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 a nd kinetics of nonionic surfactant penta(oxyethylene) dodecyl ether (C12E5) On mica as a function of temperature. Surface forces and topographical ima ges of surfactant aggregates at the liquid/solid interface were captured in the vicinity of 21 degrees C. The surfactant molecular aggregates were ima ged 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 degree s C, C12E5 adsorbed as fragments of tens of nanometers in size. The fragmen ts consist of 1-2 nm thick strongly adsorbed species and weakly adsorbed sp ecies extending as far as 15 nm from the substrate surface. The fragments g radually increased in number and attached to each other to form clusters co nnected in one dimension (strings) and two dimensions (networks). With incr easing surface coverage, the surface force changed from a purely attractive one to one showing a repulsive force barrier; At above 21 degrees C, a smo oth and continuous layer structure formed instantaneously. Scan damage in t he form of ever-expanding holes was observed in the surfactant layers. The force versus distance profiles suggest continuous bilayers formed on both m ica and AFM tip with a thickness of 4 nm. The abrupt changes in the aggrega te structure and adsorption rate point to a phase transition at the surface s, which originates from the temperature-induced dehydration of the headgro up as in the cloud point phenomenon. This correlation was also supported by the evidence that the adsorption of C12E5 was suppressed by the addition o f a salting-in compound, sodium perchlorate.