We have measured normal and lateral (friction) forces across films of saturated and unsaturated fatty acids with the Surface Forces Apparatus (SFA). When adsorbed onto mica from n-hexadecane, linoleic acid forms a dimer layer between monolayer-covered surfaces. This dimer layer, which is not observed in stearic or oleic acid, is removed at low loads. The remaining linoleic acid monolayers are thinner than the ones formed by stearic and oleic acid, and show a more complex friction response with two regimes of linear friction, higher stability at very high loads, and a dependence of the friction on sliding speed and adsorption time.

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