Reply to the ‘Comment on “Bi-layering at ionic liquid surfaces: a sum-frequency generation vibrational spectroscopy- And molecular dynamics simulation-based study”’ by M. Deutsch, O. M. Magnussen, J. Haddad, D. Pontoni, B. M. Murphy and B. M. Ocko,Phys. Chem. Chem. Phys., 2021, DOI: 10.1039/D0CP04882H

In our recent paper titled “Bi-layering at ionic liquid surfaces: a sum-frequency generation vibrational spectroscopy- and molecular dynamics simulation-based study” co-authored by T. Iwahashi, T. Ishiyama, Y. Sakai, A. Morita, D. Kim, and Y. Ouchi,Phys. Chem. Chem. Phys., 2020,22, 12565 (hereafter referred to as IW), the sum-frequency (SF) spectra for a homologous series of 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C_nmim][TFSA]n= 4, 6, 8, 10, and 12) were reported. In particular, a clear decrease in the SF signals from the [TFSA]⁻anions with increasing chain length of the [C_nmim]⁺cation (Fig. 5 of IW) was explained in terms of “head-to-head” bi-layer formation at the air/ionic liquid (IL) interface. A comment by M. Deutschet al.(hereafter referred to as DE) questioned this report, claiming that our proposed structure is not consistent with a multilayered electron density (ED) profile obtained by X-ray reflectivity (XR).