(138a) Effects of Salts and Ethanol on the Population and Morphology of Tri-Block Copolymer Micelles in Solution

Amphiphilic triblock copolymers are often used as templating agents to synthesise large-pore mesoporous silica materials. Depending on the surfactant choice and reaction conditions (temperature, pH, additives, stirring rate, etc?) mesoporous silicas with different pore sizes and structures can be prepared [1, 2]. The interactions between the surfactant and the various additives (silica, acids, salts, solvents) during the synthesis are believed to play an important role in the formation of these highly structured materials [3]. In spite of the numerous syntheses carried out by various research groups, the formation mechanism of these materials is insufficiently clear.

A more fundamental understanding of the self-assembling mechanism of the templating agent is needed in order to better control the syntheses of these materials. Here, we present a study on the self-assembly of Poly(ethylene oxide)-Poly(propylene oxide)-Poly(ethylene oxide) block copolymer in water/ethanol mixtures in the presence of KCl. Additionally, we have examined the effect that different salt cations and anions have on the morphology of the block copolymer micelles in the absence of ethanol. The studied block copolymer is a typical templating agent used in the syntheses of ordered mesoporous materials, like SBA-15 for example. Ethanol has been chosen to mimic the effect of alcohols that are typically released, or even added during syntheses. Rheometry, Dynamic Light Scattering and Cryo-EM are the techniques used in this study.

The results show that both ethanol and salts strongly affect the micelle morphology in different ways. Worm-like and spherical micelles coexist when salts with a significant ?salting out? power (like KF, KCl and CsCl) are added to the aqueous solutions. Salts like KI do not affect the micelle morphology. We show that the effectiveness of the studied inorganic salts in promoting a micellar shape transition follows the well-known Hoffmeister salt series.

The addition of ethanol to the KCl micellar solutions leads to the formation of more and/or longer worm-like micelles. The number of spherical micelles in this case is greatly reduced. Interestingly, the length of the worm-like micelles increases with increasing ethanol concentration until an optimal ethanol concentration (8 - 10 vol%) is reached, above which the length of the micelles decreases again. At higher surfactant concentrations the worm-like micelles start to overlap and form a transient network. In the presence of ethanol both the dilute and semi-dilute regimes of the worm-like micelles have been studied.

[1] Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. [2] Flodström, K.; Alfredsson, V.; Källrot, N. J. Am. Chem. Soc. 2003,125, 402. [3] Edler, K.J. Aust. J. Chem. 2005, 58, 627.