Effect of temperature on the surface phase behavior of n-hexadecyl dihydrogen phosphate in adsorption layers at the air–water interface

We present the adsorption kinetics and the surface phase behavior of n-hexadecyl dihydrogen phosphate (n-HDP) at the air–water interface by film balance and Brewster angle microscopy (BAM). A phase diagram, which shows a triple point at about 25.8 °C, is constructed by measuring the surface pressure (π)–time (t  ) adsorption isotherms. Below 25.8 °C, each of the π–tπ–t curves shows a plateau at about zero surface pressure indicating the existence of a first-order phase transition. The BAM observation confirms the order of this phase transition by presenting two-surface phases during this plateau. However, the BAM observation also shows clearly another second-order phase transition from an isotropic phase to a mosaic-textured liquid condensed (LC) phase. The initial phase is a gas (G) phase. Considering the peculiarity of the middle phase, we suggest this phase as an intermediate (I) phase. Above the triple point, the π–tπ–t curves predict the existence of two-step first-order phase transitions. Similar to the results at lower temperatures, the BAM images show two-surface phases during these first-order phase transitions together with a second-order phase transition from an isotropic phase to an LC phase. These transitions are classified as a first-order G–LE (liquid expanded) phase transition, which is followed by another first-order LE–I phase transition. The second-order phase transition is an I–LC phase transition. Contrary to these results, at 36 °C both the π–tπ–t measurements and the BAM observation present only two first-order phase transitions, which are G–LE at zero surface pressure and LE–LC transition at higher surface pressure. The shape of the domains during the main transitions shows a peculiar change from a circular at 20 °C to an elongated at 24 °C and finally to a circular shape at 36 °C. Such a change in the domain shapes has been explained considering the dehydration effect at higher temperatures as well as the nature of phases.

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