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 l inear poly(ethylene glycol) alkyl ether surfactants (CiEj = 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 pre ssure isotherms suggest a first-order phase transition between liquid expan ded (L-1) and liquid condensed (L-2) states for the least soluble amphiphil es: 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 o f 46 mN/m. Fluorescence microscopy of spread C14E1 monolayers confirms L-1- L-2 phase coexistence and reveals coexistence between liquid expanded and g aseous (G) states at low surface pressure. The dynamic assembly of these ph ases from solution, induced by flow in the subphase, is visualized with flu orescence and monitored with surface tension measurements; results compare well with spread monolayers. These observations show that as surfactant ads orbs 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 pron ounced induction period represents G-L-1 coexistence, and the intermediate plateau results from an L1-L2 transition.