Highly-Ordered Selective Self-Assembly of a Trimeric Cationic Surfactant on a Mica Surface
Original entry: William Bonificio, AP 225, Fall 2009
Highly-Ordered Selective Self-Assembly of a Trimeric Cationic Surfactant on a Mica Surface Yanbo Hou, Meiwen Cao, Manli Deng, Yilin Wang Langmuir 2008 24 (19), 10572-10574
Soft matter keywords
micelle, surfactant, DTAD, bilayer, hydrophobic, hydrophilic
The purpose of this study was to investigate a novel self assembly on a mica surface. When the researchers allowed (dodecyldimethylammonioacetoxy)diethyltriamine trichloride (DTAD) to assemble on a mica surface nanoscaled parallel stripes formed. This interesting formation is quite unique compared to islands that form on normal surfaces. A suggested mechanism for this ordering was also proposed that draws on many different forces including hydrophbicity and lattice matching.
Soft matter discussion
Future devices may have a need for nanoscale structures, so research in this area is booming even without direct applications in mind. To create these structures, many scientists have already noticed that surfactant molecules can self assemble in unique ways; much research has been done on single tailed, or two tailed surfactants. The people in this study however, have used a trimeric structure, DTAD, pictured left.
The researchers used the critical concentration to create micelles from DTAD, 0.29mM, as a basis for their study. They added concentrations of 0.08, 2, 10, and 20 mM, corresponding to 0.2, 5, 25, and 50 times its cmc, respectively to a mica surface. At the lowest concentration typical islands were observed, then as the CMC was passed, parallel stripes began forming. As the concentration was then increased the islands gave way to longer and thicker parallel stripes. These can be seen in the figure on the right. The dimensions of these structures is about 1um in length, 40-80 nm in width, and 2-4nm in height.
I will refer you to the paper to look into the proposed theory for why this structure arises. It has to do with many different factors, including hydrophobic interactions, lattice matching, and electrostatic binding. A proposed structure is shown in the image to the right.