Dynamics of surfactant sorption at the air/water interface: continuous-flow tensiometry.
Dynamics of surfactant sorption at the air/water interface: continuous-flow tensiometry.
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DOI：
10.1016/s0021-9797(02)00241-2
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发表时间：
2003-05
影响因子：
9.9
通讯作者：
T. Svitova;M. J. Wetherbee;C. Radke
中科院分区：
文献类型：
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作者：
T. Svitova;M. J. Wetherbee;C. Radke
动态界面张力仪，衡量静态液滴或气泡的轴对称液滴形状分析，提供了有用的信息表面活性剂吸附动力学。然而，传统的悬滴法不容易适用于解吸动力学的研究。因此，吸附可逆性的问题很难通过这种技术进行评估。我们扩展了经典的悬垂/固着滴动态张力计浸入固着气泡在一个连续混合的光学细胞。理想的混合条件是通过搅拌和恒定的流动通过细胞建立的。将表面活性剂水溶液供应到电池（加载）或通过用水冲洗（冲洗）从电池中除去，从而允许研究吸附和解吸动力学。良好的混合条件和消除任何传质阻力允许直接识别的吸附动力学障碍，并从外部水相的时间常数长于光电池的停留时间。单分散非离子表面活性剂乙氧基十二烷醇（C12 E5）沿着阳离子十六烷基三甲基溴化铵（CTAB）在添加盐的存在下，吸附和解吸瞬间在空气/水界面。在这些情况下，实验观察到的动态张力曲线遵循局部平衡模型，精确地加载和冲洗。因此，这些表面活性剂低于其临界胶束浓度（CMC）表现出没有检测到的吸附活化障碍的时间尺度上的顺序分钟。然而，在没有电解质的情况下，稀CTAB的吸附动力学是显着不同的KBr的存在下。在这里，CTAB解吸发生在局部平衡，但吸附速率是动力学限制，最有可能是由于静电势垒所产生的带电的表面活性剂在界面处积累。商业的，多分散的非离子表面活性剂乙氧基壬基酚（NP 9）负载与局部平衡理论的良好一致性，但显示偏离理论洗脱曲线，大概是由于溶解的，但否则水不溶性组分的缓慢解吸。聚合物非离子三嵌段共聚物Pluronic在空气/水界面上在3至14 kDa的分子量范围内表现出几乎完全的不可逆吸附。牛血清白蛋白（BSA）在空气/水界面上从稀水溶液中的吸附/解吸也观察到类似的不可逆动力学行为。新的连续流动张力计（CFT）是一种简单而强大的工具，用于研究流体/流体界面的吸附动力学，特别是对于表现出显着阻碍解吸的较大分子量表面活性剂。
Dynamic interfacial tensiometry, gauged by axisymmetric drop shape analysis of static drops or bubbles, provides useful information on surfactant adsorption kinetics. However, the traditional pendant-drop methodology is not readily amenable to the study of desorption kinetics. Thus, the question of sorption reversibility is difficult to assess by this technique. We extend classical pendant/sessile drop dynamic tensiometry by immersing a sessile bubble in a continuously mixed optical cell. Ideal–mixed conditions are established by stirring and by constant flow through the cell. Aqueous surface-active-agent solutions are either supplied to the cell (loading) or removed from the cell by flushing with water (washout), thereby allowing study of both adsorption and desorption kinetics. Well-mixed conditions and elimination of any mass transfer resistance permit direct identification of sorption kinetic barriers to and from the external aqueous phase with time constants longer than the optical-cell residence time. The monodisperse nonionic surfactant ethoxy dodecyl alcohol (C12E5), along with cationic cetyltrimethyl ammonium bromide (CTAB) in the presence of added salt, adsorbs and desorbs instantaneously at the air/water interface. In these cases, the experimentally observed dynamic-tension curves follow the local-equilibrium model precisely for both loading and washout. Accordingly, these surfactants below their critical micelle concentrations (CMC) exhibit no detectable sorption-activation barriers on time scales of order a min. However, the sorption dynamics of dilute CTAB in the absence of electrolyte is markedly different from that in the presence of KBr. Here CTAB desorption occurs at local equilibrium, but the adsorption rate is kinetically limited, most likely due to an electrostatic barrier arising as the charged surfactant accumulates at the interface. The commercial, polydisperse nonionic surfactant ethoxy nonylphenol (NP9) loads in good agreement with local-equilibrium theory but shows deviation from the theoretical washout curve, presumably due to slow desorption of solubilized but otherwise water insoluble components. The polymeric nonionic triblock copolymer Pluronic exhibits almost complete irreversible adsorption at the air/water interface over a molecular-weight range from 3 to 14 kDa. Similar irreversible dynamic behavior is observed for adsorption/desorption of the protein bovine serum albumin (BSA) from dilute aqueous solutions at the air/water interface. The new continuous-flow tensiometer (CFT) is a simple, yet powerful, tool to investigate sorption dynamics at fluid/fluid interfaces, especially for larger molecular weight surface-active agents that exhibit significant hindrance to desorption.