On the non ideality of CO2 solutions in ionic liquids and other low volatile solvents

The nonideality of CO2 solutions in ionic liquids and other low volatile solvents, with which CO2 is known to form electron donor-acceptor (EDA) complexes, is here investigated. It is shown that the deviations from the ideality observed are not related with the stability of the EDA complex formed, and inmost cases these deviations are small and dominated by entropic effects. For this reason, when the CO2 concentration is expressed in molality, the pressure versus concentration phase diagrams of CO2 in nonvolatile solvents are, within the uncertainty of the experimental data, solvent independent. Following this approach, a correlation for the solubility of CO2 in nonvolatile solvents, valid for pressures up to 5 MPa and temperatures ranging from room temperature up to 363 K, is here proposed. These results are tested with success by measuring the solubility of CO2 in [THTDP][Cl] to show that, in such heavy ionic liquid the solubility, although identical in molality units, if expressed in mol fractions, it is larger than that in [BMIM][NTf2]. SECTION Statistical Mechanics, Thermodynamics, Medium Effects T he solubility of CO2 in solvents of low volatility or nonvolatile is highly relevant for many technological applications. Enhanced oil recovery (EOR) requires the knowledge of the CO2 solubility in heavy hydrocarbons; 1 purification of vegetable or animal oils or the extraction of value-added compounds from them using supercritical technologies is related with the CO2 solubility in these oils, and their fatty acids and esters; and the Selexol process uses a mixture of dimethyl ethers of polyethylene glycol (PEG) for the removal of acid gases from feed gas streams. Recently, a great deal of works have addressed the use of ionic liquids (ILs) for CO2 capture and gas separation purposes. 7-31 In all these cases, the understanding and description of the absorption of CO2 in solvents of low volatility is of importance for the design and operation of processes or the design of new and enhanced sorbents for CO2. Equations of state (EoSs) are useful to correlate the experimental data available and extrapolate the data into conditions forwhich no experimental data is available. A number of EoSs have been used to describe the experimental data for the systems mentioned above. The EoSs available have however a number of shortcomings. They have a very limited predictive ability for systems not previously studied. Moreover, given that empirical binaryparameters are required to fit the experimental data for the most complex systems, the understanding of the solubility at a molecular level is quickly lost. The best approach to obtain direct molecular level interaction information for CO2-containing systems is through spectroscopic techniques. It is well-known that the CO2 molecule can act both as proton acceptor or electron donor according to the nature of the solvent, forming electron donor-acceptor (EDA) complexes. A number of authors have investigated the interactions between CO2 and carbonyl groups, ester groups, hydroxyl groups, ether groups and with ILs. While these interactions observed by spectroscopic techniques are of high relevance for an understanding of the solvation of CO2 in nonvolatile solvents, they are not enough to fully explain its solubility or the deviations to the ideal behavior on these systems. Although the CO2 EDAcomplexes with sp 3 O-donating atoms (such as H2O and alcohols) are more stable than complexes involving sp O-donating atoms (such as aldehydes and ketones), the solubility on the former is inferior to that observed on the latter. Kazarian et al. based on the solubility and spectroscopic evidence for the interactions of CO2 with BF4 and PF6 anions were the first to recognize that “the strength of the interactions cannot be solely responsible for the solubility of CO2 in ionic liquids”. 68 Seki et al. in a recent work show that, although the interactions of CO2 with BF4 and PF6 anion-based ILs are stronger than those with the NTf2, the solubility of CO2 on these ILs is larger than in the former, and thus the interactions alone are not enough to provide an explanation for the CO2 sorption. They recognize that “the strong Lewis acid-base interactions between the ILs and the dissolved CO2 has no promotional effect on the solubility of CO2”. 71 Received Date: January 5, 2010 Accepted Date: January 27, 2010