Surface rheological transitions in Langmuir monolayers of bi-competitive fatty acids

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.