Aggregation behavior of pyridinium based ionic liquids in water - Surface tension, 1H NMR chemical shifts, SANS and SAXS measurements

The aggregation behavior of short alkyl chain ionic liquids (ILs), namely 1-butyl, or 1-hexyl or 1-octylpyridinium and 1-octyl-2-, or -3-, or -4-methylpyridinium chlorides, in water has been assessed using surface tension, electrical conductance, H-1 NMR, small angle neutron scattering (SANS) and small angle X-ray scattering (SAXS) measurements. Critical aggregation concentrations (CACs), adsorption (at air/water interface) and thermodynamic parameters of aggregation have been reported. The values of CAC and area per adsorbed molecule decrease with the number of carbon atoms in the alkyl chain. The aggregation process is driven by both favorable enthalpy and entropy contributions. An attempt was made to examine the morphological features of the aggregates in water using SANS and SAXS methods. SANS and SAXS curves displayed diffuse structural peaks that could not be model fitted, and therefore, we calculated the mean aggregation numbers from the Q(max) assuming that IL molecules typically order into cubic type clusters. (C) 2012 Elsevier Inc. All rights reserved.