A microscopic ellipsometric model of dioctadecyldimethylammonium bromide monolayer compression has been proposed. The suggested model explains the dependence of the ellipsometric parameters on the surface density throughout the entire range of compression, comprising the liquid expanded (LE) and liquid condensed (LC) phases, as well as their coexistence region. The model uses unique microscopic parameters, such as molecular polarizabilities, Lorentz factors and geometric molecular dimensions. Molecules are treated as rods whose cross-section decreases linearly during the LE-LC phase transition. During compression the rods change their orientation from parallel to the interface to almost perpendicular. It was shown that the volume density of the LE phase is 10-15% lower than that of the LC phase.