Fiber optic sensors based on hybrid phenyl-silica xerogel films to detect n-hexane: determination of the isosteric enthalpy of adsorption.

We investigated the response of three fiber optic sensing elements prepared at pH 10 from phenyltriethoxysilane (PhTEOS) and tetraethylsilane (TEOS) mixtures with 30, 40, and 50% PhTEOS in the silicon precursor mixture. The sensing elements are referred to as Ph30, Ph40 and Ph50, respectively. The films were synthesized by the sol-gel method and affixed to the end of optical fibers by the dip-coating technique. Fourier transform infrared spectroscopy, N2 adsorption-desorption at 77 K and X-ray diffraction analysis were used to characterize the xerogels. At a given pressure of n-hexane, the response of each sensing element decreased with temperature, indicating an exothermic process that confirmed the role of adsorption in the overall performance of the sensing elements. The isosteric adsorption enthalpies were obtained from the calibration curves at different temperatures. The magnitude of the isosteric enthalpy of n-hexane increased with the relative response and reached a plateau that stabilized at approximately -31 kJ mol(-1) for Ph40 and Ph50 and at approximately -37 kJ mol(-1) for Ph30. This indicates that the adsorbate-adsorbent interaction was dominant at lower relative pressure and condensation of the adsorbate on the mesopores was dominant at higher relative pressure.