Switchable hydrophilicity solvents in the microextraction context

The concept of switchable solvents (SS) was firstly introduced by Canter in 2006, in the framework of industrial processes. The initial idea was to introduce solvents which can switch between a polar and non-polar chemical form, aiming to reduce the number of organic solvent and toxic wastes generated during extraction procedures [1]. Recently, professor Jessop and co-workers expands the application field of this kind of solvents in order to develop cheaper and greener chemical processes.

Jessop proposed gaseous CO₂ as trigger reagent to switch between the two chemical forms of the SS (see equation 1), which is pretty interesting since it is cheap, easily available and non-toxic. Moreover, it can be easily introduced and removed from the solution, thus simplifying the "switching" process. Solvents presenting this behavior were named as "switchable hydrophilicity solvents" (SHS) as they are miscible with water in the presence of CO₂ while become immiscible when CO₂ is removed_. This removal can be simply achieved by bubbling the solution with N₂ or Ar.

NR₃ + H₂O + CO₂ ⟺NR₃H⁺ + HCO₃^- (Equation 1)

SHS were firstly used as substitute of hexane in soybean oil extraction. In this case, SHS avoid the traditional separation by distillation, minimizing ht environmental impact and reducing the overall economic and energy costs of the process by substitution of the distillation process [2]. Recently, SHS has been successfully employed for removing solvent from products such as algae oil in bio fuel production [3,4], bitumen [5] and high density polystyrene [6].

Several solvents showing this behavior had been identified, most of them amidines and tertiary amines [7,8]. It is also available a small group of Ionic Liquids (IL) which present a switchable behavior [9]. One of the IL shows exactly the opposite polarity change in presence of CO₂: tetrabutylphosphonium N-tri-fluoro-methanesulfonyl-leucine became hydrophobic when CO₂ is dissolved in an aqueous phase containing the IL.

Table 1 summarizes the main SHS employed and indicates two principal properties, namely the logarithm of the octanol/water portioning coefficient (log Kow) and the negative logarithm of the acid dissociation constant (pKa) [10].

Table 1. Main studied amines presenting switchable behavior
Compound	Ratio in water (v:v)	log Kowa	pKaH
Triethylamine	1:1	1.47	10.68
N,N-Dimethylbutylamine	1:1	1.60	10.02
N,N-ethylpiperidine	1:1	1.75	10.45
N-methyldipropylamine	1:1	1.96	10.40
N,N-dimethylcyclohexylamine	1:1	2.04	10.48
N-buthylpyrrolidine	1:1	2.15	10.36
N,N-Diethylbutylamine	1:1	2.37	10.51
N,N-Dimethylhexylamine	1:1	2.51	10.18
a Calculation based on ALOGPS 2.1 software (adapted from [10])

The ideal SHS should present a log Kow value in the range from 1.2 to 2.5. Solvents presenting lower values are too hydrophilic and generate monophasic system in its neutral form. Solvents with higher values generates a biphasic systems even in the presence of CO₂. The pKa values should be higher than 9.5. In addition to these requisites, it is desirable for a SHS to be a non-volatile solvent, generally with a high molecular weight. The main reason, is that the common way to eliminate CO₂ from the solution is by bubbling another gas (such as N₂ or Ar), and it could lead to partial/complete losses of SHS.

Our research group has recently proposed the adaptation of SHS as solvents in microextraction. In this case, we used N,N-dimethylcyclohexylamine (DMCA) [11] as solvent under a homogeneous liquid-liquid microextraction (HLLE) format. The miscibilitation of DMCA and water in a 1:1 v/v ratio is accomplished using dry ice. The resulting solution is injected in a defined volume of aqueous sample where the DMCA is completely solubilized. For phases´s separation, sodium hydroxide has been proposed instead of N₂ bubbling since the evaporation of SHS can have a dramatic effect when the solvent are used in the microliter range. After the extraction, DMCA with the target analytes is recovered for further analysis.

The SHS-HLLE procedure was evaluated by extracting benz[a]anthracene from water samples. Fluorescence detection was selected as instrumental technique to take advantage of the native photoluminescence of the target analyte. However, during the optimization process, we found that DMCA quenched benz[a]anthracene fluorescence. To solve this issue, a dilution (1:1) with acetic acid (HAc) was necessary (see Figure 1). The dilution avoided quenching and produced a 35 % increase in the fluorescence intensity compared to that obtained in pure methanol.

 

Figure 1. Fluorescence of benz[a]anthracene in DMCA and DMCA:HAc media

We suggest the reading of this article to the blog´s followers (Link to the article). In the original manuscript, they will find the complete optimization as well as the analytical characterization of the proposal.

REFERENCES

[1] N. Canter Tribol. and Lubric. Tech. 62 (2006) 15-16.

[2] P. G. Jessop, L. Phan, A. Carrier, S. Robinson, C. J. Dürr, J. R. Harjani, Green Chem. 12 (2010) 809–814.

[3] A. R. Boyd, P. Champagne, P. J. McGinn, K. M. MacDougall, J. E. Melanson, P. G. Jessop, Bioresour. Technol. 118 (2012) 628–632.

[4] C. Samori, D. L. Barreiro, R. Vet, L. Pezzolesi, D. W. F. Brilman, P. Galletti, E. Tagliavini, Green Chem.15 (2013) 353–356.

[5] A. Holland, D. Wechsler, A. Patel, B. M. Molloy, A. R. Boyd, P. G. Jessop, Can. J. Chem. 90 (2012) 805–810.

[6] P. G. Jessop, L. Kozycz, Z. G. Rahami, D. Scheonmakers, A. R. Boyd, D. Wechsler, A. M. Holland, Green Chem. 13 (2011) 619–623.

[7] P. G. Jessop, L. Phan, A. Carrier, S. Robinson, C. J. Dürr, J. R. Harjani, Green Chem. 12 (2010) 809–814.

[8] P. G. Jessop, L. Kozycz, Z. G. Rahami, D. Schoenmakers, A. R. Boyd, D. Wechsler, A. M. Holland, Green Chem. 13 (2011) 619–623.

[9] Y. Kohno, H. Arai and H. Ohno, Chem. Comm. 47 (2011) 4772–4774.

[10] J. R. Vanderveen, J. Durelle, P. G. Jessop, Green Chem. 16 (2013) 1187-1197

[11] G. Lasarte-Aragonés, R. Lucena, S. Cárdenas, M. Valcárcel, Talanta 131 (2015) 645–649.

Guillermo Lasarte-Aragones studied Biochemistry in the University of Córdoba (Spain) until 2007, getting later on a Master degree in "Molecular, cellular an genetic biotechnology". In his early research, Guillermo worked at mitochondrial redoxins and molecular defenses against oxidative stress using Saccharomyces cerevisiae as model organism. Nowadays he is developing his PhD Thesis under the supervision of Prof. Valcárcel, Cárdenas and Lucena at the same University. His work is focused on the innovative uses of carbon dioxide on the development of novel microextraction techniques.

Guillermo on twitter: https://twitter.com/LasarteG 

FQM-215 Research group: https://www.uco.es/grupos/FQM-215/