Coalescence stability of water-in-oil drops: Effects of drop size and surfactant concentration
Coalescence stability of water-in-oil drops: Effects of drop size and surfactant concentration
复制标题
DOI：
10.1016/j.colsurfa.2017.07.085
复制
发表时间：
2017-10-20
影响因子：
5.2
通讯作者：
Muelheims, Kerstin
中科院分区：
文献类型：
--
作者：
Politova, Nadya I.;Tcholakova, Slavka;Muelheims, Kerstin
我们研究了（1）液滴大小;（2）表面活性剂链长和浓度;（3）油相粘度对重力压向平面油-水界面的水滴稳定性的影响。实验结果表明，在低表面活性剂浓度（周围和低于CMC）的油滴寿命控制的排水时间的油膜，即达到临界膜厚度的时间，在该膜破裂。小液滴在形成具有大界面的平面膜之前聚结，因此，它们的寿命随着液滴直径的增加而迅速减小。相反，较大的液滴在形成平面平行膜后聚结，并且它们的寿命随着液滴直径而增加。因此，当改变液滴直径时，液滴寿命通过最小值。在低的表面活性剂浓度和小液滴所获得的结果以及描述的理论模型，可在文献中。与理论预测相比，由于厚度不均匀的油膜变薄的速度更快，因此相应的较大液滴的寿命更短。表面活性剂浓度增加到CMC以上导致小液滴的稳定性显著增加。有趣的是，对于所有研究的表面活性剂和油，中间体在大小液滴中的稳定性仍然很低。在较高的表面活性剂浓度下，当使用Span 80时，大水滴的稳定性显著增加，而Span 20稳定的水滴的稳定性仍然相当低。用单个水滴获得的结果与各批油包水乳液的稳定性良好一致。
We study the effects of (1) drop size; (2) surfactant chain-length and concentration; and (3) viscosity of the oil phase on the stability of water drops, pressed by gravity towards planar oil-water interface. The experimental results show that at low surfactant concentrations (around and below the CMC) the drop lifetime is controlled by the drainage time of the oily film, viz. by the time for reaching the critical film thickness at which this film ruptures. The small drops coalesce before the formation of a planar film with the large interface and, as a consequence, their lifetime rapidly decreases with the increase of drop diameter. In contrast, the bigger drops coalesce after formation of a plane-parallel film and their lifetime increases with the drop diameter. Therefore, the drop lifetime passes through a minimum when varying the drop diameter. The obtained results at low surfactant concentrations and for small drops are described well by theoretical models, available in the literature. The lifetime of the respective larger drops is shorter, as compared to the theoretical predictions, due to the faster thinning of the oil films which are with uneven thickness. The increase of surfactant concentration above the CMC leads to a significant increase in the stability of the small drops. Interestingly, the stability of the intermediate in size drops remains low for all surfactants and oils studied. At higher surfactant concentrations, the stability of the large water drops increases significantly when Span 80 is used, while it remains rather low for drops stabilized by Span 20. The results obtained with single water drops are in a good agreement with the stability of the respective batch water-in-oil emulsions.