Subramanyam Rajeev, Maldarelli Charles
Department of Chemical Engineering, City College of the City University of New York, New York, New York 10031, USA.
J Colloid Interface Sci. 2002 Sep 15;253(2):377-92. doi: 10.1006/jcis.2002.8398.
An aqueous soluble surfactant adsorbing from solution onto an initially clean air/water interface often exhibits an induction period in the surface tension relaxation in which, as the adsorption begins, the tension remains near the clean interface value for an extended period of time before decreasing rapidly to the equilibrium value. In this study, using a model nonionic soluble surfactant, C14E6(CH3(CH2)13-(OCH2CH2)6-OH), we present direct fluorescence evidence that this induction is due to a first-order phase transition from a gaseous (G) to a liquid expanded (LE) phase that the assembling monolayer undergoes at constant surface pressure. An open channel flow cell is initially filled with water, and onto its air/water interface is spread an insoluble amphiphilic dye that fluoresces upon irradiation in the LE phase and whose fluorescence is quenched in the G phase. An aqueous solution of C14E(6) is then allowed to flow through the channel. We observe the immediate appearance of bright islands of the LE phase growing in a dark (G) background, confirming the presence of the G/LE phase transition. These islands eventually occupy the entire surface, after which the interface remains uniformly bright. We correlate this phase transition to the induction period by simultaneously measuring the tension of the interface of the open channel, and verifying that as the islands grow the tension remains at the clean value until the bright LE phase occupies the entire surface, whereupon the tension rapidly decreases. We further develop a phase transition surfactant transport model for the induction period in which surfactant diffuses toward and kinetically adsorbs onto the surface, and then rapidly equilibrates between the G and LE phases. For our model surfactant C14E6, we independently measure the surface concentration of the nucleating LE phase, the LE phase surfactant equation of state, the kinetic rate constants for adsorption into the LE phase, and the bulk diffusion coefficient. Using these measurements, we predict induction times for adsorption onto a clean surface without convection. We also measure these induction times in tension relaxation for adsorption onto a pendant bubble using axisymmetric shape analysis, and demonstrate agreement with the simulations with no adjustable constants.
一种从溶液吸附到初始清洁的空气/水界面上的水溶性表面活性剂，在表面张力松弛过程中通常会出现一个诱导期。在此期间，随着吸附开始，表面张力在较长时间内保持接近清洁界面的值，然后才迅速下降至平衡值。在本研究中，我们使用一种模型非离子可溶性表面活性剂C14E6（CH3(CH2)13-(OCH2CH2)6-OH），提供了直接的荧光证据，表明这种诱导是由于组装的单分子层在恒定表面压力下从气态（G）到液体扩张（LE）相的一级相变所致。一个开放式通道流动池最初装满水，在其空气/水界面上铺展一种不溶性两亲染料，该染料在LE相中受辐照时会发出荧光，而在G相中其荧光会被淬灭。然后让C14E(6)的水溶液流经通道。我们观察到在黑暗（G）背景中生长的明亮的LE相岛立即出现，证实了G/LE相变的存在。这些岛最终占据整个表面，此后界面保持均匀明亮。我们通过同时测量开放式通道界面的张力，并验证随着岛的生长，张力保持在清洁值，直到明亮的LE相占据整个表面，此时张力迅速下降，从而将这种相变与诱导期联系起来。我们进一步开发了一个用于诱导期的相变表面活性剂传输模型，其中表面活性剂向表面扩散并在动力学上吸附到表面，然后在G相和LE相之间迅速达到平衡。对于我们的模型表面活性剂C14E6，我们独立测量了成核LE相的表面浓度、LE相表面活性剂的状态方程、吸附到LE相的动力学速率常数以及本体扩散系数。利用这些测量结果，我们预测了在无对流情况下吸附到清洁表面的诱导时间。我们还使用轴对称形状分析测量了吸附到悬垂气泡上的张力松弛过程中的这些诱导时间，并证明与模拟结果一致，且无需调整常数。
