Characterization of nanostructures of mixed surfactant aggregation using Analytical Ultracentrifugation

Authors: Shaohua Lu and Ponisseril Somasundaran, Columbia University, New York, NY.

Venue: 81st ACS (American Chemical Society) Colloid & Surface Science Symposium, University of Delaware, Newark, DE, June 24–27, 2007 (http://www.engr.udel.edu/Colloids2007/) [external site]).

Abstract: Nanostructures of aggregation determine the behavior and performance of surfactant systems, especially surfactant mixtures. However, the characterization of nanostructures remains a challenge due to the lack of techniques. Analytical ultracentrifuge (AUC) was first successfully used to study surfactant micellar systems to obtain quantitative data on micelles, including size, shape, aggregation number, and diffusion coefficients. Various mixed surfactant systems and individual surfactants involved have been studied using AUC in this work, including both nonionic/nonionic and nonionic/ionic combinations. The aggregation numbers obtained for sugar-based n-dodecyl-â-D-maltoside (DM) and noneyl phenol ethoxylated ester (NPn) are in accordance with literature values. In contrast, those for anionic sodium dodecyl sulfate (SDS), cationic dodecyltrimethylammonium chloride (DTAC), and the cationic gemini surfactant are smaller than the literature values, because the electrostatic repulsion reduces the sedimentation velocity. Spherical micelles were identified in mixed DM/SDS and DM/DTAC systems, while cylindrical micelles were observed in mixed nonionic DM/NPn and NPn/NPm systems. It was found that the decrease of the average number of ethoxylated groups in NPn surfactants increases the size and aggregation number of the system and causes a shape transition from spherical to cylindrical. It is interesting to note that the coexistence of two different types of micelles was identified in the mixed nonionic systems, depending on the mixing ratio. The information obtained will help quantitatively understand the packing of surfactant molecules in micelles, which leads to a relationship between the nanostructure of aggregates and the chemistry of the molecules.

Related NETL Project: The overall objective of the related NETL project DE-FC26-03NT15413, “Mineral-Surfactant Interactions for Minimum Reagents Precipitation and Adsorption for Improved Oil Recovery,” is to understand the role of mineralogy of reservoir rocks in determining interactions of reservoir minerals and their dissolved species with externally added reagants (surfactants/polymers) and their effects on solid-liquid and liquid-liquid interfacial properties, such as adsorption, wettability, and interfacial tension. A further goal is to devise schemes to control these interactions in systems relevant to reservoir conditions. Particular emphasis will be placed on the type and nature of different minerals in oil reservoirs.

NETL Project Contacts
NETL – Betty Felber (betty.felber@netl.doe.gov or 918-699-2031)
Columbia U. – P. Somasundaran (ps24@columbia.edu or 212-854-2926)