Thank you for visiting nature.com. The browser version you are using has limited CSS support. For the best experience, we recommend using the latest browser version (or turning off compatibility mode in Internet Explorer). Additionally, to ensure continued support, this site will not include styles or JavaScript.
       Shale expansion in clastic reservoirs creates significant problems, leading to wellbore instability. For environmental reasons, the use of water-based drilling fluid with added shale inhibitors is preferred over oil-based drilling fluid. Ionic liquids (ILs) have attracted much attention as shale inhibitors due to their tunable properties and strong electrostatic characteristics. However, imidazolyl-based ionic liquids (ILs), widely used in drilling fluids, have proven to be toxic, non-biodegradable and expensive. Deep eutectic solvents (DES) are considered a more cost-effective and less toxic alternative to ionic liquids, but they still fall short of the required environmental sustainability. Recent advances in this field have led to the introduction of natural deep eutectic solvents (NADES), known for their true environmental friendliness. This study investigated NADESs, which contain citric acid (as a hydrogen bond acceptor) and glycerol (as a hydrogen bond donor) as drilling fluid additives. The NADES-based drilling fluids were developed in accordance with API 13B-1 and their performance was compared with potassium chloride-based drilling fluids, imidazolium-based ionic liquids, and choline chloride:urea-DES-based drilling fluids. The physicochemical properties of the proprietary NADESs are described in detail. The rheological properties, fluid loss, and shale inhibition properties of the drilling fluid were evaluated during the study, and it was shown that at a concentration of 3% NADESs, the yield stress/plastic viscosity ratio (YP/PV) was increased, the mud cake thickness was reduced by 26%, and the filtrate volume was reduced by 30.1%. Notably, NADES achieved an impressive expansion inhibition rate of 49.14% and increased shale production by 86.36%. These results are attributed to the ability of NADES to modify the surface activity, zeta potential, and interlayer spacing of clays, which are discussed in this paper to understand the underlying mechanisms. This sustainable drilling fluid is expected to revolutionize the drilling industry by providing a non-toxic, cost-effective, and highly effective alternative to traditional shale corrosion inhibitors, paving the way for environmentally friendly drilling practices.
       Shale is a versatile rock that serves as both a source and reservoir of hydrocarbons, and its porous structure1 provides the potential for both production and storage of these valuable resources. However, shale is rich in clay minerals such as montmorillonite, smectite, kaolinite and illite, which make it prone to swelling when exposed to water, leading to wellbore instability during drilling operations2,3. These issues can lead to non-productive time (NPT) and a host of operational problems including stuck pipes, lost mud circulation, wellbore collapse and bit fouling, increasing recovery time and cost. Traditionally, oil-based drilling fluids (OBDF) have been the preferred choice for shale formations due to their ability to resist shale expansion4. However, the use of oil-based drilling fluids entails higher costs and environmental risks. Synthetic-based drilling fluids (SBDF) have been considered as an alternative, but their suitability at high temperatures is unsatisfactory. Water-based drilling fluids (WBDF) are an attractive solution because they are safer, more environmentally friendly, and more cost-effective than OBDF5. Various shale inhibitors have been used to enhance the shale inhibition ability of WBDF, including traditional inhibitors such as potassium chloride, lime, silicate, and polymer. However, these inhibitors have limitations in terms of effectiveness and environmental impact, especially due to the high K+ concentration in potassium chloride inhibitors and the pH sensitivity of silicates. 6 Researchers have explored the possibility of using ionic liquids as drilling fluid additives to improve drilling fluid rheology and prevent shale swelling and hydrate formation. However, these ionic liquids, especially those containing imidazolyl cations, are generally toxic, expensive, non-biodegradable, and require complex preparation processes. To solve these problems, people started looking for a more economical and environmentally friendly alternative, which led to the emergence of deep eutectic solvents (DES). DES is a eutectic mixture formed by a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA) at a specific molar ratio and temperature. These eutectic mixtures have lower melting points than their individual components, primarily due to charge delocalization caused by hydrogen bonds. Many factors, including lattice energy, entropy change, and interactions between anions and HBD, play a key role in lowering the melting point of DES.
