Gas hydrates (GH) are non-stoichiometric substances that resemble ice and are created by trapping gas (guest) molecules in hydrogen-bonded water molecules. They develop under circumstances of high pressure and low temperature, with host and guest molecules bonded through Van der Waals forces (Harrison, 2010). Guest molecules can exist in either gaseous or liquid states. Water molecules can form cavities (cages) that possess consistent pentagonal and hexagonal surfaces. The only factor preventing hydrate cages from collapsing under their own attractive forces is the presence of a guest molecule, which can be found either within the cage or in a substantial fraction of the cages surrounding it, given that these cages are larger than crystalline ice holes (Ma et al., 2016). Within the cavities of the water molecules, the guest molecule spins freely. Some typical guest molecules are methane, ethane, propane, carbon dioxide, natural gas, etc. A typical GH structure is composed of approximately 85% water molecules that are hydrogen-bonded to form cages that confine the guest molecules (Sloan et al., 2007). Due to the high water content of GH, its characteristics are akin to those of ice. However, hydrates and ice differ significantly in their mechanical and thermal characteristics. Since the rate of water diffusion in hydrates is less than that of ice, hydrate structures are more robust as ice structures. Also, GH doesn’t transfer heat as well as ice does and can hold more heat.
Hydrate synthesis is dependent upon several main factors: reduced temperature, elevated pressure, the existence of guest molecules, and an adequate quantity of water molecules. Production occurs more through a physical process when compared to a chemical one.GH formation is a crystallization process that involves techniques for nucleation and growth crystal formation. GH nucleation is a minor phenomenon that involves only a few molecules (Khurana et al., 2017). This process is referred to as the production and expansion of hydrate nuclei to a crucial size for future growth. Ke et al. (2019) explained model of the labile cluster nucleation hypothesis is based on the notion that water clusters around dissolved gas molecules can develop to a critical radius. Nucleation is complete when a threshold size of cluster aggregation is attained, allowing hydrate growth to commence.
To promote the development of GH, chemical and mechanical methods are commonly employed. The chemical strategy is employed to promote hydrate formation under milder conditions, boost the formation rate and gas absorption, and enhance hydrate selectivity, whilst the mechanical methods aim to increase the contact area and mass transfer between water and gas (Dashti et al., 2015). Two fundamental categories of GH chemical additives—inhibitors and promoters—are frequently used to modify the thermodynamic production of gas hydrates, depending on the application. They operate by the manipulation of the equilibrium boundary conditions of the hydrate phase, either by accelerating or delaying hydrate nucleation and crystal growth (Rossi et al., 2021). The chemical promoters for gas hydrates that are now available include thermodynamic hydrate promoters (THPs) and kinetic hydrate promoters (KHPs). The hydrate phase boundary conditions are modified to higher temperatures and lower pressures using THPs. During the hydrate formation process, KHPs are also employed to enhance gas/water absorption, hydrate induction time, and formation rate (Wang et al., 2020).
For CO2 collection and sequestration, THPs and KHPs are utilized (Park et al., 2013), as well as in storage and transit of gas (Song et al., 2014; Veluswamy et al., 2018). General THPs comprise tetrahydrofuran (THF) and acetone, whereas nanoparticles (Nashed et al., 2018), sodium dodecyl sulfate (Pan et al., 2018), and other surfactants are KHPs. The numerous additives are all synthetic compounds that are either required in extremely high quantities to be effective or are hazardous and constitute a safety risk. As a result, researchers are currently looking towards greener, biodegradable, and benign additives that might potentially replace old conventional promoters and traditional inhibitors that are both ecologically prohibitive and ineffective.
As is known, amino acids are chemical substances that are commonly referred to as the building blocks (monomer units) of proteins and are an important part of the human diet. An amino acid is composed of a carboxyl (COOH) group, an amino (NH2) group, a hydrogen (H) atom, and a distinctive organic R group (or side chain). The presence of amine and carboxylic acid groups on the side chains of most amino acids imparts both hydrophilic and hydrophobic properties. The properties of this side chain dictate whether amino acids are classified as polar (hydrophilic) or nonpolar (hydrophobic) (Bhattacharjee et al., 2021). The side chain governs the chemical and physical properties of the molecule (Bavoh et al., 2017). Utilizing amino acids as growth promoters has the advantage of being biodegradable.
In the past decade, amino acids have gained increased significance as additions in gas hydrate research. They can interact with water electrostatically. Most importantly, they are ecologically friendly, biodegradable, and water soluble, and they can be purchased in large quantities at a fair cost. An essential advantage of amino acids is their natural safety and biodegradability. Amino acids are also expected to be less costly than other synthetic compounds commonly employed as gas hydrate inhibitors or promoters (Bavoh et al., 2019; Sinehbaghizadeh et al., 2022).
Potential applications of clathrate hydrate formation include the following: recovery of water from electrolyte solutions (desalination); storage of natural gas, hydrogen, and other substances in solid clathrate hydrates; recovery of water from aqueous organics (waste-water treatment and concentration of organic mixtures); gas separations; gas storage utilizing clathrate hydrates; and gas mixture separations utilizing clathrate hydrates (Englezos, 2022). Multiple dietary applications of GH have been documented in recent years. These CO2 GH can replace existing technologies such as freeze-drying, reverse osmosis, and thermal evaporation for different food products if they are applied effectively (Srivastava et al., 2021). Furthermore, water readily dissolves the CO2 gas, and the forces of contact between the CO2 molecules and water are greater, increasing its prospects for production (Misyura and Donskoy, 2020).
This study aims to comprehend the stability of CO2 GH with promoters at various temperatures. Based on literature cysteine, leucine, methionine, and valine, along with lecithin, were investigated to assess the temperature stability and gas trapping of CO2 GH at -18°C, 10°C, 20°C, and 23°C (ambient room temperature). These promoters are safe for human consumption, biodegradable, and eco-friendly. A thorough understanding of the CO2 gas's stability at different temperatures is needed to calculate the amount of GH to be used in food processing, particularly as a leavening ingredient in the baking industry. The promoters are also employed in combination to improve the stability of GH in order to develop a candidate that could serve as a replacement for the existing leavening agent in the baking industry.