Kinetic Appearance of First-Order Gas−Liquid Expanded and Liquid Expanded−Liquid Condensed Phase Transitions below the Triple Point

Phase diagram of Gibbs monolayers of mixtures containing n-hexadecyl phosphate (n-HDP) and l-arginine (l-arg) at a molar ratio of 1:2 has been constructed by measuring surface-pressure−time (π−t) isotherms with film balance and by observing monolayer morphology with Brewster angle microscopy (BAM). This phase diagram shows a triple point for gas (G), liquid expanded (LE), and liquid condensed (LC) phases at around 6.7 °C. Above this triple point, a first-order G−LE phase transition occurring at 0 surface pressure is followed by another first-order LE−LC phase transition taking place at a certain higher surface pressure that depends upon temperature. The BAM observation supports these results. Below the triple point, the π−t measurements show only one first-order phase transition that should be G−LC. All of these findings are in agreement with the general phase diagram of the spread monolayers. However, the BAM observation at a temperature below the triple point shows that the thermodynamically allowed G−LC phase transition is, in fact, a combination of the G−LE and LE−LC phase transitions. The latter two-phase transitions are separated by time and not by the surface pressure, indicating that the G−LC phase transition is kinetically separated into these two-phase transitions. The position of the LE phase below the triple point in the phase diagram is along the phase boundary between the G and LC phases.