2614
J. Phys. Chem. B 1998, 102, 2614-2617
Relating the Organization of the Molecular Tilt Azimuth to Lateral-Force Images in Monolayers Transferred to Solid Substrates Ulrike Gehlert, Jiyu Fang, and Charles M. Knobler* Department of Chemistry and Biochemistry, UniVersity of California, Los Angeles, California 90095-1569 ReceiVed: February 12, 1998
The molecular tilt azimuth in domains of a condensed phase in monolayers of glycerol esters at the air/water interface exhibits a regular star texture. Lateral-force images of these monolayers that have been transferred to a silicon substrate show that the tilt organization survives. Regular patterns are also found in images of monolayers on mica, but the organization differs from that observed at the air/water interface.
Introduction The imaging of monolayers at the air-water interface by fluorescence microscopy and Brewster-angle microscopy (BAM) has revealed a remarkable array of “textures” that reflect the spontaneous organization of the molecular tilt azimuth in tilted monolayer phases, those in which the molecules all have the same tilt with respect to the surface normal. This self-assembly is similar to that observed in liquid crystals. Among the patterns observed are stripes within continuous phases1 and “stars”, domains of an ordered phase surrounded by an isotropic (liquid or gas) phase.2 An example of a star texture is shown in Figure 1, which shows BAM images of domains in a monolayer of 1-monopalmitoyl-rac-glycerol (MPG). Each domain is divided into seven wedge-shaped segments. An analysis of such images3 demonstrates that the molecules within a segment have the same tilt azimuth; they point outward in the direction parallel to the segment bisector. There is a discrete jump in the tilt azimuth between segments. The extent to which the molecular organization in floating monolayers can be retained upon transfer of the film to solid supports is of both scientific and technical interest. If the integrity of the structures persists on transfer, then techniques such as scanning force microscopy can be employed to examine the nature of the order on length scales smaller than those that can be resolved by optical microscopy. Such patterned films might also be used for liquid crystal alignment. The tilt organization in transferred films has been studied by Santesson et al.,4 who used lateral force microscopy (LFM) to examine Langmuir-Blodgett (LB) monolayers of a thiolipid on mica substrates. Although they observed regular 6-fold patterns within star-shaped solid domains and associated the patterns with different tilt directions, they concluded that the variations in lateral force were not directly related to the molecular tilt. In a more recent LFM study of another thiolipid5 on mica, a small asymmetry in right- and left-hand scan directions was attributed to the effect of molecular tilt. Unfortunately, the tilt azimuth on the water surface is not known for the thiolipids, so neither the fidelity of the transfer process nor the relation between tilt direction and force can be determined with certainty. We have conducted a similar study of domains of glycerol esters that have been transferred to silicon and mica. These amphiphiles are well-suited for such experiments because the organization of the tilt azimuth in star domains on the water
surface has been established by Brewster-angle microscopy3 and the underlying microscopic structure has been determined by diffraction at grazing incidence.6,7 Experimental Section The films were prepared in a NIMA type 611 trough. Monolayers of 1-monopalmitoyl-rac-glycerol (MPG) and 1-monostearoyl-rac-glycerol (MSG) (Sigma, purity >99 mol %) were spread from chloroform solutions (Fischer, spectranalyzed) onto water purified with a Millipore Milli-Q-system. (18 MΩ, pH 5.7). Surface pressure-area isotherms were identical to those reported earlier.3,8 Examination of the monolayer by BAM showed the coexistence of an isotropic liquid phase with 7-fold star domains 200-700 µm in diameter as previously reported.3 Because the maximum scan range of our force microscope is 100 µm, it is preferable to work with smaller domains of the condensed phase, and we therefore added a small amount (
