Department of Chemical and Biomolecular Engineering, Rice University (MS 362), Houston, TX 77005, United States.
J Colloid Interface Sci. 2012 Nov 1;385(1):111-21. doi: 10.1016/j.jcis.2012.07.041. Epub 2012 Jul 23.
Nanoemulsions were formed spontaneously by diluting water-in-oil (W/O) or brine-in-oil (B/O) microemulsions of a hydrocarbon (octane), anionic surfactant (Aerosol-OT or AOT) and water or NaCl brine in varying levels of excess brine. The water-continuous nanoemulsions were characterized by interfacial tension, dynamic light scattering, electrophoresis, optical microscopy and phase-behavior studies. The mechanism of emulsification was local supersaturation and resulting nucleation of oil during inversion. For nanoemulsions formed at low salinities with Winsor I phase behavior, octane drops grew from initial diameters of 150-250 nm to 480-1000 nm over 24h, depending on salinity. Growth was caused by mass transfer but seemed to approach the asymptotic stage of Ostwald ripening described by the Lifshitz-Slyozov-Wagner (LSW) theory only for dilution with salt-free water. Near the higher cross-over salinity (Winsor III), the nanoemulsions showed much slower growth with droplet size consistently remaining below 200 nm over 24h and reaching 250 nm after 1 week. Birefringence indicated the presence of liquid crystal for these conditions, which could have contributed to the slow growth rate. At even higher salinity levels in the Winsor II domain, W/O/W multiple emulsions having drops greater than 1 μm in diameter were consistently recorded for the first 5-7h, after which size decreased to values below 1 μm. The number and size of internal water droplets in multiple emulsion drops was found to decrease over time, suggesting coalescence of internal droplets with the continuous water phase and mass transfer of water from internal droplets to continuous phase as possible mechanisms of the observed drop shrinkage. Electrophoresis studies showed the nanoemulsions to be highly negatively charged (zeta potentials of -60 mV to -120 mV). The high charge on octane droplets helped assure stability to flocculation and coalescence, thereby allowing mass transfer to control growth in the Winsor I and III regions.
自发形成的纳米乳剂是通过在不同程度的过盐水中稀释油包水 (W/O) 或油包盐水 (B/O) 微乳液来制备的，其中包含烃 (辛烷)、阴离子表面活性剂 (Aerosol-OT 或 AOT) 和水或 NaCl 盐水。连续的纳米乳剂通过界面张力、动态光散射、电泳、光学显微镜和相行为研究来表征。乳化的机制是在反转过程中局部过饱和和油的成核。对于在低盐度下形成的具有 Winsor I 相行为的纳米乳剂，辛烷滴的初始直径为 150-250nm，在 24 小时内增长到 480-1000nm，这取决于盐度。增长是由传质引起的，但似乎仅在用水稀释时才接近 Lifshitz-Slyozov-Wagner (LSW) 理论描述的奥斯特瓦尔德熟化的渐近阶段。在更高的跨越盐度（Winsor III）附近，纳米乳剂的生长速度非常缓慢，在 24 小时内液滴尺寸始终保持在 200nm 以下，1 周后达到 250nm。双折射表明在这些条件下存在液晶，这可能导致生长速率较慢。在 Winsor II 区域的更高盐度水平下，在最初的 5-7 小时内，始终记录到直径大于 1μm 的 W/O/W 复乳，之后尺寸减小到 1μm 以下。发现复乳液滴中的内水相液滴的数量和尺寸随时间减少，表明内相液滴与连续水相的聚结以及内相液滴中的水向连续相的传质可能是观察到的液滴收缩的机制。电泳研究表明纳米乳剂带高度负电荷（zeta 电位为-60mV 至-120mV）。辛烷液滴的高电荷有助于确保其稳定性，以防止絮凝和聚结，从而使传质能够控制 Winsor I 和 III 区域的生长。
