Some alkylated merocyanine dyes form "J-aggregates" that are self-organized functional supermolecules in their monolayers prepared upon aqueous subphases containing metallic cations such as Mg²⁺. "What drives the J-aggregation?" was an open question because the molecules' static dipoles may prefer another type of aggregate. Recently, it has been found that one of those dyes forms a J-aggregate upon pure water and a detailed infrared absorption study of this new J-aggregate has answered to that question. That is, the large dielectric constant of water enables intermolecular hydrogen bonds (or metal chelation) to drive the J-aggregation against the interaction between the dipoles.