R EFERENCES
2.1 Introduction
Fractionation of bio-oil with water is the easiest method which transforms bio-oil into two fractions: an aqueous top phase and an organic bottom phase. The bottom phase mainly contains lignin-containing fractions, while top layer is enriched with carbohy- drate derived compounds [Sipila et al., 1998, Mohan et al., 2006]. Both phases can be processed separately to extract value added chemicals. Vitasari et al. [Vitasari et al., 2011] have studied the effects of stirring rate and water-to-oil ratio on the extraction of Glycolaldehyde, Acetic acid, Acetol, Furfural, Furanone, Levoglucosan, Syringol and Guaiacol from forest residue-derived bio-oil and pine-derived bio-oil. Based on experi- mental observation, they concluded that the distribution coefficient of a compound in aqueous phase depends on its polarity and solubility. However the mechanism of dis- tribution of compounds into two phases has not been studied in detail.
It is a known fact that intermolecular forces acting between water and compounds present in bio-oil are primarily responsible for the solubility of compounds in water.
The magnitude of the intermolecular force decides the relative solubility of the com- pounds in water. Various experimental studies have been carried out for different bio systems, but many difficulties prevent the detailed understanding of the nature of these forces. In this scenario Ab initio quantum chemistry calculation plays an important role in understanding the nature of such interactions. The most simple and conventional method to calculate the interaction energy between different molecules in a system is the supermolecule approach [Morokuma, 1971, Kitaura and Morokuma, 1976, Ziegler and Rauk, 1977, Stevens and Fink, 1987, Frey and Davidson, 1989, Glendening and Streitwieser, 1994, Chen and Gordon, 1996, van der Vaart and Merz, 1999, Khali- ullin et al., 2007, Wu et al., 2009, Mitoraj et al., 2009, Mo et al., 2011, Su and Li, 2009, Kumar et al., 2013] in which the interaction energy of a pair of molecules is calculated as the difference between the energy of the pair and the energy of the indi- vidual molecules.
Many methods have been developed over the years to partition this total interaction
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Chapter-2 energy (INT) into various meaningful energy terms. These methods employ either the variational or the perturbation approach. The symmetry-adapted perturbation the- ory (SAPT) is one of the most widely used schemes in perturbation approach [Sza- lewicz and Jeziorski, 1979, Chałasi´nski and Szcze´sniak, 1988, Jeziorski et al., 1994].
Density functional theory based SAPT methods such as DFT-SAPT and DF-DFT-SAPT have also been developed [Heßelmann and Jansen, 2002a, Heßelmann and Jansen, 2002b, Heßelmann and Jansen, 2003, Heßelmann et al., 2005]. Some of the widely used schemes based on the variational approach include the Kitaura-Morokuma (KM) scheme [Morokuma, 1971, Kitaura and Morokuma, 1976] which forms the basis for many of the other energy decomposition schemes.
The KM scheme partitions the interaction energy into electrostatic (ES), exchange (EX), polarization energy (POL), charge transfer (CT) and MIX components. The different components of the interaction terms are defined as follows: (a) electrostatic: the clas- sical coulombic interaction between occupied molecular orbitals (MO's) which does not cause mixing of MO's; (b) exchange: the interaction between occupied MO's which causes electron exchange between molecules; (c) polarization: the interaction which causes the mixing between the occupied and vacant MO's within each molecule; (d) charge transfer: the interaction which causes intermolecular transfer of electrons from the occupied MO's of one molecule to the vacant MO's of the other and vice versa. Local- ized Molecular Orbital-Energy Decomposition Analysis (LMO-EDA) scheme developed by Su et al. [Su and Li, 2009] is an extension and modification of the KM scheme. The advantage of this method over KM scheme is that high-level quantum chemistry meth- ods such as MP2, CCSD and CCSD (T) can be used with it and thus makes it a robust model. LMO-EDA divides the total interaction energy (INT) into Electrostatic Energy (ES), Exchange Energy (EX), Repulsion Energy (REP), Polarization Energy (POL), and dispersion Energy (DISP) terms. In such a scheme there is no change in Electrostatic Energy (ES). However the sum of EX and REP terms of LMO-EDA is same as the EX term of KM scheme. Similarly the Polarization Energy (POL) of LMO-EDA is equal to the sum of POL, CT and MIX terms of KM scheme. The DISP term then simply becomes
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Chapter-2 the difference between the MP2 and HF interaction energies.
Due to its simplicity and robustness, the LMO-EDA scheme has been widely used for var- ious weak and strong interactions [Yu, 2013, Ma et al., 2013, Singh et al., 2014, Shen et al., 2012, Thellamurege and Hirao, 2013, Semrouni et al., 2013, El-Hamdi et al., 2013].Yu [Yu, 2013] has studied the intermolecular interactions between HCOOH and C6H6 with the LMO-EDA method and found that the dispersion energies are as impor- tant as the electrostatic energies for the total interaction energies of the five HCOOH· · · C6H6complexes. Ma et al. [Ma et al., 2013] have investigated the hydrogen bonding in- teractions in HXeCCH· · · H2O and HXeCCH· · · HF complexes by Ab initio calculation.
They also carried out LMO-EDA analysis and observed that the dominant stabilizing forces are exchange energies and electrostatic interactions in all the complexes. How- ever it was found that the polarization and dispersion interactions play a minor role to the stabilization of these complexes. Singh et al. [Singh et al., 2014] have investigated the competition between a very weak n→ π∗Ar interaction and a very strong hydrogen bond (N-H· · · N) interaction present in the complexes of 7-azaindole with a series of 2,6-substituted fluoropyridines by Ab initio calculation. The conclusion obtained from Ab initio calculation was supported by LMO-EDA analysis that the increase in the dis- persion component is much more rapid compared to that of the electrostatic component with an increase in fluorination of the fluoropyridine ring. The report of solubility pa- rameters of Bio-Oil derived chemicals from EDA calculation in the literature is sparse.
Recently Garrec et al. [Garrec et al., 2014] obtained the relative partition of peroxyl radical intermediate and hydroperoxide derivatives using a Universal Solvation Model (SMD). Further the authors conducted classical MD simulations for a hydrated phos- phatidylcholine (DLPC) bilayer containing a small number of oxidized lipids.
Thus, in this chapter solubility of selected bio-oil molecules in water has been esti- mated. Further the natural bond orbital (NBO) analysis was employed to confirm the hydrogen bonding interaction in these complexes. Thereafter the computed interaction energies were regarded as the measurement for the solubility of the bio-oil molecules in water. Finally based on interaction energy, solubility ranking of the compounds in water
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Chapter-2 was confirmed and compared with the available experimental solubility parameters.