Bending Elasticity of Nonionic Surfactant Layers

A novel approach to evaluate the bending elasticity of monolayers formed by nonionic surfactants with a rigid head group is introduced by means of considering head group repulsion as derived from the free energy of mixing rigid hydrophilic head groups with surrounding solvent molecules as well as contributions related to the hydrophobic tails. Explicit expressions for the spontaneous curvature (H-0), bending rigidity (k(c)) and saddle-splay constant ((k) over bar (c)) have been derived for the constraint of constant chemical potential of free surfactant (thermodynamically open layers) as well as the constraint of constant aggregation number (thermodynamically closed layers). Most interestingly, it is demonstrated that k(c) for thermodynamically open layers formed by a nonionic surfactant with rigid tail and head group always must be zero. However, k(c) for surfactants with a flexible tail as a function of the head group-to-tail volume ratio is found to go through a maximum at some large, positive value of k(c) and H-0 approximate to 0. Eventually, k(c) falls below zero as the head group volume increases above a certain value. Hence, we may conclude that nonionic surfactants with a rigid head group may form thermodynamically stable fluid layers or aggregates only insofar the hydrophobic part is flexible with respect to chain conformational. degrees of freedom and the head group is not too voluminous. It is found that the head group repulsion contribution to k(c)H(0) is always positive whereas the corresponding contribution to (k) over bar (c) may be positive or negative depending on whether the hydrophobic layer of the film is thicker or thinner than the hydrophilic layer.