S. J. Candau

Interactions between lysozyme molecules under precrystallization conditions studied by light scattering

Abstract Light scattering experiments were undertaken on lysozyme under various solvent conditions. When the protein is undersaturated, attractive interparticular interactions are detected. They are enhanced when the temperature is decreased, but are much weaker in NaCl solutions in which the protein crystallizes than in ammonium sulfate solutions in which it forms amorphous precipitates. When the protein in a NaCl solution is brought to supersaturation by a temperature decrease, light scattering measurements indicate the simultaneous presence of two scatter populations which can be assimilated to individual lysozyme molecules and to large particles. Kinetic experiments in which the temperature is quenched rapidly indicate that the apparent hydrodynamic radius of the large particles increases regularly with time up to a plateau value of about 2600A˚.

Elastic behaviour of salt-free polyelectrolyte gels

Poly(acrylic acid) gels ionized after preparation for different preparation concentrations and degrees of cross-linking and ionization, have been studied as a function of the swelling ratio and at swelling equilibrium. Measurements performed as a function of the polymer concentration indicated that for low swelling ratios ( < 100) the shear modulus follows a scaling law with an exponent in agreement with classical theories. For a given concentration the shear modulus has been found to decrease with the degree of ionization as qualitatively predicted by a recent model. At swelling equilibrium the shear modulus varies linearly with the osmotic pressure of the counter-ions in a large concentration domain and for various cross-linking densities. The osmotic prefactor values, found both by mechanical and by light-scattering experiments, were in agreement with various independent measurements performed on poly(acrylic acid) solutions.

Frozen-in Intensity Fluctuations of Light Scattered by Partially Ionized Gels

We report results of static light scattering for partially ionized gels at different cross-link densities and different swelling degrees and for the corresponding semi-dilute solutions. The r.m.s. fluctuations of the scattered intensity obtained when scanning through various positions in the gels is proportional to the excess of scattering of the gel with respect to the semi-dilute solution, whatever the method used to vary the ratio of the actual swelling degree to its equilibrium value, i.e. varying the cross-link density or the swelling extent. The excess of scattering is attributed to a frozen-in component associated with scattering domains with a spatial extent less than ~ 300 A.

Self-assembling surfactant systems

During the past year, the major advances have concerned the development of techniques that allow one to probe the structure of surfactant systems under flow. The techniques include the use of neutrons or light scattering, birefringence and NMR.