Fig. 3.

Schematic views of the PC head group in DMSO–water mixtures with various dimensions and planes of origin drawn to scale in (A) pure water, (B) 2–7.5 mol% DMSO, and (C) 10–20 mol% DMSO. Atoms labeled P and N represent phosphorous and nitrogen, respectively. The hydrocarbon/head group interface labeled “hc interface” is drawn at the height of the carbonyl groups. The van der Waals plane of origin, D_o,vdw, is located at the same height in A and B, because the position of the choline is relatively unchanged between the two scenarios. Brush length, L, and brush plane of origin, D_o,brush, are smaller in B than in A because of a decrease in head group-excluded volume (i.e., dehydration). D_o,vdw is smaller in C than in B, reflecting gradual collapse of the choline at large DMSO concentrations and orientation of the P-N vector parallel with the plane of the bilayer. In C, the corresponding bilayer displays little to no polymer brush character in SFA measurements.