       In previous studies, various additives were added to water-based drilling fluid to solve the shale expansion problem. For example, Ofei et al. added 1-butyl-3-methylimidazolium chloride (BMIM-Cl), which significantly reduced the mud cake thickness (up to 50%) and decreased the YP/PV value by 11 at different temperatures. Huang et al. used ionic liquids (specifically, 1-hexyl-3-methylimidazolium bromide and 1,2-bis(3-hexylimidazol-1-yl)ethane bromide) in combination with Na-Bt particles and significantly reduced the shale swelling by 86.43% and 94.17%, respectively12. In addition, Yang et al. used 1-vinyl-3-dodecylimidazolium bromide and 1-vinyl-3-tetradecylimidazolium bromide to reduce shale swelling by 16.91% and 5.81%, respectively. 13 Yang et al. also used 1-vinyl-3-ethylimidazolium bromide and reduced shale expansion by 31.62% while maintaining shale recovery at 40.60%. 14 In addition, Luo et al. used 1-octyl-3-methylimidazolium tetrafluoroborate to reduce shale swelling by 80%. 15, 16 Dai et al. used ionic liquid copolymers to inhibit shale and achieved 18% increase in linear recovery compared to amine inhibitors. 17
       Ionic liquids themselves have some disadvantages, which prompted scientists to look for more environmentally friendly alternatives to ionic liquids, and thus DES was born. Hanjia was the first to use deep eutectic solvents (DES) consisting of vinyl chloride propionic acid (1:1), vinyl chloride 3-phenylpropionic acid (1:2), and 3-mercaptopropionic acid + itaconic acid + vinyl chloride (1:1:2), which inhibited the swelling of bentonite by 68%, 58%, and 58%, respectively18. In a free experiment, M. H. Rasul used a 2:1 ratio of glycerol and potassium carbonate (DES) and significantly reduced the swelling of shale samples by 87%19,20. Ma used urea:vinyl chloride to significantly reduce the expansion of shale by 67%.21 Rasul et al. The combination of DES and polymer was used as a dual-action shale inhibitor, which achieved excellent shale inhibition effect22.
       Although deep eutectic solvents (DES) are generally considered a greener alternative to ionic liquids, they also contain potentially toxic components such as ammonium salts, which makes their eco-friendliness questionable. This problem has led to the development of natural deep eutectic solvents (NADES). They are still classified as DES, but are composed of natural substances and salts, including potassium chloride (KCl), calcium chloride (CaCl2), Epsom salts (MgSO4.7H2O), and others. The numerous potential combinations of DES and NADES open up a wide scope for research in this area and are expected to find applications in a variety of fields. Several researchers have successfully developed new DES combinations that have proven effective in a variety of applications. For example, Naser et al. 2013 synthesized potassium carbonate-based DES and studied its thermophysical properties, which subsequently found applications in the areas of hydrate inhibition, drilling fluid additives, delignification, and nanofibrillation. 23 Jordy Kim and co-workers developed ascorbic acid-based NADES and evaluated its antioxidant properties in various applications. 24 Christer et al. developed citric acid-based NADES and identified its potential as an excipient for collagen products. 25 Liu Yi and co-workers summarized the applications of NADES as extraction and chromatography media in a comprehensive review, while Misan et al. discussed the successful applications of NADES in the agri-food sector. It is imperative that drilling fluid researchers begin to pay attention to the effectiveness of NADES in their applications. recent. In 2023, Rasul et al. used different combinations of natural deep eutectic solvents based on ascorbic acid26, calcium chloride27, potassium chloride28 and Epsom salt29 and achieved impressive shale inhibition and shale recovery. This study is one of the first studies to introduce NADES (particularly citric acid and glycerol-based formulation) as an environmentally friendly and effective shale inhibitor in water-based drilling fluids, which features excellent environmental stability, improved shale inhibition ability and improved fluid performance compared with traditional inhibitors such as KCl, imidazolyl-based ionic liquids and traditional DES.
       The study will involve the in-house preparation of citric acid (CA) based NADES followed by detailed physicochemical characterization and its use as a drilling fluid additive to evaluate the drilling fluid properties and its swelling inhibition ability. In this study, CA will act as a hydrogen bond acceptor while glycerol (Gly) will act as a hydrogen bond donor selected based on the MH screening criteria for NADES formation/selection in shale inhibition studies30. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and zeta potential (ZP) measurements will elucidate the NADES-clay interactions and the mechanism underlying the clay swelling inhibition. Additionally, this study will compare CA NADES based drilling fluid with DES32 based on 1-ethyl-3-methylimidazolium chloride [EMIM]Cl7,12,14,17,31, KCl and choline chloride:urea (1:2) to investigate their effectiveness in shale inhibition and improving drilling fluid performance.
