| Abstract
| - The headgroup orientation of a merocyanine dye, 1-docosyl-4-(4-hydroxystyryl)pyridinium bromide, hasbeen studied as a function of surface pressure on an acidified and a buffered subphase at pH 4 by secondharmonic generation (SHG). The protonated form of the dye is SHG active for a fundamental wavelengthof 800 nm. Phase transitions observed in the pressure−area isotherms correlate with changes in headgrouporientation, thus yielding microscopic information on the nature of the surface phase transition. Thepolarization resolved SHG data are analyzed to recover the orientation of the merocyanine headgroup asa function of surface pressure. The complex index of refraction of the surface film has been taken intoaccount. Both the measured pressure−area isotherms and the orientation of the headgroup are found tobe dependent on the nature of the subphase. In particular, at low pressure the acidified subphase yieldsa more vertical mean orientation (24°) of the headgroup than the buffered surface (42°), while at highpressures, above the expanded-to-condensed phase transition, the mean orientation of the headgroup isnearly independent of subphase (>50°). This behavior is discussed in terms of complex formation betweenthe headgroup and the phthalate ions in the buffered subphase and shows how complexation of the adsorbatefrom the subphase can influence adsorbate orientation.
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