DETERMINATION OF ALKYL CHAIN TILT ANGLES IN LANGMUIR MONOLAYERS - A COMPARISON OF BREWSTER-ANGLE AUTOCORRELATION SPECTROSCOPY AND X-RAY-DIFFRACTION

Brewster angle autocorrelation spectroscopy (BAAS), a combination of B rewster angle microscopy and the photocorrelation technique is a new m ethod for the quantitative characterization of Langmuir monolayers. In this paper we compare tilt angle measurements in octadecanol monolaye rs performed using BAAS and grazing incidence x-ray diffraction (GIXD) . BAAS offers the possibility of tilt angle measurements in Langmuir m anolayers-without any fitted parameters-with a precision similar to GI XD, but ten times faster, GIXD, conversely, offers detailed microscopi c information not available from BAAS, The swiftness of BAAS is exploi ted for a detailed analysis of the phase transitions from the L-2' pha se to the LS (Rotator I) and LS (Rotator II) phases. The L-2'/LS (Rot I) transition is of first order whereas the L-2'/LS (Rot II) transitio n is more probably of weak first order than of second order. (C) 1998 American Institute of Physics.