Surfactant Interactions and Organization at the Gas–Water Interface (CTAB with Added Salt)

Abstract

We have studied adsorbed layers of cetyltrimethylammonium bromide (CTAB) at air-water interfaces in the presence of added electrolyte. Fast bubble compression/expansion measurements were used to obtain the surface equation of state, i.e., the surface tension vs CTAB surface concentration dependence. We show that while a simple model where the surfactant molecules are assumed to be noninteracting is insufficient to describe the measured response of the surfactant layer, a modified Frumkin equation where the local interactions between the molecular components depend on their surface concentration captures the response. The variation of the effective interactions in the surfactant layer in the model shows that the interactions in the surfactant layer change from effectively repulsive to attractive with increasing surface concentration. Molecular dynamics simulations are performed to probe the origins of the change in the interactions. The simulations indicate that already at low surface concentrations the surfactants aggregate as highly dynamic rafts with surfactant orientation parallel to the interface. Increasing the concentration leads to a change in the assembly morphology at the interface: the surfactant layer thickens and the surfactants sample a range of tilted orientations with respect to the interfacial plane. The change from transient raftlike assemblies to dynamical aggregates at the interface involves a clear increase in the degree of counterion binding: we speculate that the flip of the effective interaction parameter in the model used to interpret the experimental results could result from this. The work here presents basic steps toward a proper understanding of the molecular organization and interactions of surfactants at an air-water interface. This is crucially important in understanding macroscopic properties of surfactant-stabilized systems such as foams, emulsions, and colloidal dispersions.

Gas–liquid phase equilibrium of a model Langmuir monolayer captured by a multiscale approach

“…with the Helmholtz free energy F 0 of the repulsive 2D reference system described by U 0 = U rep , while r 2 (r,r 0 ;l) is the pair areal density of the system in which particles interact with U l according to eqn (23). This pair density is not known in general for inhomogeneous systems, 43,44 but it is related to the radial distribution function g(r,r 0 ;l) through r 2 (r,r 0 ;l) = g(r,r 0 ;l)r(r)r(r 0 ) and within the random phase approximation (RPA) [43][44][45][46][47] given by…”

“…2,9,20 Theoretical frameworks such as Landau theory of phase transitions have been used with some success in predicting condensed phases and coexistence boundaries of Langmuir monolayers 21 qualitatively. Molecular dynamics (MD) and Monte Carlo (MC) simulations 1,2,8,10,[22][23][24] of the full 3D system including the liquid phase do not only provide more quantitative information but have also broadened our insight into the structure and phase behavior of Langmuir monolayers. Current state-of-the-art atomistic simulations 25 were so far successful in predicting the structure of gas and solid phases of phospholipid Langmuir monolayers (76 nm 2 in size, with 76 to 144 surfactants at each interface).…”

Structure and Dynamics of Nanoconfined Water Between Surfactant Monolayers

“…9 This finding is consistent with the work of Yazhgur and co-workers: they have shown that CTAB monolayers are disordered, with a significant portion lying in the plane of the monolayer-water interface even at higher packing densities than those considered in this work. 34 The monolayer roughness also increases with decreasing A CTAB (increased packing density). There are clear differences between the roughness of the monolayers in the two asymmetric systems, most notably in ASYM2, in which the difference in A CTAB of the two monolayers is greater.…”

“…Figures 1(a) and (b) highlight the roughness and patchiness of the CTAB monolayers, as previously observed in studies of CTAB monolayers at the air-water interface. 19,34 These two characteristics present a challenge when analyzing the structure of the confined water layer:…”

“…Previous studies have investigated the structure of water near the interface of a CTAB monolayer through experiments [32][33][34][35][36][37] and MD simulations. 19,34,38 However, it was only recently that Dhopatkar and co-workers studied a thin layer of water under confinement by two CTAB monolayers using sum-frequency generation (SFG) spectroscopy. 9 They commented on the intricate ordering of the confined water layer, akin to that found in ice.…”