Guy Chauveteau

Salt dependence of the conformation of a single-stranded xanthan

Abstract A highly pyruvated single-stranded xanthan sample was studied by low angle light scattering and low shear viscosity in a salinity range (10 −5 M-1 M NaCl) where xanthan can exist in both ordered and disordered forms as shown by optical rotation. No aggregates have been observed in solutions prepared by direct dilution of the fermentation borth, but it was found that molecular associations can result from the polymer drying process used. No molecular weight change has been detected during the salt-induced conformational transition which is thus shown to be an intramolecular process. The salinity dependence of xanthan conformation is analysed on the basis of Yamakawas theory for a wormlike chain model with a total persistence length comprising a structural part, which is found to assume different values depending on the nature of the secondary structure (ordered or not), and an electrostatic part which is dependent on the ionic strength.

Adsorption of polyacrylamides on the different faces of kaolinites

The adsorption densities of nonionic and anionic high molecular weight polyacrylamides, respectively on the basal and lateral faces of kaolinite, are derived from measurements on four Na-kaolinite samples with different proportions of lateral-to-total surface area. For nonionic polyacrylamide, adsorption is independent of NaCl content and pH (between pH 3 and 9) and is shown to take place on both basal and lateral faces. The adsorption density on lateral faces is high; i.e.,Γ= 3500 μg/m2. Adsorption on basal surfaces takes place most likely on the aluminum hydroxide face with a low adsorption density, i.e.,Γ= 500 μg/m2. Adsorption of the anionic polyacrylamide on lateral faces increases with NaCl concentrationCs in two steps, due to charge screening up toCs 10), adsorption drops again markedly down to negligible values. The adsorption on basal surfaces increases with salinity and decreases as pH increases as expected from the contribution of electrostatic repulsions between the polymer and mineral surface.

Rheology and Transport in Porous Media of New Water Shutoff / Conformance Control Microgels

The performance of new microgels specifically designed for water shutoff and conformance control was investigated extensively at laboratory scale. These microgels are preformed, stable, fully water soluble, size controlled with a narrow size distribution, and nontoxic. They reduce water permeability by forming adsorbed layers sufficiently soft to be easily collapsed by oil/water capillary pressure, so that oil permeability is not affected significantly. Because the manufacturing process of these new microgels makes it possible to vary chemical composition, size, and crosslink density, they can be designed to meet the requirements of a given field application.

Salt-induced extension and dissociation of a native double-stranded xanthan

Abstract This study shows that xanthan molecules at room temperature may assume at least three different conformations in 0.1 m NaCl aqueous solutions in which the local structure is ordered: (1) the native compact double helix, (2) the extended double helix, and (3) the extended single helix. Experiments including viscosity, low-angle light scattering and optical rotation measurements have been carried out with a fully pyruvated and fully acetylated native laboratory sample supplied as fermentation broth. Two major conformation changes of the native double helix which were found irreversible in our experimental conditions can be induced by treatments at low ionic strength. After treatment in 10−4 m NaCl, xanthan is still a double helix in 10−1 m NaCl, but the backbone of each strand has been extended. After the sample has been in 10−5 m NaCl, the double helix has been dissociated and a single helix sample is obtained. Thus, the denaturing of xanthan is a two-step process. The first step consists of the extension of the two chains inside the double helix, and the second is a dissociation of the native double strand.

Concentration dependence of the effective viscosity of polymer solutions in small pores with repulsive or attractive walls

This work documents a very strong nonlinear dependence on concentration of the reduction in apparent viscosity which a polymer solution experiences inside a small pore. This result has been anticipated by a simple scaling argument which concludes that slip effect will be proportional to the bulk fluid viscosity. This is roughly what was observed. A bead-spring model mimics the data well in dilute solution but fails at higher concentration. An empirical depleted layer model works well at all concentrations and suggests that the depleted layer in the pore may decrease with concentration like the correlation length in the bulk solution. Better molecular modelling of these small pore phenomena at higher concentrations awaits the development of molecular models for the bulk solution rheology of these fluids. It is very clear now, however, that this viscosity reduction in small pores can be very adequately described by solely steric considerations.

