Supramolecular assembly of a thermoresponsive steroidal surfactant with an oppositely charged thermoresponsive block copolymer.

Supramolecular rearrangements are crucial in determining the response of stimuli sensitive soft matter systems such as those formed by mixtures of oppositely charged amphiphiles. Here mixtures of this kind were prepared by mixing the cationic block copolymer pAMPTMA30-b-pNIPAAM120 and an anionic surfactant obtained by the modification of the bile salt sodium cholate. As pure components, the two compounds presented a thermoresponsive self-assembly at around 30-35 °C; a micelle formation in the case of the copolymer and a transition from fibers to tubes in the case of the bile salt derivative. When both were present in the same solution they associated into mixed aggregates that showed complex thermoresponsive features. At room temperature, the core of the aggregate was comprised of a supramolecular twisted ribbon of the bile salt derivative. The block copolymers were anchored on the surface of this ribbon through electrostatic interactions between their charged blocks and the oppositely charged heads of the bile salt molecules. The whole structure was stabilized by a corona of the uncharged blocks that protruded into the surrounding solvent. By increasing the temperature to 30-34 °C the mixed aggregates transformed into rods with smooth edges that associated into bundles and clusters, which in turn induced clouding of the solution. Circular dichroism allowed us to follow progressive rearrangements of the supramolecular organization within the complex, occurring in the range of temperature of 20-70 °C.

Successful prediction of the hydrogen bond network of the 3-oxo-12alpha-hydroxy-5beta-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.

Aggregation behavior of sodium fusidate in aqueous solution.

We analyzed the freezing point depression and pNa measurements for aqueous solutions of sodium fusidate. At concentrations lower than 0.011 mol kg(-1), sodium fusidate behaves as a strong 1:1 electrolyte. At higher concentrations, sodium fusidate self-aggregates. To analyze the results two hypotheses on the monomer concentration are presented and discussed. The first one accepts that the monomer concentration, C(A), is constant and equal to 0.023 mol kg(-1). This concentration corresponds to a break point in the plot of the freezing point depression vs total sodium fusidate concentration, C(A)t. The second hypothesis accepts that C(A) increases with C(A)t following a leveling-off curve. Measurements of hydrodynamic radii and comparison with similar systems, such as sodium taurocholate, strongly support the second hypothesis. The results indicate that at concentrations lower than 0.08 mol kg(-1) the aggregation number increases from 2 to 3. Above this concentration, both the aggregation number and the fraction of bound counterions remain constant, with average values of 3.13 +/- 0.10 and 0.31 +/- 0.05, respectively. Such results indicate that for trimers only one Na+ counterion is involved per aggregate. We propose that this counterion shields the repulsion between the two nearest carboxylate groups which, according to a disklike model in which the monomers are packed with that group alternatively oriented up and down, should hold together. Values for the formation equilibrium constant of aggregates are also calculated. Its dependence with the aggregation number allows the determination of the reversible transfer of a free surfactant ion together with the associated counterions from the bulk solution to the aggregate, the resulting value being w(0) = -4.2k(B)T.