The model employed in SurfFitter, the PRISM model, is derived from basic thermodynamics equations. This was originally meant for lipid monolayers, but it is extrapolable for any insoluble surfactant monolayers. For that reason, fitting parameters are denoted as \(\mathrm{APL}\) (Area Per Lipid) instead of \(\mathrm{AMM}\) (Mean Molecular Area). It identifies the regimes of single-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 above). See the article referenced above for a more detailed explanation.

• At regions of pure phase \(\beta\), lipids occupy an average molecular area between \(\mathrm{APL}^\beta_{min}\) and \(\mathrm{APL}^\beta_{max}\), and the surface pressure increases with the monolayer compression according to the elasticity modulus \(K^\beta\) of that phase.
• At coexistence regions, for any \(\mathrm{APL}\) value, lipids in the \(\beta,\beta'\) phases occupy a constant average molecular area \(\mathrm{APL}^\beta_{min}\) and \(\mathrm{APL}^\beta_{max}\), respectively. It is the proportion of lipids at each phase that changes across compression. The surface pressure, according strictely to the Gibbs phase rule, is constant and equal to \(\pi^{\beta\beta'}\) (PRISM-I). However, there might be some deviations from ideality (light slopes in the isotherms) due to line tension at phase domains (PRISM-DC).

SurfFitter implementation of the PRISM model considers the introduction of PRISM-DC only when statistically needed. Moreover, it takes into account that not all phases might be present in the isotherm, and proposes two alternative phase identification schemes when ambiguity. In any case, the fitted parameters allow to extract additional thermodynamic information of the system: