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Fluid mixtures interacting with strong specific forces are, as a rule, complex systems, departing significantly from ideal-solution behaviour. The description of calorimetric properties of such systems by thermodynamic models of mixtures is a challenging task, even though such models may satisfactorily describe their phase behaviour and/or their volumetric properties. In this chapter, the Non-Random Hydrogen-Bonding theory, an equation of state model, is applied to the description of mixing enthalpies in two types of systems with peculiar association behaviour: mixtures of fluids capable of forming association dimers; and fluids interacting via both intra- and inter-molecular hydrogen bonds. The model sheds light on the interplay of intermolecular interactions through the calculation of the various contributions to the excess enthalpy from hydrogen bonding, dipolar, induced polar or dispersive interactions. Interesting conclusions arise from such an approach for the behaviour of the investigated solutions. The model reveals that the addition of inert solvents to carboxylic acids with small carbon numbers at ambient temperature does not dramatically alter their degree of dimerization. Consequently, the observed endothermic dissolution process is mainly attributed to weak van der Waals interactions. Furthermore, in mixtures of alkoxy-alcohols, which are typical fluids interacting via both inter- and intra-molecular hydrogen bonding the addition of an inert solvent fine tunes the mixing enthalpy through the decrease of the endothermic inter-molecular association and the simultaneous increase of the exothermic intra-molecular association.

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