ABSTRACT
The future of molecular gels may rely on a deeper theoretical understanding of the gel-to-solid phase-separation process that usually limits the lifetime of molecular gels. Stable sol phases, when available, can be the appropriate vectors to transfer the 1D self-assembled and functional morphologies to solid substrates that may be used for technological devices.
ABSTRACT
Quinuclidine grafted cationic bile salts are forming salted hydrogels. An extensive investigation of the effect of the electrolyte and counterions on the gelation has been envisaged. The special interest of the quinuclidine grafted bile salt is due to its broader experimental range of gelation to study the effect of electrolyte. Rheological features of the hydrogels are typical of enthalpic networks exhibiting a scaling law of the elastic shear modulus with the concentration (scaling exponent 2.2) modeling cellular solids in which the bending modulus is the dominant parameter. The addition of monovalent salt (NaCl) favors the formation of gels in a first range (0.00117 g cm(-3) (0.02 M) < T(NaCl) < 0.04675 g cm(-3) (0.8 M)). At larger salt concentrations, the gels become more heterogeneous with nodal zones in the micron scale. Small-angle neutron scattering experiments have been used to characterize the rigid fibers (
Subject(s)
Bile Acids and Salts/chemistry , Hydrogels/chemistry , Quinuclidines/chemistry , Cations/chemistry , Electrolytes/chemistry , Micelles , Molecular Structure , Osmolar Concentration , Rheology , Sodium Chloride/chemistryABSTRACT
The knowledge of the structure of a molecular crystal is frequently a prerequisite for the understanding of its solid state properties. Even though single-crystal diffractometry is the method of choice when it comes to crystal structure determination, methods using powder diffraction data become more and more competitive. There has been much recent interest in the development of a new generation of "direct-space" approaches that are particularly suited for molecular crystals. The crystallographic structure of a steroid derivative molecule (17,17-di-n-propyl-17a-aza-D-homo-5alpha-androstan-3beta-ol) was obtained in two independent ways: from a single crystal by laboratory X-rays and from a polycrystalline powder by high-resolution synchrotron powder diffraction. The molecule crystallizes in the orthorhombic space group P2(1)2(1)2(1) (a = 6.5346, b = 17.6006 and c = 19.6978 A). Hydrogen bonds form infinite chains of molecules parallel to the c axis.
Subject(s)
Androstanols/chemistry , Steroids/chemistry , Crystallization , Models, Molecular , Molecular Structure , Powder Diffraction , X-Ray DiffractionABSTRACT
The kinetics and mechanism of the desorption of mineral clay particles originally adsorbed to a phospholipid monolayer has been studied. The desorption was induced by the injection of citric acid solution into the aqueous subphase. To follow the related kinetic processes, grazing incidence diffuse X-ray scattering out of the specular plane (GIXOS) was used with a remarkable time resolution of the order of one minute. From the variations of the electron density profile normal to the surface, the Langmuir desorption isotherm and the related kinetic constants were determined.
ABSTRACT
Methods of measurement of the phase transition temperatures in physical molecular gels are analyzed. The "falling ball" technique, NMR, and rheology are compared. Temperature versus concentration phase diagrams can be easily obtained, and the reliability and workability of the methods are discussed. It appears that rheology is the more accurate and convenient technique while the falling ball method provides acceptable DeltaH, DeltaS values, but the melting temperature values are significantly altered. With NMR, kinetic information associated with the molecular aggregation process can also be extracted. Copyright 2000 Academic Press.
