Study On The Extraction Mechanism Of The Natural Deep Eutectic Solvents Based Extraction System In Pigment Separation

Liquid-liquid extraction is a widely used separation method in pigment separation,which has the advantages of simple construction,scalable operation and easy control.NADES-based pigment extraction and separation system（NADES-PESS）provides an environmentally friendly green separation method for liquid-liquid extraction of pigments.It has the advantages of low cost,simple synthesis,and scalable production,and has broad development prospects.NADES has excellent designability.According to the characteristics and properties of the separation target,the type or proportion of hydrogen bond donor（HBD）and hydrogen bond receptor（HBA）in NADES can be precisely adjusted to achieve efficient selective separation.However,some key issues are still unclear:（1）The construction of NADES-based PESS often requires a two-phase system,and there is a lack of quantitative selection methods for phase-forming solvents;（2）In the process of pigment extraction,the extraction mechanism of NADES-based PESS is still unclear,and it is difficult to obtain the key driving force of pigment extraction from the molecular and atomic levels of the apparent phenomenon in the experiment;（3）Due to the complexity and similarity of pigment structures,the classification of pigments by group or main structure is still limited,and there is a lack of quantitative indicators for the differences in pigment properties under different pigment structures;（4）The method of selecting the separation system for different kinds of pigments is not yet perfect;（5）There is still a lack of clear understanding of the mechanism of pigment structure on the NADES-PESS extraction process.In view of the above-mentioned key problems,combined with experiments,theoretical analysis and computer simulation,a method for quantitative selection of NADES phase-forming solvents was established,and the extraction mechanism of NADES on pigments was explored from multiple perspectives.The influence of the composition of NADES and the type of pigment on the extraction efficiency was deeply explored.The quantitative index of the difference of pigment properties corresponding to different pigment structures was established,and the regular model of the pigment structure and the partition coefficient was established.The main contents of this paper are as follows:（1）The construction of NADES-PESS is the premise of using NADES to extract and separate pigments.Selecting NADES with good extraction performance is the key to developing high-efficiency NADES-PESS.For pigments of different polarities,NADES with different hydrophilicity and hydrophobicity are designed.It is proposed to use the relative energy difference parameter（RED）to quantify the compatibility between NADES and solvents.And the RED value is used as a quantitative index for selecting a phase solvent.Through the index,the immiscible solvent can be easily and conveniently selected as the phase-forming solvent for the construction of NADES-PESS,which avoids the problem of a lot of experimental time and cost in selecting the phase-forming solvent by blind testing.（2）The research on extraction mechanism is the basis for developing new NADES-PESS and improving extraction efficiency.Taking triarylmethane（TPM）pigments as the research object and MC₁₂ NADES water two-phase system as the research system,a comprehensive experimental study and in-depth mechanism analysis of the extraction performance of NADES-PESS were carried out.It is proposed to use solvation free energy(ΔE_SFE),mixing energy(E_mix),and Hansen solubility parameters(δ_Hansen)to measure the difference in the interaction between pigments and two-phase components from the molecular level and from the perspective of thermodynamics.It is believed that the energy difference of theΔE_SFE of the pigment in the two phases is the main reason for the extraction process.It is proposed to use the E_mix value to represent the difficulty of the compatibility between pigments and two-phase components,and it is believed that a largerΔE_mix value is beneficial to the transfer of pigments in the two phases.It is proposed to use theδ_Hansen spatial distance value(Δδ_Hansen)to measure the compatibility between the pigment and the two-phase components.It is believed that the larger difference between theΔδ_Hansen value of the pigment and the two-phase components is thermodynamically beneficial to the extraction process.The difference of theΔδ_Hansen value of the pigment and the component can quantitatively express the difference of the partition of different pigments in the two-phase system.Based on density functional theory（DFT）,the differences in pigment properties under different pigment structures were quantified at the atomic level in terms of Mulliken charge and electrostatic potential distribution.It is believed that the difference in the area of hydrogen bond donor and acceptor regions and non-polar regions caused by the difference in charge distribution outside the pigment molecule under different pigment structures leads to the difference in the interaction,which is the main reason for the difference in the distribution of different pigment structures.（3）Based on the extraction mechanism of NADES,a method was established to predict the extraction capacity by theΔδ_Hansen value from the perspective of thermodynamics.From the perspective of solvation and theoretical analysis at the atomic level,the correctness of the method for predicting the extraction capacity based on theΔδ_Hansen value is verified,and the extraction and separation of Rhodamine pigments with similar structures verifies the accuracy and feasibility of the prediction method.（4）Selecting suitable NADES-PESS for different types of pigments is the core of pigment extraction and separation research.A classification strategy for establishing pigments based on theδ_Hansen value is proposed.Theδ_Hansen sphere of MC₁₂ NADES and water was used as boundaries to demarcate pigment classes.It is believed that the pigments in the intersection area ofδ_Hansen spheres have higher HRR values,which is conducive to the migration of pigments to the MC₁₂ NADES phase.NADES-PESS of smallerΔδ_Hansen with pigments and smallerΔδ_Hansenbetween its own two-phase components is thought to facilitated separation between this class of pigments.A proline-based NADES/water-alcohol two-phase system was constructed to achieve the extraction and separation of isoflavone pigments in the intersecting region ofδ_Hansenspheres,which verifies the feasibility of classifying pigment types byΔδ_Hansen value and constructing suitable NADES-PESS.（5）Establishing the quantitative relationship between pigment structure and distribution is a key task for designing efficient NADES-PESS.Using molecular dynamics to obtain two-dimensionalδ_MD(δ_{vd W}-δ_ES)instead of complex three-dimensionalδ_Hansen（δ_D-δ_P-δ_H）was used to quantify the differences in pigment properties under different pigment structures.The quantitative relationship between the molecular structure of the pigment and theδ_MD value is constructed,which solves the problem of a lot of molecular dynamics computing resources consumed in the screening work.It is proposed to use the proximity algorithm to obtain the correlation coefficient of theδ_{vd W} difference and theδ_ES difference in the calculation ofΔδ_MD,and improve the method of using theΔδ_MD value to analyze and predict the compatibility.TheΔδ_MD value was proposed to quantify the difference in the distribution of pigment structure to the extraction process.The regular model of pigment structure and partition coefficient was constructed.In this paper,NADES-PESS with high extraction efficiency was constructed for pigment structure by a quantitative method.The results are helpful to understand and master the distribution rule of pigment in NADES-PESS.It provides a new idea for the design of functional pigment extraction system and the separation of high value pigment.