Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, 8093 Zurich, Switzerland.
Langmuir. 2010 Feb 16;26(4):2761-8. doi: 10.1021/la902800x.
We demonstrated previously (Wu, H.; Zaccone, A.; Tsoutsoura, A.; Lattuada, M.; Morbidelli, M. Langmuir 2009, 25, 4715) that, for a colloid stabilized by charges from both polymer chain-end groups and adsorbed sulfonate surfactants, when the surfactant surface density reaches a certain critical value, the shear-induced gelation becomes unachievable at room temperature, even at an extremely large Peclet number, Pe = 4.6 x 10(4). This is due to the presence of the short-range, repulsive hydration force generated by the adsorbed surfactant. In this work, we investigate how such hydration force affects the shear-induced gelation at higher temperatures, in the range between 303 and 338 K. It is found that a colloidal system, which does not gel at room temperature in a microchannel at a fixed Pe = 3.7 x 10(4), does gel when temperature increases to a certain value. The critical initial particle volume fraction for the gelation to occur decreases as temperature increases. These results indicate that the effect of the hydration force on the gelation decreases as temperature increases. Moreover, we have observed that at the criticality only part of the primary particles is converted to the gel network and the effective particle volume fraction forming the gel network does not change significantly with temperature. The effective particle volume fraction is also independent of the surfactant surface coverage. Since the effective particle volume fraction corresponds to space filling requirement of a standing gel network, which is mainly related to the clusters structure, this result indicates that at a given shear rate the cluster structure does not change significantly with the surfactant surface coverage. On the other hand, since the cluster morphology is a strong function of the shear rate, we have observed that when the Peclet number is lowered from Pe = 3.7 x 10(4) to 1.7 x 10(4), the effective particle volume fraction reduces from 0.19 to 0.12 at 313 K.
我们之前已经证明（Wu, H.; Zaccone, A.; Tsoutsoura, A.; Lattuada, M.; Morbidelli, M. Langmuir 2009, 25, 4715），对于同时由聚合物链端基团和吸附磺酸盐表面活性剂提供电荷稳定的胶体，当表面活性剂的表面密度达到一定临界值时，即使在非常大的 Peclet 数 Pe = 4.6 x 10(4)下，室温下也无法实现剪切诱导凝胶化。这是由于吸附的表面活性剂产生的短程、排斥水化力所致。在这项工作中，我们研究了这种水化力如何在较高温度下影响剪切诱导凝胶化，温度范围在 303 到 338 K 之间。结果发现，在微通道中，在固定的 Pe = 3.7 x 10(4)下，胶体系统在室温下不会凝胶，但当温度升高到一定值时会凝胶。发生凝胶化的临界初始颗粒体积分数随着温度的升高而降低。这些结果表明，水化力对凝胶化的影响随着温度的升高而降低。此外，我们还观察到，在临界点只有部分初级颗粒转化为凝胶网络，形成凝胶网络的有效颗粒体积分数随温度变化不大。有效颗粒体积分数也与表面活性剂的表面覆盖率无关。由于有效颗粒体积分数对应于站立凝胶网络的空间填充要求，主要与簇结构有关，因此，这一结果表明，在给定的剪切速率下，簇结构随表面活性剂表面覆盖率的变化不大。另一方面，由于簇形态是剪切速率的强函数，我们观察到当 Peclet 数从 Pe = 3.7 x 10(4)降低到 1.7 x 10(4)时，在 313 K 时有效颗粒体积分数从 0.19 降低到 0.12。
