Abdelhafidh Gharbi

A correspondence between Grunberg–Nissan constant d′ and complex varieties in water/methanol mixture

The aim of this study was to analyse the change in the complex structure formed in water/methanol mixture under temperature effect and/or mixture composition by a simple method. First, the dynamic viscosity of mixture was measured by rheoviscosimetry, and then the variation of the total activation energy versus methanol molar fraction was determined and discussed. The Grunberg–Nissan constant d′ was calculated in a large mixture composition and over the 20–40°C temperature range. By comparing the complex formulas in water/methanol mixture (H2O) m (CH3OH) n reported in literature at 20°C and the Grunberg–Nissan constant d′ variation versus methanol molar fraction X A, a correspondence between Grunberg–Nissan constant d′ and the complex varieties was established. According to this hypothesis, the change in the complex varieties for different temperatures was determined by showing a reduction of the complex varieties of water/methanol complex at high temperatures (35–40°C) in the mixture.

Study of PEO—PPO—PEO Copolymers Conformational Changes: Viscosity and Dynamic Light Scattering Measurements

The self-assembly behaviour of poly(ethylene oxide)-b-poly(propylene oxide)-b-poly (ethylene oxide) copolymers, (EO)13(PO)30(EO)13 (Pluronic L64), and (EO)70 (PO)30(EO)70 (Pluronic F68) in water and in p-xylene has been elucidated by using viscosity and dynamic light scattering measurements to investigate the effects of hydrophilic chains length on their conformational changes. The viscosity measurements were performed for a range of temperature varying from 27°C to 60°C and concentration from 4 to 60 mg/ml. The variation of the viscosity and the conformational changes in aqueous solution depends on the kinds of interactions and the balance between excluded volume effects and hydrophobic interactions. Some dynamic light scattering measurements were also performed at room temperature for the same range of concentration to provide more information on the micellar structures in aqueous and organic solution.

Effect of Temperature on Cononsolvency of Polyvinylpyrrolidone in Water/Methanol Mixture

The behavior of polyvinylpyrrolidone in mixed water/methanol solvents was studied by rheoviscosimetry over a temperature range of 20°C–40°C. For the lower temperatures of this range, the intrinsic viscosity variation of the polymer vs. methanol molar fraction shows structural transitions (coil–globule–coil). This transition, which is usually attributed to the cononsolvency phenomenon, agrees with our previously published results obtained by dynamic light scattering. For higher temperatures, near 40°C, the intrinsic viscosity increase shows an expansion of the polymer over the alcohol molar fraction range 0.2 < X A < 0.5. This last result can be attributed to the water/alcohol complex destruction under temperature increase. The “excess viscosity” of the polymer-mixed solvents vanishes with increasing temperature and becomes positive at 40°C. So, the polymer chain tends to transit from a globular to an ideal chain in the middle composition range of the mixed solvents.

A Model to Study the Behavior of a Polar Polymer in the Mixture of Polar Solvents

The aim of the present work was to purpose a simple model to study the behavior of a polar polymer in a mixture of two polar solvents where the mixed solvents are considered as an effective solvent. This effective solvent interacts with the polymer. The ternary system polymer/solvent1/solvent2 is described by an effective solvent interacting with polymer model (E.S.I.P. model). We propose to calculate the polymer–mixed-solvents interaction parameter on the basis of the solubility parameter concept. New expressions for the second virial coefficient and the preferential adsorption coefficient are given. The proposed model is used to explain the structural transition observed in the system of polyvinylpyrrolidone (PVP)/water/methanol mixture at 25°C and to calculate the preferential adsorption parameter. The enthalpy contribution to the PVP/water interaction parameter was found to dominate the PVP/water interaction parameter. Thus, we consider the dominance of the enthalpy contribution to the polymer–solvent interaction parameter as a fundamental hypothesis when polymer and solvent interact. In addition, according to this hypothesis, the calculated PVP/methanol interaction parameter shows that methanol is a good solvent for PVP at 25°C, as reported in the literature. The E.S.I.P model and the dominance of the enthalpy contribution to the polymer-interaction parameter are used to calculate the polymer–mixed-solvents interaction parameter. Then, the second virial coefficient of the PVP in mixed solvents is deduced, showing a change in the solvent quality in agreement with our previous experimental studies. Finally, the variation of the preferential adsorption parameter versus methanol molar fraction X A is calculated showing different behavior of the polymer versus the mixture composition. The obtained results are in agreement with previous studies by light scattering and rheoviscosimetry of the ternary system PVP/water/methanol at 25°C.

