP). Gelation of unheated milk was a monotonous process that started at pH 4.9. Heat-treated milk from SMP (16 and 14 g per 100 ml) acidified by GDL (3 g per 100 ml) at 43 °C gave non-monotonous kinetics of gelation with two phases corresponding to the mechanisms induced by denatured whey proteins at pH>5 and by casein–casein interactions at pH 4.8–4.9. For heat-treated milk, measurement of bound water gave two stages of decrease in water mobility. Additional hydration of SMP during acidification gave 0.15–0.2 g and 0.8 g bound H₂O per gram of SMP for unheated and heat-treated milk, respectively.     

Temperature-induced association and gelation of aqueous solutions of pectin. A dynamic light scattering study

Dynamic light scattering (DLS) measurements were carried out on aqueous solutions of low-methoxyl pectin at different temperatures and polymer concentration. Low temperature and increased polymer concentration promote the formation of multichain aggregates. The time correlation data obtained from the DLS experiments revealed, for all polymer solutions, the existence of two relaxation modes, one single exponential at short times followed by a stretched exponential at longer times. In the semidilute regime, a temperature reduction induced enhanced chain associations in the solutions with high values of the slow relaxation time and a strong wave vector dependence of the slow mode. These features could be rationalized in the framework of the coupling model of Ngai. At low temperatures (10 °C), gelation occurs in the semidilute regime and a transparent gel is formed. In this state, the profile of the correlation function changes and nonergodic signs are observed. The conjecture is that the association complexes and the gel network are stabilized through intermolecular hydrogen bonds, which are broken-up at higher temperatures. The hydrogen-bonded structures are formed in a process where the polymer chains have been “zipped” together in a cooperative manner.     

State of the hydroxide ion in water and aqueous solutions

Conclusion Analysis of these experimental facts leads to the conclusion that in water and aqueous solutions of alkali metal hydroxides it is extremely probable that the hydroxide ion exists in the form H₃O₂ ^–. The marked displacement of the extrapolated chemical shift of the proton of the H₃O₂ ^– ion towards weak fields and the displacement of the frequency of the bending vibrations of the OH bond towards higher frequencies for hydroxide solutions indicate strong hydrogen bonding between the OH^– ion and the H₂O molecule. The comparatively low heat of hydration of the OH^– ion (111 cal/mole) compared with the heat of hydration of the H₊ ion (276 cal/mole) cannot, as has been shown, serve as proof that there is no strong electrostatic bond between the OH^– ion and a water molecule. All the heat of hydration is used up in the formation of this bond; this can be regarded as additional confirmation of the hydrophobic nature of the ion produced. The experimental data on the absolute value of the chemical shift of the proton of the H₃O₂ ^– ion indicate the important role played by the excited state of the proton in this complex. This conclusion agrees with the spectroscopic data.M. V. Lomonosov Moscow State University. Translated from Zhurnal Strukturnoi Khimii, Vol. 12, No. 6, pp. 969–974, November–December, 1971.     

One-dimensional model for water and aqueous solutions. I. Pure liquid water

Two simplified one-dimensional models for waterlike particles are studied. One is referred to as the primitive model which is a simplified version of a model introduced by Ben-Naim in 1992 [Statistical Thermodynamics for Chemists and Biochemists (Plenum, New York, 1992)]. The second, referred to as the primitive cluster model, is a simplified version of the model used by Lovett and Ben-Naim in 1969 [J. Chem. Phys. 51, 3108 (1969)]. The two models are shown to be nearly equivalent and both exhibit some of the most characteristic behavior of liquid water. It is argued that a key feature of the molecular interactions--the correlation between the strong binding energy and low local density--is essential for the manifestation of the anomalous behavior of liquid water. It is also essential for the understanding of the outstanding behavior of liquid water.     

1H NMR spectroscopic investigations on the micellization and gelation of PEO-PPO-PEO block copolymers in aqueous solutions

The effects of temperature, polymer composition, and concentration on the micellization and gelation properties of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers in aqueous solutions were investigated by 1H NMR spectroscopy. It was found that the temperature-dependent behavior of PPO blocks, observed as changes in chemical shift, half-height width, and integral value, could be attributed as an intrinsic tool to characterize the transition states during unimer to micelle formation. The 1H NMR spectral analysis revealed that the hydrophobic part, PPO, of the Pluronic polymers plays a more significant role in the temperature-induced micellization, whereas the transitional behavior of Pluronic polymer, i.e., from micellization to liquid crystals formation, resulted in the drastic broadening of the spectral signals for the PEO, indicating that the PEO segments play a more significant role in the crystallization process. It was also observed that the temperature-dependent changes in the half-height width of the PEO -CH2- signal are sensitive to the liquid crystalline phase formation, which could be attributed to the close packing of spherical micelles at high polymer concentrations or temperatures.     

Modelling aqueous solutions near the critical point of water

We review the thermodynamic properties of dilute solution near the critical point of the solvent. Two examples are discussed, a solution of a non-electrolyte and a solution of an electrolyte. The limiting behavior of the electrolyte solutions is modelled with a Debye-Huckel term in the Helmholtz free energy. The partial molar properties, in particular the volume and isobaric thermal expansion are examined in detail. The derivation of these properties is introduced by considering the geometry of the thermodynamic surfaces near to and far from the critical point of the solvent. We conclude that the properties of solutions near the solvent critical point are dominated by that feature; solution properties cannot be adequately modelled without including the functional forms associated with the critical point.     

Rheological characterisation of bis-urea based viscoelastic solutions in an apolar solvent

The rheological properties of hydrogen bonded reversible aggregates in apolar solvent have been investigated. The zero shear viscosity, the plateau modulus and the terminal viscoelastic relaxation time exhibit power laws dependencies with the concentration that have been compared with the behaviours expected for reversible polymers. It appears that the dynamics originate from scission/recombination mechanisms. Moreover, the growth of the aggregates with concentration follows the laws observed and predicted for worm-like micelles at low concentrations allowing the length of the aggregates to be estimated 500 nm at 3 g L(-1). However, at the highest concentrations investigated, deviations from the power laws suggest the presence of some intrinsic chain stopper poisons in the solutions.     

Effective extractants for the extraction of lithium from aqueous solutions containing sodium and potassium compounds

The extraction power of newly obtained pure methoxy-1,3-diketones in diluents and in their mixtures with electron-donating additives during the extraction of lithium from aqueous solutions containing sodium and potassium was investigated. High separation factors were obtained; no appreciable amounts of sodium and potassium were found in the extract after total extraction of the lithium.Institute of Chemical Kinetics and Combustion, Siberian Branch, Russian Academy of Sciences, 630090 Novosibirsk. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 6, pp. 1304–1310, June, 1992.     

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