       Citric acid (monohydrate), glycerol (99 USP), and urea were purchased from EvaChem, Kuala Lumpur, Malaysia. Choline chloride (>98%), [EMIM]Cl 98%, and potassium chloride were purchased from Sigma Aldrich, Malaysia. The chemical structures of all chemicals are shown in Figure 1. The green diagram compares the main chemicals used in this study: imidazolyl ionic liquid, choline chloride (DES), citric acid, glycerol, potassium chloride, and NADES (citric acid and glycerol). The eco-friendliness table of the chemicals used in this study is presented in Table 1. In the table, each chemical is rated based on toxicity, biodegradability, cost, and environmental sustainability.
       Chemical structures of the materials used in this study: (a) citric acid, (b) [EMIM]Cl, (c) choline chloride, and (d) glycerol.
       Hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) candidates for the development of CA (natural deep eutectic solvent) based NADES were carefully selected according to the MH 30 selection criteria, which are intended for the development of NADES as effective shale inhibitors. According to this criterion, components with a large number of hydrogen bond donors and acceptors as well as polar functional groups are considered suitable for the development of NADES.
       In addition, the ionic liquid [EMIM]Cl and choline chloride:urea deep eutectic solvent (DES) were selected for comparison in this study because they are widely used as drilling fluid additives33,34,35,36. In addition, potassium chloride (KCl) was compared because it is a common inhibitor.
       Citric acid and glycerol were mixed in different molar ratios to obtain eutectic mixtures. Visual inspection showed that the eutectic mixture was a homogeneous, transparent liquid without turbidity, indicating that the hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) were successfully mixed in this eutectic composition. Preliminary experiments were conducted to observe the temperature-dependent behavior of the mixing process of HBD and HBA. According to the available literature, the proportion of eutectic mixtures was evaluated at three specific temperatures above 50 °C, 70 °C and 100 °C, indicating that the eutectic temperature is usually in the range of 50–80 °C. A Mettler digital balance was used to accurately weigh the HBD and HBA components, and a Thermo Fisher hot plate was used to heat and stir the HBD and HBA at 100 rpm under controlled conditions.
       The thermophysical properties of our synthesized deep eutectic solvent (DES), including density, surface tension, refractive index, and viscosity, were accurately measured over a temperature range from 289.15 to 333.15 K. It should be noted that this temperature range was chosen primarily due to the limitations of existing equipment. The comprehensive analysis included an in-depth study of various thermophysical properties of this NADES formulation, revealing their behavior over a range of temperatures. Focusing on this specific temperature range provides insights into the properties of NADES that are of particular importance for a number of applications.
       The surface tension of as-prepared NADES was measured in the range from 289.15 to 333.15 K using an interfacial tension meter (IFT700). NADES droplets are formed in a chamber filled with a large volume of liquid using a capillary needle under specific temperature and pressure conditions. Modern imaging systems introduce appropriate geometric parameters to calculate the interfacial tension using the Laplace equation.
       An ATAGO refractometer was used to determine the refractive index of freshly prepared NADES over the temperature range of 289.15 to 333.15 K. The instrument uses a thermal module to regulate the temperature to estimate the degree of refraction of light, eliminating the need for a constant-temperature water bath. The prism surface of the refractometer should be cleaned and the sample solution should be evenly distributed over it. Calibrate with a known standard solution, and then read the refractive index from the screen.
       The viscosity of as-prepared NADES was measured over the temperature range of 289.15 to 333.15 K using a Brookfield rotational viscometer (cryogenic type) at a shear rate of 30 rpm and a spindle size of 6. The viscometer measures viscosity by determining the torque required to rotate the spindle at a constant speed in a liquid sample. After the sample is placed on the screen under the spindle and tightened, the viscometer displays the viscosity in centipoise (cP), providing valuable information on the rheological properties of the liquid.
