Microcalorimetric titration of a tetra-p-sulfonated calix[4]arene with alkylammonium ions in aqueous solution

Abstract

25,26,27,28-Tetrahydroxycalix[4]arene-5,11,17,23-tetrasulfonate (CATS) consists of four para-sulfonated phenolic rings, which are connected via methylene groups in the meta-positions to a circular system. CATS was titrated with alkylammonium ions, H(CH₂)_nNH₃⁺, n=3–7, in aqueous solution containing 0.01 mol l⁻¹ phosphate buffer at pH 7.1. At this pH CATS carries five negative charges, one of its hydroxy groups and the four sulfonate groups are deprotonated. The measurements were conducted at 288.15, 298.15 and 308.15 K, using an isothermal heat conduction microcalorimeter. Assuming a 1:1 binding model, the titration curves were fitted to concentration binding constants, K_c, and enthalpy changes, ΔH°. From these parameters, values for ΔG°, ΔS° and ΔC_p^°, were derived. As typical for interactions involving dehydration of non-polar surfaces, the temperature dependences in ΔH° and ΔS° compensate each other, resulting in an essentially temperature-invariant ΔG°. Typical are also the large and negative ΔC_p^° values having a linear dependence of the alkane chain length of the ammonium ions.