Shrestha Lok Kumar, Shrestha Rekha Goswami, Varade Dharmesh, Aramaki Kenji
Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai 79-7, Hodogaya-ku, Yokohama 240-8501, Japan.
Langmuir. 2009 Apr 21;25(8):4435-42. doi: 10.1021/la8039225.
Formation of reverse micelles in surfactant/oil binary systems without water addition and the tunable parameters for the structure control of such micelles are presented. The small-angle X-ray scattering (SAXS) technique has been used for the structural characterization of micelles. It has been found that the nonionic surfactant glycerol monoisostearate (abbreviated as iso-C18G1) forms reverse micelles in different organic solvents such as cyclohexane, n-decane, and n-hexadecane without the addition of water. The structure (shape and size) of the reverse micelles has been found to depend on the solvent nature (alkyl chain length of oil), composition, temperature, and added water. Phase behavior study has shown that iso-C18G1 forms isotropic single-phase solutions in the aforementioned oils at 25 degrees C. At lower temperatures (<20 degrees C) II phases (dispersion of solid or liquid crystal phase) has been observed. SAXS data were evaluated by the generalized indirect Fourier transformation (GIFT) method, which has drawn a clear picture on the structural variations of the reverse micellar aggregates. Small globular types of micelles are found in the iso-C18G1/cyclohexane system. On the other hand, elongated ellipsoidal prolatelike or rodlike micelles are found in iso-C18G1/decane or iso-C18G1/hexadecane systems. The underlying mechanism of this structural evolution may be explained in terms of the transfer free energy of hydrophilic glycerol moiety from hydrophilic to hydrophobic environment of oils with different chain lengths. Besides, the penetration of oils to the lipophilic chain of the surfactant in reverse micellar systems differs depending on the chain length of oils. Lowering temperature and increasing surfactant concentration similarly lead to micellar growth while the cross-section structure remains essentially unchanged. Addition of trace water induced micellar growth, which is accompanied by the rapid swelling of the micellar core. The results obtained by this study demonstrate that the solvent nature, temperature, composition, and water addition can be the tunable parameters for the size, shape, and internal structure control of the iso-C18G1-based reverse micelles.
介绍了在不加水的表面活性剂/油二元体系中反胶束的形成以及用于此类胶束结构控制的可调参数。小角X射线散射（SAXS）技术已用于胶束的结构表征。已发现非离子表面活性剂单异硬脂酸甘油酯（简称为异-C18G1）在不加水的情况下能在不同的有机溶剂如环己烷、正癸烷和正十六烷中形成反胶束。已发现反胶束的结构（形状和大小）取决于溶剂性质（油的烷基链长度）、组成、温度和添加的水。相行为研究表明，异-C18G1在25℃时在上述油中形成各向同性单相溶液。在较低温度（<20℃）下观察到II相（固体或液晶相的分散体）。SAXS数据通过广义间接傅里叶变换（GIFT）方法进行评估，该方法清晰地描绘了反胶束聚集体的结构变化。在异-C18G1/环己烷体系中发现了小球状类型的胶束。另一方面，在异-C18G1/癸烷或异-C18G1/十六烷体系中发现了细长的椭球形长棒状或棒状胶束。这种结构演变的潜在机制可以根据亲水性甘油部分从不同链长的油的亲水环境转移到疏水环境的自由能来解释。此外，在反胶束体系中，油对表面活性剂亲油链的渗透因油的链长而异。降低温度和增加表面活性剂浓度同样会导致胶束生长，而横截面结构基本保持不变。添加微量水会导致胶束生长，同时胶束核心会迅速膨胀。本研究获得的结果表明，溶剂性质、温度、组成和水的添加可以作为控制基于异-C18G1的反胶束的大小、形状和内部结构的可调参数。
