It was shown that the low-frequency edge of the exciton absorption band of J-aggregates of 1-methyl-1'-octadecyl-2,2'-cyanine iodide (analogue of PIG) experiences a change from Gaussian to Lorentzian in frozen, binary DMF-H2O solution, upon decrease of water concentration. In a Langmuir-Blodgett film, the shape of the low-frequency edge of exciton absorption band was neither Gaussian nor Lorentzian, in contrast. A model of diagonal and off-diagonal disorder in the molecular chains is proposed to explain the observed band shapes, based on selective solvation of amphiphilic molecules by water and DMF solvent molecules.