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Contrasting p K a of Protonated Bis(3‐aminopropyl)‐Terminated Polyethylene Glycol “Jeffamine” and the Associated Thermodynamic Parameters in Solution and Covalently Attached to Graphite Surfaces
Author(s) -
Abiman Poobalasingam,
Wildgoose Gregory G.,
Crossley Alison,
Jones John H.,
Compton Richard G.
Publication year - 2007
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200700942
Subject(s) - protonation , potentiometric titration , titration , enthalpy , chemistry , gibbs free energy , graphite , deprotonation , monolayer , polyethylene glycol , molecule , solvent , organic chemistry , thermodynamics , ion , biochemistry , physics , electrode
The p K a value of protonated Jeffamine (bis(3‐aminopropyl) terminated polyethylene glycol) in solution and attached as a monolayer to graphite surfaces has been determined using potentiometric titration. The protonated Jeffamine was found to have a p K a value of 9.7 in solution at 25 °C, whereas this value decreases to 7.1 when it is attached to a graphite surface. Potentiometric titrations from 25 to 40 °C allowed us to determine the surface p K a of the protonated Jeffamine at each temperature studied and hence to determine the enthalpy, entropy and Gibbs energy changes associated 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 thermodynamic parameters controlling the shift in surface p K a value indicates that this process is controlled by entropic contribution arising from the ordering/disordering of solvent molecules at the carbon–water interface. This suggests that the long chain Jeffamine molecules are oriented on the carbon surface rather than existing in the bulk solution.