Department of Chemical & Biological Engineering, ‡Department of Biology, and §Department of Chemistry, Center for Biotechnology and Interdisciplinary Studies, Rensselaer Polytechnic Institute (RPI) , Biotechnology Building, 110 Eighth Street, Troy, New York 12180, United States.
Langmuir. 2017 Jun 13;33(23):5760-5768. doi: 10.1021/acs.langmuir.7b00480. Epub 2017 Jun 1.
Surfactants are ubiquitous constituents of commercial and biological systems that function based on complex structure-dependent interactions. Sophorolipid (SL) n-alkyl esters (SL-esters) comprise a group of modified naturally derived glycolipids from Candida bombicola. Herein, micellar self-assembly behavior as a function of SL-ester chain length was studied. Surface tensions as low as 31.2 mN/m and critical micelle concentrations (CMCs) as low as 1.1 μM were attained for diacetylated SL-decyl ester (dASL-DE) and SL-octyl ester, respectively. For deacetylated SL-esters, CMC values reach a lower limit at SL-ester chains above n-butyl (SL-BE, 1-3 μM). This behavior of SL-esters with increasing hydrophobic tail length is unlike other known surfactants. Diffusion-ordered spectroscopy (DOSY) and T relaxation NMR experiments indicate this behavior is due to a change in intramolecular interactions, which impedes the self-assembly of SL-esters with chain lengths above SL-BE. This hypothesis is supported by micellar thermodynamics where a disruption in trends occurs at n-alkyl ester chain lengths above those of SL-BE and SL-hexyl ester (SL-HE). Diacetylated (dA) SL-esters exhibit an even more unusual trend in that CMC increases from 1.75 to 815 μM for SL-ester chain lengths of dASL-BE and dASL-DE, respectively. Foaming studies, performed to reveal the macroscopic implications of SL-ester micellar behavior, show that the observed instability in foams formed using SL-esters are due to coalescence, which highlights the importance of understanding intermicellar interactions. This work reveals that SL-esters are an important new family of green high-performing surfactants with unique structure-property relationships that can be tuned to optimize micellar characteristics.
表面活性剂是商业和生物系统中无处不在的成分，它们基于复杂的结构依赖性相互作用发挥功能。槐糖脂（SL）n-烷基酯（SL-酯）是一组从 Candida bombicola 衍生的天然改性糖脂。本文研究了 SL-酯链长作为功能的胶束自组装行为。二乙酰化 SL-癸基酯（dASL-DE）和 SL-辛基酯的表面张力低至 31.2 mN/m 和临界胶束浓度（CMC）低至 1.1 μM。对于去乙酰化的 SL-酯，CMC 值在 SL-酯链长超过正丁基（SL-BE，1-3 μM）时达到下限。这种随着疏水性尾链长度增加的 SL-酯的行为与其他已知的表面活性剂不同。扩散有序光谱（DOSY）和 T 弛豫 NMR 实验表明，这种行为是由于分子内相互作用的变化，这阻碍了 SL-BE 以上链长的 SL-酯的自组装。这种假设得到了胶束热力学的支持，其中在 n-烷基酯链长超过 SL-BE 和 SL-己基酯（SL-HE）时，趋势发生了中断。二乙酰化（dA）SL-酯表现出更为异常的趋势，即对于 SL-酯链长分别为 dASL-BE 和 dASL-DE，CMC 从 1.75 增加到 815 μM。为揭示 SL-酯胶束行为的宏观影响而进行的发泡研究表明，使用 SL-酯形成的泡沫中观察到的不稳定性是由于聚结引起的，这突出了理解胶束间相互作用的重要性。这项工作表明，SL-酯是一类具有独特结构-性能关系的新型绿色高性能表面活性剂，其结构可进行调整以优化胶束特性。
