Evaporation of an emulsion trapped in a yield stress fluid.

The present work deals with emulsions of volatile alkanes in an aqueous clay suspension, Laponite, which forms a yield stress fluid. For a large enough yield stress (i.e. Laponite concentration), the oil droplets are prevented from creaming and the emulsions are thus mechanically stabilized. We have studied the evaporation kinetics of the oil phase of those emulsions in contact with the atmosphere. We show that the evaporation process is characterized by the formation of a sharp front separating the emulsion from a droplet-free Laponite phase, and that the displacement of the front vs. time follows a diffusion law. Experimental data are confronted to a diffusion-controlled model, in the case where the limiting step is the diffusion of the dissolved oil through the aqueous phase. The nature of the alkane, as well as its volume fraction in the emulsion, has been varied. Quantitative agreement with the model is achieved without any adjustable parameter and we describe the mechanism leading to the formation of a front.

High-resolution 29Si solid-state NMR study of silicon functionality distribution on the surface of silicas.

29Si solid-state NMR allows us to distinguish silicon atoms involved in siloxane bridges (Q4), single silanol (Q3) and geminal silanol groups (Q2) on the surface of silica. The relative proportions of each species can be obtained by Magic Angle Spinning (MAS) associated with 1H-->29Si crosspolarization (CP/MAS). This preliminary study deals with the silicon functionality of MCM-41, a purely siliceous, amorphous and mesoporous silica. From the 29Si spectra the relative proportions are estimated to be 3.7% Q2 27.3% Q3 and 69.0% Q4, while the ratio Q2/Q2 + Q3) is 0.12. These results are found to be in perfect agreement with a binomial probability distribution of sites.

Spin-5/2 Hahn echoes in solids.

The Hahn echoes for a spin I = 5/2 system were derived by considering the first-order quadrupole interaction throughout the experiment. Consequently, our results are valid for any ratio of the quadrupole coupling, omega Q, to the amplitude of the RF pulse, omega RF. The use of simplified density operators reduced the computation time dramatically. Furthermore, this approach allows a better understanding of the origin of the Hahn echoes in half-integer quadrupole spins. The tau 4 = tau 2 Hahn echoes were illustrated with the 27Al nucleus in a polycrystalline sample of KAl(SO4)2.12H2O. The quadrupole coupling constant and the asymmetry parameter of 27Al were determined by two methods: from the lineshape of the powder pattern and from the variation of the central line integrated intensity as a function of the second pulse duration.