Self-assembled mesoscopic surface domains of fluorocarbon–hydrocarbon diblocks can form at zero surface pressure: tilting of solid-like hydrocarbon moieties compensates for cross-section mismatch with fluorocarbon moieties

Abstract

At low molecular areas, fluorocarbon–hydrocarbon diblocks (C_nF_2n+1C_mH_2m+1, FnHm), when spread as Langmuir monolayers on water, form organized monodisperse circular self-assembled domains, one molecule high and tens of nanometers in diameter. Whether such domains form at high molecular areas (low surface pressures) could until now not be established. Furthermore, the common assumption was that the inner core hydrocarbon chains within these domains were in the liquid state in order to compensate for the difference in the cross-section area between the perfluoroalkyl (∼30 Ų) and alkyl (∼20 Ų) chains. Our IRRAS investigation of F8H16 now establishes (1) that these diblock surface domains do exist at the air/water interface at large molecular areas (zero surface pressure), (2) that they remain essentially unchanged throughout film compression, and (3) that the H16 moieties are actually stretched in an all-trans configuration and tilted by ∼30° with respect to the normal to the monolayer in order to satisfy the greater space requirement of the F8 moieties. Consequently, the core of the domains is in an ordered, crystalline-like state, and the domains can be visualized as solid particles at the air/water interface.