Molecular understanding of interfacial polarization and its effect on ionic liquid hydrogen bonds

doi:10.11949/0438-1157.20230489

Abstract

Abstract:

The structure and behavior of ionic liquid (IL) at the electrode interface have an important impact on their practical chemical applications such as supercapacitors and immobilized catalysts. In these applications, the interactions between IL with solid surfaces are common. Understanding the effect of the interfacial interactions on hydrogen bonds of IL is crucial for their practical chemical applications. However, due to the diversity of IL and solid surfaces, as well as the coexistence and coupling of many types of interactions on the surface, the effects of interfacial polarization on hydrogen bonds and the quantitative relationship between them are not clear. In this work, the first-principles calculations were performed to investigate the molecular mechanism of interfacial polarization and its influence on the hydrogen bonds for seven imidazole-based IL at the two-dimensional (2D) solid surfaces. The 1-ethyl-3-methylimidazolium (Emim]⁺) was selected as the cation for the study, used as electrolytes widely owing to their excellent conductivity, quasi-planar structure, and enhanced electrochemical properties. It was paired with seven anions, including [Cl]^-, [Br]^-, thiocyanate ([SCN]^-), dicyanamide ([DCA]^-), dicyanoboranuide ([B(H₂CN₂)]^-), tricyanomethanide ([TCM]^-) and tetracyanoborate ([TCB]^-). The typical three 2D materials, including graphene (Gra), boron nitride (BN), and molybdenum disulfide (MoS₂) were chosen as the solid surfaces. The results showed that charge transfer and orbital interaction would occur when ionic liquids are adsorbed on these three 2D surfaces, which further resulted in significant interfacial polarization. Its strength also increased with the decrease of the adsorption energy of IL on the 2D surfaces. Moreover, the change of IL hydrogen bonds on surfaces was described by the bond length, bond angle, bond order, and bond energy. It was found that the interfacial polarization would weaken the strength of HBs, where the strength of hydrogen bonds reduced notably as the IL lost more charge. And the infrared spectra of IL on solid surfaces were also calculated to confirm the evolution of hydrogen bonds with the interfacial polarization. Finally, the canonical correlation analysis was applied to construct a correlating model involving different factors related to the hydrogen bonds at the solid surface, proving that interface polarization was negatively correlated with the hydrogen bond strength. These quantitative results on the hydrogen bonds of IL can not only help understand the molecular mechanism of IL-solid interface but also be beneficial for the rational design of IL toward high-performance applications.

Key words: ionic liquid, green chemical engineering, interface interaction, 2D materials, hydrogen bond

摘要：

离子液体在电极界面处的结构及行为对其在超级电容器、固载催化剂等实际化工应用中有重要的影响。采用第一性原理计算结合理论分析研究了7种咪唑类离子液体在常见二维固体石墨烯、氮化硼、二硫化钼表面极化的分子机理及其对离子液体氢键的微观作用机制。结果表明，当离子液体在这三种二维表面吸附时会发生电荷转移和轨道相互作用，导致了显著的表面极化作用，且吸附能和电荷转移数值越大，表面极化作用越强。进一步分析了二维表面离子液体氢键的键长、键角、键序和键能，发现表面极化作用会显著削弱离子液体氢键。对于不同离子液体红外光谱的计算结果也验证了氢键被削弱的趋势。最后，通过SPSS软件对表面极化作用和离子液体氢键强度间的关系进行了定量解析，发现表面极化作用与氢键强度呈负相关关系。本文关于离子液体氢键的定量分析不仅有助于理解离子液体-固体表面作用的分子机理，而且可为离子液体在实际化工过程的应用提供理论支撑。

关键词: 离子液体, 绿色化工, 界面作用, 二维材料, 氢键

CLC Number:

TQ 021

Junfeng LU, Huaiyu SUN, Yanlei WANG, Hongyan HE. Molecular understanding of interfacial polarization and its effect on ionic liquid hydrogen bonds[J]. CIESC Journal, 2023, 74(9): 3665-3680.

陆俊凤, 孙怀宇, 王艳磊, 何宏艳. 离子液体界面极化及其调控氢键性质的分子机理[J]. 化工学报, 2023, 74(9): 3665-3680.

Figures/Tables 16

Fig.1 (a) The structures of seven anions and the cation of [Emim]+；(b) The stable adsorption models of [Emim][SCN] on Gra

Fig.2 The analysis of polarization between the IL and 2D interfaces

Fig.3 The analysis of HB in IL

Fig.4 The IR analysis of IL

Fig.5 The evolutions of the interaction between the IL and the Gra surface with the Dca-sub

Fig.6 The analysis of the relationship between the strength of hydrogen bond and the polarization interaction with the SPSS

Fig.A1 The different structures of [Emin][Br] and the corresponding energy

Fig.A2 The theory of atoms in molecules (AIM) graphs of IL [The lines connecting the atoms are the bond paths in the electron density distribution, and the small dots represent the bond critical points (BCP), while their corresponding energy is listed at the bottom of IL and the minimal energy values are marked in gray, others small dots represent the positions of ring critical points (RCP)]

