What makes substances hydrophilic or hydrophobic?

Sourav PanNovember 7, 2024

Answered step-by-step

Substances are classified as hydrophilic or hydrophobic based on their interactions with water, which are influenced by their molecular structure and polarity.

Hydrophilic Substances

Hydrophilic substances are often described as “water-loving.” They have a strong affinity for water due to their polar nature or the presence of charged functional groups. Key characteristics include:

• Polarity: Hydrophilic molecules typically contain polar bonds, which result in partial positive and negative charges. This allows them to form hydrogen bonds with water molecules, facilitating their dissolution in water. Common examples include salts, sugars, and alcohols.
• Contact Angle: When a droplet of water is placed on a hydrophilic surface, it tends to spread out, resulting in a contact angle of less than 90 degrees. This indicates strong adhesion between the water molecules and the surface.
• Dissolution: Hydrophilic substances readily dissolve in water because the polar water molecules stabilize the solute through interactions that involve hydrogen bonding.

Hydrophobic Substances

In contrast, hydrophobic substances are termed “water-fearing.” They do not mix well with water and exhibit the following characteristics:

• Nonpolarity: Hydrophobic molecules are typically nonpolar, meaning they do not have significant charge separation within their structure. This lack of polarity prevents them from forming hydrogen bonds with water. Examples include oils and fats.
• Contact Angle: When water droplets are placed on hydrophobic surfaces, they tend to bead up and maintain a contact angle greater than 90 degrees. This indicates that the attraction between the water molecules is weaker than the cohesive forces among the hydrophobic molecules themselves.
• Aggregation in Water: In aqueous environments, hydrophobic substances tend to aggregate or cluster together to minimize their exposure to water. This behavior is driven by the hydrophobic effect, where nonpolar molecules prefer to interact with each other rather than with polar water molecules.

Summary of Factors

The distinction between hydrophilic and hydrophobic substances can be summarized as follows: