Abstract
It is shown that the air-liquid interface can be made to display the same rich curvature phenomena as common lyotropic liquid crystal systems. Through mixing an insoluble, naturally occurring, branched fatty acid, with an unbranched fatty acid of the same length, systematic variation in the packing constraints at the air-water interface could be obtained. The combination of atomic force microscopy and neutron reflectometry is used to demonstrate that the water surface exhibits significant tuneable topography. By systematic variation of the two fatty acid proportions, ordered arrays of monodisperse spherical caps, cylindrical sections, and a mesh phase are all observed, as well as the expected lamellar structure. The tuneable deformability of the air-water interface permits this hitherto unexplored topological diversity, which is analogous to the phase elaboration displayed by amphiphiles in solution. It offers a wealth of novel possibilities for the tailoring of nanostructure.
Highlights
The presence of two-dimensional surface micelles at the air–water interface was suggested over a century ago for insoluble surfactant monolayers studied using the Langmuir technique,[25] which has since been rationalized using thermodynamic arguments based on the non-linearity observed in first order phase transitions in pressure–area isotherms.[26,27]experimental methods to validate such interfacial structures were lacking at the time
By analogy to a lyotropic liquid crystal, relaxation of the packing constraint should lead to two observations
An overall decrease in the ratio of branched molecules should lead to a larger radius of curvature of surface domains observed with atomic force microscopy (AFM)
Summary
The presence of two-dimensional surface micelles at the air–water interface was suggested over a century ago for insoluble surfactant monolayers studied using the Langmuir technique,[25] which has since been rationalized using thermodynamic arguments based on the non-linearity observed in first order phase transitions in pressure–area isotherms.[26,27]experimental methods to validate such interfacial structures were lacking at the time. The presence of two-dimensional surface micelles at the air–water interface was suggested over a century ago for insoluble surfactant monolayers studied using the Langmuir technique,[25] which has since been rationalized using thermodynamic arguments based on the non-linearity observed in first order phase transitions in pressure–area isotherms.[26,27]. Vibrational sum frequency spectroscopy (VSFS) has been used to infer the transition from 2D “cartwheel” micelles to a monolayer at higher surface coverage.[29,30] Recently, large circular aggregates have been observed by using the Langmuir–Blodgett technique[31,32,33] to deposit insoluble monolayers of partially fluorinated long chain fatty acids from the air–water interface onto a solid substrate, followed by characterisation of the monolayer structure with AFM.[34]. This study was followed by several examples of similar surface patterning of short-lived 2D domains transferred from the air–water interface to solid substrates.[35,36,37,38] These domains consisted of circular, monodisperse monolayer patches, rather than the 2D cartwheel micelles mentioned above
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