       A portable density meter DMA 35 Basic was used to determine the density of freshly prepared natural deep eutectic solvent (NDEES) in the temperature range of 289.15–333.15 K. Since the device does not have a built-in heater, it must be preheated to the specified temperature (± 2 °C) before using the NADES density meter. Draw at least 2 ml of sample through the tube, and the density will be immediately displayed on the screen. It is worth noting that due to the lack of a built-in heater, the measurement results have an error of ± 2 °C.
       To evaluate the pH of freshly prepared NADES in the temperature range of 289.15–333.15 K, we used a Kenis benchtop pH meter. Since there is no built-in heating device, NADES was first heated to the desired temperature (±2 °C) using a hotplate and then measured directly with a pH meter. Completely immerse the pH meter probe in NADES and record the final value after the reading has stabilized.
       Thermogravimetric analysis (TGA) was used to evaluate the thermal stability of natural deep eutectic solvents (NADES). Samples were analyzed during heating. Using a high-precision balance and carefully monitoring the heating process, a plot of mass loss versus temperature was generated. NADES was heated from 0 to 500 °C at a rate of 1 °C per minute.
       To begin the process, the NADES sample must be thoroughly mixed, homogenized, and have surface moisture removed. The prepared sample is then placed in a TGA cuvette, which is typically made of an inert material such as aluminum. To ensure accurate results, TGA instruments are calibrated using reference materials, typically weight standards. Once calibrated, the TGA experiment begins and the sample is heated in a controlled manner, usually at a constant rate. Continuous monitoring of the relationship between sample weight and temperature is a key part of the experiment. TGA instruments collect data on temperature, weight, and other parameters such as gas flow or sample temperature. Once the TGA experiment is complete, the collected data is analyzed to determine the change in sample weight as a function of temperature. This information is valuable in determining temperature ranges associated with physical and chemical changes in the sample, including processes such as melting, evaporation, oxidation, or decomposition.
       The water-based drilling fluid was carefully formulated according to API 13B-1 standard, and its specific composition is listed in Table 2 for reference. Citric acid and glycerol (99 USP) were purchased from Sigma Aldrich, Malaysia to prepare the natural deep eutectic solvent (NADES). In addition, the conventional shale inhibitor potassium chloride (KCl) was also purchased from Sigma Aldrich, Malaysia. 1-ethyl, 3-methylimidazolium chloride ([EMIM]Cl) with a purity of more than 98% was selected due to its significant effect in improving the rheology of drilling fluid and shale inhibition, which was confirmed in previous studies. Both KCl and ([EMIM]Cl) will be used in the comparative analysis to evaluate the shale inhibition performance of NADES.
       Many researchers prefer to use bentonite flakes to study shale swelling because bentonite contains the same “montmorillonite” group that causes shale swelling. Obtaining real shale core samples is challenging because the coring process destabilizes the shale, resulting in samples that are not entirely shale but typically contain a mixture of sandstone and limestone layers. In addition, shale samples typically lack the montmorillonite groups that cause shale swelling and are therefore unsuitable for swelling inhibition experiments.
       In this study, we used reconstituted bentonite particles with a diameter of approximately 2.54 cm. The granules were made by pressing 11.5 grams of sodium bentonite powder in a hydraulic press at 1600 psi. The thickness of the granules was accurately measured before being placed in a linear dilatometer (LD). The particles were then immersed in drilling fluid samples, including base samples and samples injected with inhibitors used to prevent shale swelling. The change in granule thickness was then carefully monitored using the LD, with measurements recorded at 60-second intervals for 24 hours.
       X-ray diffraction showed that the composition of bentonite, especially its 47% montmorillonite component, is a key factor in understanding its geological characteristics. Among the montmorillonite components of bentonite, montmorillonite is the main component, accounting for 88.6% of the total components. Meanwhile, quartz accounts for 29%, illite for 7%, and carbonate for 9%. A small part (about 3.2%) is a mixture of illite and montmorillonite. In addition, it contains trace elements such as Fe2O3 (4.7%), silver aluminosilicate (1.2%), muscovite (4%), and phosphate (2.3%). In addition, small amounts of Na2O (1.83%) and iron silicate (2.17%) are present, which makes it possible to fully appreciate the constituent elements of bentonite and their respective proportions.
