We study by molecular dynamics simulations the hydration of beta-cyclodextrin. Our simulations show that within these barrel-shaped molecules hydrophobicity dominates, while at the top and bottom sides of the barrel interactions with water are mostly hydrophilic in nature. These results agree with crystallographic data at 120 K and, in particular, with the spontaneous hydration process of a cyclodextrin crystal in wet atmosphere. The predicted structure of the hydration shells is discussed and compared with previous molecular mechanics calculations which report an overall hydrophobic behavior. Moreover, the temperature dependence of the hydration process is discussed.

Twelve systems formed with lithium fluoride and alkali halides were studied in order to put into evidence possible demixing phenomena. Liquiddiquid equilibra were found in the seven mixtures containing KBr, KI, RbBr, Rbl, CsCl, CsBr, and Csl; however, the miscibility gaps could be fully detected only for LiF + CsCl, LiF + KBr and LiF + RbBr. The tendency to demix in these systems, formed by typically ionic components, agrees with the reciprocal Coulomb effect rule. On the basis of the solid liquid and liquiddiquid equilibria, the LiF excess potentials were evaluated and compared with those calculated according to current theories.

Miscibility gaps for the solid systems AgCl + NaCl and AgBr+NaBr have been measured by a high temperature X-ray technique.For the two studied systems the solubility curves are very nearly symmetrical in respect to the compositions zNaCl = -513 and £NaBr = -506, while the upper critical temperature are 198 °C for AgCl + NaCl and 285 °C for AgBr+NaBr.The thermodynamic properties of the two solid systems have been calculated using only experimental solubility data. Values of activity and of enthalpy of mixing were estimated and compared with those reported in literature.In 1965 KLEPPA and MESCHEL 1 measured the heats of formation of solid solutions in the systems AgCl + NaCl and AgBr + NaBr. On the basis of a previous work 2 reporting for AgCl + NaCl a miscibility gap (MG) with a critical temperature near 175 °C, the authors 1 stated: "since the positive enthalpies of formation of the bromide solutions are about 20% smaller than those for the corresponding chlorides" for the system AgBr + NaBr "a critical temperature somewhat below 175 °C is predicted".More recently for the solid system AgBr + NaBr, Japanese authors 3 found, by study of galvanic cells, a MG confirmed by X-ray diffraction measurements. According to these authors the critical temperature for an equimolar mixture is between 300 °C and 350 °C.Owing to latter findings that contrast with Kleppa and Meschel's prediction we decided to experimentally reexamine the extension of the MG in the solid phase for both systems AgCl + NaCl and AgBr + NaBr. In order to describe as accurately as possible the limits of the MG, high temperature X-ray diffraction measurements were carried out giving particular care to the sample preparation. Through these data, we attempted a description of the general thermodynamic properties of these systems. The results were compared, when possible, with those reported in literature. Finally, for a complete description of the phase diagrams of the two systems, solid-liquid (SL) curves were determined by DTA measurements.Reprint requests to Prof. Experimental a) Apparatus and MaterialsThe apparatus for DTA measurements has already been described 4 . For X-ray measurements a Philips apparatus, employing Ni-filtered CuKa radiation fitted with a high temperature camera (MRC mod. X-86 N-II), was properly modified 5 to improve temperature homogeneity and control. Thus it was possible to reach temperatures up to 600 °C, controlled within +1°C.NaCl and NaBr were C. E r b a RP; AgCl and AgBr were obtained by precipitation from AgN03 (C. Erba RP). All salts were dried following the usual literature methods. b) ProceduresThe mixtures to be analyzed by X-ray diffraction were prepared by melting the components in a quartz tube and quenching the melt in liquid oxygen in order to obtain a uniform mixture. The finely powdered mixture transferred on the sample holder of the camera was held at 350 °C under N2 for about 6 hours. After the existence of a solid solution was confirmed, the sample was slowly cooled to the desired temperature and there held f...

A b initio SCF-LCAO-MO computations using a minimal basis set have been carried out on the acetylcholine ion in different conformations. The rotational barrier around the C9-C8-01-C2 dihedral angle ( T~) has been also calculated: the curve presents a broad minimum. SCF calculations have been carried out on the system Ach-water at 468 different separations and/or orientations; the interaction energies were then fitted to a pair potential function used in Monte Carlo (MC) simulations of Ach water solution. The rdfs indicate a global first hydration shell with about 38 waters. Details about the hydration shell are presented. The overall MC results indicate that for the Ach ion in water solution at room temperature relative free rotation is possible around T~, and that Ach is well represented as a large positively charged hydrated globe with a small mobile tail free of strong interactions with the surrounding water molecules.