Partition coefficients of biological compounds between ionic liquids and water. their prediction through the solvation parameter model

J. M.PADRÓ; R. PELLEGRINO VIDAL; G. DÍAZ; A. PONZINIBBIO; M. RETA

Florianópolis

Congreso; XIV Congreso Latinoamericano de Cromatografía y Técnicas Relacionadas (COLACRO); 2012

The room-temperature ionic liquids (RTILs) are organic salts formed by organic cations (imidazolium, pyridinium, pirrolidinium, phosphonium) asymmetrically substituted by alkyl groups and inorganic anions such as chloride, hexafluorophosphate (PF6-), tetrafluoroborate (BF4-), among others. The RTILs are considered as potential ?green solvents? due to the better compatibility with the environment as compared with the typical organic solvents, to its low vapor pressures, very low flammability and acceptable viscosity. Together with its favorable solvation properties towards organic and inorganic analytes, the RTILs are actually very used in analytical chemistry, specifically, in sample preparation procedures. The most common cations used in liquid-liquid microextractions with RTILs are N-alkylpyridinium and N,N´-dialkylimidazolium. The phosphonium-based cations [PR1R2R3R4]+ have some advantages as compared with the nitrogen-based cations previously mentioned, such as better solvation properties, higher immiscibility with water and, some of them, lower densities. This property can be an advantage if automation wants to be applied since no necessity of phase separation is required [1]. In this work, the partition coefficients between different RTILs and water, PIL/w, for different probe molecules (training set) as well as for compounds of biological interest (test set) were accurately measured. The trihexyl(tetradecyl)phosphonium chloride, ?Cyphos 101?, tetradecyl(trihexyl)phosphonium bromide, ?Cyphos 102?, trihexyl(tetradecyl)phosphonium dicyanamide, ?Cyphos 105?, (trihexyl)tetradecylphosphonium bis(trifluoromethylsulfonyl)imide, ?Cyphos 109?, 1-octyl-3-methylimidazolium hexafluorophosphate, [OMIM][PF6], 1-octylpyridinium tetrafluoroborate, [OPYR][BF4] were used. [OMIM][PF6] and [OPYR][BF4] were synthesized in our laboratory [2]. In order to elucidate the chemical interactions involved in the partitioning process, the ?salvation parameter model? was used. This model considers a lineal relationship (through a multiparametric equation) between the log PIL/w and different solute-solvent interactions, each one characterized by a solute parameter or descriptor which measure polarity-polarizability, hydrogen-bond donor, hydrogen-bond?acceptor, dispersive interactions and cohesivity. By using the training set, the obtained multiparametric coefficients were used to predict the PIL/w values for compounds of the test set. Solute descriptors for the test set were obtained from the literature, when available, or calculated through commercial software. An excellent agreement between calculated and experimental log PIL/w values was obtained, which demonstrate that the obtained multiparametric equations are robust and allow predicting partitioning for any organic molecule in the biphasic systems studied. Cavity formation and hydrogen-bond acceptor interactions domains the partition process. The former term is favorable to better extractions and the latter disfavor extractions towards the RTIL phase. Polarity, hydrogen-bond acidity and basicity for the different studied RTILs were measured by using solvatochromic probes (ET30..etc..) in order to interpret the regression coefficients. It is observed that polarity is similar for the different RTILs but hydrogen-bond acidity is higher for the imidazolium-based RTILs, which can be attributed to the acidic proton of the imidazolium ring. Also, the basicity of the phosphonium-based RTILs is much higher than the imidazolium counterparts, which can be attributed to the anion hydrogen-bond acceptor capability. This characteristic is very favorable to extract protonable analytes.