       This comprehensive study section details the rheological and filtration properties of drilling fluid samples prepared using natural deep eutectic solvent (NADES) and used as a drilling fluid additive at different concentrations (1%, 3% and 5%). The NADES based slurry samples were then compared and analysed with slurry samples consisting of potassium chloride (KCl), CC:urea DES (choline chloride deep eutectic solvent:urea) and ionic liquids. A number of key parameters were covered in this study including viscosity readings obtained using a FANN viscometer before and after exposure to aging conditions at 100°C and 150°C. Measurements were taken at different rotation speeds (3 rpm, 6 rpm, 300 rpm and 600 rpm) allowing for a comprehensive analysis of the drilling fluid behaviour. The data obtained can then be used to determine key properties such as yield point (YP) and plastic viscosity (PV), which provide insight into the fluid performance under various conditions. High pressure high temperature (HPHT) filtration tests at 400 psi and 150°C (typical temperatures in high temperature wells) determine the filtration performance (cake thickness and filtrate volume).
       This section utilizes state-of-the-art equipment, the Grace HPHT Linear Dilatometer (M4600), to thoroughly evaluate the shale swelling inhibition properties of our water-based drilling fluids. The LSM is a state-of-the-art machine consisting of two components: a plate compactor and a linear dilatometer (model: M4600). Bentonite plates were prepared for analysis using the Grace Core/Plate Compactor. The LSM then provides immediate swelling data on these plates, allowing for a comprehensive evaluation of the shale’s swelling inhibition properties. Shale expansion tests were conducted under ambient conditions, i.e., 25°C and 1 psia.
       Shale stability testing involves a key test often referred to as the shale recovery test, shale dip test or shale dispersion test. To begin this evaluation, shale cuttings are separated on a #6 BSS screen and then placed on a #10 screen. The cuttings are then fed to a holding tank where they are mixed with a base fluid and drilling mud containing NADES (Natural Deep Eutectic Solvent). The next step is to place the mixture in an oven for an intense hot rolling process, ensuring that the cuttings and mud are thoroughly mixed. After 16 hours, the cuttings are removed from the pulp by allowing the shale to decompose, resulting in a reduction in cuttings weight. The shale recovery test was conducted after the shale cuttings had been held in drilling mud at 150°C and 1000 psi. inch within 24 hours.
       To measure the recovery of the shale mud, we filtered it through a finer screen (40 mesh), then washed it thoroughly with water, and finally dried it in an oven. This painstaking procedure allows us to estimate the recovered mud compared to the original weight, ultimately calculating the percentage of shale mud successfully recovered. The source of the shale samples is from Niah District, Miri District, Sarawak, Malaysia. Before the dispersion and recovery tests, the shale samples were subjected to a thorough X-ray diffraction (XRD) analysis to quantify their clay composition and confirm their suitability for testing. The clay mineral composition of the sample is as follows: illite 18%, kaolinite 31%, chlorite 22%, vermiculite 10%, and mica 19%.
       Surface tension is a key factor controlling the penetration of water cations into shale micropores via capillary action, which will be studied in detail in this section. This paper examines the role of surface tension in the cohesive property of drilling fluids, highlighting its important influence on the drilling process, especially shale inhibition. We used an interfacial tensiometer (IFT700) to accurately measure the surface tension of drilling fluid samples, revealing an important aspect of fluid behavior in the context of shale inhibition.
       This section discusses in detail the d-layer spacing, which is the interlayer distance between aluminosilicate layers and one aluminosilicate layer in clays. The analysis covered wet mud samples containing 1%, 3% and 5% CA NADES, as well as 3% KCl, 3% [EMIM]Cl and 3% CC:urea based DES for comparison. A state-of-the-art benchtop X-ray diffractometer (D2 Phaser) operating at 40 mA and 45 kV with Cu-Kα radiation (λ = 1.54059 Å) played a critical role in recording the X-ray diffraction peaks of both wet and dry Na-Bt samples. The application of the Bragg equation enables the accurate determination of the d-layer spacing, thereby providing valuable information on the clay behavior.
       This section utilizes the advanced Malvern Zetasizer Nano ZSP instrument to accurately measure zeta potential. This evaluation provided valuable information on the charge characteristics of dilute mud samples containing 1%, 3%, and 5% CA NADES, as well as 3% KCl, 3% [EMIM]Cl, and 3% CC:urea-based DES for comparative analysis. These results contribute to our understanding of the stability of colloidal compounds and their interactions in fluids.
