Language
Country/Area
No result found
The surprising transformation of superheated water: How do its properties change as the temperature rises?
Superheated water is liquid water under pressure at a temperature between its regular boiling point (100°C or 212°F) and its critical temperature (374°C or 705°F). This kind of water is stabilized by overpressure, or by heating water in a closed container to maintain a liquid state under the balance of saturated vapor pressure. Its characteristics are significantly different from water in an atmospheric pressure environment. Research shows that when water is heated to a superheated state, many of its unusual properties undergo surprising changes.
"Water's hydrogen bonds are broken during the heating process, making the water less polar, which causes the water to start to behave more like an organic solvent."
Changes in properties of superheated water
As water temperature increases, superheated water shows more significant property changes than other substances. The viscosity and surface tension of water decrease with increasing temperature, while diffusivity increases. When the water temperature increases, the self-ionization of water will also increase. Its pKw value is about 11 at 250°C, showing that the concentration of hydrogen ions (H3O+) and hydroxyl radicals (OH−) in the water increases, but the pH remains the same. Maintain at neutral.
Analysis of abnormal behavior
Water is a polar molecule with positive and negative charges separated in the center. When heated, the thermal motion of the hydrogen-bonded structure destroys the overall polarity of water, causing the relative dielectric constant of water to decrease as the temperature increases. At 205°C, the relative dielectric constant drops to 33, similar to that of methanol at room temperature. This phenomenon causes the water to begin to resemble a water-methanol mixture, affecting its solubility and chemical reactivity.
Solubility changes
Organic compounds
As temperature increases, the solubility of organic molecules often increases significantly, in part due to changes in polarity. In addition, certain substances considered insoluble at conventional temperatures may become soluble in superheated water. For example, the solubility of PAHs increases by five orders of magnitude at 25°C compared to 225°C.
Salts
Despite a decrease in relative permittivity, many salts remain soluble until close to the critical point. For example, sodium chloride has a solubility of up to 37 wt% at 300°C. However, as the critical point approaches, its solubility decreases significantly.
Gas
The solubility of gases in water usually decreases as temperature increases, but rises again after a certain temperature. The solubility of oxygen in superheated water is particularly enhanced, enabling its application in wet oxidation processes.
Corrosive effects
Superheated water above 300°C may be more corrosive than room temperature water. This means that under these conditions, special care must be taken in the selection of equipment materials, often requiring the use of corrosion-resistant alloys.
Energy requirements
The energy required to heat water is significantly less than the energy required to evaporate it, making the use of heat exchangers to recover energy more feasible. For example, the energy required to heat liquid water from 25°C to 250°C is approximately 976 kJ/kg, which is significantly less than the 2869 kJ/kg required to convert it into steam.
Extraction and chemical reactions
Superheated water is widely used in extraction and chemical reaction processes. For example, it can quickly and selectively extract valuable components from plants, and can effectively chemically convert organic materials into fuel products, which is of great significance for environmental protection.
Application of chromatography technology
In reversed-phase high-performance liquid chromatography, a commonly used mobile phase is a methanol-water mixture. Since the polarity range of water is stable with temperature changes, this allows its properties to be effectively utilized in chromatographic separations for the separation and analysis of various organic compounds.
The transformation of the properties of superheated water not only reveals the unique potential of water as a solvent in science and industry, but also prompts us to think about how water behaves under various environmental conditions and how its potential uses may affect future technologies. And environmental sustainability?
