Neutron and X-ray Reflectivity from Polyisobutylene-Based Amphiphiles at the Air-Water Interface

Abstract

We have performed X-ray and neutron reflectivity experiments on amphiphile films spread at the air-water and air-saturated ammonium nitrate solution interfaces as a function of film compression. We have examined ca. 750, 1100, and 1700 molecular weight monodisperse polyisobutylene amide (PIBSA), 1100 imide (PIBSIM), and also palmitic acid for the pure amphiphiles and binary mixtures. We have used deuterated and hydrogenous components so that a given film may be examined in up to seven contrasts. All of the films form a monolayer, with molecular area determined by tail group size, which increases slightly in thickness on compression for PIB derivatives. There are differences in film thicknesses as shown by a dependence on compression and substructure; this is understood in terms of the increasing tendency of the tail to adopt a coiled conformation as molecular weight increases. PIBSA films consist of packed micrometer scale disks containing up to 10% open water (polynya) area. Palmitic acid-containing films contain a greater polynya area. All films lose up to half of the original amount of amphiphile from the monolayer when compressed. The almost reversible loss of material followed the trend palmitic acid < 750 amide < 1100 amide < 1100 imide, empirically in order of hydrophile-lipophile balance (HLB) number. This pattern suggests an amphiphile loss into submicrometer aggregates at the interface, rather than dispersion into the aqueous phase. Binary mixtures exhibit differential competition of the components to remain in the monolayer with the trend: palmitic acid > 750 amide > 1100 amide > 1100 imide, opposite to the aggregation tendency as expected. The preference to remain at the surface may be physically related to more effective packing of the headgroup molecules on the surface when head and tail areas are better matched in size. Mixtures also spontaneously segregate laterally and form micrometer scale domains. This tendency also follows the HLB number, with better mixing for those components with similar HLB values. Lateral segregation is encouraged by film compression. Surprisingly, properties of films on water and 50 wt % ammonium nitrate subphase show little difference with respect to these trends.