       The clay samples were examined before and after exposure to natural deep eutectic solvent (NADES) using a Zeiss Supra 55 VP field emission scanning electron microscope (FESEM) equipped with energy dispersive X-ray (EDX). The imaging resolution was 500 nm and the electron beam energy was 30 kV and 50 kV. FESEM provides high-resolution visualization of the surface morphology and structural features of the clay samples. The objective of this study was to obtain information about the effect of NADES on the clay samples by comparing the images obtained before and after exposure.
       In this study, field emission scanning electron microscopy (FESEM) technology was used to investigate the effect of NADES on clay samples at the microscopic level. The aim of this study is to elucidate the potential applications of NADES and its effect on clay morphology and average particle size, which will provide valuable information for research in this field.
       In this study, error bars were used to visually describe the variability and uncertainty of the mean percent error (AMPE) across experimental conditions. Rather than plotting individual AMPE values ​​(since plotting AMPE values ​​can obscure trends and exaggerate small variations), we calculate error bars using the 5% rule. This approach ensures that each error bar represents the interval within which the 95% confidence interval and 100% of the AMPE values ​​are expected to fall, thereby providing a clearer and more concise summary of the data distribution for each experimental condition. Using error bars based on the 5% rule thus improves the interpretability and reliability of graphical representations and helps provide a more detailed understanding of the results and their implications.
       In the synthesis of natural deep eutectic solvents (NADES), several key parameters were carefully studied during the in-house preparation process. These critical factors include temperature, molar ratio, and mixing speed. Our experiments show that when HBA (citric acid) and HBD (glycerol) are mixed at a molar ratio of 1:4 at 50°C, a eutectic mixture is formed. The distinguishing feature of the eutectic mixture is its transparent, homogeneous appearance, and the absence of sediment. Thus, this key step highlights the importance of molar ratio, temperature, and mixing speed, among which the molar ratio was the most influential factor in the preparation of DES and NADES, as shown in Figure 2.
       The refractive index (n) expresses the ratio of the speed of light in a vacuum to the speed of light in a second, denser medium. The refractive index is of particular interest for natural deep eutectic solvents (NADES) when considering optically sensitive applications such as biosensors. The refractive index of the studied NADES at 25 °C was 1.452, which is interestingly lower than that of glycerol.
       It is worth noting that the refractive index of NADES decreases with temperature, and this trend can be accurately described by formula (1) and Figure 3, with the absolute mean percentage error (AMPE) reaching 0%. This temperature-dependent behavior is explained by the decrease in viscosity and density at high temperatures, causing the light to travel through the medium at a higher speed, resulting in a lower refractive index (n) value. These results provide valuable insights into the strategic use of NADES in optical sensing, highlighting their potential for biosensor applications.
       Surface tension, which reflects the tendency of a liquid surface to minimize its area, is of great importance in assessing the suitability of natural deep eutectic solvents (NADES) for capillary pressure-based applications. A study of surface tension in the temperature range of 25–60 °C provides valuable information. At 25 °C, the surface tension of citric acid-based NADES was 55.42 mN/m, which is significantly lower than that of water and glycerol. Figure 4 shows that the surface tension decreases significantly with increasing temperature. This phenomenon can be explained by an increase in molecular kinetic energy and a subsequent decrease in intermolecular attractive forces.
       The linear decreasing trend of surface tension observed in the studied NADES can be well expressed by equation (2), which illustrates the basic mathematical relationship in the temperature range of 25–60 °C. The graph in Figure 4 clearly depicts the trend of surface tension with temperature with an absolute mean percentage error (AMPE) of 1.4%, which quantifies the accuracy of the reported surface tension values. These results have important implications for understanding the behavior of NADES and its potential applications.
       Understanding the density dynamics of natural deep eutectic solvents (NADES) is crucial to facilitate their application in numerous scientific studies. The density of citric acid-based NADES at 25°C is 1.361 g/cm3, which is higher than the density of the parent glycerol. This difference can be explained by the addition of a hydrogen bond acceptor (citric acid) to glycerol.
       Taking citrate-based NADES as an example, its density drops to 1.19 g/cm3 at 60°C. The increase in kinetic energy upon heating causes the NADES molecules to disperse, causing them to occupy a larger volume, resulting in a decrease in density. The observed decrease in density shows a certain linear correlation with the increase in temperature, which can be properly expressed by formula (3). Figure 5 graphically presents these characteristics of the NADES density change with an absolute mean percentage error (AMPE) of 1.12%, which provides a quantitative measure of the accuracy of the reported density values.
