Photoinduced Phase Separation and Miscibility in the Condensed Phase of a Mixed Langmuir Monolayer

Abstract

We report our studies on the mixed Langmuir monolayer of mesogenic molecules, p-(ethoxy)-p-phenylazo phenyl hexanoate (EPPH) and octyl cyano biphenyl (8CB), employing the techniques of surface manometry and Brewster angle microscopy. Our studies show that the mixed monolayer exhibits higher collapse pressures for certain mole fractions of EPPH in 8CB as compared to individual monolayers. Also, a considerable reduction in the area per molecule is seen in the mixed monolayer, indicating a condensed phase. We have also studied the photostability of the mixed monolayer at different initial surface pressures. The mixed monolayer, under alternate cycles of UV and visible illumination, exhibits changes in surface pressures. This is due to the photoinduced transformation of EPPH isomers in the mixed monolayer. Our in-situ Brewster angle microscope studies for 0.5 mole fraction of EPPH in 8CB show a phase separation in the UV and a miscible phase in the visible, at low surface pressures ( approximately 5 mN/m). At higher surface pressures ( approximately 10 mN/m), under UV illumination, we find a phase separation which does not revert to a miscible phase under visible illumination.

Photochemical Phase Transition versus Photochemical Phase Separation

“…For the system studied, significant photochemical phase separation at the micrometer scale was observed from about 10% of azo dopant. Note that the phenomenon reported here is different from the photoinduced phase separation in azobenzene-containing Langmuir monolayers and is different from the nanophase segregation of an azo dopant from within the layers to locations between the layers of a smectic LC upon the trans−cis isomerization …”

Liquid Crystals at Interfaces

“…If the mixed monolayer is irradiated at 360 nm, the EPPH molecules undergo isomeric transition from trans to cis and, due to shape incompatibility (rod‐ and bow‐shaped molecules), they expel out of the mixed monolayer. On the other hand, upon irradiation at 440 nm, the cis isomers transform to trans , leading to complete miscibility of the 8CB and EPPH molecules 61…”

Molecular Interactions at Interfaces

“…It is well known that the Ch molecules form a stable monolayer at the A-W interface Slotte & Mattjus (1995); Lafont et al (1998). Though -SH group is weakly acidic in nature Bilewicz & Majda (1991), the amphiphilic balance of the TCh for the formation of a There are numerous reports in literature indicating the formation of mixed monolayer of a non-amphiphilic component doped into the monolayer of amphiphilic molecules (Silva et al, 1996;Wang et al, 2000;Viswanath & Suresh, 2004). In all these cases, the monolayer has been stabilized upto certain value of surface pressure due to hydrophobic interaction of the tail groups of the different components.…”