Monolayers and bilayers of lipid mixtures self-assembled on mercury form spontaneously gel-phase (solid ordered, so) and liquid-ordered (lo) microdomains, thanks to the fluidity imparted to these films by the liquid metal support. The differential capacity of the hydrocarbon tail region of monolayers of mixtures of two lipid components of high and low transition temperature Tm, increases during the transition from the liquid disordered (ld) phase to the coexistence of ld and so phases. Addition of cholesterol to this binary mixture causes a decrease in differential capacity. This behavior is explained by regarding the capacity as a measure of the total perimeter of the so microdomains, due to the mismatch between these microdomains and the ld phase. Cholesterol removes this mismatch by converting the anisotropic so microdomains into isotropic lo microdomains (rafts). This allows differential capacity measurements by electrochemical impedance spectroscopy to follow phase transitions in lipid mixtures. The coexistance of ld, lo and so phases is confirmed by images of a distal lipid monolayer self-assembled on top of a thiolipid monolayer tethered to a mercury microcap, by using two-photon fluorescence lifetime imaging microscopy (2P-FLIM).