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

Abstract

Atomic force microscopy (AFM) was used to study the aggregation structure and kinetics of nonionic surfactant penta(oxyethylene) dodecyl ether (C12E5) 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, C12E5 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 temperatureinduced dehydration of the headgroup as in the cloud point phenomenon. This correlation was also supported by the evidence that the adsorption of C12E5 was suppressed by the addition of a salting-in compound, sodium perchlorate.

Direct Force Measurement of Comb Silicone Surfactants in Alcoholic Media by Atomic Force Microscopy

“…The surfactants were either highly compressed into an incompressible state or were pushed out of the contact zone. Previous study of polyoxyethylene surfactants suggests that most physically adsorbed surfactant molecules are removed from the contact zone (15). Here we ignore the thickness of the surfactant film if any is left in the contact zone after the jump-in, and we determine the adsorbed film thickness by the separation distance at the onset of the steric repulsion.…”

“…Because of its high spatial resolution and ease of operation, increasing numbers of polymer adsorption studies have been carried out by AFM. For example, steric repulsions were measured between adsorbed micelles and hemimicelles of nonionic dodecyl polyoxyethylene surfactants C 12 E n (n = 5-23) (13)(14)(15). C 12 E n film was believed to be removed from the contact zone at a discontinuity point during compression, called the jump-in point.…”

Impact dynamics of surfactant laden drops: dynamic surface tension effects

“…2b inset). Indeed the above equation is seen to quantitatively describe the behavior of Triton (a fast surfactant), taking 4 nm for the molecular size (Dong and Mao 2000). However, a typical slow surfactant such as Agral shows a qualitatively different behavior of s(c) (Fig.…”

Resolving the Thickness and Micromechanical Properties of Lipid Bilayers and Vesicles Using AFM

“…Zero separation is determined from the constant compliance region at high force where deflection is linear with the expansion of the piezoelectric crystal or by the end of the jump-in process. Discontinuity, called the jump-in point, has been a typical feature in force curves measured on lipid bilayers and adsorbed surfactant films above a threshold force (Patrick et al, 1997;Dong and Mao, 2000;Dufrêne et al, 1998;Loi et al, 2002;Franz et al, 2002). This threshold force has been used as the upper limit of the image force in the soft-contact AFM imaging mode most useful for organic, polymeric, and biological samples (Manne et al, 1994).…”