Instituto de Química Física de los Materiales, Medio Ambiente y Energía and Departamento de Química Inorgánica Analítica y Química Física, University of Buenos Aires, School of Sciences, Ciudad Universitaria, Pabellón 2, Ciudad Autónoma de Buenos Aires C1428EHA, Argentina.
Department of Pharmaceutical Sciences, College of Pharmacy, University of Nebraska Medical Center, Omaha, Nebraska 68198-6125, United States.
J Phys Chem B. 2020 Apr 16;124(15):3221-3227. doi: 10.1021/acs.jpcb.0c01301. Epub 2020 Apr 1.
Charged amphiphiles in solution usually self-assemble into flat nanoribbons that spontaneously twist into different shapes. The role of electrostatics in this process is still under strong debate. This work studies the electrostatic free energy of twisting a nanoribbon at the level of the nonlinear Poisson-Boltzmann approximation. It is shown that helicoid-shaped ribbons are more stable than flat ribbons, while other shapes under consideration (cylindrical helixes and bent ribbons) are always less stable than the flat ribbon. The unexpected electrostatics-driven twisting of the ribbon into a helicoid is ascribed to the increase in its perimeter with increasing degree of twisting, as charges near the edge of the ribbon are electrostatically more stable than those near its center. This argument successfully explains the effects of salt concentration and the width of the ribbon on the optimal twisting period and allows us to approximately describe the problem of ribbon twisting in terms of two dimensionless variables that combine the helicoid twisting period, the Debye length of the solution, and the width of the ribbon. The magnitude of the electrostatic twisting energy predicted by our calculations is comparable to that of restoring elastic forces for typical ribbons of self-assembled amphiphiles, which indicates that electrostatics plays an important role in determining the equilibrium shape of charged nanoribbons.
溶液中的带电两亲体通常自组装成扁平纳米带，这些纳米带会自发地扭曲成不同的形状。在这个过程中，静电的作用仍然存在激烈的争论。本工作在非线性泊松-玻尔兹曼近似水平上研究了扭曲纳米带的静电自由能。结果表明，螺旋形纳米带比扁平纳米带更稳定，而其他考虑的形状（圆柱螺旋和弯曲纳米带）总是比扁平纳米带不稳定。纳米带由于静电作用而扭曲成螺旋形的原因是，随着扭曲程度的增加，其周长增加，这使得纳米带边缘处的电荷比中心处的电荷更稳定。这一论点成功地解释了盐浓度和纳米带宽度对最优扭曲周期的影响，并允许我们用两个无量纲变量来近似描述纳米带扭曲的问题，这两个变量结合了螺旋扭曲周期、溶液的德拜长度和纳米带的宽度。我们的计算预测的静电扭曲能量与典型自组装两亲体纳米带的恢复弹性力相当，这表明静电在确定带电纳米带的平衡形状方面起着重要作用。
