Temperature and compression rate independent domain shape in Langmuir monolayers of di-n-dodecyl hydrogen phosphate at the air-water interface

Thermodynamic and morphological properties of Langmuir monolayers of di-n-dodecyl hydrogen phosphate (DDP) have been studied by film balance and Brewster angle microscopy (BAM) over a wide range of temperature between 5 and 40 degrees C. From pi-A isotherms, a generalized phase diagram consisting of gas (G), liquid expanded (LE) and liquid condensed (LC) phases is constructed for the DDP monolayers. The BAM images show the formation of gas bubble in the bright background of LE phase during G-LE phase transitions and fingering LC domains during LE-LC phase transitions. The shapes of these domains are independent of temperature, showing a sharp contrast to the temperature-dependent monolayer morphologies of amphiphilic systems where the shape of the LC domains changes either from compact circular to fingering or from irregular or spiral to compact patterns with increasing temperature. In addition, the domains do not show any change in their shapes with decreasing the compression rate. Since the two-alkyl chains are directly attached by covalent bonds to the phosphate group, the rearrangement of the molecules needs to move the whole molecules including the hydration sphere. The difficulty related to such a movement of the molecules causes the fingering domains, which are independent of external variables. Although the domains are formed in a fingering shape, the equilibrium shape can be attained by about 120 min at 15 degrees C indicating a rather slow relaxation rate.