Can linear micelles bridge between two surfaces?

Abstract

Surfactants are extensively used as stabilizers of colloidal particles, even though the use of high surfactant concentrations can induce a loss of the stability of the dispersion. The depletion mechanism is believed to be responsible for this instability. In this paper, we show that there exists an alternative interpretation, namely that wormlike micelles can bridge between two surfaces. Such a stalk-like object connecting two adsorbed bilayers is (in first order) stable when the endcap (free) energy of the wormlike micelle (in solution) is higher than the connection (free) energy of the stalk with the surface layer. As an example, we consider an aqueous solution of nonionic C(12)E(6) surfactants and use a molecularly realistic self-consistent field approach to evaluate the free-energy of bridge formation. It appears impossible to connect linear micelles to hydrophobic surfaces onto which a monolayer of surfactants exists, and stalks only occur with an exponentially low probability for very hydrophilic surfaces. However, at a wide regime of moderately hydrophilic surfaces the stalks are thermodynamically stable. In this regime, the adsorbed bilayers are typically only marginally stable. We identify a range of parameters for which such adsorbed bilayer ruptures around the stalk and then the wormlike micelle essentially connects (head-on) to the bare surface. The strength of interaction is of the order of the endcap energy which easily exceeds 10 k(B)T. The range of interactions is expected to be large as it is set by the characteristic size of the linear micelles in solution. The regime of moderately hydrophilic surfaces is relevant experimentally, and we conclude that surfactant-induced flocculation may well be the result of stalks. The depletion mechanism is only expected for systems with extremely hydrophobic and with very hydrophilic particles.