Mesure de forces moléculaires entre bicouches par diffusion de rayons X à pression osmotique contrôlée

Nous présentons des exemples de mesure absolue de pressions répulsives entre bicouches de tensioactifs dans des phases lamellaires. Nous comparons les deux différentes options expérimentales possibles : la diffusion X sous incidence rasante à pression de vapeur contrôlée et la diffusion X aux petits angles en transmission après équilibre de dialyse.We demonstrate one some exemples the compatibility and generality of small angle X-ray scattering experiments made under controlled osmotic pressure. lntroducted by Parsegian twenty years ago, this method remains the simplest way to access directly to molecular repulsive forces in stiff bilayers or lamellar liquid crystals

Insertion of small anisotropic clay particles in swollen lamellar or sponge phases of nonionic surfactant

we have established experimentally the region of existence of smectic and sponge lyotropic microstructures in the presence of synthetic hydrophilic clay particles. Taking into account the adsorption isotherm, we determined the extension of the single-phase regions by visual inspection and looking at low-$q$ behaviour of the small-angle neutron scattering. Conditions of stability such as geometric constraints associated with fluctuation quenching are discussed

SANS structural determination of a nonionic surfactant layer adsorbed on clay particles

We study adsorption of two nonionic surfactants (C12E5 and C12E8) on a dispersed suspension of negatively charged laponite particles. First, we quantify adsorption, i.e. the amount of adsorbed molecules per gram of dried solid. Then, we show that contrast variation experiments under controlled conditions along the adsorption isotherm of the surfactant on dispersed laponite particles allow to determine the average thickness of a nonionic surfactant layer adsorbed on a solid anisotropic particle

Effects of Alkaline Cations on Self-assembly of Cetylpyridinium Surfactants

International audienc

Small angle X-ray scattering by TiO2/ZrO2 mixed oxide particles and a synroc precursor

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Highly Oriented, Charged Multilamellar Membranes Osmotically Stressed by a Polyelectrolyte of the Same Sign

International audienceWe report an X-ray reflectivity study on highly charged lamellar phases of the surfactant didodecyldimethylammonium halides (DDA+X- where X- = Cl- or Br-). A few thousand bilayers are aligned on a flat solid surface with a mosaicity below 0.01° and are swollen under controlled osmotic pressure (Π) from periodicities (d) of a few nanometers up to several tens of nanometers by means of direct immersion in the osmotic stressor reservoir. The high-pressure regime (small d) is obtained by water vapor of controlled relative humidity, while lower pressures (high d) are imposed by means of a novel osmotic stress technique. Accordingly, the solid supported sample is immersed in a high mass polyelectrolyte solution with the same sign and counterions than the membranes under study. Thus, water equilibrating time is reduced from weeks to minutes due to the absence of a semipermeable dialysis membrane. Measurements of the specular and nonspecular X-ray reflectivity are demonstrated to be possible through the osmotic reservoir even with a laboratory X-ray tube which greatly simplifies the establishment of the equation of state Π(d). With the same setup, bilayer undulations and compressibility fluctuations can be studied by peak line shape analysis of high-resolution synchrotron data

Elasticity of fluctuating charged membranes probed by X-ray grazing-incidence diffuse scattering

International audienceWe have investigated the structure, fluctuations and elasticity parameters of charged fluid membranes composed of didodecyldimethylammonium halide bilayers (DDAX) with different counterions (X = Cl− or Br−) by non-specular (diffuse) X-ray scattering. The samples have been prepared as highly aligned thick stacks on silicon support and have been swollen at controlled osmotic pressures. From line-shape analysis of the diffuse scattering, we can determine the fluctuation amplitudes of the membranes in the Łα phase and the characteristic correlation lengths. According to the Caillé model, the measured structural parameters have been expressed in terms of the exponent η and smectic penetration length λ, from which the smectic compression modulus B (J/m3) and the bending modulus K (J/m) have been determined, as a function of the swelling state. We show that the results are in quantitative agreement with the mean-field electrostatic Poisson-Boltzmann calculations, controlled by the bilayers surface charge density (surfactant head groups with free or condensed counterions)