The model employed in SurfFitter, derived from basic thermodynamics equations, identifies the regions of pure phase and phase coexistence in the isotherm taking as fitting parameters all the \(\mathrm{APL}\),\(\pi\) values that mark the boundaries between phases (white dots in the figure). See the article referenced above for a more detailed explanation.

• At regions of pure phase \(\beta\), lipids occupy an average moleculararea betweeen \(\mathrm{APL}^\beta_{min}\) and \(\mathrm{APL}^\beta_{max}\). The surface pressure increases with the monolayer compression according o the elasticity modulus \(K^\beta\) of that phase.
• At coexistence regions, for any \(\mathrm{APL}\) value, lipids in the \(\beta,\beta'\) phases occupy an average molecular area \(\mathrm{APL}^\beta_{min}\) and \(\mathrm{APL}^\beta_{max}\), respectively, while the surface pressure is constant and equal to \(\pi^{\beta\beta'}\).

The recorded isotherm might sometimes not capture some of the phases. For example, if the monolayer collapses before reaching the solid or condensed phase, if the isotherm was recorded exactly at the G-E-C coexistence temperature (\(T_0\)), or below (which would imply that the E phase is not present in the monolayer). When the model suspects that it is facing one of this cases, it might ignore the fitting of some of the \(\mathrm{APL}\), \(\pi\) parameters. In any case, the fitted parameters allow to extract additional thermodynamic information of the system: