Langmuir 1993,9, 2145-2150
2145
Ellipsometric and Fluorescence Microscopic Investigations of a Cyclam Derivative at the Air/Water Interface D. Ducharme,? C. Salesse,*pgR. M. Leblanc,',* P. Meller,* C. Mertesdorf,* and H. Ringsdorfl Centre de recherche en photobiophysique, Universitk du Qukbec il Trois-RiviBres, Trois-RiviBres, Qukbec, Canada G9A 5H7,and Imtitut fGr Organische Chemie, University of Mainz, 0-6500 Mainz, Germany Received March 17,1993
In this study, a cyclam derivative bearing four aliphatic chain substituents shows,as many amphiphiles, liquid-expanded as well as solid phases. In contrast to the classical amphiphiles, the T-A isotherm shows a bumplike shape at the beginning of the phase transition for which the amplitude is a function of the compression speed. Ellipsometry, which is very sensitive to the monolayer physical state changes, and fluorescence microscopy, which has contributed significantly to the understanding of the phenomena occurring in the phase transition region,have been usedto investigate the monolayer behavior of N,N'~,""tetramyristoyl-lJ4,8,11-tetraazacyclotetradecane at the air/water interface. This study shows that in the liquid-expanded state, independent of the compression speed, the f i i is homogeneous and remains as such until either the maximum amplitude of the bump is reached or the beginning of the plateau sets in. Thereafter, the phase transition and solid state show domains for which the sizes, shapes, and orientation are dependent upon the compression speed. Homogeneous diamondlike shape domains with preferred orientation appear at low compressionspeeds (1.0and 3.5 Az/(molecule*min))whereas random orientation of heterogeneous domains prevails at higher compression rates (17 AZ/(molecule.min)). Ellipsometric measurements are also characterized by their dependence upon the compression rates that change the optical properties of the surface. Increased light intensity at compensation is explained with surface anisotropy.
Introduction Monolayer studies of disklike mesogens at the aidwater interface have attracted interest due to their potential application as new materials. Only a fewreports on discotic liquid crystalline compounds with hydrocarbon chains attached to the discoid aromatic cores by ester or ether linkages have been published.lg One of these studies on disklike moleculesof benzenehexa-n-alkanoatederivatives concluded that, in the low compressibility region, all hydrophilic ester anchoring groups simultaneously face the water surface, resulting in a flat arrangement of the benzene ring with its short axis perpendicular to the air/ water interface.l Recent investigations on triphenylene, anthraqqinone, and benzene derivatives claim that in the low compressibility phase the aromatic ring systems are oriented verticallywith their short axis parallel to the water surface? However, due to their chemical structures, these compounds are expected to lack distinct amphiphilic propertie~.~ Enhanced amphiphilic character of cyclic compounds has recently been achieved with synthesized poly-Nacylated derivatives of macrocyclic polyamine^.^^^ Like many amphiphiles, derivatives with aliphatic chain substituents exhibit in sequence liquid-expanded (LE) as well t Department de chimie, CEGEP de Shawinigan, Shawinigan, Quebec, Canada G9N 6V8. t University of Mainz. Present address: Universit&du Quebec B Trois-RiviBres. Universit4 du Quebec B Trois-Rivihres. (1) Rondelez, F.; Koppel, D.; Sadashiva, B. K. J . Phys. (Park) 1982, 43, 1371. (2) Albrecht,O.;Cufnming,W.;Kreuder,W.;Lascheweky,A.;Ringsdorf, H. Colloid Polym. Scz. 1986,264,659. (3) Suresh, K. A.; Blumstein, A.; Rondelez, F. J . Phys. (Paris) 1985, 46, 453. (4) Mertesdorf, C.; Ringsdorf, H. Lip. Ctyst. 1989, 4 , 1. (6)Malthbte,J.; Poupinet, D.; Vilanove, R.; Lehn,J. M. J. Chem.SOC., Chem. Commun. 1989,15,1016.
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as solid (S)phases. However, in contrast to the classical amphiphiles, their *-A isotherm is characterized by the appearanceof a "bump" at the onset of the phase transition The amplitudeof this bump between the LE and S is dependent upon the compression speed. A firstcorder transition can be obtained at a very low compressionspeed (
