Determination of thermodynamic parameters in reversed phase chromatography for polyethylene glycols and their methyl...

The thermodynamic parameters (entropy and enthalpy change) and their increment per repeat unit have been determined in RP chromatography of PEG and its mono- and dimethyl ethers in different mobile phases. The different approaches for their determination and the problems related to the characteristic volumes (void volume, interstitial volume, pore volume) of the column are discussed. Very different dependences or the thermodynamic parameters were observed in aqueous mobile phases containing ACN, acetone, or methanol as organic modifier. In the first two mobile phases a linear dependence on the number of repeat units n is found, and the additional entropy and enthalpy changes per repeat unit are positive. In methanol-water, the enthalpy change is negative with a non-linear dependence on n, and the entropy change is independent on n and close to zero. The contribution of the end groups is almost the same in the first two mobile phases, but much smaller in methanol-water.