The Mean Activity Coefficients of Fullerenehexamalonates Th-C66(COO-M+)12 in Aqueous Solutions.

The Poisson-Boltzmann equation has been solved for the spherical cell model of a fullerenehexamalonate solution to obtain the contributions of the macroion and its counterions to the total electrostatic free energies. The contribution of the macroion is large and positive, whereas the contribution of the counterions is small and negative. From these data the concentration dependence of the activity coefficients of the macroion and counterions have been calculated, as well as the mean activity coefficient of the fullerenehexamalonate electrolyte. Calculated values for the mean activity coefficient have been compared with the existing experimental data and a good agreement has been observed.

Electric transport and ion binding in solutions of fullerenehexamalonic acid Th-C66(COOH)12 and its alkali and calcium salts.

The transport numbers of the macroion constituent in aqueous solutions of fullerenehexamalonic acid, C66(COOH)12, and its lithium, sodium, cesium, and calcium salts were determined by the indirect moving boundary method. By combining the transport data with the literature data for the electrical conductivity of the same solutions, the fraction of bound counterions to the fullerenehexamalonate macroion was calculated. It was found that about 20-40% of counterions are associated with the macroion in the concentration range from about 0.1 to 0.002 mol COO-/L. Consequently, under the influence of a potential field, these counterions move toward the anode as an integral part of the migrating macroion in opposition to the electrical field. Therefore, the absolute values of the macroion constituent transport numbers are relatively high. The experimental values of the fractions of bound counterions were compared with the theoretical predictions based on the solution of the Poisson-Boltzmann equation for the spherical cell model, and a reasonable agreement was obtained.

Analysis of sample of highly water-soluble Th-symmetric fullerenehexamalonic acid C66(COOH)12 by ion-chromatography and capillary electrophoresis.

Ion chromatography (IC) was used to establish isomer purity of the highly water-soluble sample of fullerenehexamalonic acid, Th-symmetric hexakis-adduct C66(COOH)12. Sharp and symmetric peaks were obtained by IC using 1.0 M potassium hydroxide as eluent and applying gradient elution program. The identity of the two largest peaks in the chromatogram was assigned to Th-C66(COOH)12 and C66H(COOH)11. The developed IC procedure can be used for the semi-quantitative determination of the extent of the partial decarboxylation of the sample. As an alternative analytical technique, a CE procedure was introduced and its performance against IC was compared for this particular case.

Enthalpy changes upon dilution and ionization of poly(L-glutamic acid) in aqueous solutions.

The enthalpy changes accompanying the dilution and ionization of poly(L-glutamic acid) in water have been measured at 25 degrees C for two degrees of polymerization (DP = 115 and DP = 480) at various degrees of ionization, alpha, for a concentration range from about 0.2 to 0.002 monomol/L. The heat of dilution displays an unusual dependence on the degree of ionization, which is in sharp contrast to the behavior of other weak carboxylic polyelectrolytes, such as poly(acrylic acid). The exothermic heat effects observed at low values of alpha become endothermic for the region where the helix-coil transition is most pronounced, and for high degrees of ionization, they are exothermic again. Evidently, an endothermic heat effect, produced by an additional conformational transition in the dilution process, is superimposed on the exothermic enthalpy of dilution, and it overweighs the latter in the region of alpha where the conformational transition is prevailing. The calorimetric titration curve, which gives the dependence of the heat of ionization, deltaH(i), on alpha, has a maximum and is typical for poly(carboxylic acids) which undergo pH-induced conformational transition, such as poly(methacrylic acid). The values of deltaH(i) obtained at two polymer concentrations indicate that the enthalpy of ionization depends on the polypeptide concentration.