Direct Study of C12E5 Aggregation on Mica by Atomic Force Microscopy Imaging and Force Measurements

Atomic force microscopy (AFM) was used to study the aggregation structure and kinetics of nonionic surfactant penta(oxyethylene) dodecyl ether (C 12 E 5 ) on mica as a function of temperature. Surface forces and topographical images of surfactant aggregates at the liquid/solid interface were captured in the vicinity of 21 °C. The surfactant molecular aggregates were imaged by choosing an imaging force in the steric repulsion region so that the AFM tip was just outside the surface of the aggregates. At below 21 °C, C 12 E 5 adsorbed as fragments of tens of nanometers in size. The fragments consist of 1−2 nm thick strongly adsorbed species and weakly adsorbed species extending as far as 15 nm from the substrate surface. The fragments gradually increased in number and attached to each other to form clusters connected in one dimension (strings) and two dimensions (networks). With increasing surface coverage, the surface force changed from a purely attractive one to one showing a repulsive force barrier. At above 21 °C, a smooth and continuous layer structure formed instantaneously. Scan damage in the form of ever-expanding holes was observed in the surfactant layers. The force versus distance profiles suggest continuous bilayers formed on both mica and AFM tip with a thickness of 4 nm. The abrupt changes in the aggregate structure and adsorption rate point to a phase transition at the surfaces, which originates from the temperature-induced dehydration of the headgroup as in the cloud point phenomenon. This correlation was also supported by the evidence that the adsorption of C 12 E 5 was suppressed by the addition of a salting-in compound, sodium perchlorate.