Interactions of l-arginine with Langmuir monolayers of di-n-dodecyl hydrogen phosphate at the air–water interface

The surface phase behavior of di-n-dodecyl hydrogen phosphate (DDP) in Langmuir monolayer and its interactions with l-arginine (l-arg) have been investigated by measuring π–Aπ–A isotherms with a film balance and observing monolayer morphology with a Brewster angle microscopy (BAM). The DDP monolayers on pure water show a first-order liquid expanded–liquid condensed (LE–LC) phase transition and form fingering LC domains having uniform brightness at different temperatures. At 15 °C, the π–Aπ–A isotherms on pure water and on different concentration solutions of l-arg show a limiting molecular area at ∼0.50nm2/molecule. With increasing the subphase concentration of l-arg up to 4.0×10−4M, the LE and the LE–LC coexistence regions shift to larger molecular areas and higher surface pressures, respectively. With a further increase in the concentration of l-arg beyond this critical concentration, these isotherms show little or no more expansion. These results have been explained by considering the fact that the l-arg undergoes complexation with the DDP to form l-arg–DDP that remains in equilibrium with the components at the air–water interface. As the concentration of l-arg in the subphase increases, the equilibrium shifts towards the complex. At a concentration of l-arg⩾4.0×10−4M, the DDP monolayers get saturated and show the characteristics of the new amphiphile, l-arg–DDP. BAM is applied to confirm the above results. When the concentration of the l-arg is <4.0×10−4M, domains always start forming at an area of ∼0.64nm2/molecule, which is the critical molecular area for the phase transition in the DDP monolayers on pure water. In contrast, when the monolayers are formed on a solution containing ⩾4.0×10−4Ml-arg, comparatively smaller size domains are formed after the appearance of a new cusp point at ∼0.55nm2/molecule. With an increase in the concentration of l-arg in the subphase, the size of the domains decreases indicating that the fraction of the DDP gradually decreases, whereas the fraction of the complex gradually increases. In addition, a very simple procedure for determination of the stability constant, which is 2.6×104M−1 at 15 °C, has been suggested.

Expansion of π–Aπ–A isotherms of the DDP monolayers in the presence of l-arg at 15 °C.Figure optionsDownload as PowerPoint slide