E. Rodríguez Núñez

Activation energies for the epoxy system BADGE n = 0/m‐XDA obtained using data from thermogravimetric analysis

In this article we study the kinetics of thermal degradation of the epoxy system BADGE n = 0/m-XDA using different kinetic methods with data from thermogravimetric analysis (TGA) in dynamic conditions. Activation energies obtained using different integral methods (Flynn-Wall-Ozawa and Coats-Redfern Methods) are in good agreement with the value obtained using the Kissinger method (204.44 kJ/mol). The solid-state decomposition mechanism followed by this epoxy system is a decelerated Rn type (phase boundary controlled reaction). We have also calculated activation energies using the Van Krevelen and the Horowitz-Metzger methods. These last methods corroborate the decelerated behavior.

Successful prediction of the hydrogen bond network of the 3-oxo-12α-hydroxy-5β-cholan-24-oic acid crystal from resolution of the crystal structure of deoxycholic acid and its three 3,12-dihydroxy epimers

Crystal structures of p-xylene-crystallized deoxycholic acid (3alpha,12alpha-dihydroxy-5beta-cholan-24-oic acid) and its three epimers (3beta,12alpha-; 3alpha,12beta-; and 3beta,12beta-) have been solved. Deoxycholic acid forms a crystalline (P21) complex with the solvent with a 2:1 stoichiometry whereas crystals of the three epimers do not form inclusion compounds. Crystals of the 3beta,12beta-epimer are hexagonal, whereas the 3alpha,12beta-and 3beta,12alpha-epimers crystallize in the P2(1)2(1)2(1) orthorhombic space group. The three hydrogen bond sites (two hydroxy groups, i. e. O3-H, and O12-H, and the carboxylic acid group of the side chain, O24bO24a-H) simultaneously act as hydrogen bond donors and acceptors. The hydrogen bond network in the crystals was analyzed and the following sequences have been observed: two chains (abcabc... or acbacb... ) and two rings (abc or acb), which constitute a complete set of all the possible sequences which can be drawn for an intermolecular hydrogen bond network formed by three hydrogen bond donor/acceptor sites forming crossing hydrogen bonds. The orientation of O3-H (alpha or beta) determines the sequence of the acceptor and the donor groups involved in the pattern: O24a --> O12 --> O3 --> O24b when it is alpha and O24a --> O3 --> O12--> O24B when it is beta. These observations were used to predict the hydrogen bond network of p-xylene-crystallized 3-oxo,12alpha-hydroxy-5beta-cholan-24-oic acid. This compound has two hydrogen bond donor and three potential hydrogen bond acceptor sites. According to the previous sequence set, this compound should crystallize in the monoclinic P21 system, should form a complex with the solvent, O24b should not participate in the hydrogen bond network, and the chain sequence O24a --> O12 --> O3 would be followed. All predictions were confirmed experimentally.

Inhibition by Cyclodextrins of Nitrosation Reactions

Nitrite esters (RONO) are effective nitrosating reagents in aqueous basic media.1-5 Previous studies5 of their reaction with different alifatic and heterocyclic secondary amines, to yield N-nitrosoamines, have shown that the following rate equation

Curing kinetic of the epoxy system badge n = 0/1,2 DCH by fourier transform infrared spectroscopy (FTIR)

v = k_2 [{\text{amine}}][RONO]/(1 + [H^ + ]/K_1 )

Curing kinetic of the epoxy system badgen= 0/1,2 DCH by fourier transform infrared spectroscopy (FTIR): Curing Kinetic of the Epoxy System Badgen=0

(1) is obeyed. It suggests that the rate limiting step is the attack of the nitrite ester on the free amine.

Unusual Pyrene Excimer Formation during Sodium Deoxycholate Gelation

Cyclodextrins are known to form inclusion complexes with a variety of molecules, a property used to increase the bioavailability of poorly soluble drugs1. Bile salts are biosurfactants involved in the cholesterol metabolism, and used as drugs in gallstone diseases treatments2.