Lyophilic and lyophobic (lýo meaning “dissolve”, philéo – “love” and phóbos — “fear”) are quality characteristics of intermolecular interaction between the substance and the dispersion medium. If the substance and the dispersion medium are similar in molecular structure or the substance molecules strongly interact with the medium, for example, forming hydrogen bonds, we speak of lyophilic properties, and if the interaction between the substance and the environment is weak, we speak of lyophobic properties. If the dispersion medium is water the terms “hydrophilic” and “hydrophobic” are used (from Greek hydro meaning “water”, see hydrophobic interaction).

As a general rule, the hydrophobic part of amphiphilic compounds is a long straight hydrocarbon chain CH₃ (CH₂)_n, where n>4, while the hydrophilic part is a polar functional group such as COOH or a small ion, for example, COO^-, or N (CH₃)₃⁺. The existence of the polar and nonpolar parts of the molecule contributes to particle aggregation with the formation of micelles, bilayers and other structures (see Fig.). Surfactants, lipids, as well as many peptides, proteins, and polymers have amphiphilic properties.

As the amphiphilic substances in a solution are capable of forming different supramolecular structures – monolayers, micelles, liposomes and others (see Fig.) – they are widely used in the synthesis of nanoparticles of various nature, films and membranes. Besides, they are often used as a protective shell for nanoparticles.