Physical and Theoretical Chemistry Department, University of Oxford, South Parks Road, Oxford, United Kingdom.
Langmuir. 2010 Dec 7;26(23):17958-68. doi: 10.1021/la1031834. Epub 2010 Nov 2.
The use of small angle neutron scattering, SANS, neutron reflectivity, NR, and surface tension to study the mixing properties of the biosurfactant rhamnolipid with a conventional anionic surfactant, sodium dodecyl 6-benzene sulfonate, LAS, is reported. The monorhamnose rhamnolipid, R1, mixes close to ideally with LAS at the air-water interface, whereas for mixtures of LAS with the dirhamnose rhamnolipid, R2, the LAS strongly partitions to the air-water interface relative to R2, probably because of the steric hindrance of the larger R2 headgroup. These trends in the binary mixtures are also reflected in the ternary R1/R2/LAS mixtures. However, for these ternary mixtures, there is also a pronounced synergy in the total adsorption, which reaches a maximum for a LAS/rhamnolipid mole ratio of about 0.6 and a R1/R2 mol ratio of about 0.5, an effect which is not observed in the binary mixtures. In solution, the R1/LAS mixtures form relatively small globular micelles, L(1), at low surfactant concentrations (<20 mM), more planar structures (lamellar, L(α), unilamellar/multilamellar vesicles, ulv/mlv) are formed at higher surfactant concentrations for R1 and LAS rich compositions, and a large mixed phase (L(α)/L(1) and L(1)/L(α)) region forms at intermediate surfactant compositions. In contrast, for the R2/LAS mixtures, the higher preferred curvature of R2 dominates the phase behavior. The predominant microstructure is in the form of small globular micelles, except for solution compositions rich in LAS (>80 mol % LAS) where more planar structures are formed. For the ternary mixtures, there is an evolution in the resulting phase behavior from one dominated by L(1) (R2 rich) to one dominated by planar structures, L(α), (R1, LAS rich), and which strongly depends upon the LAS/rhamnolipid and R1/R2 mole ratio.
利用小角中子散射（SANS）、中子反射率（NR）和表面张力研究生物表面活性剂鼠李糖脂与传统阴离子表面活性剂十二烷基苯磺酸钠（LAS）的混合性质。单鼠李糖鼠李糖脂（R1）在气/水界面与 LAS 近乎理想混合，而对于 LAS 与二鼠李糖脂（R2）的混合物，LAS 相对于 R2 强烈地分配到气/水界面，这可能是由于较大的 R2 头基的空间位阻。这些二元混合物中的趋势也反映在三元 R1/R2/LAS 混合物中。然而，对于这些三元混合物，总吸附也存在明显的协同作用，最大协同作用出现在 LAS/鼠李糖脂摩尔比约为 0.6 和 R1/R2 摩尔比约为 0.5 时，而在二元混合物中没有观察到这种协同作用。在溶液中，R1/LAS 混合物在低表面活性剂浓度（<20 mM）下形成相对较小的球形胶束（L(1)），在较高表面活性剂浓度下形成更平面的结构（层状，L(α)，单层/多层囊泡，ulv/mlv），对于 R1 和 LAS 丰富的组成，形成一个大的混合相（L(α)/L(1)和 L(1)/L(α)）区，而在中间表面活性剂组成下，形成一个大的混合相（L(α)/L(1)和 L(1)/L(α)）区。相比之下，对于 R2/LAS 混合物，R2 的较高优势曲率主导了相行为。主要的微观结构是小球形胶束的形式，除了富含 LAS（>80 mol % LAS）的溶液组成，其中形成了更多的平面结构。对于三元混合物，其相行为的演变从以 L(1)（R2 丰富）为主导到以平面结构 L(α)（R1、LAS 丰富）为主导，这强烈依赖于 LAS/鼠李糖脂和 R1/R2 摩尔比。
