We present the monolayer phase behavior of several slightly water-soluble linear poly(ethylene glycol) alkyl ether surfactants (C i E j = CH3(CH2) i - 1−(O(CH2)2) j OH) and the relationship between the adsorbed monolayer phases, their transitions, and the rate of surface tension reduction. Surface pressure isotherms suggest a first-order phase transition between liquid expanded (L1) and liquid condensed (L2) states for the least soluble amphiphiles:  C12E0, C14E1, and C16E2, with transition surface pressures near 20 mN/m at room temperature. In addition, C14E1 isotherms show a possible vertical LS state at higher pressure but below its equilibrium spreading pressure of 46 mN/m. Fluorescence microscopy of spread C14E1 monolayers confirms L1−L2 phase coexistence and reveals coexistence between liquid expanded and gaseous (G) states at low surface pressure. The dynamic assembly of these phases from solution, induced by flow in the subphase, is visualized with fluorescence and monitored with surface tension measurements; results compare well with spread monolayers. These observations show that as surfactant adsorbs to an initially clean air−water interface and phase transitions occur, coexistence gives rise to tension plateaus consistent with those measured for spread monolayers by surface compression. We confirm these results with C14E1 pendant bubble dynamic tension measurements, where the observed pronounced induction period represents G−L1 coexistence, and the intermediate plateau results from an L1−L2 transition.

Recent equilibrium force measurements on aqueous films of surfactant above the critical micelle concentration show oscillations for film thicknesses up to 50 nm. To model this phenomenon we express the micellar contribution to the disjoining pressure in terms of thickness-dependent inhomogeneous micelle number density distributions through the film. Density functional theory is used to calculate micelle density profiles, presuming the micelles to behave as charged spheres interacting with each other, and with the film interfaces, through screened-Coulomb potentials. The background electrolyte permits dilute micellar solutions to act as concentrated systems exhibiting pronounced layering in the film. For a 0.1 M sodium dodecylsulfate (SDS) film we find up to five micellar layers for a film thickness equal to ten micelle diameters (d), the layer separation scaling with the effective diameter (deff/d=1.86) which includes the micelle Debye atmosphere. The peaks are largest near the interfaces and decay toward the bulk density at the film midplane. The corresponding disjoining pressures show oscillations with the same distance scaling between the branches as in the density profiles; these values are consistent with experiment. With decreasing film thickness, the (meta-)stable disjoining pressure regions represent micellar layers in the film being forced closer together, raising the pressure until the interior layer is expelled, allowing more space between the remaining micellar layers at that thickness. Repulsive (positive) disjoining pressures result from layer separations less than the corresponding bulk value whereas attractive (negative) regions represent more distance between layers than that in the bulk. The 0.2 M SDS disjoining pressure isotherm exhibits one additional layer than the 0.1 M case for thicknesses up to 50 nm. The pressure magnitudes of the former case are about twice that of the latter. Addition of ionic salts greatly inhibits the long-range micellar structuring. For SDS foam films, predicted disjoining pressures are much higher than measured values. Comparison with cetyltrimethyl-ammonium bromide (CTAB) micellar films in the surface forces apparatus, however, shows near quantitative agreement. The nature of the confining interfaces thus plays a key role in supporting the internal micellar structuring.

Thin liquid films stabilized by surjfactants above the critical micelle concentration exhibit stratification or stepwise dynamic thinning. A continuum hydrodynamic model is outlined for stepwise film thinning that incorporates equilibrium micellar structuring through self-consistent oscillato y disjoining pressures and effective viscosities. Effective viscosities as functions of thickness are evaluated with an extension of the local average density model, considering dilute colloidal suspension shear viscosities and solvent effects. To establish local shear viscosities, structured DFT micellar profiles, coarse-grained densities, and disjoining pressure are used. Ionic micelles and other colloidal systems with repulsive interactions show structured effective viscosities that are generally less than the corresponding homogeneous solution shear viscosity, bounded by the pure solvent viscosity and that of the bulk micellar solution. For 0.1 and 0.244 sodium dodecylsulfate micellar solutions, the effective viscosities are less than 5 and lo%, respectively, below the homogeneous jluid viscosi&, except at small thicknesses, indicating that the micellar film thins faster than a pure water film of the same thickness.Calculated thinning curves closely resemble experimental observations in the stepwise thinning behavior, displaying decreasing slopes and increased step durations at later times. Despite the micellar structuring within the film, the ionic micelles do not contribute appreciably to the viscous resistance of the thinning film. Rather, Reynolds 'film thinning is obeyed, with the equilibrium oscillatory disjoining pressures driving the stepwise dynamics. The shear viscosity of the ionic micellar film is well approximated by that of the bulk solution.