Egger Holger, McGrath Kathryn M
School of Chemical and Physical Sciences, MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand.
J Colloid Interface Sci. 2006 Jul 15;299(2):890-9. doi: 10.1016/j.jcis.2006.03.022. Epub 2006 Mar 20.
Controlling stability and aging of emulsions is important from commercial and scientific perspectives. Achieving such control comes through gaining an understanding of the relationship between emulsion constituents and microstructure and how these influence the kinetics and mechanism of destabilisation. We present here an investigation determining the rate of destabilisation as a function of time for a series of water/n-alkane/Triton X-100 oil-in-water emulsions. The time dependence of the emulsions was investigated using static light scattering, PFG-NMR and measurement of gross phase separation. By changing the chain length of the oil from hexane to tetradecane, an almost five orders of magnitude variation in emulsion lifetime could be achieved, while maintaining most of the other chemical and physical characteristics of the emulsions. Further, we show that while Ostwald ripening is the dominant destabilisation mechanism, two distinct regimes are evident. Initially, we observed an enhanced Ostwald ripening regime due to the presence of oil-swollen micelles in the aqueous continuum, that is a depletion flocculation mechanism is followed. The presence of oil-swollen micelles was confirmed using PFG-NMR. The micelles aid the gross oil transport between the discrete oil domains. Upon phase separation the oil-swollen micelles are predominantly removed from the emulsion along with the excess water resulting in a concomitant reduction in the ripening rate, producing the more general Ostwald ripening cubic dependence of droplet radius as a function of time for the lower molecular weight oils. The oils with higher molecular weight (decane and above), however, were observed to switch over to destabilisation via creaming. PFG-NMR was shown to be a powerful technique to fully probe emulsion microstructure as a function of time with droplet size and spacing being directly obtained from the data.
从商业和科学的角度来看，控制乳液的稳定性和老化非常重要。要实现这种控制，需要了解乳液成分与微观结构之间的关系，以及它们如何影响去稳定化的动力学和机制。我们在此展示了一项研究，该研究确定了一系列水/正构烷烃/ Triton X - 100水包油乳液的去稳定化速率与时间的函数关系。使用静态光散射、脉冲场梯度核磁共振（PFG - NMR）和总相分离测量来研究乳液的时间依赖性。通过将油的链长从己烷改变为十四烷，可以在保持乳液的大多数其他化学和物理特性的同时，使乳液寿命实现近五个数量级的变化。此外，我们表明，虽然奥斯特瓦尔德熟化是主要的去稳定化机制，但有两个明显的阶段。最初，我们观察到由于水相中存在油溶胀胶束而导致奥斯特瓦尔德熟化增强阶段，即随后是耗尽絮凝机制。使用PFG - NMR证实了油溶胀胶束的存在。这些胶束有助于离散油域之间的总油传输。在相分离时，油溶胀胶束主要与过量的水一起从乳液中去除，导致熟化速率随之降低，对于较低分子量的油，产生了更普遍的奥斯特瓦尔德熟化中液滴半径与时间的立方关系。然而，观察到较高分子量的油（癸烷及以上）通过乳析转变为去稳定化。PFG - NMR被证明是一种强大的技术，可以根据时间全面探测乳液微观结构，直接从数据中获得液滴尺寸和间距。
