Nanoconfinement of Liquids

Figure 1: Distance between two surfaces (D) determined by multiple beam interferometry, with high accuracy (±30pm). Force (F) determined by Hooke’s law.

Ionic Liquids (by Mengwei Han, Ge Song)

Fundamental research on the behavior of liquids under nanoconfinement is ongoing, specially in the areas of Ionic liquids, aqueous solutions and oils. The mere geometric constraint induced by a smooth solid surface in the liquid is known to promote molecular layering in the absence of any repulsive or attractive interaction, and it is most pronounced for spherical, rigid and non-flexible molecules.

Force measurements can be used to determine the structure of the confined liquid. An extended Surfaces Forces Apparatus (eSFA) is  used  for this research (Figure 1). Figure 2 shows the measured solvation force between mica surfaces in [C2mim][FAP] in dry nitrogen (redish colors). The steps in the solvation force indicate the layering of the ionic liquid in films confined between mica surfaces. We note that while the structural force is oscillatory in origin, a single force measurement cannot show the oscillations but rather steps or discontinuities in the force profile; in some laboratories, it is, however, common practice to superpose multiple force measurements with retractions from different distances to illustrate the underlying oscillations.

Figure 2: Force isotherms between mica surfaces across [C2mim][FAP] at 0 (empty circles) and 37 % RH (full triangles).

The curves in blue/green were obtained after equilibration in ambient air (37%RH) and show that the ionic liquid still form layers in nanoconfinement, although they differ in size and composition. Figure 3 shows the dynamics of the squeezing of an ionic liquid layer, as measured with an eSFA. The ionic liquid ([C2mim][FAP] was exposed to 37% RH (ambient air), and although the water uptake was small, the squeezing dynamics completely changed. It was proposed that the water molecules are at the water/IL interface and they enhance slip during squeezing-out IL-layers. The structure and dynamics of ionic liquids in nanoconfinement is relevant for the use of the liquids as lubricants and as electrolytes for energy storage.

Force measurements can be also carried out with an Atomic Force Microscope (AFM) but differences are observed when compared to SFA force measurements, which is not a surprise.  The confinement size is larger in SFA experiments due to a ~3 orders of magnitude larger contact radius (R~10-20 mm), which means that a larger number of molecules per layer is collectively squeezed-out in SFA measurements compared to AFM. The confinement effect by a sharp AFM tip (radius ~10 nm) is thus questionable since the contact geometry involves less than 10 ion pairs per layer. Therefore, it is often assumed that the sharp AFM tip only resolves the interfacial structure on a single surface. In fact, a systematic study of the influence of the tip chemistry on the structural force has demonstrated that tip chemistry and size (~10-150 nm) do not affect number, size and position of the layers, but the tip size affects the push-through force. Further, the applied Hertzian pressure in AFM experiments is typically in the order of 1 GPa (1 nN) while it is ~10 MPa (100 mN/m) in SFA measurements. Hence, the boundary layer, which is not generally accessible by SFA (see the finite final separation at the maximum applied force in Figure 1), can be easily probed by AFM force spectroscopy with a sharp tip.

Figure 3: Film thickness vs. time during the squeezing-out of a layer of [C2mim][FAP]; the film thickness decreases from 2.7 nm to 2 nm under dry conditions over 2 minutes, while when exposed to 37%RH, the squeezing prcess is 25 times faster. B) It is proposed that the water molecules are at the water/IL interface and they enhance slip during squeezing-out IL-layers.

Publications:

  1. RM Espinosa-Marzal, A Arcifa, A Rossi, ND Spencer, Microslips to “avalanches” in confined, molecular layers of ionic liquids, The journal of physical chemistry letters 5 (1), 179-184, 2013.
  2. RM Espinosa-Marzal, A Arcifa, A Rossi, ND Spencer, Ionic liquids confined in hydrophilic nanocontacts: structure and lubricity in the presence of water, The Journal of Physical Chemistry C 118 (12), 6491-6503, 2014.
  3. LA Jurado, H Kim, A Arcifa, A Rossi, C Leal, ND Spencer, RM Espinosa-Marzal, Irreversible structural change of a dry ionic liquid under nanoconfinement, Physical Chemistry Chemical Physics 17 (20), 13613-13624, 2015.
  4. LA Jurado, H Kim, A Rossi, A Arcifa, JK Schuh, ND Spencer, C Leal, Randy H Ewoldt, Rosa M Espinosa-Marzal, Effect of the environmental humidity on the bulk, interfacial and nanoconfined properties of an ionic liquid, Physical Chemistry Chemical Physics 18 (32), 22719-22730, 2016

 

 

 

Collapsing Electrical Double Layers of Aqueous Electrolytes Solutions in the SFA (by Zita Zachariah)

Figure 4. Schematic illustration of the surface forces (a, b, c) measured during approach and the pull-off forces measured during retraction of mica surfaces submerged in aqueous NaNO3 (blue), KNO3(red) and CsNO3(green).

We have investigated nanoconfined monovalent aqueous electrolytes between mica surfaces in order probed the role of the ions and their hydration shells on the adhesion between the surfaces. As shown in Figure 1, we see that the occurrence of film-thickness transitions (arrows) during approach of the surfaces is accompanied by sharp decreases in the pull-off forces at specific ion concentrations. This sudden drop of the pull-off force, which we call the π-transition, is an indicator of a change at the interface and the “confined” electrical double layer (EDL). Hence it is dependent on the ions present in the EDL and we see that, for example, the collapse of EDLs containing the strongly hydrated Na ion (blue in Figure 1) takes place in two steps (πlayer and πmultiple layer) depending on the electrolyte concentration when compared to the weakly hydrated Cs ions (green in Figure 1) which already sit on the surface and do not layer.

Publications:

  1. Zita Zachariah, Rosa M. Espinosa-Marzal, Nicholas D. Spencer, Manfred Heuberger. Stepwise collapse of highly overlapping Electrical Double Layers. Physical Chemistry Chemical Physics, 18, 24417-24427, 2016.
  2. Manfred Heuberger, Zita Zachariah, Nicholas D. Spencer, Rosa M. Espinosa-Marzal. Driving the Collective Dehydration of Ions in Nano-Pores, submitted (2016).
  3. Zita Zachariah, Rosa M. Espinosa-Marzal, Manfred Heuberger. Ion-specific hydration in nanoconfined Electrical Double Layers, Journal of Colloid & Interface Science, accepted, 2017.