Study On Physicochemical Properties,applications And Hydrogen Bonding Of Protic Ionic Liquids Comprising2-ethylhexylethylenediaminium Cation And Acylalaninate Anion

Ionic liquids(ILs)are ionic compounds with melting points below 100?,comprising organic cations coupled with inorganic or organic anion.They have unique physicochemical properties,e.g.,low melting point,low volatility,wide liquid range,electrochemical window,high thermal stability,good electrical conductivity,good light-transmittance and high refractive index,etc.In particular,amino acid ionic liquids(AA–ILs)have attracted wide attention from researchers since the raw materials for the syntheses are natural amino acids and their derivatives.In our study,the amino acid protic ionic liquids(AA-PILs)composed of 2-N-ethylhexylethylenediaminium as cation and acylalaninate as anion were synthesized and the physicochemical properties(e.g.,density,viscosity,and conductivity)were investigated.The hydrogen-bonding between AA and PILs,water and PILs were also studied here.In order to extend the studies of AA-PILs to application fields,studied on the CO₂absorption to AA-PIL/water binary system and on the solubilities of Cu Cl₂,Co Cl₂and Ni Cl₂in DL-[HEt Hex][Hexala]were studied.This research is composed of the following four parts.1?Physicochemical properties of AA-PILs and solubility of transition metal ions in AA-PILs:In order to study the effect of chirality on physicochemical properties of AA-PILs,D and L-[HEt Hex][Hexala]were synthesized according to the method of DL-[HEt Hex][Hexala]in our previous work.D,L and DL-[HEt Hex][Hexala]are easily soluble in polar solvents(e.g.,water,methanol,acetone,n-butanol,ethanol,and propanol),and non-polar solvents(e.g.,benzene,ether,and chloroform).Therefore,AA-PILs have moderate hydrophilic–lipophilic balance.Respectively,For D,L and DL-[HEt Hex][Hexala],the decomposition temperatures were 174.6,185.2 and 155.2?,and the glass transition temperatures were-35.6,-37.2 and-34.2?.In addition,the density,viscosity and conductivity of D,L and DL-[HEt Hex][Hexala]were measured in the range of303.15-353.15 K.The results showed that the density and viscosity decrease,while conductivity increases with increasing the temperature.At the same temperature,there are no significant difference in the density,viscosity and conductivity between D,L and DL-[HEt Hex][Hexala].For example,at 308.15 K,the density,viscosity and conductivity are 1.0217±0.0009 g·cm^-3,1390±90 m Pa·s,10.1±2.6?S cm^-1,respectively for the three stereoisomers.The chelate-ring moiety in both the cationic and anionic units(ethylenediaminium group for the cation and amino-carboxylic group for the anion)is advantageous to encapsulate transition metal ions and it is expected to apply to analytical chemistry.The solubilities of Cu Cl₂,Co Cl₂and Ni Cl₂to[HEt Hex][Hexala]were investigated using the inductively coupled plasma emission spectrometer(ICP-7000).The solubilities were 74.5,65.2,and 17.6 mmol?kg^-1,for Cu Cl₂,Co Cl₂and Ni Cl_2,respectively.2?Study on absorption of CO₂using[HEt Hex][Octala]/water systems:[HEt Hex][Octala]/water systems with different molar ratios(1:10,1:15,1:20,and1:25)were prepared and used for the CO₂absorption.p H value,dynamic viscosity,density and electric conductivity of[HEt Hex][Octala]/water systems were measured before and after the CO₂absorption.By the CO₂absorption,the p H value and electric conductivity decrease while viscosity and density increase.It means that[HEt Hex][Octala]has a good absorption capacity for CO₂.3?Hydrogen-bonding between the[HEt Hex]cation and the[Acylala]anion:In order to reveal the physicochemical properties based on the microstructure of[HEt Hex][Acylala](Acyl=butanoyl,octanoyl,decanoyl and dodecanoyl),the hydrogen-bonding between the anion and the cation of[HEt Hex][Acylala]were studied by the density functional theory(DFT).The five stable possible structures(S1-S5)were obtained at M06-2X/6-311G(d,p)level.The results of IR spectra indicate that vibration spectra of N-H bond disappeared,and the stronger vibration spectra of O-H bond appeared in the region of 2400?2815 cm^-1.Since,stronger O-H…N type hydrogen bond is formed between[HEt Hex]molecule and[Acylala]molecules.The second-order interaction energies and the hydrogen-bonding energy are in the range of 43.58-57.83 kcal?mol^-1and17.9-29.8 kcal?mol^-1,respectively.Therefore,the hydrogen bond between[HEt Hex]and[Acylala]is classified as the strong hydrogen bond.When the same cation(2-N-ethylhexylethylenediamine)was used,there was no significant change for the strength of the hydrogen-bonding with increasing the chain length of anion(from butyl to dodecyl).The intermolecular hydrogen-bonding energies for[HEt Hex][Acylala]were in the range of 22.9-29.8 kcal?mol^-1.4?Hydrogen-bonding between[HEt Hex][Acylala]and water molecules[HEt Hex][Acylala](Acyl=butanoyl,hexanoyl)is hydrophilic,and their physicochemical properties such as density,viscosity and conductivity were largely influenced by their water contents.Therefore,in order to study the relationship between the physicochemical properties and their micro structures,the hydrogen bonding between the water molecules and[HEt Hex][Acylala]was investigated.The optimized configurations of[HEt Hex][Butala]-n H₂O(n=1,2,6)A1-A10 and[HEt Hex][Hexala]-n H₂O(n=1,2,6)S1-S11 were obtained using the density functional theory(DFT)at M06-2X/6-311G(d,p)level.The results of the interaction energy,vibration spectra,the second-order perturbation and electron density show that N—H…O,O—H…N,and O—H…O type hydrogen bond was formed between[HEt Hex][Acylala]and n H₂O.The hydrogen bonding between AA-PILs and water molecules increase with increasing the number of water molecules.The hydrogen-bonding energies E_HB(sum)were in the range of 7.3–83.3 kcal?mol^-1.Therefore,H-bonding between[HEt Hex]and[Acylala]is regarded as the strong hydrogen bond.