Fig.A3 The electron localization function (ELF) of isolated IL and IL on 2D interfaces

Table A2 The charge of Gra， cation （CA）， and anion （AN） in IL（The negative values represent gaining charge）

Item	Charge/e
Item	[Emim][Cl]	[Emim][Br]	[Emim][SCN]	[Emim][DCA]	[Emim][B(H₂CN₂)]	[Emim][TCM]	[Emim][TCB]
Gra	-0.03	-0.03	-0.03	-0.02	-0.01	-0.02	-0.01
CA	0.77	0.78	0.80	0.83	0.83	0.85	0.86
AN	-0.73	-0.74	-0.78	-0.81	-0.82	-0.83	-0.85
IL	0.03	0.03	0.03	0.02	0.01	0.02	0.01

Table A3 The charge of BN， cation （CA）， and anion （AN） in IL（The negative values represent gaining charge）

Item	Charge/e
Item	[Emim][Cl]	[Emim][Br]	[Emim][SCN]	[Emim][DCA]	[Emim][B(H₂CN₂)]	[Emim][TCM]	[Emim][TCB]
BN	-0.06	-0.06	-0.06	-0.05	-0.05	-0.06	-0.05
CA	0.78	0.79	0.82	0.85	0.85	0.87	0.88
AN	-0.72	-0.73	-0.76	-0.80	-0.80	-0.81	-0.82
IL	0.06	0.06	0.06	0.05	0.05	0.06	0.05

Table A4 The charge of MoS2， cation （CA）， and anion （AN） in IL（The negative values represent gaining charge）

Item	Charge/e
Item	[Emim][Cl]	[Emim][Br]	[Emim][SCN]	[Emim][DCA]	[Emim][B(H₂CN₂)]	[Emim][TCM]	[Emim][TCB]
MoS₂	-0.27	-0.31	-0.21	-0.13	-0.12	-0.14	-0.11
CA	0.85	0.86	0.86	0.88	0.88	0.90	0.91
AN	-0.58	-0.55	-0.65	-0.75	-0.77	-0.76	-0.80
IL	0.27	0.31	0.21	0.13	0.12	0.14	0.11

Table A5 The charge of Gra， cation （CA）， anion （AN）， and IL under different Dca-sub（The negative values represent gaining charge）

IL	D_ca-sub/Å	Charge/e
IL	D_ca-sub/Å	Gra	CA	AN	IL
[Emim][Cl]	2.376	-0.277	0.656	-0.379	0.277
	3.376	-0.031	0.751	-0.720	0.031
	4.376	-0.023	0.773	-0.749	0.023
	5.376	-0.010	0.768	-0.758	0.010
[Emim][Br]	2.248	-0.188	0.679	-0.491	0.188
	3.047	0.001	0.725	-0.726	-0.001
	4.046	-0.021	0.783	-0.762	0.021
	5.045	-0.011	0.784	-0.773	0.011
[Emim][SCN]	2.311	-0.198	0.678	-0.481	0.198
	3.311	-0.020	0.787	-0.766	0.020
	4.311	-0.018	0.827	-0.808	0.018
	5.311	-0.007	0.828	-0.821	0.007
[Emim][DCA]	2.332	-0.026	0.702	-0.676	0.026
	3.530	-0.018	0.829	-0.811	0.018
	4.030	-0.017	0.847	-0.829	0.017
	5.030	-0.007	0.846	-0.838	0.007
[Emim][B(H₂CN₂)]	2.310	0.057	0.707	-0.764	-0.057
	2.810	0.057	0.746	-0.803	-0.057
	3.810	-0.014	0.839	-0.825	0.014
	4.810	-0.009	0.852	-0.843	0.009
[Emim][TCM]	2.365	0.019	-0.727	0.708	-0.019
	3.165	-0.012	-0.808	0.819	0.012
	4.165	-0.016	-0.854	0.870	0.016
	5.165	-0.006	-0.864	0.870	0.006
[Emim][TCB]	2.152	-0.065	0.744	-0.680	0.065
	2.652	0.070	0.747	-0.817	-0.070
	3.952	-0.016	0.873	-0.857	0.016
	4.952	-0.008	0.885	-0.877	0.008

Fig.A4 The AIM analysis of ILs at different distances from the surface [The unit of E is kcal/mol; The specific values of Dca-sub1 to Dca-sub4 are listed in Table A5; The lines connecting the atoms are the bond paths in the electron density distribution, and the numbered dots represent the bond critical points (BCPs), while their corresponding energies are listed at the bottom of IL, others small dots represent the positions of ring critical points (RCPs)]

Table A1 The charge of the cation （CA） and anion （AN） in isolated IL（The negative values represent gaining charge）

Item	Charge/e
Item	[Emim][Cl]	[Emim][Br]	[Emim][SCN]	[Emim][DCA]	[Emim][B(H₂CN₂)]	[Emim][TCM]		[Emim][TCB]
CA	0.76	0.77	0.81	0.84	0.84	0.87	0.88
AN	-0.76	-0.77	-0.81	-0.84	-0.84	-0.87	-0.88

Fig.A5 The infrared spectra of [Emim][SCN] with different Dca-sub

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