Studies on kinetics of isomerization of gamma oryzanol at air-water interface

Highlights: • The phase transition of gamma oryzanol ($\gamma$-Or) Langmuir monolayer is studied. • Calculations of rate constants of trans-cis isomerization of $\gamma$-Or at interface. • The rate constant of photo/thermal isomerization show Arrhenius temperature dependence. We report the studies on Langmuir monolayer of gamma oryzanol ($\gamma$-Or) in $trans$ and $cis$ states and kinetics of photo-isomerization and thermal isomerization of $\gamma$-Or at air-water interface. The surface manometry studies show that the monolayer of $trans-\gamma$-Or on compression exhibits gas and liquid expanded phases with a small region of gas-liquid expanded coexistence region. The monolayer collapses at a surface pressure of 34 mNm${}^{-1}$. In contrast to this, the $cis-\gamma$-Or monolayer on compression exhibits an extended gas-liquid expanded coexistence region between gas and liquid expanded phases before collapsing at a surface pressure of 38 mNm${}^{-}1$. The studies on isomerization of $\gamma$-Or in the Langmuir monolayer show that the photo-isomerization process follows first order kinetics and thermal isomerization follows second order kinetics. The rate constants of photo-isomerization and thermal isomerization of $\gamma$-Or at air-water interface obtained at different temperatures show Arrhenius temperature dependence. The activation energy for the photo-isomerization was found to be 85.34 kJ/mol and for thermal isomerization it was found to be 51.33 kJ/mol.