Occurrence, Shape, and Dimensions of Large Surface Hemimicelles Made of Semifluorinated Alkanes. Elongated versus Circular Hemimicelles. Pit- and Tip-Centered Hemimicelles

• Published in: Journal of the American Chemical Society Vol. 127; no. 29; pp. 10412 - 10419
• Main Authors: Zhang, Guifang, Marie, Pascal, Maaloum, Mounir, Muller, Pierre, Benoit, Nicole, Krafft, Marie Pierre
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 27-07-2005
• Subjects: Chemistry; Colloidal state and disperse state; Exact sciences and technology; General and physical chemistry; Micelles. Thin films; Ethylene oxide polymer; Monolayer; Atomic force microscopy; Solution structure; Non ionic surfactant; Mixed micelle; Fluorocarbon; Air water interface; Experimental study; Amphiphilic compound; Langmuir layer
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja0510515

The formation of large (∼20−35 nm) surface hemimicelles in monolayers of semifluorinated alkanes, C n F2 n +1C m H2 m +1 (FnHm), observed after transfer onto silicon wafers, is a general phenomenon. F6H16 and F8H14 exclusively form highly monodisperse circular hemimicelles, organized in a hexagonal array. The other FnHm investigated form both circular and elongated hemimicelles. The longer FnHm is, the larger the area fraction of elongated micelles; both the hydrocarbon block (H-block) and the fluorocarbon block (F-block) affect this area fraction. The length of the elongated micelles increases with the total length of the diblocks. The diameter of the circular micelles increases with the length of the H-block but, unexpectedly, not with that of the F-block. Model calculations account for these observations. Close examination of the circular micelles showed that they generally present a pit or a tip at their center. The width of the elongated micelles is comparable to the radius of the circular micelles, suggesting that the latter arise from a partition of elongated micelles, followed by coalescence of the edges of the resulting fragments. The elongated micelles become shorter and fewer when surface pressure increases, further suggesting a conversion of elongated into circular micelles. This conversion is reversible. The surface pressure−molecular area isotherms do not present any feature that forebears the existence of hemimicelles. The obtaining of stable surface patterns from simple, “nonpolar” molecular fluorocarbon/hydrocarbon diblocks opens a new approach for producing featured nanostructures from organic templates.