Conferring selectivity to chemical sensors via polymer side-chain selection: thermodynamics of vapor sorption by a set of polysiloxanes on thickness-shear mode resonators

Entropy of mixing is shown to be the driving interaction for the endothermic physisorption process of organic vapor partitioning into seven systematically side-chain-modified (polar, acidic, basic, polarizable side groups and groups interacting via H-bridges) polysiloxanes on thickness-shear mode resonators. Each sensor was exposed to seven analytes, selected for their diversity of functional groups. This systematic investigation of sorption yields benchmarking data on physisorption selectivity: response data and modeling reveal a direct correlation of partition coefficients with interactions between specific polymer side chains and analyte functional groups. Partition coefficients were determined for every polymer/analyte pairing over the 273-343 K range at 10 K intervals; from partition coefficient temperature dependence, overall absorption enthalpies and entropies were calculated. By subtracting the enthalpy and entropy of condensation for a given pure analyte, its mixing entropy (primarily combinatorial) and mixing enthalpy (associated with intermolecular interactions) with each polymer matrix were determined. These two crucial thermodynamic parameters determine the chemical selectivity patterns of the polymers for the analytes. Simple molecular modeling based on the polymer contact surface share of the modified side group or the introduced functional group reveals a direct correlation between the partition coefficients and the side-group variation.