Visco-elastic properties of thin surfactant films studied with the tribological surface force apparatus

Normal force and visco-elastic measurements were performed using the tribological surface force apparatus, where the distance between molecular smooth surfaces was controlled to within 1 Angstrom. During the measurements the surfaces were imaged using FECO optical interference technique, this gives continuous information about the local surface geometry and the surface separation. The static and dynamic behaviour of thin surfactant films in aqueous solution of hexadecyltrimethyl-ammonium salicylate (C16TASal) was investigated. The saltfree surfactant system, 0.15 mM C16TASal displays a jump (from 60 Angstrom) into a hard wall contact at approximately 20-30 Angstrom. Upon addition of salt a hard wall contact is found at approximately 60 Angstrom. The frequency dependence on the shear modulus was investigated both at the hard wall and larger separations (up to 300-400 Angstrom). The general trend for both systems is that the elasticity modulus dominates over the loss modulus. Upon addition of salt the structure is more close packed resulting in a higher resistance against compression (a higher elasticity modulus, G'). Shear induced structures where observed after shear at high contact pressures.