KTH Royal Institute of Technology, School of Chemical Science and Engineering, Department of Chemistry, Surface and Corrosion Science, SE-100 44 Stockholm, Sweden.
J Colloid Interface Sci. 2012 Sep 1;381(1):89-99. doi: 10.1016/j.jcis.2012.05.015. Epub 2012 May 18.
Self-assembly in mixtures of two single-chain cationic surfactants, with different tail lengths (CTAB and DTAB) as well as of a single-chain (DTAB) and a double-chain (DDAB) cationic surfactant, with identical tail lengths, have been investigated with small-angle neutron scattering (SANS) and rationalised in terms of bending elasticity properties. The growth behaviour of micelles with respect to surfactant composition appears completely different in the two surfactant mixtures. DTAB form small oblate spheroidal micelles in presence of [NaBr]=0.1 M that transform into prolate spheroidal mixed CTAB/DTAB micelles upon adding moderate amounts of CTAB, so as to give a mole fraction y=0.20 in solution. Most unexpectedly, upon further addition of CTAB the mixed CTAB/DTAB micelles grow with an almost equal rate in both length and width directions to form tablets. In contrast to this behaviour, mixed DDAB/DTAB micelles grow virtually exclusively in the length direction, in presence of [NaBr]=0.1 M, to form elongated ellipsoidal (tablet-shaped) and subsequently long wormlike micelles as the fraction of DDAB in the micelles increases. Mixed DDAB/DTAB micelles grow to become as long as 2000Å before an abrupt transition to large bilayer structures occurs. This means that the micelles are much longer at the micelle-to-bilayer transition as compared to the same mixture in absence of added salt. It is found that the point of transition from micelles to bilayers is significantly shifted towards higher fractions of aggregated DTAB as an appreciable amount of salt is added to DDAB/DTAB mixtures, indicating a considerable reduction of the spontaneous curvature with an increasing [NaBr]. By means of deducing the various bending elasticity constants from our experimental results, according to a novel approach by ours, we are able to conclude that the different growth behaviours appear as a consequence of a considerably lower bending rigidity, as well as higher saddle-splay constant, for DDAB/DTAB surfactant mixtures in presence of [NaBr]=0.1 M, as compared to mixtures of CTAB/DTAB in [NaBr]=0.1 M and DDAB/DTAB in absence of added salt.
两种单链阳离子表面活性剂（CTAB 和 DTAB）混合物以及单链（DTAB）和双链（DDAB）阳离子表面活性剂混合物的自组装已通过小角中子散射（SANS）进行了研究，并根据弯曲弹性特性进行了合理化。在两种表面活性剂混合物中，胶束的生长行为完全不同。在[NaBr]=0.1 M 的存在下，DTAB 形成小的扁长球体胶束，当加入适量 CTAB 时，这些胶束会转变成拉长的 CTAB/DTAB 混合胶束，从而在溶液中形成摩尔分数 y=0.20。最出人意料的是，当进一步加入 CTAB 时，混合 CTAB/DTAB 胶束在长度和宽度方向上以几乎相同的速度生长，形成平板。与这种行为相反，在[NaBr]=0.1 M 的存在下，混合 DDAB/DTAB 胶束实际上仅在长度方向上生长，形成拉长的椭圆形（平板状），随后随着胶束中 DDAB 分数的增加形成长的蠕虫状胶束。混合 DDAB/DTAB 胶束生长到 2000Å 长之前，会突然转变为大双层结构。这意味着与没有添加盐的相同混合物相比，在胶束到双层的转变点处，胶束要长得多。发现，随着向 DDAB/DTAB 混合物中加入相当数量的盐，从胶束到双层的转变点显著向更高的聚集 DTAB 分数移动，表明随着[NaBr]的增加，自发曲率显著降低。根据我们的新方法，从我们的实验结果推导出各种弯曲弹性常数后，我们能够得出结论，与 CTAB/DTAB 在[NaBr]=0.1 M 以及 DDAB/DTAB 在没有添加盐的混合物相比，DDAB/DTAB 表面活性剂混合物在[NaBr]=0.1 M 存在下具有较低的弯曲刚性以及较高的鞍形曲折常数，这是导致不同生长行为的原因。
