Thermodynamic study of aqueous micellar solutions of biologically active bisquaternary ammonium chlorides

Thermodynamic properties of aqueous solution of bisquaternary ammonium salts, which are derivatives of N,N-bisdimethyl-1,2-ethanediamine ( bis-Cn-BEC) , of general formula /CnH2n/1OOCCH2(CH3)2N!CH2CH2N!(CH3)2CH2COOCn-H /2Cl0(bis-Cn-BEC, where the subscript n stands for the number of carbon atoms of the alkyl chain bound to the carboxyl group) are here reported and compared with those of the corresponding monomers. Dilution enthalpies have been measured by means of a flow type microcalorimeter at 313 K, and densities have been measured by means of a vibrating tube densimeter at 298 K. Apparent and partial molar quantities have been obtained from the experimental data and expressed as a function of molality assuming the infinite dilution as standard state. The changes in enthalpy and in volume upon micellization have been evaluated by assuming the pseudo-phase transition model. From the calculated enthalpy changes at 313 K and that previously reported at 298 K, the changes in heat capacity for micellization has been evaluated. The data suggest the hypothesis that the alkyl chain in the dimers are already partially associated in solution. Moreover, the trends of the cmc as a function of the chain length for monomers and dimers suggest that the association of the chains probably leaves out the first three methylene groups of the alkyl chains bound to the carboxylic groups. The packing parameter, P, gives 0.37 for bis-C10-BEC, and the formation of rod-like micelles is therefore suggested.