Analysis of suspended layer of alkali ions on a surface of silica sol by synchrotron X-ray reflectivity and scattering

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Budker INP

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Oral X-ray structural analysis


Yuriy Volkov (FSRC "Crystallography and Photonics" RAS)


Solutions of SiO$_2$ nanoparticles (with typical diameter of 5--25 nm) in water stabilized by a small amount of alkali ions --- known as silica sols --- exhibit strong gradient of the surface potential at the sol/air interface. It has been shown previously that under such boundary conditions charged silica nanoparticles ($Q\sim 1000$ $\bar e$) form a macroscopically flat layer near the surface, its depth proportional to the Debye screening length within the bulk solution, while alkali cations are accumulated in a thin suspended film directly at the surface [1]. We present new systematic data on the structural properties of suspended cation layers for silica sols enriched with Na$^+$, K$^+$, Cs$^+$ and Rb$^+$ ions, based on the measurements of X-ray reflectivity (XRR), grazing-incidence diffuse scattering (XDS) and small-angle X-ray scattering (SAXS). Measurements were performed at beamline ID31 (ESRF, Grenoble) with photon wavelength $\lambda = 0.1747\pm 0.0003$ Å ($E \approx 71$ keV), beam size $10\times 250$ $\mu$m, peak intensity $I_{max} \sim 10^{19}$ photons/s. SAXS data were analyzed by fitting the particles' structure factors in frames of rigid spheres approximation using ATSAS software suite [2]. XRR and XDS data were analyzed in frames of a self-consistent model-independent approach [3], which allowed us to simultaneously extract depth-graded distribution of volumetric electron density $\rho(z)$ as well as power spectral density functions of in-plane surface roughness $\bar C(\nu)$, including "hidden" M$^+$-layer/substrate interface, without any a priori assumptions on either sample internal structure or statistical properties of surface roughness. Fitting of SAXS results indicates that characteristic diameter distributions of SiO$_2$ nanoparticles increase with dopation by Cs$^+$ and Rb$^+$ for $0.6\pm 0.1$ nm compared to Na$^+$, which indicates additional adsorption of alkali ions at the nanoparticles. $\rho(z)$ distributions extracted from XRR data show characteristic density peaks near external interface corresponding to the suspended ions layer with thickness $d \approx 7\ldots 12$ Å; estimation of two-dimensional surface concentration of ions yields $\Theta = (5\pm 1)\cdot 10^{18}$ m$^{-2}$, which corresponds to known theoretical predictions for two-dimensional Wigner crystal [1,4]. Effective rms heights of surface roughness obtained from XDS data give $\sigma = (3.0\pm 0.2)$ Å within spatial frequency range $\nu = 10^{-5}\ldots 10^{-1}$ nm$^{-1}$, which corresponds to theoretical values in frames of capillary waves theory [5]. However, extracted power spectral density functions $\bar C(\nu)$ are found to diverge substantially from the capillary waves-based prediction in frequency range $\nu < 10^{-4}$ nm$^{-1}$. This can be interpreted according to the "roughness scaling" model [6] as a superposition of two different roughness distribution, which indicates possible presence of at least two different structural phases across the surface. Additionally, preliminary experiments for reproduction of a whispering gallery (WG) phenomenon on liquid silica sol has been conducted; the data obtained in the present work are planned to be applied to further quantitative assessment and modeling of X-ray wave propagation in WG case. The present work has been supported by the Federal Agency of Scientific Organizations (Agreement No 007-ГЗ/Ч3363/26). [1] A.M. Tikhonov // J. Chem. Phys. 130, 024512 (2009). [2] P.V. Konarev, M.V. Petoukhov et al. // J. Appl. Cryst. 39, 277 (2006). [3] I.V. Kozhevnikov, L. Peverini, E. Ziegler // Phys. Rev. B 85, 125439 (2012). [4] Y. Burak, D. Andelman, H. Orland // Phys. Rev. E 70, 016102 (2004). [5] F. Buff, R. Lovett, F. Stillinger // Phys. Rev. Lett. 15, 621 (1965). [6] S. Majaniemi, T. Ala-Nissila, J. Krug // Phys. Rev. B 53, 8071 (1996).

Primary authors

Dr Alexei Tikhonov (Kapitza Institute for Physical Problems RAS) Yuriy Volkov (FSRC "Crystallography and Photonics" RAS)


Boris Roshchin (FSRC "Crystallography and Photonics" RAS) Maria Blanco (European Synchrotron Research Facility) Veijo Honkimäki (European Synchrotron Research Facility) Prof. Victor Asadchikov (FSRC "Crystallography and photonics" RAS) Dr Vladimir Volkov (FSRC "Crystallography and Photonics" RAS)

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