Pan R, Green J, Maldarelli C
Levich Institute and Department of Chemical Engineering, City College of New York, New York, New York
J Colloid Interface Sci. 1998 Sep 15;205(2):213-230. doi: 10.1006/jcis.1998.5559.
The paper focuses on the measurement of the rate constants for the kinetic steps of adsorption and desorption of surfactant between an air/water surface and the aqueous bulk sublayer adjacent to the surface. Kinetic constants are determined in nonequilibrium experiments in which either a clean surface is contacted with a bulk solution and surfactant diffuses toward and adsorbs onto the interface, or the area of an established monolayer in equilibrium with an underlying solution is changed, and surfactant exchanges between the surface and bulk. The dynamic tension change due to the surfactant exchange is measured, and compared to predictions of kinetic-diffusive transport models in order to infer the kinetic coefficients as well the diffusion coefficients. Model comparisons for highly surface active surfactants have resolved only the diffusion coefficient as the transport was found to be diffusion controlled; kinetic constants have only been established for less active materials such as alcohols or bolaform surfactants. In this study, we demonstrate that kinetics can be differentiated from diffusion in clean interface adsorption and re-equilibration if high bulk concentrations of the surfactant are used, or in re-equilibration, if the surface is compressed sufficiently. We first establish theoretically that mass transfer shifts from diffusion-limited to mixed as the bulk concentration increases in clean interface adsorption, or the surface compression is increased in re-equilibration. We then experimentally verify this idea by using the polyethoxylated surfactant C12E6 (C12H25 (OCH2CH2)6-OH) and by measuring dynamic surface tensions in clean interface adsorption and re-equilibration, respectively by the shape analysis of pendant bubbles. We find values of 6 x 10(-10) m2/s for the diffusion coefficient, and 1.4 x 10(-5) m/sec and 1.4 x 10(-4) s-1 for the adsorption and desorption rate constants, respectively, in a Frumkin kinetic formulation. While the adsorption constant is comparable to previously measured values for the less surface active surfactants, the desorption rate constant is a few orders of magnitude smaller. This indicates that the more surface active materials may have much smaller desorption rate constants than had been previously anticipated based on the studies of the less surface active materials. Copyright 1998 Academic Press.
本文重点研究了表面活性剂在气/水表面与邻近表面的水相本体亚层之间吸附和解吸动力学步骤的速率常数的测量。动力学常数是在非平衡实验中确定的，在这些实验中，要么使清洁表面与本体溶液接触，表面活性剂向界面扩散并吸附到界面上，要么改变与下层溶液处于平衡状态的已形成单分子层的面积，表面活性剂在表面和本体之间进行交换。测量由于表面活性剂交换引起的动态张力变化，并与动力学扩散传输模型的预测进行比较，以推断动力学系数以及扩散系数。对于高表面活性表面活性剂的模型比较仅解析出了扩散系数，因为发现传输是由扩散控制的；仅针对活性较低的材料（如醇类或bolaform表面活性剂）确定了动力学常数。在本研究中，我们证明，如果使用高本体浓度的表面活性剂，或者在重新平衡时，如果表面充分压缩，则在清洁界面吸附和重新平衡过程中可以将动力学与扩散区分开来。我们首先从理论上确定，在清洁界面吸附中，随着本体浓度的增加，或者在重新平衡中，随着表面压缩的增加，传质从扩散限制转变为混合控制。然后，我们通过使用聚乙氧基化表面活性剂C12E6（C12H25 (OCH2CH2)6-OH）并分别通过悬垂气泡的形状分析测量清洁界面吸附和重新平衡过程中的动态表面张力，对这一观点进行了实验验证。在弗鲁姆金动力学公式中，我们分别得到扩散系数的值为6×\(10^{-10}\) m²/s，吸附和解吸速率常数分别为1.4×\(10^{-5}\) m/sec和1.4×\(10^{-4}\) s⁻¹。虽然吸附常数与先前测量的低表面活性表面活性剂的值相当，但解吸速率常数要小几个数量级。这表明，与基于低表面活性材料的研究先前预期的相比，表面活性更高的材料可能具有小得多的解吸速率常数。版权所有1998年学术出版社。
