What causes some substances to be hygroscopic?

Some materials naturally attract and hold water from the air. This property, called hygroscopicity, depends on features like polarity, porosity, and molecular structure. These materials either absorb water into their bulk or adsorb it onto their surface. At the molecular level, interactions such as hydrogen bonding or forming hydrates make water uptake an energy-favorable process.

Key Factors and Mechanisms

1. Molecular Structure and Polarity

• Polar molecules (e.g., salts, certain polymers) strongly attract water, which is also polar.
• This polarity leads to effective absorption of moisture.

2. Porosity and Surface Area

• Materials with tiny pores or large surface areas (like silica gel) draw in water vapor.
• Capillary action helps water move into narrow spaces within the material.

3. Chemical Interactions

• Some compounds form hydrates, where water becomes part of the substance’s structure.
• These hydrates are more stable than free water vapor, locking the moisture in.

4. Thermodynamic Favorability

• Water absorption lowers the overall energy (Gibbs energy) of the system.
• The process is energetically favorable, which explains why it happens naturally.

Examples of Hygroscopic Materials

• Ionic compounds: Many salts, such as sodium hydroxide or calcium chloride.
• Porous materials: Silica gel, commonly used for keeping products dry.
• Polymers: Plastics like PET, PA, and PC that must be dried before processing.
• Everyday substances: Honey, sugar, and wood, which all absorb moisture from the air.