Centre for Surface Science, Department of Chemistry, Jadavpur University, Kolkata 700 032, India.
J Colloid Interface Sci. 2013 Mar 15;394:329-36. doi: 10.1016/j.jcis.2012.12.004. Epub 2012 Dec 28.
Amphiphile adsorption at the air/water interface lowers the surface tension (γ) of the solution. After a critical surfactant concentration (C), γ becomes constant (with a break in the γ-logC plot), which is considered the critical micelle concentration (CMC). At very low amphiphile concentration, γ decreases slowly, forming a plateau, then decreases sharply and often nonlinearly by a co-operative adsorption process till the second plateau is reached at CMC. To get the Gibbs surface excess (Γ) of the amphiphile relative to water, a polynomial equation of appropriate degree needs to be used, since the drop in γ progresses with continuous changing slope, which maximizes at CMC and becomes zero afterward. Recent research has evidenced that a complete saturated Gibbs monolayer may not always form at CMC; there may be formation of multilayer of micelles below the Gibbs monolayer, which cannot be assessed by ST measurements. A method like neutron reflectometry (NR) can evaluate the Γ beyond CMC. A procedure for determining Γ(max) from tensiometric results is herein proposed. Amphiphiles do sometimes show a linear decline in γ with logC followed by a break with a plateau at CMC. There, a single slope leading to a single surface excess quantity is obtained for the Gibbs equation at all concentrations up to CMC. Possible reasons for such results are given. Current conflicting ideas and criticisms on the issue of Gibbs equation and determination of Γ and Γ(max) have been addressed.
两亲物质在气/水界面的吸附会降低溶液的表面张力 (γ)。在达到临界表面活性剂浓度 (C) 后，γ 变得恒定（在 γ-logC 图中出现断裂），这被认为是临界胶束浓度 (CMC)。在非常低的两亲物质浓度下，γ 缓慢下降，形成一个平台，然后通过协同吸附过程急剧下降，通常是非线性的，直到达到 CMC 时达到第二个平台。为了得到两亲物质相对于水的吉布斯表面过剩（Γ），需要使用适当阶数的多项式方程，因为 γ 的下降与连续变化的斜率有关，在 CMC 处达到最大值，之后变为零。最近的研究表明，在 CMC 处并不总是形成完整的饱和吉布斯单层；可能在吉布斯单层以下形成了多层胶束，这无法通过 ST 测量来评估。类似中子反射测量（NR）的方法可以评估 CMC 之后的 Γ。本文提出了一种从张力测量结果确定 Γ(max)的方法。有时，两亲物质的 γ 会随 logC 线性下降，然后在 CMC 处出现断裂和平台。在 CMC 之前的所有浓度下，吉布斯方程都可以获得一条单一斜率，从而获得单一的表面过剩量。给出了产生这种结果的可能原因。针对吉布斯方程和 Γ 和 Γ(max)的确定问题，已经解决了当前存在的矛盾观点和批评。
