Thermodynamic modelling of the effect of pH upon aggregation transitions in aqueous solutions of the poloxamine, T701

Differential scanning calorimetry has been used to investigate the effect of pH upon the temperature interval over which aggregation occurs in aqueous solutions of the ethylene oxide (EO)/propylene oxide (PO) block co-polymeric compound tetronic T701 (molecular formula (EO4PO13)2NCH2CH2N(PO13EO4)2). Self association, in this system, occurs at elevated temperatures because the PO blocks become increasingly non-polar and, as a consequence, self associate because of the hydrophobic effect. pH titration shows that the central ethylene diamine moiety is di-basic with pKa values of 3.8 and 8. Thus, in aqueous solution when the pH value is below 8 a necessary pre-condition for self association is deprotonation. A theoretical model is developed—based upon this hypothesis—which attempts to replicate, with some success, the effect of pH upon the calorimetrically observed T701 self association process.