Study Of The Structure Of Water In A Typical Organic-Water Binary System And Its Low-Temperature Structural Phase Transition

Water is the source of life,and its unique structure and properties have made it a hot spot for research in many fields such as physics,chemistry,and biomedicine.Water usually exists in nature in the form of pure water,aqueous solutions of acid and alkaline salts,and aqueous solutions of organic substances.Research on binary mixed systems of organic matter/water has been widely carried out,due to the complex interaction between hydrophile groups and water molecules in organic matter.There are still controversies about the structure of water in the binary mixed system of organics/water and the interaction between water molecules and organic molecules:a part of them believes that water in the mixed system exists around the hydrophobic groups of organic matter in the structure of ice-like water;while the opposing view is that water is inhomogeneously mixed in the organic matter at the molecular level.It is found that the form of water present at room temperature and pressure directly affects its phase transition process at low temperature,e.g.,bulk water forms hexagonal ice(Ih ice)at low temperature,while nano water easily forms cubic ice(Ic ice).Therefore,it is expected to achieve the designation of water structure at room temperature and pressure by studying the low-temperature phase transition process of water in organic/water binary mixed systems,and provide a new approach to explore the structure of water in complex systems.In this paper,the structures of water and its low-temperature structural phase transitions in methanol/water,isopropanol/water and acetone/water mixed systems at different molar ratios were investigated by using Raman spectroscopy system.The details are as follows.The structures of water molecules in binary mixed systems of methanol/water,isopropanol/water and acetone/water with different molar fractions of organic matter at room temperature and pressure were studied.The results showed that the tetrahedral hydrogen-bonded mesh structure of water molecules was expanded in the binary methanol/water mixed system when 00.5,the hydrogen bonding network of water molecules becomes loose and eventually exists in solution as clusters of different sizes.The structure change process of water in the binary mixed systems of isopropanol/water and acetone/water is very similar to that of the binary mixed system of methanol/water,and effective differentiation of the water structure in the binary mixed system cannot be achieved.The phase transition of water molecules in the binary mixed systems of methanol/water,isopropanol/water and acetone/water was studied at low temperatures(293K-103K).At low molar fractions of organic matter,water molecules in the binary mixed systems of methanol/water(0methanol<0.4),isopropanol/water(0isopropanol<0.4)and acetone/water(0acetone<0.4)all underwent a phase transition from liquid water to I_c ice and then to I_h ice,which is consistent with the low-temperature phase transition process experienced by bulk phase water(4)The water molecules in the binary mixture undergo a phase transition from liquid water to I_c ice to I_h ice,which is consistent with the low-temperature phase transition process experienced by bulk phase water,except that the phase transition temperature is reduced.This result indicates that the water molecules in the binary mixed system at lower molar fractions have a similar structure to that of the bulk phase water.In the low-temperature phase transition study(293K-103K),the water molecules in the methanol/water,isopropanol/water,and acetone/water binary systems underwent different phase transitions as the molar fraction of organic compounds increased.In the methanol/water binary system,when X_methanol>0.5,the water molecules in the binary mixed system formed I_c ice and could not further form I_h ice;in the isopropanol/water binary system,the phase transition process of I_c ice then to I_h ice was experienced in the whole molar fraction range;in the acetone/water binary system,X_acetone>0.5,the water molecules in the binary mixed system In the acetone/water binary system,X_acetone>0.5,the water molecules in the binary mixture crossed the Ic ice phase transition process and formed I_h ice phase directly.This is due to the relatively longer carbon chains and more hydrophobic groups of isopropanol and acetone,and the water molecules in the system are more likely to aggregate as water clusters,so that water can form I_h ice in the entire molar fraction range.However,the carbon chain of methanol is shorter,with fewer hydrophobic groups and more hydrophilic alcohol hydroxyl groups.When the molar fraction of methanol is higher,water molecules are more likely to form heterogeneous clusters with methanol molecules,and the size of water molecule clusters becomes smaller and cannot form I_h ice.The above results indicate that the length of the carbon chain of organic matter directly affects the structure and existence form of water molecules.