CHARACTERIZATION OF POLYMERIC SURFACE-ACOUSTIC-WAVE SENSOR COATINGS AND SEMIEMPIRICAL MODELS OF SENSOR RESPONSES TO ORGANIC VAPORS

Responses from an array of four polymer-coated surface acoustic wave s ensors exposed to a series of 39 organic vapors were used to investiga te sensor response models based on vapor boiling point, solubility par ameters, and solvation parameters in conjunction with linear solvation energy relationships. As part of this effort, sensor response data we re used to estimate the solubility parameters and solvation parameters of the sensor coatings by adaptation of methods originally developed for use with gas-liquid chromatographic retention data. Values of thes e parameters were found to be consistent with the structures of the co atings though in some cases different from those determined by other m ethods. Discrepancies were attributed to differences in the conditions used for the determinations. Sensor responses were linear over the co ncentration ranges examined and could be summarized using the empirica lly determined partition coefficient, K(e), for each vapor-coating pai r. Linear correlations were found between log K(e) and vapor boiling p oint, and the slopes of the regressions lines were similar to those ex pected for ideal behavior. The strength of the correlations decreased with increasing coating polarity, and it was necessary to divide the v apors into two or three broad chemical classes in order to obtain sati sfactory results. Improved correlations were found by use of Hildebran d solubility parameters in a model based on regular solution theory wh ich attempts to account for nonideal vapor-coating interactions. The u se of solvation parameters in linear solvation energy relationships, h owever, provided the strongest correlations, with modeled K values fal ling within a factor of 2 of experimental values in all cases and with in +/-25% of experimental values in 83% of the cases. Application of t hese models to the prediction of sensor array response patterns appear s promising.