Abstract：
Rhamnolipids are glycolipid surfactants composed by a hydrophilic head of either one (mono-RL) or two (di-RL) rhamnose moieties coupled to hydroxyaliphatic chains that can be of different lengths. In spite of their importance in different fields of applications, as bioremediation processes for instance, self-aggregation physico-chemical properties of RLs are not unique. This because a variety of aggregates morphologies (shape and size) can either exist or coexist in aqueous dispersion due to mono-RL:di-RL molar ratio, hydrophobic tails length, pH and the presence of co-surfactants and additives. Recently, a theorethical approach reported the self-assembling morphologies of either pure mono or di-RL in aqueous environment, predicting the formation of spherical to ellipsoidal micelles to worm-like and disk-like aggregates depending on RL concentration and fatty acid chain length. In order to add new information to those previously available, the present work investigated the self-assembling properties of mono-RL-C10-C10 and di-RL-C10-C10 separately in aqueous dispersion by small angle X-Ray scattering (SAXS). A novel approach was applied to the data analysis coupling the scattering length density profiles of the RLs chemical groups and Monte Carlo simulations. Such an approach allowed us to infer about the preferred mono-RL and di-RL conformations that fit better in the self-assembling morphologies. In this way, we show that mono-RL-C10-C10 self-assembles into lamella-like aggregates coexisting with 30 % of multi-lamella aggregates (circa of 5 closed stacked lamella) from a concentration ranging from 10 to 50 mM, with hydrophobic thickness of about 12 Å, a hydrated polar head thickness of 10 Å, and an area per glycolipid of 76 Å2. On the other hand, di-RL prefers to self-associate into flexible cylinder-like aggregates, from 70 mM to 110 mM concentration, with hydrophobic radius on the order of 7.5 Å, a hydrated polar shell of 21.5 Å, with hydropobic/polar interface of 110 Å2 per glycolipid. Interestingly, the parameters obtained from the best fitting to the experimental data associated to the volume fraction distribution of the chemical groups within the aggregates revealed that the hydrophobic chains are more disordered in mono-RL planar aggregates than in di-RL worm-like aggregates, as well as the hydration properties. Further, the addition of 100 mM NaCl in di-RL aqueous dispersion leads to the formation of longer worm-like aggregates. Taking together, this work opens a new avenue regarding characterization of biosurfactants self-assembling properties by using SAXS, also contributing to prepare more efficient biosurfactant dispersions depending on the desired applications in industrial sectors and bioremediation.
摘要：
鼠李糖脂是糖脂表面活性剂,其由一个(单-RL)或两个(二-RL)鼠李糖部分的亲水头与可以具有不同长度的羟基脂族链偶联组成。尽管它们在不同应用领域的重要性，例如，作为生物修复过程，自聚集的物理化学性质的RL不是唯一的。这是因为由于单-RL:二-RL摩尔比,各种聚集体形态(形状和大小)可以存在或共存于水分散体中,疏水尾巴长度，pH和辅助表面活性剂和添加剂的存在。最近,一种理论方法报道了纯mono或di-RL在水环境中的自组装形态，根据RL浓度和脂肪酸链长度，预测球形到椭圆形胶束到蠕虫状和盘状聚集体的形成。为了向以前可用的信息添加新信息，本工作通过小角度X射线散射（SAXS）分别研究了单RL-C10-C10和双RL-C10-C10在水分散体中的自组装性能。将一种新颖的方法应用于将RL化学基团的散射长度密度分布与蒙特卡罗模拟相结合的数据分析。这种方法使我们能够推断更好地适合自组装形态的优选的单RL和di-RL构象。这样,我们表明，mono-RL-C10-C10自组装成层状聚集体与30％的多层聚集体（大约5个封闭堆叠的薄片）共存，浓度范围从10到50mM，疏水厚度约为12埃，水合的极头厚度为10。每个糖脂的面积为76µ2。另一方面,di-RL更喜欢自缔合成灵活的圆柱状聚集体，从70mM到110mM浓度，疏水半径约为7.5，一个21.5的水合极性壳，每个糖脂的亲水/极性界面为110µ2。有趣的是,从与聚集体内化学基团的体积分数分布相关的实验数据的最佳拟合获得的参数表明，疏水链在单RL平面聚集体中比在di-RL蠕虫样聚集体中更无序，以及水合性能。Further,在di-RL水分散体中添加100mMNaCl会导致形成更长的蠕虫状聚集体。一起,这项工作为使用SAXS表征生物表面活性剂的自组装特性开辟了一条新途径，还有助于制备更有效的生物表面活性剂分散体,这取决于在工业部门和生物修复中的所需应用。
