Khannanov Artur, Gareev Bulat, Batalin Georgii, Amirova Liliya M, Dimiev Ayrat M
Kazan National Research Technical University , K. Marx Str. 10 , Kazan 420111 , Russian Federation.
Langmuir. 2019 Oct 15;35(41):13469-13479. doi: 10.1021/acs.langmuir.9b01882. Epub 2019 Oct 1.
Despite enormous interest toward graphene oxide (GO) from the research community, surprisingly, little is known about its solutions. In particular, the questions related to the structure of the GO/liquid interface have not been yet properly addressed. In this report, we use a simple but efficient experimental approach to investigate the distribution of the four metal cations Na, Cs, Ni, and Gd at the GO/water interface. We demonstrate that the concentration of the cations decreases exponentially with the distance from the GO surface. Such distribution for colloid systems was theoretically predicted and commonly accepted for a century but, to the best of our knowledge, has been never proved experimentally. We further demonstrate that the shape of the counterion distribution profiles depends on the pH of solution and on the fine chemical structure of GO. In particular, organic sulfates and vinylogous acids that are ionizable at different pH levels are responsible for the difference in the shapes of the concentration profiles. Unlike classical colloid systems, the diffuse layer in the GO solutions is rather broad (30-55 nm), and the concentration gradient is registered even at distances of >55 nm from the GO surface, which is typically considered as the bulk solution. The latter observation is explained by the immobilized character of the GO flakes in the nematic phase, impeding the flow of liquid and the migration of hydrated metal cations. This helps to establish and maintain the long-range concentration gradient in the space between the two parallel neighboring GO flakes. Based on the new findings and on the previously reported data, we formulate some basic principles of GO solutions.
尽管研究界对氧化石墨烯（GO）有着浓厚的兴趣，但令人惊讶的是，人们对其溶液却知之甚少。特别是，与GO/液体界面结构相关的问题尚未得到妥善解决。在本报告中，我们采用一种简单而有效的实验方法来研究四种金属阳离子Na、Cs、Ni和Gd在GO/水界面的分布。我们证明，阳离子浓度随着与GO表面距离的增加呈指数下降。这种胶体系统的分布在理论上已被预测且一个世纪以来普遍被接受，但据我们所知，从未得到实验证明。我们进一步证明，抗衡离子分布曲线的形状取决于溶液的pH值和GO的精细化学结构。特别是，在不同pH水平下可电离的有机硫酸盐和乙烯型酸是造成浓度曲线形状差异的原因。与经典胶体系统不同，GO溶液中的扩散层相当宽（30 - 55纳米），即使在距离GO表面>55纳米处（通常被视为本体溶液）也能记录到浓度梯度。后一观察结果可通过GO薄片在向列相中的固定特性来解释，这阻碍了液体流动和水合金属阳离子的迁移。这有助于在两个平行相邻的GO薄片之间的空间中建立和维持长程浓度梯度。基于这些新发现和先前报道的数据，我们阐述了一些GO溶液的基本原理。
