SurfFitter analyzes isotherm data applying the PRISM model, which is built on basic thermodynamic relations. The PRISM model, although initially meant for lipid monolayers, is valid for monocomponent Langmuir monolayers of any insoluble surfactant. PRISM 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.

• At regions of single phase \(\beta\), surfactants occupy an average molecular area between \(\mathrm{APL}^\beta_{min}\) and \(\mathrm{APL}^\beta_{max}\), and surface pressure increases with monolayer compression according to the elasticity modulus \(K^\beta\).
• At regions of coexistence of \(\beta-\beta'\) phases, the molecular area for each phase is constant across compression: \(\mathrm{APL}^\beta_{min}\) and \(\mathrm{APL}^{\beta'}_{max}\), respectively. However, the proportion of molecules in each phase evolves with \(\mathrm{APL}\). Surface pressure should remain constant at coexistence according to the Gibbs phase rule strictely (PRISM-I approach), but can the formain of phase domains might deviate the isotherm from this behavior (PRISM-DC approach). This approach introduces additional fitting parameters (\(s\) and \(a\)) that account for the line tension between phases.

Additionally, based on the isotherm initial shape SurfFitter discriminates the presence of four phases in the isotherm (S-C-E-G) or only three (C-E-G and S-C-G). In any case, the PRISM model fitted parameters allow to extract additional thermodynamic information of the system:

S-solid, C-condensed, E-expanded, G-gas, b-bulk water, s-surface water.