Lipids, including fats, are considered hydrophobic due to their molecular structure. The primary reason for this property lies in the nature of the components that make up lipids, particularly their long hydrocarbon chains.
Hydrophobicity refers to the tendency of a substance to repel water. In the case of lipids, their structure consists primarily of long chains of carbon and hydrogen atoms, which do not form hydrogen bonds with water molecules. Instead of interacting with water, the nonpolar hydrocarbon chains aggregate together. This aggregation is driven by the entropy of the surrounding water molecules, which prefer to remain in an ordered state rather than interact with hydrophobic substances.
Additionally, the majority of fats are triacylglycerols, which have a glycerol backbone esterified to three fatty acid chains. These fatty acids can be saturated (no double bonds) or unsaturated (one or more double bonds). The presence of double bonds in unsaturated fatty acids introduces kinks in the chains, preventing the lipids from packing closely together. However, regardless of their structure, the nonpolar nature of these molecules leads to their hydrophobic characteristics.
In contrast, substances that are hydrophilic, like sugars or proteins, contain polar groups that can interact with water through hydrogen bonding. This fundamental difference in molecular structure and interaction with water explains why fats and lipids as a whole are hydrophobic.