Domain Structures and Phase Transitions in Langmuir Monolayers
- PDF / 1,845,170 Bytes
- 8 Pages / 420.48 x 639 pts Page_size
- 41 Downloads / 213 Views
DOMAIN STRUCTURES AND PHASE TRANSITIONS
IN LANGMUIR MONOLAYERS
XIA QIU, JAIME RUIZ-GARCIA AND CHARLES M. KNOBLER University and Biochemistry, Department of Chemistry California, Los Angeles, CA 90024
of
ABSTRACT A variety of recent measurements has shown that the phase diagrams of Langmuir monolayers of relatively simple amphiphiles such as fatty acids and their methyl and ethyl esters are remarkably complex. Nine condensed phases have been identified; their structures can be related to those of known smectic phases. A key distinction between phases is the orientation of the molecular tilt azimuth with respect to the local hexagonal order of the head groups. When monolayers are examined by fluorescence microscopy, regions of uniform tilt can be observed if the exciting radiation is polarized with respect to the surface normal. The tilt regions form patterns similar to those observed in freely suspended films of smectic liquid crystals. The patterns can be changed by compressing the film and by changing the temperature. Transitions between different phases can be observed. INTRODUCTION The traditional discussion [1] of the phase diagrams in Langmuir monolayers -- insoluble monolayers at the air/water interface -has been in terms of four phases: gas, liquid expanded, solid and liquid condensed. The first three phases were thought of as the two-dimensional analogues of their three-dimensional counterparts, while the microscopic nature of the liquid condensed phase, which intrudes between the liquid expanded and solid phases, remained controversial. Indeed, the existence of this phase had been doubted by some; unequivocal proof of coexistence between liquid expanded and liquid condensed phases has only recently been conclusively established [2]. These four phases can be identified with relatively easily distinguished features in surface pressure-area isotherms. A number of other features are often observed, but these have usually been dismissed as artifacts. Some experimenters such as Stenhagen [3] and Lundquist [4] took the view that the minor features in isotherms were real and used them to construct complex monolayer phase diagrams. Their work was largely overlooked until x-ray studies showed that there are a variety of distinct condensed phases in Langmuir monolayers of common amphiphiles [5-9]. Bibo, et al. [10] carried out extensive isotherm measurements that agreed well with those described by the early workers. They also analyzed the features of the diffraction studies and showed that they are largely consistent with the Stenhagen-Lundquist phase diagrams. They proposed that the condensed monolayer phases had the character of two-dimensional smectic liquid crystals and that each monolayer phase could be identified with a known thermotropic smectic phase. Four order parameters are needed to characterize the phases: (1) Positional order (PO); (2) Bond orientational Mat. Res. Soc. Symp. Proc. Vol. 237. Q1992 Materials Research Society
264
order; (3) Tilt order (TO) - correlations of the molecular tilt azimu
Data Loading...
