Study On The Hydrogen-bonding Interactions Between Two Components Of Choline-Based Deep Eutectic Solvent

The use of a large amount of volatile organic solvents in the traditional chemical industry has caused serious pollution to the environment.This demands people to search for and develop green,non-toxic solvents instead of traditional organic solvents.Because deep eutectic solvent(DES)has low melting point,low vapor pressure,strong dissolving ability,excellent conductive,non-toxic or low toxicity and low price,it is considered to be a good substitute for traditional organic solvents.Due to its good properties,DES shows a good application prospect in many fields of chemistry and chemical engineering.However,in these applications,the study on the interaction between the DES components is very scarce,which restricts the understanding of physical-chemical properties and microstructure of DES for peoples,and further limits the design and development of new kinds of DES.Therefore,the investigation of interactions between two components of choline-based deep eutectic solvents is very important.As a part of the project supported by the National Natural Science Foundation of China(No.21573060),the classical choline chloride(ChCl)and different aliphatic alcohols,carboxylic acids and phenolic compounds were selected to the study of their hydrogen bonding(HB)interaction through many different methods.The main contents are as follows:1.The hydrogen-bonding interactions between ChCl and alcohols(1,2-butanediol,1,3-butanediol,1,4-butanediol,2,3-butanediol,ethylene glycol,1,3-propanediol,1,4-butanediol,1,5-pentadiol,1,6-hexanediol)were studied by FTIR,FIR,1H NMR,35 Cl NMR and quantum chemistry calculation at 293 K.The effect of the alkyl chain length of alcohols and mole ratios onthe hydrogen-bonding interaction between alcohols and Ch Cl was analyzed,and the relationship of molecular interactions with its properties was discussed.It was found that the hydrogen-bonding interaction is present between H atom of hydroxyl group and Cl atom in Ch Cl.This HB interaction decrease with the carbon number between two hydroxyl groups increasing,which leads to the increase of surface tension but the decrease of polarity of DES.In addition,the ability of alcohols to weaken the HB interaction among ChCl moleculars become weak with the increase of content of ChCl in mixtures,and the HB interaction among the same kinds of compounds is enhanced.2.The DESs composed of ChCl with oxalic acid(malonic acid,succinic acid,glutaric acid and adipic acid)were chosen as the research object.FTIR,FIR,1H NMR and 35 Cl NMR have been used to investigate the HB interactions between ChCl and carboxylic acid at 293 K.The influence of alkyl chain length in carboxylic acid on the HB interaction between carboxylic acid and ChCl was analyzed.The results have shown that there is HB interaction between H atom of carboxyl group and Cl atom in Ch Cl,which is stronger than that among the same kinds of compounds.Furthermore,this HB interaction decrease with the carbon number between two carboxyl groups increasing,leading to the increase of melting point of DES.3.The HB interactions of ChCl with phenol,o-cresol,m-cresol or p-cresol were investigated by FTIR,FIR,1H NMR and 35 Cl NMR at 293 K.The effect of the location of hydroxyl groups relative to methyl group in phenolic compounds on the HB interaction between phenolic compounds and ChCl was discussed.It was found that the formation capability of hydrogen bonding of phenolic compounds with ChCl was increased in the sequence: phenol> p-cresol>m-cresol> o-cresol.This order is then same to that of acid strength for four kinds of phenoliccompounds,but inverse to the order of melting points of DES.These results suggest that the stronger the HB interaction between phenolic compounds and ChCl is,the more effectively the damage of HB interaction between the same molecules is,which causes the melting points of DES decrease.When the molar ratio of phenolic compounds to ChCl is 2:1,the melting point of DES is lowest.