Fibrous crystals: Relaxation processes and dilational properties at oil–water interface

The dilational viscoelastic properties of interfacial films containing fibrous crystals were investigated by means of two methods: the interfacial tension response to sinusoidal area variations and the relaxation of an applied stress. The results obtained show that fibrous crystals (length > 50 μm) could increase the dilational modulus of interfacial films on a jet kerosene/water interface. The dilational modulus increased with time due to the increase of sedimenting quantities and decreased with concentration after 15 h of pre-equilibrium time due to the enhancement of particle exchange between the bulk and the interface. The nature of films containing solid particles strongly depends on their shape. The ethyl metal phosphates, which are short and rigid, may pack tightly at the interface and form long-range ordered structure. As a result, interfacial films containing ethyl metal phosphates had higher dilational modulus and showed more elastic behaviour. The results obtained by interfacial tension relaxation measurements show that there exist two main relaxation processes in all films: the fast relaxation process with a characteristic relaxation time of several seconds, which may correspond to the exchange of alkyl metal phosphates between the bulk and the interface, and the slower relaxation process with a characteristic relaxation time of several tens of seconds, which may correspond to the interparticle interaction. However, only the films containing ethyl metal phosphates have the third relaxation process, which is the slowest process with a characteristic time of several hundreds of seconds. This slowest process may correspond to the reorganization of whole interfacial film. Our results may be used to account for the ability of solid particles in stabilizing emulsions.