Figure 25.

(a) Fatty acids consist of a (hydrophilic) carboxilic acid head group and a (hydrophobic) hydrocarbon tail. The longer the hydrocarbon tail, the lower its solubility in water and the stronger the van der Waals attractions with adjacent fatty acids. Saturated hydrocarbon tails pack well with each other, whereas unsaturated tails (e.g. oleic acid, with a double bond at the ninth carbon) are ‘kinked’ and frustrate packing. (b) Generalized isotherm of an insoluble monolayer of saturated fatty acids, adapted from Kaganer et al. (1999). Monolayers form a gaseous phase at extremely low concentration (inset), which condenses to form a disordered, liquid expanded (LE) phase when compressed. At higher surface concentrations, a phase transition occurs from the LE phase to one of various liquid condensed (LC) phases with different liquid crystalline order, and even further phase transitions at higher concentrations (here to an untilted, condensed phase). (c) Cartoon showing transition between a disordered, low-density phase (e.g. LE or gaseous) to phase coexistence with a higher-density phase (e.g. LE/LC or gas/LE). (d) Fluorescence micrograph showing gas/LE phase coexistence of the phospholipid DPPC (courtesy of Dr. Ian Williams). (e) Polarized micrograph of LE-LC phase coexistence between methyl eicosanoate(C₂₀). Within the LC domain, the six different brightness levels correspond to six distinct orientations of the packed tails, which in turn reveal the hexagonal headgroup lattice (from Knobler & Desai (1992)).