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 endothermi c physisorption process of organic vapor partitioning into seven systematic ally side-chain-modified (polar, acidic, basic, polarizable side groups and groups interacting via H-bridges) polysiloxanes on thickness-shear mode re sonators. Each sensor was exposed to seven analytes, selected for their div ersity of functional groups. This systematic investigation of sorption yiel ds benchmarking data on physisorption selectivity: response data and modeli ng reveal a direct correlation of partition coefficients with interactions between specific polymer side chains and analyte functional groups. Partiti on coefficients were determined for every polymer/analyte pairing over the 273-343 K range at 10 K intervals; from partition coefficient temperature d ependence, overall absorption enthalpies and entropies were calculated. By subtracting the enthalpy and entropy of condensation for a given pure analy te, its mixing entropy (primarily combinatorial) and mixing enthalpy (assoc iated with intermolecular interactions) with each polymer matrix were deter mined. These two crucial thermodynamic parameters determine the chemical se lectivity patterns of the polymers for the analytes. Simple molecular model ing based on the polymer contact surface share of the modified side group o r the introduced functional group reveals a direct correlation between the partition coefficients and the side-group variation.