Lipophilicity of acetylcholine and related ions examined by ion transfer voltammetry at a polarized room-temperature ionic liquid membrane

Ion transfer voltammetry at a polarized room-temperature ionic liquid (IL) membrane was used to evaluate the standard Gibbs energy of ion transfer from water to IL. This quantity was considered to be a measure of the ion lipophilicity, which is one of the factors playing a role in the extraction and transport processes in the two-phase liquid and liquid membrane systems. On this basis, the lipophilicity of several biologically active ions was compared, namely of neurotransmitter acetylcholine (ACH+) and several related ions including choline (CH+, precursor for ACH+), muscarine (MUS+, agonist of the muscarinic ACH+ receptors), protonated atropine (ATH+, antagonist of the muscarinic ACH+ receptors), protonated scopolamine (SAH+, antagonist of the muscarinic ACH+), and the tetramethylammonium ion (TMA+) representing their charged moiety. Cyclic voltammetric measurements were carried out using a 4-electrode cell with the IL membrane composed of highly hydrophobic tridodecylmethylammonium tetrakis[3,5-bis(trifluoromethyl)phenyl] borate. Analysis of the voltammetric data provided the values of the standard Gibbs energy of ion transfer (in kJ mol−1 in parentheses), which follow the order of ions TMA+ (14.6)