       Viscosity is the attractive force between different layers of a liquid in motion and plays a key role in understanding the applicability of natural deep eutectic solvents (NADES) in various applications. At 25 °C, the viscosity of NADES was 951 cP, which is higher than that of glycerol.
       The observed decrease in viscosity with increasing temperature is mainly explained by the weakening of intermolecular attractive forces. This phenomenon results in a decrease in the viscosity of the fluid, a trend clearly demonstrated in Figure 6 and quantified by Equation (4). Notably, at 60°C, the viscosity drops to 898 cP with an overall mean percent error (AMPE) of 1.4%. A detailed understanding of the viscosity versus temperature dependence in NADES is of great importance for its practical application.
       The pH of the solution, determined by the negative logarithm of the hydrogen ion concentration, is critical, especially in pH-sensitive applications such as DNA synthesis, so the pH of NADES must be carefully studied before use. Taking citric acid-based NADES as an example, a distinctly acidic pH of 1.91 can be observed, which is in sharp contrast to the relatively neutral pH of glycerol.
       Interestingly, the pH of the natural citric acid dehydrogenase soluble solvent (NADES) showed a non-linear decreasing trend with increasing temperature. This phenomenon is attributed to the increased molecular vibrations that disrupt the H+ balance in the solution, leading to the formation of [H]+ ions and, in turn, a change in the pH value. While the natural pH of citric acid ranges from 3 to 5, the presence of acidic hydrogen in glycerol further lowers the pH to 1.91.
       The pH behavior of citrate-based NADES in the temperature range of 25–60 °C can be appropriately represented by equation (5), which provides a mathematical expression for the observed pH trend. Figure 7 graphically depicts this interesting relationship, highlighting the effect of temperature on the pH of NADES, which is reported to be 1.4% for AMPE.
       Thermogravimetric analysis (TGA) of natural citric acid deep eutectic solvent (NADES) was systematically carried out in the temperature range from room temperature to 500 °C. As can be seen from Figures 8a and b, the initial mass loss up to 100 °C was mainly due to the absorbed water and the hydration water associated with citric acid and pure glycerol. A significant mass retention of about 88% was observed up to 180 °C, which was mainly due to the decomposition of citric acid to aconitic acid and the subsequent formation of methylmaleic anhydride(III) upon further heating (Figure 8 b). Above 180 °C, a clear appearance of acrolein (acrylaldehyde) in glycerol could also be observed, as shown in Figure 8b37.
       Thermogravimetric analysis (TGA) of glycerol revealed a two-stage mass loss process. The initial stage (180 to 220 °C) involves the formation of acrolein, followed by significant mass loss at high temperatures from 230 to 300 °C (Figure 8a). As the temperature increases, acetaldehyde, carbon dioxide, methane, and hydrogen are formed sequentially. Notably, only 28% of the mass was retained at 300 °C, suggesting that the intrinsic properties of NADES 8(a)38,39 may be defective.
       To obtain information about the formation of new chemical bonds, freshly prepared suspensions of natural deep eutectic solvents (NADES) were analyzed by Fourier transform infrared spectroscopy (FTIR). The analysis was performed by comparing the spectrum of the NADES suspension with the spectra of pure citric acid (CA) and glycerol (Gly). The CA spectrum showed clear peaks at 1752 1/cm and 1673 1/cm, which represent the stretching vibrations of the C=O bond and are also characteristic of CA. In addition, a significant shift in the OH bending vibration at 1360 1/cm was observed in the fingerprint region, as shown in Figure 9.
       Similarly, in case of glycerol, the shifts of OH stretching and bending vibrations were found at wavenumbers of 3291 1/cm and 1414 1/cm, respectively. Now, by analyzing the spectrum of as-prepared NADES, a significant shift in the spectrum was found. As shown in Figure 7, the stretching vibration of C=O bond shifted from 1752 1/cm to 1720 1/cm and the bending vibration of -OH bond of glycerol shifted from 1414 1/cm to 1359 1/cm. These shifts in wavenumbers indicate the change in electronegativity, which indicates the formation of new chemical bonds in the structure of NADES.