Adsorption of rhodamine 6G and choline on gold electrodes: a molecular dynamics study

Impact of the Crystal Structure of Silica Nanoparticles on Rhodamine 6G Adsorption: A Molecular Dynamics Study

Understanding the mechanism of adsorption of Rhodamine 6G (R6G) to various crystal structures of silica nanoparticles (SNPs) is important to elucidate the impact of dye size when measuring the size of the dye-SNP complex via the time-resolved fluorescence anisotropy method. In this work, molecular dynamics (MD) simulations were used to get an insight into the R6G adsorption process, which cannot be observed using experimental methods. It was found that at low pH, α-Cristobalite structured SNPs have a strong affinity to R6G; however, at high pH, more surface silanol groups undergo ionization when compared with α-Quartz, preventing the adsorption. Therefore, α-Quartz structured SNPs are more suitable for R6G adsorption at high pH than the α-Cristobalite ones. Furthermore, it was found that stable adsorption can occur only when the R6G xanthene core is oriented flat with respect to the SNP surface, indicating that the dye size does not contribute significantly to the measured size of the dye-SNP complex. The requirement of correct dipole moment orientation indicates that only one R6G molecule can adsorb on any sized SNP, and the R6G layer formation on SNP is not possible. Moreover, the dimerization process of R6G and its competition with the adsorption has been explored. It has been shown that the highest stable R6G aggregate is a dimer, and in this form, R6G does not adsorb to SNPs. Finally, using steered molecular dynamics (SMD) with constant-velocity pulling, the binding energies of R6G dimers and R6G complexes with both α-Quartz and α-Cristobalite SNPs of 40 Å diameter were estimated. These confirm that R6G adsorption is most stable on 40 Å α-Quartz at pH 7, although dimerization is equally possible.

Graphene Oxide and Its Derivatives as Adsorbents for PFOA Molecules

Effective, low-cost adsorbents are needed to remove perfluoroalkyl and polyfluoroalkyl substances (PFAS) from water sources. Carbon-based materials are promising PFAS adsorbents. Here, we explore the potential of graphite oxide (GO) and its derivatives as PFAS adsorbents by studying the adsorption of perfluorooctanoic acid (PFOA), a model PFAS molecule, on GO surfaces with O/C ratios up to 16.7% using molecular dynamics simulations. An adsorption free energy of approximately -30 kJ/mol (or -310 meV) is obtained for pristine graphene in pure water, and adsorbed PFOA molecules diffuse rapidly. As the O/C ratio increases, hydrophobic interactions' contribution to PFOA adsorption diminishes, but that by electrostatic interactions becomes important. Overall, adsorption is weakened, but favorable adsorption still occurs at an O/C ratio of 16.7%. The in-plane diffusion coefficient of adsorbed PFOA molecules decreases by more than 45 times as the O/C ratio increases to 8.3% but increases significantly when the O/C ratio increases further to 16.7%. Adding salt improves the adsorption owing to the salting-out and screening effects but slows the diffusion of adsorbed PFOA molecules, and these effects are more pronounced at low O/C ratios. These results show that GOs are effective PFOA adsorbents. Such effectiveness, along with GO's potentially low cost and the possibility of regenerating spent GO by removing adsorbed PFOA molecules through a mild electrical potential, makes GO a promising adsorbent for PFOA and similar molecules. The insights from the present study can help the rational design of GOs to realize their full potential.