Are acetonitrile and water mutually soluble under all conditions_

Acetonitrile is a common organic solvent. In reversed-phase liquid chromatography, acetonitrile-water acts similarly to methanol-water and is often used as the mobile phase. Acetonitrile is a polar solvent, similar to methanol. It is generally accepted that acetonitrile can be miscible with water under any conditions.

Many of the so-called conclusive findings will become better understood as scientific research progresses. This paper summarises the results of critical learning in this open environment. It lists the experimental conditions that can lead to phase separation of acetonitrile-water homogeneous solutions and the four related extraction methods:

• Acetonitrile brine extraction
• Acetonitrile sugar extraction
• Acetonitrile low-temperature extraction
• Solvent-induced phase change extraction

The scattered and fragmented acetonitrile-based extraction methods in the literature are summarised and shared to improve the knowledge base on acetonitrile properties and apply these properties in practical work.

1 Several cases in which acetonitrile and water are not mutually soluble

1.1 Salts and acetonitrile-water systems

In a homogeneous, transparent solution of acetonitrile and water, an appropriate amount of inorganic or organic salt will result in phase separation between acetonitrile and water. Since acetonitrile is less dense than water, the upper phase is rich (with a small amount of water), and the lower phase is poor (mainly water). This salt-induced phase separation of acetonitrile is known as the salting out of acetonitrile. Adding salt produces a strong hydration effect, which binds more water molecules and significantly reduces the number of free water molecules that would otherwise interact with the acetonitrile, resulting in phase separation. This way, substances that would otherwise be dissolved in the aqueous phase can be extracted into the acetonitrile-rich phase during phase separation. The main salts used as initiators for phase separation are sodium chloride, calcium chloride, zinc sulfate, ammonium sulfate, magnesium sulfate, tetrabutylammonium perchlorate, etc. The type of salt affects the extraction recovery. It is important to note that organic solvents that can produce phase separation in solutions that are otherwise miscible with water, induced by salts, are not limited to acetonitrile but also ethanol, isopropanol, acetone, etc. However, acetonitrile is favored as a matrix for samples that can be well-matched to the acetonitrile-based mobile phase for later reversed-phase liquid chromatography.

1.2 Sugar and acetonitrile-water systems

Sugar-induced phase separation between acetonitrile and water was reported by Wang at the University of Illinois, USA, in 2008. When monosaccharides (glucose, xylose, arabinose, or fructose) or disaccharides (sucrose or maltose) were added to the acetonitrile-water solution above a certain threshold, significant phase separation was observed. Polysaccharides do not have such a property of triggering phase separation. Other organic solvents commonly miscible with water (e.g., methanol, ethanol, n-propanol, isopropanol, acetone) do not have such a property of being phase separated from water by sugar initiation. The phase separation results in an acetonitrile-rich upper phase and an acetonitrile-poor lower phase. The extraction method based on this phenomenon is known as acetonitrile sugar extraction.

1.3 Low temperature and acetonitrile-water systems

In 1999 Yoshida and colleagues reported a low temperature-induced separation of the acetonitrile and aqueous phases. When a solution of acetonitrile and water (volume ratio: 1:1) was placed at -20 °C for 20 min, phase separation of acetonitrile and water was observed. The extraction method based on this phenomenon is known as low-temperature extraction with acetonitrile. The temperature at which phase separation occurs is generally controlled to be between -40°C and -40°C, as the melting point of acetonitrile is -45°C. The acetonitrile-rich phase is the liquid phase, and the water-rich phase is mostly the solid phase in the experimental temperature range. Experiments confirm that most common organic solvents miscible with water at room temperature cannot separate from water at low temperatures. Compared to the other two acetonitrile-based extraction methods, low-temperature extraction with acetonitrile does not require the introduction of a third-party phase-initiating reagent, so the system is relatively simple and does not contaminate the subsequent separation. In addition, low-temperature extraction avoids the chemical changes in temperature-sensitive molecules caused by higher temperatures, making the results more reliable.

1.4 Hydrophobic solvents and acetonitrile-water

Adding a hydrophobic, miscible organic solvent (e.g., chloroform, methylene chloride, toluene) to the homogeneous acetonitrile-water system induces phase separation in the acetonitrile-water system, and the extraction method based on this is known as solvent-induced phase change extraction.

2, Conclusion

“Are acetonitrile and water mutually soluble under all conditions?” This question is a simple one and has long been settled in existing textbooks. However, with the rapid development of information technology, it is now possible to access the literature more easily and to keep up to date with the latest knowledge.

Reference: Chemistry Education (Vol. 38, No. 20, 2017)