The greenhouse effect represents one of the most significant environmental challenges currently facing humanity. The burning of fossil fuel is the main cause of greenhouse gas emissions. Carbon dioxide (CO2) is a byproduct during the combustion of gasoline and coal. It can be converted into valuable materials, such as propylene carbonate [1]. Ionic liquids (ILs) are a type of liquid salt composed of ions with a melting point below 100 °C. They exhibit high thermal stability, extremely low vapor pressure, high gas solubility, and good structure adjustability. These properties make ILs have significant potential for CO2 absorption [[2], [3], [4]]. Furthermore, the applications of ILs in the biotechnology [5], catalysis [6], and chemical industry [7] benefit from the diversity of their physicochemical properties. However, conventional pyridine-based and imidazole-based ILs have potential toxicity and poor biodegradability [8,9]. In order to overcome these shortcomings, choline amino acid ionic liquids ([Ch][AA]) have been proposed as an environmentally friendly alternative. Those ionic liquids are composed of choline cation ([Ch]+) and amino acid anions ([AA]-), and are characterized by their non-toxic and renewable properties [[10], [11], [12]].

The physicochemical properties of [Ch][AA] are closely related to its ionic structure. Different anions combined with [Ch]+ to form different [Ch][AA] and network structures, which directly affect their physicochemical properties [13]. Therefore, it is crucial to reveal the nanostructure in [Ch][AA] and the relationship between its structure and properties, which is essential for effectively promoting its applications and development. Density functional theory (DFT) calculations and molecular dynamics (MD) simulations are effective tools for studying the behavior of ILs at the nanoscale [14]. It has been shown that the chain length of anion has a significant impact on the interactions and structure of ILs [15]. Del et al. [16] investigated the molecular structure of [Ch][AA] by using DFT and found that the polarity of [Ch][AA] formed by different anions affects its hydrophilicity, and the thermodynamic properties of the ionic liquids are affected by the lengths of their anionic side chains. Arashnezhad et al. [17] used DFT calculations to analyze the interactions of three ammonium-based ILs. They found that as the cationic chain length increases, the interaction energies between cations and anions decrease, while the contribution of dispersive interactions increases significantly. Moosavi et al. [18] investigated the microscopic interaction mechanism of [Ch][AA] through DFT calculations and MD simulations. Their study revealed that the carboxy oxygen (Oa) atom on [AA]- forms hydrogen bonds with the hydroxyl hydrogen (Hc1) atom and methyl hydrogen (Hc2) atom of [Ch]+. Khorrami et al. [19] found that a hydrogen bond was formed between the methyl Hc2 atom and the hydroxyl oxygen (Oc) atom of the adjacent cations, which stabilizes the structural arrangement of cations in [Ch][AA]. Zheng et al. [20] analyzed the microstructures and interactions of six [Ch][AA] using DFT calculations and MD simulations, and found that both Oc-Hc1···Oa and Cc-Hc2···Oa hydrogen bond interactions increase with the increase of anion alkyl chain length, with the Oc-Hc1···Oa hydrogen bond playing a major role in ILs. Additionally, they observed that as the anionic chain length increases, the aggregation phenomenon of anions intensifies. Furthermore, Donne et al. [21] pointed out that the aggregation phenomenon of anions in ILs is mainly caused by the strong hydrogen bond interactions between amino acid anions, which contain both hydrogen bond donors and acceptors. These hydrogen bond interactions form dimers among the anions, thereby enhancing the aggregation between the anions. Therefore, an in-depth understanding of the relationship between the interactions and structures of [Ch][AA] is essential for promoting its applications in various fields.