Temperature, concentration and salt effect on F68 tri-block copolymer in aqueous solution: Rheological study

The rheology of the aqueous solution of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO−PPO−PEO) triblock copolymer, Pluronic F68 in the presence of KF was studied in the temperature range from 15 to 60°C. The variation of the shear stress according to the shear rate shows that independently from the temperature and concentration, the F68 solutions exhibit a Newtonian behavior. The results show that the Critical Micelle Temperature of Pluronic F68 in a KF aqueous solution decreases with the increase in the salt concentration.

Rheological Properties and Reverse Micelles Conditions of PEO-PPO-PEO Pluronic F68: Effects of Temperature and Solvent Mixtures

The rheological properties of Pluronic F68 were dissolved in various water/organic liquid mixtures over a wide range of temperatures, all at a concentration of 20 mg/mL. We have considered the following binary mixtures: Pluronic F68/water, F68/p-xylene, and F68/phenol. Various conformational transitions were detected and interpreted. We have also shown that these mixtures retain a Newtonian behavior independently of temperature and conformational changes. For ternary F68/p-xylene/water, F68/phenol/water, and F68/water/phenol mixtures, the behaviour of the solution is intimately related to the temperature and the amount of water and organic solvent added.

Polyacrylic acids–bovine serum albumin complexation: Structure and dynamics

The study of the mixture of BSA with polyacrylic acids at different masses versus pH allowed highlighting the existence of two regimes of weak and strong complexation. These complexes were studied in diluted regime concentration, by turbidimetry, dynamic light scattering (DLS), zeta-potential measurements and nuclear magnetic resonance (NMR). We have followed the pH effect on the structure and properties of the complex. This allowed refining the interpretation of the phase diagram and understanding the observed phenomena. The NMR measurements allowed probing the dynamics of the constituents versus the pH. The computational method was used to precisely determine the electrostatic potential of BSA and how the polyelectrolyte binds to it at different pH.

Complex formation between ovalbumin and strong polyanion PSSNa: Study of structure and properties

The mixture system of long-chain polyelectrolyte complexed with a globular protein was investigated based on dynamic light scattering and turbidimetric measurements. We have discussed at different pH values the influence of high salt concentration and mass ratio (protein:PSSNa) on the behavior of the mixture. In dilute concentration regime, the PSSNa chain contracts at pHc by patch binding. We found two critical values of mass ratio: The first corresponds to the maximum shrinking of PSSNa. The second indicates the system that became more stable where the number of proteins attached to the PSSNa chain was constant. The screen of electrostatic interaction shows a high contribution of hydrophobic interaction at large salt concentration to form the coacervates. By building phase diagram, the continuity of pHφ1 in over whole range of salt concentrations and the widening of pH window (pHφ1-pHφ2) were observed. At certain salt concentrations, we can obtain the coexistence of two types of complex particles formed by electrostatic and hydrophobic interactions.

Determination of the Flory Exponent by Study of Steady Shear Viscosity

The aim of the present work was to determine first the behavior of a neutral polymer, polyvinylpyrrolidone (Mw = 3,6 105 g/mol), in the dilute and semidilute regimes in water by a rotational concentric cylinder viscometer and then to deduce the Flory exponent. The absolute viscosity of the samples was directly determined from the shear stress–shear rate rheograms over a temperature range of 20°C–40°C and a polymer concentration range of 0.114–1.438 g/dl. The rheological nature of the polymer solutions was systematically analyzed, showing Newtonian behavior. Then, the intrinsic viscosity [η] was determined according to the Schultz–Blaschke equation. The overlapping concentration C* was determined by plotting the double logarithmic plots of reduced viscosity ηr versus polymer concentration C, yielding . The Flory exponents, ν, were deduced from viscosity measurements in the semidilute regime (ν = 0.544). In addition, the hydrodynamic radius, RH, was measured for different polymer molar mass (Mw (g/mol) = 10,000, 40,000, 50,000, and 360,000) by a dynamic light scattering technique at 25°C, yielding a Flory exponent at this temperature of 0.521. The obtained value was thus in agreement with that obtained by the rheological method in the semidilute regime with a relative deviation close to 4.5%.

Sponge phase behaviour in concentrated surfactant-alcohol-brine system

The sponge phase monodomain extending from the brine corner to the alcohol corner of an ionic surfactant-alcohol-brine phase diagram is first detailed and then investigated by electrolytic conductivity and refractive index measurements. Upon progressive dimunition of the brine fraction of the sponge phase, the ionic conductivity suffers from two gumps corresponding to two respective critical concentrations, while the variation of the refractive index remains linear but exhibits two slope changes. According to the phase diagram observations we propose one scenario, in agreement with theoretical predictions, to explain these results in terms of continuous structural transformations of the amphiphilic membrane from the swollen sponge phase to the inverse micellar phase.