Experimental investigation of the reversible collapse of a capped amphiphile Langmuir monolayer

Surface isotherms, Brewster angle microscopy (BAM), surface potential measurements, and ab initio calculations were used to examine Langmuir monolayer films of an amphiphile with a bulky group that caps the terminus of an alkanol chain. BAM images of the collapsed state differ significantly from those of long-chain paraffinic amphiphiles. The collapsed phase BAM images show that aggregates of these amphiphiles extrude to form a three-dimensional phase. These clusters are able to easily reform the pre-collapse monolayer upon release of the applied surface pressure to pre-collapse values. This system may be described over all accessible surface pressures as a two-dimensional liquid-crystal analog at temperatures near and above room temperature. The behavior of this amphiphile is suggested to be related to structural elements of the molecule, particularly the bulky capping group, that may define a class of such systems.