Figure 1

(a) Advancing contact angle $\Theta$ of a water droplet on the mica surface: ▲—immersion in water; △—immersion in aqueous $0.01M$ NaCl; ●—immersion in water for mica incubated for 30 sec [rather than 2 h [5] ] in the surfactant solution. Mica sheets were never reimmersed: each data point corresponds to a new mica sheet [7]. (b) Tapping-mode AFM images (fluid cell, Nanoscope III, Digital Instruments) of a coated mica surface under water [30 sec incubation in surfactant solution [5] ], 5 min after coverage of the surface by the water. Dark areas are holes, $1.5\pm 0.2\mathrm{nm}$ deep, in the surfactant monolayer. (c) As (b), but following 30 min immersion in water. (d) Coated mica following 11 min immersion in $0.01M$ NaCl, showing similar bilayer islands to (c), with cross section indicated below image. Bottom cartoon: the initially smooth monolayer [left, as in (b)] rearranges on immersion in water or $0.01M$ NaCl to patches of bilayer [right, as in (c), (d)] which bear a net positive charge, surrounded by the negatively charged bare mica.Reuse & Permissions

Figure 2

SFB-measured force-distance profiles $F(D)/R$ vs $D$ between curved mica surfaces (mean radius of curvature $R$) a closest distance $D$ apart as described previously [8, 9]. (a) Forces between bare (*) and coated (all other symbols) [5] mica surfaces across water [6], fitted by a DLVO model [1, 2, dot-dashed curves], using parameters of surface potential and effective ion concentration within the range of literature reports [24, 27]. ○: profiles taken within 95 min of adding water to the cell [surfaces hydrophobic, Fig. 1a]. Filled symbols are recorded between 2 and 17 h after adding water [surfaces hydrophilic, Fig. 1a]. The dotted curve is the predicted vdW interaction between the surfaces according to Ref. 13. The cartoon inset shows the negatively charged mica surface regions on one surface largely facing the positively charged bilayer rafts on the other. (b) As (a) but in aqueous $0.01M$ NaCl. The shaded band shows the range of attractions measured in salt-free water from Fig. 2a, while the data (●, ▲, ◆) are for immersion periods of 40–90 min in the salt solution. The curves are the calculated DLVO double-layer profile for this salt concentration (offset by twice the thickness of a surfactant bilayer) and a surface potential 40 mV, indicating uncompensated charge on the mica surfaces. Inset: profiles in the vicinity of the jump on an expanded scale, showing the DLVO prediction up to the peak where vdW attraction overcomes the double-layer repulsion. The broken curve in the inset indicates the hydration repulsion observed between charged bare mica surfaces across NaCl solutions of similar concentrations [25, 28].Reuse & Permissions