Phase Behavior of Sparingly Soluble Polyethoxylate Monolayers at the Air−Water Surface and Its Effect on Dynamic Tension

We present the monolayer phase behavior of several slightly water-soluble linear poly(ethylene glycol) alkyl ether surfactants (C i E j = CH 3 (CH 2 ) i - 1 −(O(CH 2 ) 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 (L 1 ) and liquid condensed (L 2 ) states for the least soluble amphiphiles: C 12 E 0, C 14 E 1, and C 16 E 2, with transition surface pressures near 20 mN/m at room temperature. In addition, C 14 E 1 isotherms show a possible vertical LS state at higher pressure but below its equilibrium spreading pressure of 46 mN/m. Fluorescence microscopy of spread C 14 E 1 monolayers confirms L 1 −L 2 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 C 14 E 1 pendant bubble dynamic tension measurements, where the observed pronounced induction period represents G−L 1 coexistence, and the intermediate plateau results from an L 1 −L 2 transition.