José Pedro WOJEICCHOWSKI1,2, Dinis O. ABRANCHES1, Ana M. FERREIRA1, Marcos R. MAFRA2, João A.p. COUTINHO2
1CICECO - Aveiro Institute of Materials, Chemistry Department, University of Aveiro, Aveiro, Portugal
2Department of Chemical Engineering, Federal University of Paraná, Curitiba, Brazil
Knowledge on the thermophysical and solvation properties (e.g., polarity) of ionic liquids (ILs) and deep eutectic solvents (DESs) are essential for their design and application. Solvatochromic parameters are suitable to describe the polarity of solvents, and can be measured using the Kamlet-Taft (K-T) parameters. This method is based on dipolarity/polarizability (π*), basicity (β), and acidity (α). Despite the usefulness of K-T parameters, they are not yet available for a wide range of unconventional solvents. Moreover, their experimental measurement for all DESs is a costly endeavor. Therefore, the aim of this work was to develop alternatives to estimate K-T values, using predictive models based on descriptors of the Conductor-like Screening Model for Real Solvents (COSMO-RS) to estimate the K-T parameters of DESs. A comparison between the polarity of common organic compounds, ILs, and DESs was also performed.
The K-T parameters were modeled using linear regression analysis based on the following descriptors: i) Misfit interaction energy (EMF); ii) van der Waals interaction energy (EvdW); iii) Moment HB acceptor (MHBA) and, iv) Moment HB donor (MHBD). Initially, the method was developed and tested for 175 organic compounds to validate their applicability. After the preliminary study, the procedure was extended to a dataset of 44 DESs.
Considering the nature of the MHBA, MHBD, and EvdW descriptors, they were used to predict α and β. For π* modeling, MHBA and MHBD retain their importance as they represent the charged areas of the molecule. However, using EMF instead of EvdW improves the performance of the π* model. The R² of the α, β, and π* models were 0.86, 0.63, and 0.42, respectively. The results showed that COSMO-RS descriptors can be used to develop models for solvatochromic parameters for organic solvents. Therefore, the approach was extended to DESs.
The models used to predict the K-T parameters were based on the same structure used for organics. However, since DESs are not pure compounds, composition-weighted averages of the individual descriptors were used. The correlations developed have physical meaning, i.e., for the α correlation, the coefficient associated with MHBD was positive. The R² values between experimental and predicted α, β and π* parameters were 0.93, 0.86 and, 0.89, respectively. This work was a pioneer in the development of correlations for all solvatochromic parameters for DESs. In addition, through the representation of π* vs β (Figure 1), it was found that DESs and ILs do not replace organic solvents, but rather are solvents that extend the polarity range of available solvents, highlighting their ability as designer solvents.
In summary, this work proposed new predictive models to estimate K-T parameters for DESs based on COSMO-RS descriptors. The comparison of polarity between DES, ILs and organic solvents indicated that these non-conventional solvents should be considered as complementary to conventional organic solvents.
Acknowledgments: This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020, UIDP/50011/2020 & LA/P/0006/2020, financed by national funds through the FCT/MCTES (PIDDAC).
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