No abstract

A technique for the manipulation of Langmuir-monolayers at the air/water interface is presented. Optical tweezers are combined with fluorescence and Brewster angle microscopy to produce a system capable of mechanical and thermal handling of monolayers on a micrometer scale. The viscoelastic, electrical, and thermodynamic properties of the monolayer can be determined by analysis of the perturbations of its structure. A method for force and heat calibration of the tweezers is given. Both kinds of micromanipulations are applied to the star shaped condensed domains in liquid expanded surroundings of bis[8-(1,2-dipalmitoyl-snglycero-3-phosphoryl)-3,6-dioxaoctyl]disulfide ("thiolipid") monolayers. The stability of the stars and the consequence of their interlocking on the interfacial flow are investigated.

Surface pressure-temperature phase diagrams have been determined by Brewster-angle microscopy for Langmuir monolayers of heneicosanoic acid with the esters methyl and ethyl heneicosanoate and octadecanoic acid with methyl, ethyl, and propyl octadecanoate. The behavior is similar to that found previously in mixtures of an acid and an alcohol. In each case with increasing ester concentration the L 2 /L 2 Ј phase boundary moves toward lower pressure and higher temperature while the L 2 /Ov boundary moves toward lower pressure and lower temperature. The L 2 Ј and Ov phases eventually merge and the boundary with the L 2 phase moves to zero pressure. The phase diagram of eicosyl acetate is similar to that of the fatty acids. We attribute the variations in the diagrams to the extent of hydrogen bonding between the head group and the subphase.