Nanofilm molecular structures at the mica-water interface

Amphiphilic molecules such as surfactants and surface-active polymers can readily adsorb at the solid-aqueous interface and form various organised structures. Understanding the characteristics and properties of such adsorbed molecular structures at the solid surface is important to many chemical and industrial applications and natural processes including personal care products, detergency and biolubrication.

Molecularly smooth muscovite mica, although widely used as a model substrate in surface-sensitive techniques such as atomic force microscopy (AFM), electron microscopy and surface force apparatus, up to now has not been considered as a suitable substrate for X-ray reflectometry (XRR) studies on nanofilms due to the difficulties in obtaining flatness over a sufficiently large area of mica. We have overcome this difficulty using a liquid cell (Figure 1a) that implements a simple “bending mica” method [1], where the enhanced rigidity of the mica sheet along its bending axis would facilitate sufficient flatness to perform XRR measurements along the axis. Experimentally, a detailed consideration of the bending curvature on the reflectivity has guided the setup to account for the divergence of the reflected beam (Figure 1b). In addition, the effect of the crystal truncation rods due to mica’s layered crystal structure (Figure 1c) on the overall reflectivity must be accounted for in the data analysis. This challenge is met by modelling mica’s electron density distribution as a dual layer (Figure 1d).

Figure 1. a) Key components of the liquid cell, with a liquid chamber formed when two 50 µm mica windows are clamped between the stainless-steel plates. A freshly cleaved mica substrate of 30 – 50 µm is clamped, by two small plates, onto an underlying cylinder of radius R = 7.5 cm, and the XRR measurement is performed along the bending axis, as shown in the inset. b) Perspective view of the reflection of an incident X-ray beam on a cylindrical surface. The blue beam is specularly reflected from a plane tangent to the apex, whereas the red one diverges due to the surface curvature. ΔZm and ΔYm indicate the maximum vertical and horizontal deviations at the extreme end of the reflected beam, respectively. At the beamlines ID10B and BM28, the slits in front of the detector are adjusted to accommodate such divergence. c) Muscovite mica crystal structure, with each layer containing an Al octahedral sheet covered by two (Si, Al) tetrahedral sheets, and mica’s monoclinic crystal lattice unit cell shown on the top right. d) The Gaussian distributions of the electron density of the muscovite mica unit cell for σ = 0.5 Å, where σ is the distribution width at 1/e of the maximum of the Gaussian distribution, and the dual layer model consisting of two slabs used to model the electron distribution of the mica unit cell.

This method has been successfully employed on BM28 (XMaS) and ID10B to reveal detailed structural information of a number of soft nanofilms as exemplified in Figure 2.  Hexadecyltrimethylammonium bromide (C₁₆TAB), a commonly used cationic surfactant, is found to form an interdigitated bilayer 32.0 Å in thickness with a full surface coverage at the highly charged mica-water interface (Figure 2a,b) at its critical micelle concentration (cmc). This result contrasts with previous AFM imaging data which showed the formation of cylindrically shaped surface aggregates. It implies that the localised force field or nanoconfinement might have induced the morphology of surface aggregates as observed with AFM, which is different from the intrinsic aggregation structure formed under quiescent conditions in XRR. This has general implications for our fundamental understanding of surfactant adsorption mechanisms at the solid-liquid interface, and in turn, how surfactant layers may tailor surface chemistry and mediate inter-surface interactions. As another example, the interaction between poly(ethyleneimine) (PEI), a positive charged polymer, and cesium perfluorononanoate (CsPFN), a fluorinated anionic surfactant at its critical micelle concentration, is shown to lead to the formation of a CsPFN bilayer 18.0 Å in thickness atop the PEI layer. The CsPFN fluorocarbon surfactant tails are found to tilt at 30.6° with respect to normal, with a full surface coverage. Such detailed structural information will be beneficial to optimising the synergetic effect of co-adsorption of surfactant-polymer complexes and achieving desired surface structures for applications such as detergency, paints, cosmetics and lubrication.

Figure 2. a) Experimental (open circles) and fitted (red curve) reflectivity for C16TAB at 1 cmc. The thickness of the adsorbed C16TAB bilayer is found to be 32.1 Å, a value that is considerably shorter than two fully extended C16TAB molecules (~ 47.4 Å). b) This suggests possible interdigitation of the alkyl chains, i.e. they penetrate into each other. c) Experimental (open circles) and fitted (solid curve) reflectivity curves for CsPFN-PEI complex at CsPFN concentration of 1 cmc. d) Results show the formation of a fluorinated bilayer with full surface coverage and a thickness of 18.0 Å atop a PEI layer adsorbed on the mica surface. The fluorocarbon tails are tilted at 30.6° with respect to normal.

The liquid cell used here can be applied to study the structure of a wide range of systems at the mica-water interface. These include the interactions between polyelectrolytes and lipids, structuring of ionic liquids, conformations of polymer brushes and lipid liposomes. The results will provide molecular details of these soft nanofilms at the highly charged mica-water interface previously unattainable with optical techniques such as ellipsometry, optical reflectometry and Fourier transform infrared spectroscopy. Such information will further improve our fundamental understanding of soft matter adsorption at the solid-liquid interface, relevant to a number of industrial applications and biological processes.

Principal publication and authors
Synchrotron XRR study of soft nanofilms at the mica-water interface, W.H. Briscoe (a), F. Speranza (a), P. Li (b), O. Konovalov (c), L. Bouchenoire (d,e), J. van Stam (f), J. Klein (g), R.M.J. Jacobs (h), and R.K. Thomas (b), Soft Matter 8, 5055-5068 (2012).
(a) School of Chemistry, University of Bristol (U.K.)
(b) Physical and Theoretical Chemistry Laboratory, University of Oxford (U.K.)
(c) ESRF
(d) XMaS, the UK-CRG, ESRF, Grenoble (France)
(e) Department of Physics, Oliver Lodge Laboratory, University of Liverpool (U.K.)
(f) Department of Chemistry and Biomedical Sciences, Karlstad University (Sweden)
(g) Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot (Israel)
(h) Department of Chemistry, Chemistry Research Laboratory, University of Oxford (U.K.)

References
[1] W.H. Briscoe et al., Journal of Colloid and Interface Science 306, 459-463 (2007).

Top image: X-ray reflectometry using a “bending mica” method yields interfacial structural details of soft nanofilms in aqueous media.