Phase diagram of n-tetradecyl phosphate in Gibbs monolayers

A phase diagram for Gibbs monolayers of n-tetradecyl phosphate (n-TDP) has been constructed from the surface pressure–time isotherms and the direct observation of the monolayer morphology using Brewster angle microscopy (BAM). Three different phases, which are gas (G), liquid expanded (LE) and liquid condensed (LC) are observed during the formation of the monolayers under different conditions. All of these phases coexist at the triple point of 6.5 °C of n-TDP monolayers. Below this triple point, a first-order G-LC phase transition has been found to occur. Above the triple point, a first-order G-LE phase transition is followed by another first-order LE-LC phase transition. BAM images confirmed the order of phase transitions and the nature of phases. The critical temperature for the LC phase formation has been found to be 36.5 °C. All the findings are in line with the present understanding of the spread monolayers.