Phase transitions of untilted fatty-acid Langmuir monolayers during isobaric heating and cooling: Interfacial rheology results

We study the two-dimensional melting of fatty acid Langmuir monolayers via the mechanical properties of their untilted phases, compact solid (CS), solid (S), and superliquid (LS). Taking the dynamic moduli as order parameters in isobaric temperature sweeps of behenic and arachidic acid monolayers, the CS-S and S-LS transitions appear as continuous transitions. However, a discontinuity appears in a high-order crossed susceptibility that involves the derivative of the dynamic moduli with respect to the control parameter (temperature). The critical value of the shear storage modulus at the melting temperature agrees with the prediction of the KTHNY theory, but the characteristic exponent does not.