3.4. LSER and vapor/polymer K-matrix
A polymer coated SAW sensor when exposed to a volatile organic vapor attains thermodynamic equilibrium by sorption of
vapor analytes into the polymer coating. The process of sorption is driven by the difference between Gibb’s free energy per mole (chemical potential) of analyte molecules in vapor phase and in polymer phase. The partitioning of vapor analytes occurs at the vapor–polymer interface in accordance with the lowering of overall free energy of the system. This is the process of solvation–analyte molecules serve as solute and polymer matrix as solvent. The quantitative specification of partitioning is expressed by defining the partition coefficient K as ratio of the analyte concentration in polymer phase CP to that in the vapor phase CV. That is, K = CP/CV. The Abraham’s LSER (linear solvation energy relationship) models log10K as a linear combination of terms representing contributions from different elementary interaction mechanisms
by which the analyte molecules are sorbed in the polymer matrix [74,109–111]. Specifically, an analyte (solute) in contact with a polymer (solvent) is seen to be interacting by five component mechanisms:(i) polarizability interaction where solute and solvent nonpolar electronic structure induce mutual polarization (calledinduced-dipole/induced-dipole or dispersioninteraction),(ii) dipolarity interaction where dipole moments associated with solute and solvent undergo orientational alignment plus dipole-induced