Thermodynamic properties of dilation of chondroitin 4-sulfate having tetraalkylammonium counterions

The dilution enthalpies, Delta(dil)H, of tetraalkylammonium chondroitin 4-sulfate (R(4)NChSA) and polyacrylate (R(4)NPAA) having methyl (Me), ethyl (Et), propyl (Pr) and butyl (Bu) as alkyl groups (R) were measured to elucidate the effects of hydrophobic counterions. The Delta(dil)H of R(4)NChS-A and R(4)NPAA were negative (exothermic) and decreased as the carbon number of tetraalkylammonium cation (R(4)N(+)) increased. These results were found to possess the same effects on Delta(dil)H as these of bromides. The non-electrostatic terms of dilution enthalpies Delta(dil)H(n) of R(4)NChS and R(4)NPAA were evaluated by using the electrostatic terms of Delta(dil)H obtained from Manning's theory. The Delta(dil)H(n) of R(4)NChS-A and R(4)NPAA decreased, i.e., their exothermic tendencies increased as the carbon number of R(4)N(+) increased, and the rate of decrease of the R(4)NPAA was more than that of the R(4)NChS-A. These results are discussed in relation to the increase of the hydrated water structure around R(4)N(+) on dilution.