JOURNAL OF CHEMICAL PHYSICS, Volume 108, NUMBER 11, 15 MARCH 1998, p. 4640
Determination of alkyl chain tilt angles in Langmuir monolayers:
A comparison of Brewster angle autocorrelation spectroscopy and x-ray diffraction
C. Lautz, Th. M. Fischer, M. Weygand, and M. Lösche
Institute of Experimental Physics I, University of Leipzig. D 04103 Leipzig, Germany
P. B. Howes and K. Kjaer
Department of Solid State Physics, Riso National Laboratory, DK-4000 Roskilde, Denmark
(Received 29 September 1997: accepted 9 December 1997)
Brewster angle autocorrelation spectroscopy (BAAS), a combination of Brewster angle microscopy and the photocorrelation technique is a new method for the quantitative characterization of Langmuir monolayers. In this paper we compare tilt angle measurements in octadecanol monolayers performed using BAAS and grazing incidence x-ray diffraction (GIXD). BAAS offers the possibility of tilt angle measurements in Langmuir monolayers - without any fitted parameters - with a precision similar to GIXD, but ten times faster. GIXD, conversely, offers detailed microscopic information not available from BAAS. The swiftness of BAAS is exploited for a detailed analysis of the phase transitions from the L2' phase to the LS (Rotator I) and LS (Rotator II) phases. The L2'/LS (Rot I) transition is of first order whereas the L2'/LS (Rot II) transition is more probably of weak first order than of second order.