Calorimetry in Liquid Phase

Finally, it should be noted that calorimetric measurements can also be used to monitor adsorption phenomena at the solid-liquid interface (in a solvent). This method has been used to measure the adsorption heats evolved upon injection of dilute solutions of pyridine in alkanes ( -hexane, cyclohexane) onto an acidic solid itself in a slurry with -hexane. The amount of free base in solution is measured separately with a UV-Vis spectrometer, leading to an adsorption isotherm that is measured over the range of base addition used in the calorimetric titrations. The combined data from the calorimetric titration and adsorption measurements are analyzed simultaneously to determine equihbrium constants, quantities of sites per gram and acid site strengths for different acid sites on the solid. 

The measurements are performed in a non-interacting hydrocarbon solvent (e.g. cyclohexane) whose molecular mass is close to that of the donor (e.g. pyridine) in order to cancel out contributions from a dispersion component to the measured enthalpy [25]. As an example, the acid strength of tungsten oxide supported on a silica gel has been determined by this method [26]. 

A similar technique has been used to determine the acidic character of niobium oxide and niobyl phosphate catalysts in different solvents (decane, cyclohexane, toluene, methanol and isopropanol) using aniline and 2-phenyl-ethylamine as probe molecules [27, 28]. The heat evolved from the adsorption reaction derives from two different contributions the exothermic enthalpy of adsorption and the endothermic enthalpy of displacement of the solvent, while the enthalpy effects describing dilution and mixing phenomena can be neglected owing to the differential design and pre-heating of the probe solution. 

The titration of acid sites in liquids of different polarities and proticities (decane, cyclohexane, toluene, methanol and isopropanol) makes it possible to discriminate the acid site strength distribution more accurately than from the more conventional gas-solid phase titration with ammonia. 

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