Unveiling Microscopic Structures of Charged Water Interfaces by Surface-Specific Vibrational Spectroscopy

A sum-frequency spectroscopy scheme is developed that allows the measurement of vibrational spectra of the interfacial molecular structure of charged water interfaces. The application of this scheme to a prototype lipid-aqueous interface as a demonstration reveals an interfacial hydrogen-bonding water layer structure that responds sensitively to the charge state of the lipid headgroup and its interaction with specific ions. This novel technique provides unique opportunities to search for better understanding of electrochemistry and biological aqueous interfaces at a deeper molecular level.

Figure 1

(a) Illustration of a charged interfacial structure probed by SFVS. (b) Fractional surface densities of unionized COOH and negative charges $\sigma /N_S$ of the lignoceric acid monolayer on water versus $p\mathrm{H}$ (dots), where $N_S$ is the surface density of the monolayer. Lines are guides for the eyes. The inset shows the calculated $\sigma /N_S$ in the low-$p\mathrm{H}$ range. (c),(d) OH stretching spectra of $|{\chi }_{S,\mathrm{eff}}^{(2)}{|}^2$ and $\mathrm{Im}{\chi }_{S,\mathrm{eff}}^{(2)}$ for the lignoceric acid monolayer-water interface at various $p\mathrm{H}$, respectively.

Figure 2

(a) Spectra of complex ${\chi }_{S,\mathrm{DL}}^{(2)}$ of the diffuse layer and (b) spectra of complex ${\chi }_B^{(3)}$ of bulk water in the OH stretching region at various $p\mathrm{H}$ deduced from the spectra of lignoceric acid monolayer-water interfaces in Figs. 1 and 1. The red curve in (b) for ${\chi }_B^{(3)}(\omega )$ is a weighted average of the spectra for different $p\mathrm{H}$. The green curve is an average of ${\chi }_B^{(3)}(\omega )$ obtained from a set of ${\chi }_{S,\mathrm{eff}}^{(2)}(\omega )$ spectra presented in Fig. S9 of the Supplemental Material [27], taken at $p\mathrm{H}$ 6 with different NaCl concentrations in water.

Figure 3

(a) OH stretching spectra of the BIL of the lignoceric acid monolayer-water interface at three $p\mathrm{H}$ values with different interface charging conditions: $p\mathrm{H}$ 2.5, 10.6, and 12 corresponding to the fractional density of ${\mathrm{COO}}^{-}$ $({\mathrm{COO}}^{-}\cdots {\mathrm{Na}}^{+})=0\%$ (0%), 27% (9%), and 39% (52%) of the monolayer, respectively. The shadowed regions denote uncertainty, and the spectra are vertically shifted for clarity. (b),(c) Side-view snapshots of the MD trajectories for the neutral and fully deprotonated fatty acid–water interfaces, respectively. Major bonding configurations at the interfaces are emphasized in opaque. Detailed MD results are given in Ref. [27].

Figure 4

(a) Measured OH stretching spectra of $\mathrm{Im}{\chi }_{S,\mathrm{eff}}^{(2)}$ of an octadecanol monolayer mixed with small fractions of lignoceric acid on 0.1 mM NaCl solution (dots). Lines are guides to the eyes. (b) Weighted average of $\mathrm{Im}{\chi }_B^{(3)}(\omega )$ deduced from $\mathrm{Im}{\chi }_{S,\mathrm{eff}}^{(2)}(\omega )$ in (a) in comparison with the average $\mathrm{Im}{\chi }_B^{(3)}$ spectrum (red curve) given in Fig. 2.