Phase transitions among aqueous soluble surfactants at the air-water interface and its effect on dynamic surface tension and spreading and retention of drops impacting on a hydrophobic surface.

摘要

The phase behavior of surfactant molecules at an interface describes their states of aggregation and molecular ordering. For insoluble surfactants, extended research has identified a varied polymorphism of gaseous (G), liquid expanded (LE) and condensed (LC) and solid states (S). Little attention has been devoted to the phase behavior of monolayers formed by self-assembly from solution at the air/water interface. The object of this thesis is to study the phase behavior of soluble surfactant monolayers and establish how the surfactant structure dictates which phases form. We study a homologous series of n-alkyl poly (ethylene glycol) ether nonionic surfactants CiE j (CH3 (CH2)i-1 (OCH2CH2) j-OH) with i = 12 or 14 and j = 0,1,2,4 and 6.;Using fluorescence microscopy, we will demonstrate that all the surfactants in this series occupy gaseous and liquid expanded phases, and that the transition is first order and occurs at a tension close to the clean air/water tension.;Dynamic surface tension reductions measured for adsorption onto an initially clean interface exhibit induction periods in which the tension remains constant before decreasing rapidly. Here we establish that the induction period is caused by a G/LE phase transition that the monolayer undergoes as adsorption from solution proceeds. The objective is to experimentally prove this hypothesis, and obtain an expression for the induction time in terms of the properties of the phase transition.;Our studies of the monolayer phase behavior will also demonstrate that only the members of the series with zero or one ethoxy group can assemble into a LC state. The close packing causes the maximum reduction in tension, and the development of a hydrocarbon phase which space fills with little free volume. This suggests that these surfactants may be useful in reducing the contact angle formed when an aqueous drop beads up and causes it to rebound when it impacts on a hydrophobic surface. It is demonstrated that binary surfactant systems can form condensed phases rapidly and achieve low equilibrium contact angles. When drops laden with these surfactant mixtures impact on a hydrophobic surface, then the rapid lowering of the contact angle eliminates the rebound by allowing the drop to spread.