Time-resolved atomic force microscopy and contact angle measurements reveal that the self-assembly of octadecyltrichlorosilane-based monolayers on the oxidized Si(100) surface can be initiated by three distinct mechanisms : island growth at low temperatures (T < 16 degrees C), homogeneous growth at high temperatures (T greater than or equal to 40 degrees C), and a mixed regime at intermediate temperatures. These results support the notion of a transient Langmuir film at the substrate-solution interface : the observed growth patterns are consistent with nucleation of liquid-condensed domains coexisting with a two-dimensional gas at low temperature, or with a liquid-expanded phase at intermediate temperatures.