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

Abstract

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.

Dependence of Phase Behavior of Some Non-ionic Surfactants at the Air–Water Interface on Micellization in the Bulk

“…1b). Such phase behavior is due to the existence of gas; a (G)-LE phase transition as has been reported previously (40). After the plateau region of the G-LE coexistence state, the surface pressure starts to increase gradually and that leads to a cusp point followed by a plateau region for the LE-LC phase transition.…”

Theory and Experiment on the Measurement of Kinetic Rate Constants for Surfactant Exchange at an Air/Water Interface

“…These authe induction period is due to a G/ LE phase coexistence is presented in Pollard et al (59 ).…”

“…For clean interface gaseous state at even lower concentrations. Recent studies adsorption, we assume that at t Å 0, the surface concentration using fluorescence microscopy to visualize phase domains (G(t)) is equal to zero, and the bulk concentration (C (r, of soluble polyethoxylates (59,60) confirms that the polye-t)) is uniformly C 0 . For re-equilibration following compresthoxylates first adsorb into a gaseous state, and then, at in-sion, we assume the surface concentration at t Å 0 is G i (not creasingly higher surface concentrations, pass through a equal to G e ) and the bulk concentration is uniformly C 0 .…”

Fluorescence Evidence That a Phase Transition Causes the Induction Time in the Reduction in Dynamic Tension during Surfactant Adsorption to a Clean Air/Water Interface and a Kinetic–Diffusive Transport Model for the Phase-Induced Induction

“…The study of Pollard et al (49) provides more direct evidence that the initial induction period in dynamic tension reduction can be caused by a G/LE phase transition. These authors studied the poly (ethylene glycol) n-alkyl ether or poly ethylene-oxide C 14 E 1 (CH 3 (CH 2 ) 13 (OCH 2 CH 2 ) 1 OH), which is slightly soluble in water.…”

“…The polar group is, however, large enough so that, as verified by fluorescence microscopy in their study, directly spread monolayers exist in the G, LE, and LC states rather than just the G and LC states as was the case with hexadecanoic acid. To demonstrate the connection between the phase behavior and the dynamic tension, Pollard et al (49) used an open channel flow cell in which surfactant adsorbed from a flowing solution onto an initially clean surface in which a fluorescing dye was previously spread and a Wilhelmy plate was situated to measure the surface tension. During the initial adsorption the LE phase was found to nucleate in a gaseous background, and grow at the constant tension of the clean surface.…”