Due to the significant difference in physical properties between water and organic solvents, a completely different approach and selection of materials are needed in the formulation of paints and the design of painting.
The Unique Properties of Water as a Solvent
1. High Cohesive Energy Between Molecules
Water molecules are strongly bonded by hydrogen bonds, giving them a cohesive energy between 10-40 kJ/mol. This is much greater than the van der Waals forces in hydrocarbons, which are around 1 kJ/mol. Due to these strong intermolecular forces, water has high melting and boiling points and significant latent heat of vaporization. Also, because it has high cohesive energy and a small molar volume, its SP value is much higher compared to other organic solvents.
2. Regular Molecular Arrangement
Even as a liquid, water has a regular tetrahedral structure where molecules shift around through thermal motion. As it becomes ice, this motion stops and the molecules spread apart, making ice less dense than water. This results in buoyancy, a phenomenon not seen with other solvents.
3. Hydrophobic Interactions
Hydrophobic substances in water are accommodated in a slightly distorted tetrahedral structure, forming a cage-like space. To minimize this entropically unfavorable structure, hydrophobic substances tend to clump together, creating what is known as hydrophobic interactions, a unique phenomenon of water.
4. Considerations When Using Water as a Solvent
As shown below, since water and organic solvents (like toluene) have very different physical properties, the composition of paints and the design of painting must take a completely different approach and material selection.
・Difference in Solubility Parameters: Ordinary binder resins don’t dissolve in water, so emulsion resins are commonly used instead.
・Boiling Point, Latent Heat of Evaporation, Relative Evaporation Rate: Due to high latent heat compared to the boiling point, evaporation is slow, making dripping and sagging likely.
・Surface Tension: Poor wetting properties on materials, leading to repulsion or indentation.
・Dielectric Constant: Special attention is needed when used in electrostatic processes (like electrostatic painting).
・Main Forces of Adsorption: While solvents use acid-base interactions, water relies on hydrophobic interactions, requiring different requirements for pigment surfaces and the selection of dispersants.