Ion-Specific Induced Charges at Aqueous Soft Interfaces

Ionic specificity effects, i.e., ions of the same valence leading to different macroscopic effects, are studied by considering a Langmuir monolayer of arachidic acid over a solution containing either ${\mathrm{Fe}}^{3+}$ or ${\mathrm{La}}^{3+}$. We systematically vary $p\mathrm{H}$ levels as a way to control the interfacial surface charge and characterize the system by surface-sensitive x-ray scattering and spectroscopic techniques. We show that the critical surface pressure at the tilted ($L2$) to untilted ($LS$) transition is ionic specific and varies with $p\mathrm{H}$. While the maximum density of surface bound ${\mathrm{La}}^{3+}$ per head group of arachidic acid is $\sim 0.3$, the amount necessary to neutralize the surface charge, for ${\mathrm{Fe}}^{3+}$ it is nearly 0.6 and it is accompanied with a significant accumulation of the coions ${\mathrm{Cl}}^{-}$ as revealed by surface x-ray spectroscopy. We account for the experimental observations by a statistical mechanical model including ion specificity.

Figure 1

Isotherms of AA on pure water and on (a) ${\mathrm{FeCl}}_3$ of ${10}^{-3}M$ and (b) ${\mathrm{LaCl}}_3$ of ${10}^{-3}M$ solutions at various $p\mathrm{H}$ values (arrows indicate ${\pi }_c$). (c) $L\mathrm{\text{−}}S$ critical surface pressure ${\pi }_c$ as a function of $p\mathrm{H}$ for ${\mathrm{Fe}}^{3+}$ and ${\mathrm{La}}^{3+}$. The solid line for ${\mathrm{La}}^{3+}$ is calculated with Eq. (2) and the dashed curves provide the limiting cases with reasonable variations of parameters used in the theoretical model. The two vertical dashed lines indicate the $p{\mathrm{H}}_c$ (left) and $p{\mathrm{H}}_M$ (right) (solid line for ${\mathrm{FeCl}}_3$ data is for display only).

Figure 2

(a) Normalized XR from monolayer on water and subphase at different $p\mathrm{H}$. The symbols ○, □, and ▽ represent the experimental data of XR from the monolayer sitting on pure water, ferric chloride subphase of $p\mathrm{H}=1.5$ and 2.6. Solid lines through these symbols are calculated reflectivities based on two-slab model (curves shifted vertically for clarity, vertical dash line indicates first minimum in curves). (b) ED profiles simulated by the best-fit parameters. (c) Fluorescence signal below critical angle (integrated over $Q_z=0.01–0.021{\AA }^{-1}$). (d) Fluorescence signal above critical angle (integrated over $Q_z=0.022–0.03{\AA }^{-1}$). (e) Integrated Fe emission line intensity over 6.2–6.6 keV with curve fit as function of $Q_z$. (f) $p\mathrm{H}$ dependence of iron accumulation at surface (dashed line is a guide to the eye).

Figure 3

Fluorescence spectrum showing chlorine characteristic emission line of subphase (a) $p\mathrm{H}=2.6$, (b) $p\mathrm{H}=2.3$, and (c) $p\mathrm{H}=1.5$. The symbols ○ and □ represent the fluorescence with and without monolayer, respectively. The signals are integrated over $Q_z=0.01–0.021{\AA }^{-1}$. Each point is obtained by binning three consecutive channels.

Figure 4

Two scenarios of ion binding. (a) ${\mathrm{La}}^{3+}$ ions bind to charged surface by mainly electrostatic forces, and (b) bound $\mathrm{Fe}(\mathrm{OH}{)}_{3-z}^{z+}$ complexes to AA attract ${\mathrm{Cl}}^{-}$ coions.