Chain-length anomaly in the two-dimensional ordering of the cationic surfactants CnTAB at the graphite/water interface, revealed by advanced calorimetric methods

Surfactants adsorbed at the graphite/aqueous solution interface form half-cylindrical surface micelles templated by a flat, ordered monolayer. Here, the differential heats of adsorption of even-numbered alkyltrimethylammonium bromides C(n)TAB (n = 6-16) were measured in the monolayer region by frontal-flow calorimetry and pulsed-flow calorimetry. The monolayer self-assembles exothermically and with a negative change of heat capacity. Pure water and pure surfactant domains coexist in the submonolayer region. The differential heat of adsorption is independent of the surface coverage and increases linearly with n, but a step occurs between n = 10 and n = 12. The adsorption is reversible below this critical chain length, but is nonreversible above it. This anomaly has been interpreted in terms of a reorientation of the headgroup from nonbonding to bonding with respect to the surface, and the formation of two different two-dimensional crystal structures on the surface. Pulsed-flow calorimetry, ideally suited for the study of nonreversible adsorption processes, has a high potential for the investigation of a variety of solid/liquid interfacial phenomena that cannot be studied by other current calorimetric methods.