Tuning Solvent Miscibility: A Fundamental Assessment on the Example of Induced Methanol/n-Dodecane Phase Separation

Abstract

In this work, we assess the fundamental aspects of mutual miscibility of solvents by studying the mixing of two potential candidates, methanol and n -dodecane, for nonaqueous solvent extraction. To do so, 1 H NMR spectroscopy and molecular dynamics simulations are used jointly. The NMR spectra show that good phase separation can be obtained by adding LiCl and that the addition of a popular extractant (tri- n -butyl phosphate) yields the opposite effect. It is also demonstrated that in a specific case the poor phase separation is not due to the migration of n -dodecane into the more polar phase, but due to the transfer of the extractant into it, which is especially relevant when considering industrial applications of solvent extraction. With the aid of molecular dynamics simulations, explanations of this behavior are given. Specifically, an increase of all hydrogen-bond lifetimes is found to be consequent to the addition of LiCl which implies an indirect influence on the methanol liquid structure, by favoring a stronger hydrogen-bond network. Therefore, we found that better phase separation is not directly due to the presence of LiCl, but due to the “hardening” of the hydrogen-bond network.

Understanding the Complex Surface Interplay for Extraction: A Molecular Dynamics Study

“…Indeed, our previous work suggests that the increased phase separation is not directly related to the nature of LiCl, but rather stems from as trengthening of the hydrogen-bond network in the methanolp hase. [19] Thus, we expect similar results foro ther alkali metal halides. For industrial application of solvente xtraction,o ne of the main requirements is neat phase separation between the solvents.…”

“…Its presence hinders the migration of DD molecules into MeOH,i nl ine with what we found in our previous work. [19] The effect of TBP was less strong;n evertheless, we point out that, as expected, for higher concentrations of TBP the migration seems to be facilitated, as shown by the barso nt he right sideo fb oth panels. Hence, with regardt ot he migration of DD into MeOH,L iCl might againb e used to decreasem utual miscibility,w hereas the amount of TBP should be carefully evaluated.…”

“…The presence of LiCl, which slows down the dynamics of the MeOH phase (as described in our previous work), [19] hindered the diffusion of TBP molecules into the MeOH phase and increasedt he concentration of TBP at the interface. Also the orientation of interfacialT BP was affected, especially for higher concentrations of TBP.I nf act, with LiCl, when the TBP concentration was increased, TBP migration into MeOH was hindered, and this constraint led to ar earrangemento ft he TBP layer,i n which TBP molecules oriented towards each other to maximize the interaction of the polar and nonpolar moieties.…”

“…[17,18] Although thesep rocesses can be carriedo ut easily on al aboratory scale, on an industrial scale they may be challenging and several conditions should be fulfilled. [5,19] Amongt hose,i n solvent extraction the transition region between the immiscible liquid phases is of paramount importance,a si tc an either facilitateo rh indert he migration of the target compound betweenthe phases and even increase or decreasethe selectivity. For these reasons, overthe years several studies have been undertaken to characterize the interface between solvents for classical solvente xtraction.…”

“…Previously,w einvestigated the mixture of methanol( MeOH)a nd n-dodecane (DD) in order to discuss phase separation, and studied how the system can be altered and the leadingp rinciples for optimizing the system. [19] In the present work, we employed the same mixture to characterize the interface between two immiscible molecular solvents by means of molecular dynamics simulations.W ea ssess at a molecular level to what extent the solvents form distinct phases and study the structuralf eatures of the interface between them.I no rder to do so, we also added as urface-active ligand,n amely tri-n-butylp hosphate (TBP), and as alt, namely LiCl, which was shown to improvet he phase separation of these solvents, to the system.T his is af irst step towards the understanding of how the interface structure can influence solvent extractionp rocesses and therefore of how thesep rocesses can be improved.…”