Fig. 1.

Forces, F, measured between mica-supported gel-phase DPPC bilayers in DMSO–water mixtures at 22 °C and pH 6.0 ± 0.2. Solid colored lines corresponding to 0 mol%, 2 mol%, and 7.5 mol% DMSO runs are fits comprised of polymer brush and van der Waals forces. Force curves for 10% and 20% DMSO were unable to be fitted with the polymer brush equation owing to their steepness in the repulsive regime, but corresponding hand-drawn (dotted) lines were included for consistency. (A) Static measurements of repulsive forces on a semilog plot. Values for the fits at D < 1.3 nm were drawn by hand (dotted) because of mathematical divergence of the van der Waals force near D_o,vdw. The steep upturns of forces above 20 mN/m in the 0–7.5 mol% data series occurred when the surfaces began deforming noticeably, resulting in inaccurate R values. (B) Adhesion forces, F_ad, measured on slow separation of the surfaces. Dashed lines show a reduced van der Waals force resulting from screening of the Hamaker constant by the zwitterionic head groups (28). The solid gray line corresponds to a van der Waals force (D_o,vdw = 0 nm, A = 2.3 × 10⁻²⁰ J), which may be operationally used to describe adhesion forces in this system but was not used in the full fits of the force curves.