Institute for Mechanical Process Engineering and Mechanics, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany.
Langmuir. 2010 May 18;26(10):7045-53. doi: 10.1021/la9043705.
We have studied the effect of counterion binding efficiency on the linear viscoelastic properties of wormlike micelles formed from hexadecyltrimethylammonium bromide (CTAB) in the presence of different nonpenetrating inorganic salts: potassium bromide (KBr), sodium nitrate (NaNO(3)), and sodium chlorate (NaClO(3)). We have varied the salt/surfactant ratio R at fixed surfactant concentration of 350 mM. Results are compared to data for the system cetylpyridinium chloride (CPyCl) and the penetrating counterion sodium salicylate (NaSal) (Oelschlaeger, C.; Schopferer, M.; Scheffold, F.; Willenbacher, N. Langmuir 2009, 25, 716-723). Mechanical high-frequency rheology and diffusing wave spectroscopy (DWS) based tracer microrheology are used to determine the shear moduli G' and G'' in the frequency range from 0.1 Hz up to 1 MHz (Willenbacher, N.; Oelschlaeger, C.; Schopferer, M.; Fischer, P.; Cardinaux, F.; Scheffold, F. Phys. Rev. Lett. 2007, 99, 068302, 1-4). This enables us to determine the plateau modulus G(0), which is related to the cross-link density or mesh size of the entanglement network, the bending stiffness kappa (also expressed as persistence length l(p) = kappa/k(B)T) corresponding to the semiflexible nature of the micelles, and the scission energy E(sciss), which is related to their contour length. The viscosity maximum shifts to higher R values, and the variation of viscosity with R is less pronounced as the binding strength decreases. The plateau modulus increases with R at low ionic strength and is constant around the viscosity maximum; the increase in G(0) at high R, which is presumably due to branching, is weak compared to the system with penetrating counterion. The scission energy E(sciss) approximately = 20 k(B)T is independent of counterion binding efficiency irrespective of R and is slightly higher compared to the system CPyCl/NaSal, indicating that branching may be significant already at the viscosity maximum in this latter case. The micellar flexibility increases with increasing binding efficiency of counterions according to the Hofmeister series. The persistence length values for systems CTAB/KBr, CTAB/NaNO(3), and CTAB/NaClO(3) are 40, 34, and 29 nm, respectively, independent of R, and are significantly higher than in the case of CPyCl/NaSal.
我们研究了抗衡离子结合效率对十六烷基三甲基溴化铵（CTAB）在不同非穿透无机盐（溴化钾（KBr）、硝酸钠（NaNO3）和氯酸钠（NaClO3））存在下形成的蠕虫状胶束的线性粘弹性质的影响。我们在固定表面活性剂浓度为 350mM 的情况下改变盐/表面活性剂比 R。结果与系统十六烷基吡啶氯（CPyCl）和穿透性抗衡离子水杨酸钠（NaSal）（Oelschlaeger，C.；Schopferer，M.；Scheffold，F.；Willenbacher，N. Langmuir 2009, 25, 716-723）的数据进行了比较。机械高频流变学和基于扩散波光谱（DWS）的示踪剂微流变学用于在 0.1Hz 至 1MHz 的频率范围内确定剪切模量 G'和 G''（Willenbacher，N.；Oelschlaeger，C.；Schopferer，M.；Fischer，P.；Cardinaux，F.；Scheffold，F. Phys. Rev. Lett. 2007, 99, 068302, 1-4）。这使我们能够确定平台模量 G(0)，它与交联密度或缠结网络的网格尺寸有关，与胶束的半柔性相对应的弯曲刚度 kappa（也表示为持久长度 l(p) = kappa/k(B)T），以及与它们的轮廓长度有关的断裂能 E(sciss)。粘度最大值向更高的 R 值移动，并且随着结合强度的降低，粘度随 R 的变化不太明显。在低离子强度下，平台模量随 R 增加，并且在粘度最大值附近保持不变；高 R 时 G(0)的增加（可能归因于分支）与穿透性抗衡离子的系统相比较弱。断裂能 E(sciss)≈20k(B)T 与抗衡离子结合效率无关，无论 R 如何，均独立于抗衡离子结合效率，并且与系统 CPyCl/NaSal 相比略高，表明在后者情况下，分支可能在粘度最大值时就很重要。根据豪夫迈斯特序列，随着抗衡离子结合效率的增加，胶束的柔韧性增加。CTAB/KBr、CTAB/NaNO3 和 CTAB/NaClO3 系统的持久长度值分别为 40、34 和 29nm，与 R 无关，并且明显高于 CPyCl/NaSal 系统。
