Lamellar solids, formation

Both of the above hexagonal phase formation mechanisms must be treated as speculative. They are not placed in the broader context and try to account for particular observations without analysis of alternative explanations. In particular, it is quite likely that both precursors (lamellar solid and silica-clad rods) are metastable intermediates that dissolve and serve only as the source of nutrients for the growing MCM-41 framework. The arguments in favor of both pathways are valid essentially only with the hexagonal phase. For example, one can readily justify close-packing of silica-clad rods into hexagonal MCM-41. However, it is not as self-evident to rationalize the formation of cubic and lamellar phases on the same premise. Lastly, neither mechanism addresses the issue of phase transformation. 

The role of cholesterol in promoting lamellar phase formation in mixtures with l-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine in excess water was investigated using multinuclear solid-state NMR and X-ray scattering. ... 

In this work, we have evidenced the formation of a lamellar solid from the tribochemical reaction of a borated additive and a succinimide additive. The result is the formation of h-BN in the tribofilm. The tribofilm is mainly composed of an amorphous borate matrix containing highly-dispersed h-BN nanoparticles in the form of sheets less than 10 nm wide and 5 nm thick. The originality of this h-BN tribochemical reaction lies in the nature of the intermolecular reaction between the two additives. At the opposite, M0S2 formation fiom MoDTC is the result of intramolecular chemical reaction. 

In this work, we have evidenced the formation of a lamellar solid fiom the tribochemical reaction of a borated additive and a succinimide additive. The result is the formation of h-BN in the tribofilm. Results can be summarized as followed ... 

Contaminant precipitation involves accumulation of a substance to form a new bulk solid phase. Sposito (1984) noted that both adsorption and precipitation imply a loss of material from the aqueous phase, but adsorption is inherently two-dimensional (occurring on the solid phase surface) while precipitation is inherently three-dimensional (occurring within pores and along solid phase boundaries). The chemical bonds that develop due to formation of the solid phase in both cases can be very similar. Moreover, mixtures of precipitates can result in heterogeneous solids with one component restricted to a thin outer layer, because of poor diffusion. Precipitate formation takes place when solubility limits are reached and occurs on a microscale between and within aggregates that constitute the subsurface solid phase. In the presence of lamellar charged particles with impurities, precipitation of cationic pollutants, for example, might occur even at concentrations below saturation (with respect to the theoretical solubility coefficient of the solvent). 

Whatever the precursor, the formation of an intermediate solid phase was always observed. It can be inferred from X-ray diffraction (Fig. 9.2.7) and infrared spectroscopy that this intermediate phase shows a lamellar, incompletely ordered structure (turbostratic structure) built up with parallel and equidistant sheets like those involved in the lamellar structure of the well-crystallized hydroxides Ni(OH)2 or Co(OH)2, these sheets are disoriented with intercalation of polyol molecules and partial substitution of hydroxide ions by alkoxy ions (29). The dissolution of this solid phase, which acts as a reservoir for the M(I1) solvated species, controls the concentration of these species and then plays a significant role in the control of the nucleation of the metal particles and therefore of their final morphological characteristics. For instance, starting from cobalt or nickel hydroxide as precursor in ethylene glycol, the reaction proceeds according to the following scheme (8) ... 

In the SC lipids form two crystalline lamellar phases.27 The mixture of both phases produces the optimal barrier to water loss from SC. The balance between the liquid crystalline and the solid crystal phases is determined by the degree of fatty acid unsaturation, the amount of water, and probably by other yet undiscovered factors. A pure liquid crystal system, produced by an all-unsaturated fatty acid mixture, allows a rapid water loss through the bilayers with a moderate barrier action. The solid system produced with an all-saturated fatty acid mixture causes an extreme water loss due to breaks in the solid crystal phase.6,23 Studies with mixtures prepared with isolated ceramides revealed that cholesterol and ceramides are very important for the formation of the lamellar phases, and the presence of ceramide 1 is crucial for the formation of the long-periodicity phase.27 The occurrence of dry skin associated with cold, dry weather for example, may result from an extensive, elevated level of skin lipids in the solid state. Therefore, a material that maintains a higher proportion of lipid in the liquid crystalline state may be an effective moisturizer.6... 

---