Tcholakova Slavka, Denkov Nikolai D, Danner Thomas
Laboratory of Chemical Physics & Engineering, Faculty of Chemistry, Sofia University, 1 James Bourchier Avenue, 1164 Sofia, Bulgaria.
Langmuir. 2004 Aug 31;20(18):7444-58. doi: 10.1021/la049335a.
A systematic experimental study of the effect of several factors on the mean drop diameter, d32, during emulsification, is performed with soybean oil-in-water emulsions. These factors are (1) type of used emulsifier; (2) emulsifier concentration, CS; and (3) ionic strength of the aqueous solution. Three different types of emulsifier, anionic (sodium dodecyl sulfate, SDS), nonionic (polyoxyethylene-20 cetyl ether, Brij 58), and protein (whey protein concentrate), are studied. For all of the studied systems, two well-defined regions are observed in the dependence of d32 on CS: at low surfactant concentration, d32 increases significantly with the decrease of CS (region 1), whereas d32 does not depend on CS at high surfactant concentration (region 2). The model, proposed by Tcholakova et al. (Langmuir 2003, 19, 5640), is found to describe well the dependence of d32 on CS in region 1 for the nonionic surfactant and for the protein emulsifier at high electrolyte concentration, 150 mM NaCl. According to this model, a well defined minimal surfactant adsorption (close to that of the dense adsorption monolayer) is needed for obtaining an emulsion. On the other hand, this model is found inapplicable to emulsions stabilized by the ionic surfactant, SDS, and by the nonionic surfactant, Brij 58, at low electrolyte concentration. The performed theoretical analysis of drop-drop interactions, in the emulsification equipment, shows that a strong electrostatic repulsion between the colliding drops impedes the drop-drop coalescence in the latter systems, so that smaller emulsion drops are obtained in comparison with the theoretically predicted ones. The results for SDS-stabilized emulsions in region 1 are explained by a quantitative consideration of this electrostatic repulsion. The drop size in region 2 (surfactant-rich regime) is described very well by the Kolmogorov-Hinze theory of turbulent emulsification.
利用水包油型大豆乳液，对乳化过程中影响平均液滴直径d32的几个因素进行了系统的实验研究。这些因素包括：（1）所用乳化剂的类型；（2）乳化剂浓度CS；（3）水溶液的离子强度。研究了三种不同类型的乳化剂，即阴离子型（十二烷基硫酸钠，SDS）、非离子型（聚氧乙烯-20十六烷基醚，Brij 58）和蛋白质型（乳清浓缩蛋白）。对于所有研究的体系，在d32对CS的依赖关系中观察到两个明确的区域：在低表面活性剂浓度下，d32随CS的降低而显著增加（区域1），而在高表面活性剂浓度下，d32不依赖于CS（区域2）。发现Tcholakova等人（《Langmuir》2003年，第19卷，5640页）提出的模型能够很好地描述非离子表面活性剂以及高电解质浓度（150 mM NaCl）下蛋白质乳化剂在区域1中d32对CS的依赖关系。根据该模型，为了获得乳液，需要有明确的最小表面活性剂吸附量（接近致密吸附单层的吸附量）。另一方面，发现该模型不适用于离子表面活性剂SDS以及低电解质浓度下非离子表面活性剂Brij 58稳定的乳液。对乳化设备中液滴间相互作用进行的理论分析表明，在后者体系中，碰撞液滴之间强烈的静电排斥阻碍了液滴间的聚并，因此与理论预测相比，得到了更小的乳液液滴。通过对这种静电排斥的定量考虑，解释了区域1中SDS稳定乳液的结果。区域2（富含表面活性剂状态）中的液滴尺寸可以很好地用湍流乳化的Kolmogorov-Hinze理论来描述。
