Narayan Shweta, Barman Sourav, Moravec Davis B, Hauser Brad G, Dallas Andrew J, Zasadzinski Joseph A, Dutcher Cari S
Department of Mechanical Engineering, University of Minnesota - Twin Cities, Minneapolis, MN, USA.
Department of Chemical Engineering and Materials Science, University of Minnesota - Twin Cities, Minneapolis, MN, USA.
Soft Matter. 2021 May 12;17(18):4751-4765. doi: 10.1039/d1sm00064k.
Micrometer-sized water droplets dispersed in diesel fuel are stabilized by the fuel's surface-active additives, such as mono-olein and poly(isobutylene)succinimide (PIBSI), making the droplets challenging for coalescing filters to separate. Dynamic material properties found from interfacial rheology are known to influence the behavior of microscale droplets in coalescing filters. In this work, we study the interfacial dilatational properties of water-in-fuel interfaces laden with mono-olein and PIBSI, with a fuel phase of clay-treated ultra-low sulphur diesel (CT ULSD). First, the dynamic interfacial tension (IFT) is measured using pendant drop tensiometry, and a curvature-dependent form of the Ward and Tordai diffusion equation is applied for extracting the diffusivity of the surfactants. Additionally, Langmuir kinetics are applied to the dynamic IFT results to obtain the maximum surface concentration (Γ∞) and ratio of adsorption to desorption rate constants (κ). We then use a capillary pressure microtensiometer to measure the interfacial dilatational modulus, and further extract the characteristic frequency of surfactant exchange (ω0) by fitting a model assuming diffusive exchange between the interface and bulk. In this measurement, 50-100 μm diameter water droplets are pinned at the tip of a glass capillary in contact with the surfactant-containing fuel phase, and small amplitude capillary pressure oscillations over a range of frequencies from 0.45-20 rad s-1 are applied to the interface, inducing changes in interfacial tension and area to yield the dilatational modulus, E*(ω). Over the range of concentrations studied, the dilatational modulus of CT ULSD with either mono-olein or PIBSI increases with a decrease in bulk concentration and plateaus at the lowest concentrations of mono-olein. Characteristic frequency (ω0) values extracted from the fit are compared with those calculated using equilibrium surfactant parameters (κ and Γ∞) derived from pendant drop tensiometry, and good agreement is found between these values. Importantly, the results imply that diffusive exchange models based on the equilibrium relationships between surfactant concentration and interfacial tension can be used to infer the dynamic dilatational behavior of complex surfactant systems, such as the water-in-diesel fuel interfaces in this study.
分散在柴油燃料中的微米级水滴通过燃料中的表面活性添加剂（如单油酸甘油酯和聚异丁烯琥珀酰亚胺（PIBSI））得以稳定，这使得水滴对于聚结过滤器来说难以分离。已知从界面流变学中发现的动态材料特性会影响聚结过滤器中微尺度水滴的行为。在这项工作中，我们研究了含有单油酸甘油酯和PIBSI的燃料 - 水界面的界面膨胀特性，燃料相为经粘土处理的超低硫柴油（CT ULSD）。首先，使用悬滴张力测量法测量动态界面张力（IFT），并应用沃德和托尔代扩散方程的曲率相关形式来提取表面活性剂的扩散率。此外，将朗缪尔动力学应用于动态IFT结果，以获得最大表面浓度（Γ∞）和吸附与解吸速率常数之比（κ）。然后，我们使用毛细管压力微张力计测量界面膨胀模量，并通过拟合一个假设界面与本体之间存在扩散交换的模型来进一步提取表面活性剂交换的特征频率（ω0）。在该测量中，直径为50 - 100μm的水滴固定在与含表面活性剂的燃料相接触的玻璃毛细管尖端，在0.45 - 20 rad s-1的频率范围内对界面施加小振幅毛细管压力振荡，引起界面张力和面积的变化，从而得到膨胀模量E*(ω)。在所研究的浓度范围内，含有单油酸甘油酯或PIBSI的CT ULSD的膨胀模量随着本体浓度的降低而增加，并在单油酸甘油酯的最低浓度处达到平稳。从拟合中提取的特征频率（ω0）值与使用从悬滴张力测量法得出的平衡表面活性剂参数（κ和Γ∞）计算得到的值进行比较，发现这些值之间具有良好的一致性。重要的是，结果表明基于表面活性剂浓度与界面张力之间平衡关系的扩散交换模型可用于推断复杂表面活性剂体系的动态膨胀行为，例如本研究中的柴油 - 水界面。
