Perkin Susan, Kampf Nir, Klein Jacob
Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford, OX1 3QZ, United Kingdom.
J Phys Chem B. 2005 Mar 10;109(9):3832-7. doi: 10.1021/jp047746u.
Using contact angle measurements, surface force balance experiments, and AFM imaging, we have investigated the process of self-assembly of surfactants onto mica and the subsequent stability of those layers in pure water. In the case of cetyltrimethylammonium bromide (CTAB), the stability of a monolayer when immersed in pure water is found to be dependent on initial immersion time in surfactant, which is likely to be caused by an increase in the proportion of ion-exchange to ion-pair adsorption when incubated in surfactant for longer periods of time. Infinite dilution of the surfactant solution before withdrawal of the sample is found to have little effect on the stability of the resulting layer in pure water. The nature of the counterion is found to affect dramatically the stability of a self-assembled surfactant monolayer: cetyltrimethylammonium fluoride (CTAF) forms a layer that is much more stable in water than CTAB, which is likely to be due to faster and more complete ion-exchange with the mica surface for CTAF. Surface force balance experiments show that when the hydrophobic monolayer is immersed in pure water it does not simply dissolve into the water; instead it rearranges, possibly to patches of bilayer or hemimicelles. The time scale of this rearrangement agrees well with the time scale of the change from a hydrophobic to more hydrophilic surface observed using contact angle measurements. AFM imaging has also in some cases shown an evolution from an even monolayer to patches of bilayer.
通过接触角测量、表面力平衡实验和原子力显微镜成像，我们研究了表面活性剂在云母上的自组装过程以及这些层在纯水中的后续稳定性。对于十六烷基三甲基溴化铵（CTAB），发现单层在浸入纯水中时的稳定性取决于在表面活性剂中的初始浸入时间，这可能是由于在表面活性剂中孵育较长时间时离子交换与离子对吸附比例的增加所致。发现在取出样品前对表面活性剂溶液进行无限稀释对所得层在纯水中的稳定性影响很小。发现抗衡离子的性质极大地影响自组装表面活性剂单层的稳定性：十六烷基三甲基氟化铵（CTAF）形成的层在水中比CTAB稳定得多，这可能是由于CTAF与云母表面的离子交换更快且更完全。表面力平衡实验表明，当疏水单层浸入纯水中时，它不会简单地溶解在水中；相反，它会重新排列，可能形成双层或半胶束斑块。这种重新排列的时间尺度与使用接触角测量观察到的从疏水表面到更亲水表面变化的时间尺度非常吻合。在某些情况下，原子力显微镜成像也显示了从均匀单层到双层斑块的演变。
