IITB-Monash Research Academy, Indian Institute of Technology Bombay , Main Gate Road, Powai, Mumbai 400076, India.
Department of Metallurgical engineering and Materials Science, Indian Institute of Technology Bombay , Main Gate Road, Powai, Mumbai 400076, India.
Langmuir. 2017 Oct 3;33(39):10311-10321. doi: 10.1021/acs.langmuir.7b02320. Epub 2017 Sep 19.
Graphene oxide (GO) is widely known as an amphiphile having hydrophilic oxygen functionality and unoxidized graphitic patches as the hydrophobic domains. Exploiting this amphiphilicity, GO serves as a surfactant to stabilize oil-water interfaces. While there are numerous reports on GO as a surfactant, most of these reports concern oil-in-water (O/W) emulsions, and there are very few on the formation of water-in-oil (W/O) emulsions. We prepared W/O emulsions using partially reduced graphene oxide (prGO) as a surfactant. The partial reduction introduces a subtle hydrophilic-lipophilic balance (HLB), which favors the formation of the W/O emulsion. The morphological features and rheological characteristics of the W/O emulsion with 75:25 water-to-oil ratio were investigated and analyzed in detail. The W/O emulsion was found to have polydispersity with wide range of droplet sizes varying between 2 to 500 μm. Using confocal microscopy, the role of parameters such as extent of reduction, continuous phase volume fraction and the concentration of GO on the stability, microstructure and variation of droplet size distribution of the W/O emulsion were carefully monitored. With prGO concentration as large as 0.05% (w/w), highly concentrated emulsion will form, and are stable up to 20 days from formation; destabilization occurred from sedimentation and subsequent coalescence as the partially reduced GO was limited by its dispersion ability in the oil-phase studied here. Understanding the mechanisms behind the transient stability will enable the development of novel emulsion compositions containing GO as a multifunctional additive.
氧化石墨烯（GO）是一种众所周知的两亲性物质，具有亲水性含氧官能团和未氧化的石墨状区域作为疏水区。利用这种两亲性，GO 可用作表面活性剂来稳定油水界面。虽然有许多关于 GO 作为表面活性剂的报道，但大多数报道都涉及油包水（O/W）乳液，而关于形成水包油（W/O）乳液的报道却很少。我们使用部分还原氧化石墨烯（prGO）作为表面活性剂来制备 W/O 乳液。部分还原会引入细微的亲水-亲油平衡（HLB），有利于 W/O 乳液的形成。我们详细研究和分析了水油比为 75:25 的 W/O 乳液的形态特征和流变特性。发现 W/O 乳液具有多分散性，粒径范围很宽，介于 2 至 500μm 之间。通过共聚焦显微镜，仔细监测了还原程度、连续相体积分数和 GO 浓度等参数对 W/O 乳液稳定性、微观结构和粒径分布变化的影响。即使 prGO 的浓度高达 0.05%（w/w），也会形成高浓度乳液，并且从形成到 20 天都保持稳定；部分还原 GO 受到其在研究中油相分散能力的限制，会发生沉降和随后的聚结，从而导致不稳定性。了解瞬态稳定性背后的机制将使含有 GO 的新型乳液成分的开发成为可能，GO 作为多功能添加剂。
