Surface Rheological Transitions in Langmuir Monolayers of Bi-Competitive Fatty Acids

Abstract

This paper discusses a rheological and structural transition that is observed for Langmuir films of 12-hydroxystearic acid. This molecule presents an interesting bi-competitive absorption between primary and secondary hydrophilic groups on either end of an alkane chain, which leads to a sharp transition from an expanded phase to a crystalline condensed morphology as surface pressure is increased. This abrupt morphological transition is accompanied by a strong change in the interfacial rheology of the films. These observations are obtained using an instrument that allows the acquisition of Brewster angle microscopy (BAM) simultaneously with the measurement of dynamic interfacial moduli. Below the transition, it is demonstrated that the films are purely viscous and characterized by a Newtonian rheology. However, above a transition pressure, the films behave as solidlike and crystalline with high elasticity. It is argued that the transition occurs through an evolution from a flat molecular configuration on the surface with both hydrophilic ends adsorbed at the water interface to a vertical, straightened state that has the secondary hydrophilic group lifted out of the subphase. The crystalline phase is highly non-Newtonian with a frequency-dependent dynamic surface viscosity.

Smart Foams: Switching Reversibly between Ultrastable and Unstable Foams

“…These layers are most likely within a condensed-like state with the alkyl chains in close packing. It is well known that the presence of fatty acids adsorbed at an interface provide interfacial layers of high dilational moduli E reaching easily hundreds of mN/m [25,27,28] as the molecules are insoluble. Moreover, it is also known that hydroxylated fatty acids can pack at interface even more efficiently than usual fatty acids [25] .…”

“…Moreover, it is also known that 12-HSA can pack at interfaces even more efficiently than common fatty acids. [25] Such high dilational moduli (or low compressibility) are indeed the main reason why coarsening is almost stopped: the Gibbs criterion states that coarsening stops if E > g, where g is the surface tension. In fact, if the bubble interfaces cannot be continuously compressed, the smallest bubbles cannot vanish towards the largest ones, and coarsening is blocked.…”

Smart Foams: Switching Reversibly between Ultrastable and Unstable Foams

“…To achieve this goal, we use a fatty acid system made of the 12 hydroxy stearic acid (12‐HSA) as foam stabilizer. Fatty acids exhibit low surface tension24, 25 and have promising surface properties 26. Moreover, the 12‐HSA is known to form highly elastic layers at the air/water interface25 (and even crystalline structures at high surface concentrations) because of strong intermolecular interactions induced by hydrogen bonding between hydroxy groups.…”

“…Fatty acids exhibit low surface tension24, 25 and have promising surface properties 26. Moreover, the 12‐HSA is known to form highly elastic layers at the air/water interface25 (and even crystalline structures at high surface concentrations) because of strong intermolecular interactions induced by hydrogen bonding between hydroxy groups. Besides, fatty acids are model systems for investigating the properties of insoluble Langmuir interfacial monolayers 27.…”

Two-Particle Interfacial Microrheology at Polymer−Polymer Interfaces

“…PEG with a molecular weight of 200 (PEG200) (Acros Organics) is nearly Newtonian, with a viscosity of 0.052 Pa 3 s. PDMS and PEG are stable and completely immiscible at room temperature. Negatively charged sulfate-modified polystyrene (S-PS) fluorescent microparticles (Molecular Probes) with a density of 1.055 g/cm 3 were used as tracer particles. The diameter of the particles ranges from 0.04 to 1.0 μm although particles with a diameter of 0.2 μm were used as primary tracers unless noted.…”