Application Of SAFT Association Model For Spectroscopic Properties Of The Associated System

Molecular association is a spontaneous equilibrium process by which molecules associate to form complexes or aggregates under specific conditions. These phenomena are widely found in biological and non-biological systems. The study of properties of associating systems is a not only traditional but also preceding field, which attracts chemists at all times.The triangle of experiment, theory and simulation is well known. Recently, spectroscopy was utilized to probe microscopic properties such as intermolecular interactions and solution structure. Spectroscopies such as IR, NMR, Raman, UV/vis, fluorescence spectroscopy, X-ray diffraction, small-angle X-ray scattering, neutron-scattering, MS and SEM have become the fourth vertex on the molecular thermodynamics tetrahedron.In this paper, a new association model based on the Statistical associating fluid theory (SAFT) was proposed to correlate NMR chemical shift of associating systems. 57 sets of data for 17 associating systems were correlated. Mole fractions of various hydrogen bonds were calculated and the characteristic chemical shifts were obtained. The SAFT association model can provide a intuitionistic explaining for the variation of chemical shift with composition of mixtures without assuming how many aggregates are formed. It is simper than chemical association models and the model parameters it needs are fewer. Furthermore, the SAFT association model is applicable for kinds of associating systems. Especially, it is better for dealing with complex associating systems such as aqueous systems.The SAFT association theory was also introduced into the study on IR spectrum. The 1-2-n association model was applied to correlate the IR absorbance data of OH stretching vibration in the alcohol-hydrocarbon systems. But the mole fractions of various aggregates were calculated by SAFT without introducing the equilibrium constants of aggregates. The results of correlation by this method are all good for three absorption bands of ethanol-hexane system and methanol-hexane system. So the feasibility of 1-2-n association model for alcohol-hydrocarbon systems was proved. And it was also proved that the method of calculating mole fractions of aggregates by SAFT is capable.The SAFT association theory is introduced into the study on Raman spectrum and weak hydrogen bonds, too. A SAFT association model was proposed to describe the variation of C-Hstretching vibration frequency of Raman spectrum with composition of mixtures. The C-H stretching vibration frequency data for five aqueous systems were correlated. The results of correlation indicated that SAFT association model described Raman spectrum properties of mixtures very well. Furthermore, the association parameters and characteristic frequency of Raman spectrum of C-H...O weak hydrogen bonds were obtained by the means of correlation, which are important for investigating the properties of weak hydrogen bonds.