表面活性剂在空气/水界面上动态吸附的改性Langmuir-Hinselwood动力学

C.H. Chang, E.I. Franses
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引用次数: 88

摘要

Langmuir - hinselwood (L-H)方程是符合Langmuir平衡等温线的最简单的动力学方程。该动力学方程不能很好地描述辛醇、十二烷基硫酸钠和其他表面活性剂的动态表面张力数据。建立了新的地下吸附速率动力学方程(dl/dt = kaLc(θ,t)(1−θ) exp(−Bθ)−kdL Γ exp(−Bθ))。其中,θ为表面覆盖分数Γ/Γm, c(θ,t)为亚表面浓度,kaL, kdL和B为常数)包括动力学的改变,但不包括平衡等温线的改变。新方程较好地描述了界面单层对附加表面活性剂的捕获效率,并可以描述吸附和解吸的激活障碍,或主要由单层与溶解表面活性剂之间的吸引相互作用引起的协同吸附。该方程用于一维扩散/吸附/脱附混合动力学模型。对于辛醇和庚醇,初始吸附速率由本征吸附/脱附动力学(缓慢吸附/脱附)控制。随着表面覆盖率的增加,动态吸附越来越接近扩散控制极限(相对于扩散的快速吸附/解吸)。这表明醇分子在单分子层中具有吸引和合作的相互作用。对于二-2-乙基己基磺基琥珀酸钠(DESS或AOT)和SDS,吸附比扩散控制模型预测的要慢得多。混合动力学模型中修正的L-H方程能很好地拟合数据。吸附效率系数kaL exp(−bθ)随SDS浓度cSDS或NaCl浓度cs的增大而增大,表明吸附受静电屏障的影响较大。当cs = 0, cSDS = 1.7 ~ 5.9 mM (θc<0.4)时,表面电势在150 ~ 230 mV范围内,与经典双层理论一致。对于θc >0.4和高盐浓度时,参数B可能涉及大量的空间或其他相互作用。
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Modified Langmuir—Hinselwood kinetics for dynamic adsorption of surfactants at the air/water interface

The Langmuir—Hinselwood (L-H) equation is the simplest kinetic equation which is consistent with Langmuir's equilibrium isotherm. This kinetic equation cannot describe well the dynamic surface tension data for octanol, sodium dodecyl sulfate (SDS), and other surfactants. A new kinetic equation for the rate of adsorption from the subsurface (dl/dt = kaLc(θ,t)(1−θ) exp(−Bθ)−kdL Γ exp(−Bθ). where θ is the fractional surface coverage Γ/Γm, c(θ,t) is the subsurface concentration, and kaL, kdL, and B are constants) includes modification of the kinetics but not of the equilibrium isotherm. The new equation describes better the capture efficiency of the interfacial monolayer for additional surfactant, and can describe activation barriers for adsorption and desorption, or cooperative adsorption caused by primarily attractive interactions between the monolayer and the dissolved surfactant. This equation was used in a new model of mixed kinetics for one-dimensional diffusion/adsorption/desorption. For octanol and heptanol, the initial adsorption rate is controlled by intrinsic adsorption/desorption kinetics (slow adsorption/desorption). With increasing surface coverage, dynamic adsorption gets closer to the diffusion-controlled limit (fast adsorption/desorption relative to diffusion). This indicates attractive and cooperative interactions of alcohol molecules in the monolayer. For sodium di-2-ethylhexylsulfosuccinate (DESS or AOT) and SDS, adsorption is much slower than predicted by diffusion-controlled models. The modified L-H equation in a mixed-kinetics model can fit the data well. The capture efficiency factor, kaL exp(−bθ), increases with increasing SDS concentration cSDS or NaCl concentration cs, indicating that adsorption is strongly affected by electrostatic barriers. For cs = 0 and cSDS = 1.7 to 5.9 mM (for θc<0.4), the estimated surface electrical potential is in the range 150–230 mV, and is consistent with classical double-layer theory. For θc > 0.4 and a high salt concentration, the parameter B may involve substantial steric or other interactions.

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