Influence of Microgels in Polysaccharide Solutions on Their Flow Behavior Through Porous Media

Microgels existing in xanthan solutions, which are partly responsible for the poor injectability of such solutions, are retained around the injection wells. Therefore, most of the oil is swept by microgel-free solutions. This paper shows that even a few microgels can strongly modify flow behavior in porous media. Consequently, their careful elimination is required in laboratory tests for proper design of a polymer flood.

Gelation of polymer solutions under shear flow

Abstract Gelling a polymer solution under constant shear rate was recently found to be an attractive procedure for preparing size-controlled microgels. In the theoretical approach developed in this paper, the microgels formed during gelling are modelled as non-covalent ‘star’ polymers. This approach provides scaling laws to predict the effects of the shear rate applied during gelation on: (1) the gelling kinetics in three flow regimes; (2) the microgel size dependence in weak and strong flows; and (3) the rheological and elastic properties of the microgel solutions. The reported experiments were carried out by gelling semi-dilute high molecular weight polymer solutions by a crosslinker small enough to diffuse inside macromolecules and thus able to form both intra and intermolecular cross-links. All available experimental results are in agreement with our theoretical predictions, but further experiments are needed for a complete validation of the proposed theory.

Mesophase formation in high molecular-weight xanthan solutions

After a short review of theoretical background on mesophase formation in polymer solutions, this paper describes the liquid crystal phase transition and the corresponding rheological properties for aqueous solutions of a high-molecularweight xanthan sample (Mw ≈ 1.8 ⋅ 106). The formation of mesophases has been studied using polarizing microscopy and viscometry. The effects of the presence of salts, bacteria cells and proteins have been investigated. The variations in the viscosity, due to mesophase formation, are in qualitative agreement with the predictions of Mathesons theory, but the onset of the ordered phase occurs at very low polymer concentrations and the diphasic domain is much broader than predicted by thermodynamic models. These characteristics of the phase transition are related to the very high molecular weight of the sample studied and can be explained mainly by the effects of cooperative interactions between xanthan chains and of chain flexibility reducing translational entropy.

Adsorption of hydrolyzed polyacrylamides onto amphoteric surfaces

Abstract The adsorption of hydrolyzed polyacrylamide (HPAM) dissolved in distilled water onto sorbents with (SiO)2AlOH and SiOH surface sites was investigated as a function of pH and degree of hydrolysis τ (concentration ratio [COOH]/([CONH2] + [COOH]) of carboxylic to acrylamide plus carboxylic groups). (SiO)2AlOH is amphoteric and the densities of charged (SiO)2AlOH2+, (SiO)2AlO−, and non-charged (SiO)2AlOH species were calculated as functions of pH by potentiometric titration and electrophoretic mobility measurements. It was shown that the adsorption of noncharged polyacrylamide (τ = 0) was proportional to the density of (SiO)2AlOH sites. Adsorption occurred through AlOH…OC(NH2) hydrogen bonds and SiOH sites were inactive. At very low charge densities of the polymer, controlled by pH and the degree of hydrolysis τ, the driving force of adsorption became electrostatic in nature. The adsorption of HPAM was interpreted quantitatively on both sides of the pzc of the sorbent, taking into account the effects of the electrostatic potential of the sorbent and the density of (SiO)2AlOH2+ and (SiO)2AlO− surface sites.

Wall effects in the flow of flexible polymer solutions through small pores

Effective viscosities of dilute and semidilute flexible solutions flowing through small cylindrical pores were determined in the Newtonian regime for various pore diameters. The low viscosities relative to the bulk were associated with a depletion phenomenon due to a steric exclusion of macromolecules from the pore wall. Using a two-fluid flow model, the depletion layer thickness was determined and discussed as a function of polymer concentration, ionic strength, and molecular weight. This thickness, which was constant and close to the macromolecule gyration radius in dilute regime, was found to decrease rapidly with polymer concentration in the semidilute regime.