# Intermolecular Interactions in Mixed Choline Acetate and Maleic Acid Systems
## 1 Introduction  
In recent years, the development of alternative and innovative solvents has gained significant attention to achieve more sustainable chemical processes; these solvents include ionic liquids (ILs) and deep eutectic solvents (DESs) [1; 2; 3; 4]. Despite their structural and compositional differences, both classes of solvents exhibit a range of unique and desirable physicochemical properties. These include intrinsic low vapor pressures, reduced environmental volatility, high thermal and chemical stability, as well as tunable polarity and solvating abilities [5; 6; 7; 8; 9; 10; 11]. Moreover, ILs and DESs are recognized for their low toxicity [12] compared to conventional organic solvents, and their biodegradability enhances their environmental compatibility, making them attractive for applications in green chemistry [13; 14]. Their ability to dissolve a broad spectrum of materials [15; 16; 17; 18] is further complemented by a wide liquidous range, enabling their use at different temperatures while avoiding the risk of crystallization or phase separation [19]. The ongoing research on the customizability of ILs and DESs has further highlighted their potential, as their physicochemical properties can be fine-tuned by varying the cation/anion pair in ILs or the hydrogen bond donor/acceptor composition in DESs. This allows for the precise optimization of solvent characteristics for specific applications, such as enhancing solubility, improving reaction rates, or facilitating separation processes [20]. It becomes clear that the unique and distinctive chemical-physical properties of these systems are profoundly influenced by the intricate intermolecular interactions that arise between the components. Therefore, a precise and in-depth chemical-physical characterization of these interactions is crucial for gaining a comprehensive understanding of the macroscopic behavior of these liquids, enabling us to fine-tune their properties for targeted applications. In this work, we propose mixtures based on choline acetate (ChAc), classifiable as an ionic liquid, combined with the hydrogen bond donor (HBD) maleic acid (MA) (Figure 1), with the aim of developing low-melting binary mixtures. The capability of maleic acid to act as H-bond donor in the formulation of deep eutectic solvents can be explained considering the by-carboxylic nature of this acid, which facilitates strong H-bonding interactions [21]. Choline is a widely utilized cation in the formulation of ionic liquids and deep eutectic solvents, with a variety of anions being paired with it. Among these, choline chloride (ChCI) is by far the most extensively studied combination. In contrast, systems based on ChAc have received comparatively less attention. Exploring the physicochemical properties of ChAc--where the acetate anion acts as a weak organic base--in combination with organic acids such as maleic acid could provide valuable insights into their interactions and expand the potential applications of these systems. Following our previous research [22], we propose three different molar ratios: a choline acetate-rich mixture (ChAc-MA 2-1), an equimolar mixture (ChAc-MA 1-1), and a maleic acid-rich mixture (ChAc-MA 1-2). Indeed, a study of the system based on HBD concentration is crucial for a deeper investigation of the secondary interactions. Varying the amount of acid, for instance, can directly impact the number of hydrogen bonds within the mixtures, offering valuable insights into how these interactions shape the overall behavior. As observed by our previous work [22], the increasing amount of acid leads to greater cross-linking through hydrogen bond interactions, as evidenced by mid-IR spectroscopy results reporting variable shifts in the OH and C=O stretching bands as a function of acid concentration, and by radial distribution functions obtained from the analysis of classical molecular dynamics trajectories. The phenomenon of cross-linking through hydrogen bonding in deep eutectic solvents represents a fascinating avenue of study.