In addition, temperature affects the interparticle interactions and kinetic properties of ILs [22]. Jiang et al. [23] found that with the increases in temperature, the diffusion coefficients of both cations and anions increase, and the conductivity of the system also increases, as demonstrated by MD simulations. Matsumiya et al. [24] found through MD simulations that the increase in temperature promotes the mobility of ions, and weakens the interaction strength between cations and anions, leading to a decrease in the viscosity of the system. In ILs, hydrogen bond and van der Waals (vdW) forces mainly determine the mutual attractions between cations and anions. Therefore, it is essential to investigate the hydrogen bond interactions in ILs. Bailey et al. [25] found that the hydrogen bond interactions between cations and anions are the primary cause for the high viscosity of ILs. As the temperature increases, the hydrogen bond interactions between cations and anions weaken, leading to a decrease in the viscosity. Laloš et al. [26] pointed out that increasing the temperature weakens the interactions between [P2225]+ and [TFSA]-, thereby reducing the viscosity of ILs. This indicates that temperature has a substantial influence on the inter-particle interactions and kinetic properties, which subsequently affects macroscopic properties such as viscosity and conductivity. Therefore, it is essential to investigate the influence of temperature on the inter-particle interactions and kinetic properties of ILs, which is crucial for understanding the physical and chemical properties of ILs. In addition, Noorani et al. [27] found experimentally that [Ch][AA] with different anion chain lengths and different temperatures have different CO2 absorption properties.

Although some progress has been made in the study of the structural properties of [Ch][AA] [20,28], the impact of temperature on the interactions and kinetic properties of [Ch][AA] has not been systematically studied. In addition, it was found that the operating temperature of CO2 capture in absorption towers is usually in the range of 40–60 °C [29]. The electrochemical applications of ionic liquid electrolytes often reach operating temperatures of 60–80 °C [30]. Therefore, MD simulations of [Ch][AA] were carried out at 298.15 K–358.15 K, which can help to realize the effective connection and translation from theoretical studies to practical applications.

Furthermore, the diversity of anions in [Ch][AA] leads to significant differences in their structure-properties. These differences may affect their behavior in chemical reactions and their applications in biomedicine, chemistry, materials science and other fields. Consequently, studying the [Ch][AA] with different anions is crucial for understanding their behavior and developing novel applications. Moosavi et al. [18] investigated the structures and kinetic properties of [Ch][Ala], [Ch][β-Ala] and [Ch][Phe] by DFT calculations and MD simulations. Zheng et al. [20] used the same methodology to investigate structures and kinetic properties of [Ch][Gly], [Ch][Ala], [Ch][But], [Ch][Nva], and [Ch][NIe]. However, fewer studies have been conducted on [Ch][Ser] and [Ch][Val], which play important roles in CO2 absorption and drug delivery [[31], [32], [33]]. Therefore, DFT calculations and MD simulations of choline serine IL ([Ch][Ser]) and choline valine IL ([Ch][Val]) were investigated. Choline glycine ionic liquid IL ([Ch][Gly]), choline L-alanine IL ([Ch][Ala]) and [Ch][Val] are ILs with sequentially increasing anionic alkyl chain lengths, which allows the effect of alkyl chain length on the internal interactions of [Ch][AA] liquids to be explored. Compared to previous studies, the effect of hydroxyl groups on the hydrogen bonding network of [Ch][AA] was revealed by introducing hydroxyl groups and comparing the differences between [Ch][Ala] and [Ch][Ser].

In this research, DFT calculations and MD simulations were combined to comprehensively analyze the influence of anionic chain length and temperature on the nanostructures, kinetic properties, and interactions of [Ch][AA] ([Ch][Gly]), [Ch][Ala], [Ch][Ser], and [Ch][Val]. The interaction between [Ch]+ and [AA]- was characterized by DFT calculations. Furthermore, MD simulations of [Ch][AA] were conducted at 298.15 K–358.15 K (with an interval of 20 K) to investigate the complex coupling between temperature, nanostructure, and local interactions. These findings provide insights into the influence of anionic chain length and temperature on the “structure-property” relationships of [Ch][AA], and provide important guidance for the applications of [Ch][AA] in industrial practice.