Hydrogen bond
The quantity and stability of the hydrogen bonds contained in the fluid in the supercritical state and the normal state are changed. Temperature and pressure are the influencing factors of hydrogen bond stability and quantity change, but the degree of influence is different. For example, the hydrogen bond stability of ethanol decreases with the increase in temperature, and the influence of pressure is relatively small.
Ion product
The ion product is affected by temperature and pressure, resulting in a difference in the ion product between the supercritical state and the normal state. In the supercritical region, as the temperature and pressure increase, the ion product increases and is much higher than normal. In the supercritical state, the density increases with the increase of temperature and pressure, resulting in an increase in ion product, which is several orders of magnitude higher than that in the normal state.
Esterification
Esterification, also known as alcoholysis, is the replacement of an alcohol in a glyceride with another alcohol. Compared with an alcohol solution, the esterification reaction rate of supercritical alcohol is extremely high. Sasaki’s research shows that: under the condition that at least one of the oil and the yeast is in a supercritical state, a small amount of alkaline catalyst is added to react within a certain temperature period. Compared with ordinary transesterification, supercritical transesterification has the characteristics of high yield and short reaction time. This is due to the enhanced dissolving ability of supercritical alcohols in non-polar oils, and its surface tension is approximately 0, which can form a homogeneous reaction. In addition, the increase of the supercritical alcohol ion product can dissociate to generate more alcohol oxygen ions, and the concentration of alcohol ion group is usually positively correlated with the degree of esterification reaction, so the esterification reaction is more likely to occur
Restoration
Alcohols are less reductive in the standard state than in the supercritical state. Under the condition of no catalyst, alcohol solution does not undergo a dehydrogenation reaction, but supercritical alcohol can undergo a dehydrogenation reaction and has stronger reducibility to unsaturated bonds. Supercritical high temperatures and high-pressure conditions may be the reason for the enhancement of alcohol reducibility. Daimon studied the reduction of aldehydes with supercritical 2-propanol, and the results showed that 2-propanol is an effective reducing agent for the conversion of aldehydes to alcohol. It was also found that the reduction rate increased with increasing temperature.