Contrasting pKa of protonated Bis(3-aminopropyl)-terminated polyethylene glycol "Jeffamine" and the associated thermodynamic parameters in solution and covalently attached to graphite surfaces

The pKa value of protonated Jeffamine (bis(3-aminopropyl) terminated polyethylene glycol) in soln. and attached as a monolayer to graphite surfaces has been detd. using potentiometric titrn. The protonated Jeffamine was found to have a pKa value of 9.7 in soln. at 25°C, whereas this value decreases to 7.1 when it is attached to a graphite surface. Potentiometric titrns. from 25 to 40°C allowed us to det. the surface pKa of the protonated Jeffamine at each temp. studied and hence to det. the enthalpy, entropy and Gibbs energy changes assocd. with the deprotonation of the amino-terminated surface bound Jeffamine groups. It was found that the enthalpic contribution is negligibly small and the evaluation of these thermodn. parameters controlling the shift in surface pKa value indicates that this process is controlled by entropic contribution arising from the ordering/disordering of solvent mols. at the carbon-water interface. This suggests that the long chain Jeffamine mols. are oriented on the carbon surface rather than existing in the bulk soln. [on SciFinder(R)]