Langmuir 2005, 21, 3165-3171
3165
Salt Softening of Polyelectrolyte Multilayer Capsules J. Heuvingh,† M. Zappa,‡ and A. Fery*,† Max-Planck Institute for Colloids and Interfaces, Department Interfaces, Am Muhlenberg, D14424 Potsdam, Germany, and Eidgeno¨ ssische Technische Hochschule Zurich, BioInterface Group, Wolfgang-Pauli-Strasse 10, CH - 8093 Zu¨ rich, Switzerland Received October 22, 2004. In Final Form: January 13, 2005 The changes in the morphology and the mechanical properties of hollow polyelectrolyte multilayer capsules made from poly(styrenesulfonate)/poly(allylamine hydrochloride) in response to added salt were investigated. We found that capsules shrink in response to salt exposure. The effect depends strongly on the nature of the salt added and follows trends of the Hoffmeister series, with weakly hydrated cations inducing the strongest shrinking. For NaCl, we have investigated additional effects on capsule mechanical properties that are occurring above a 3 M salt concentration and we found that the morphological changes are accompanied by a pronounced softening of the capsule wall material, which we can quantify by analyzing the force response of capsules in the prebuckling regime. This shows that salts can act as plasticizers in the multilayers and induce annealing effects.
Introduction The formation of thin polymer films by layer-by-layer deposition of polyelectrolytes has received great interest in the last 10 years as a highly versatile method for producing thin films with well-defined thickness from a broad range of materials with little requirements for the substrate.1-3 The process is based on the fact that the adsorption of a polyelectrolyte on a charged surface can result in surface charge reversal. Thus, a coated surface can serve as a substrate for the adsorption of an oppositely charged polyelectrolyte, and for many systems, the process of alternating adsorption can be continued to any desired layer number and thus film thickness. Due to the selfrepulsion of the charged polymers, the thickness increase per adsorbed layer is typically in the nanometer range and well defined. The production of hollow capsules made from polyelectrolyte multilayers is one of the most spectacular examples of the versatility of the layer-bylayer approach: such capsules can be produced by coating colloidal particles and subsequently dissolving the particles under conditions that do not destabilize the multilayer itself4,5 which is possible for various substrates.6 Such polyelectrolyte multilayer capsules can have applications in various fields (e.g., encapsulation of fragrances, flavors, or pharmaceutical agents) and are also interesting biomimetic systems, since their dimensions and wall thicknesses are comparable to those of cells. The effect of salts on polyelectrolyte multilayer capsules is of importance, since salts are present in many of the applications mentioned above and also play an important role in biological systems. Therefore, we have investigated * To whom correspondence should be addressed. E-mail:
[email protected]. † Max-Planck Institute for Colloids and Interfaces. ‡ Eidgeno ¨ ssische Technische Hochschule Zurich. (1) Decher, G.; Hong, J. D. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1430-1434. (2) Decher, G.; Schmitt, J. Prog. Colloid Polym. Sci. 1992, 89, 160. (3) Decher, G. Science 1997, 277, 1232-1237. (4) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S. A.; Mo¨hwald, H. Angew. Chem. 1998, 37, 2202-2205. (5) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Science 1998, 282, 11111114. (6) Mo¨hwald, H.; Donath, E.; Sukhorukov, G. In Multilayer Thin Films; Decher, G., Schlenoff, J. B., Eds.; Wiley VCH: New York, 2003; pp 363-392.
here the effect of changes in the salt concentration on both the morphology and the mechanical properties of polyelectrolyte multilayer capsules made from poly(allylamine hydrochloride) (PAH) and poly(styrenesulfonate) (PSS), the most widely used polyelectrolyte multilayer system. It is well-known that the thickness of polyelectrolyte multilayers can be fine-tuned by choosing the salt concentration during the multilayer buildup2,7,8 (see ref 9 for a recent review). This can be explained by changes of the polyelectrolyte conformation in solution from stretched to more coiled upon an increase of the salt concentration due to screening of the charged polyelectrolytes from each other. The effect of changes in salt concentration on already formed multilayers is much less understood and strongly depends on the particular system: when not in solution, where molecules can change their conformations freely, the molecules in a multilayer are complexed with polyelectrolytes of opposite charge. Therefore, their mobility can be strongly decreased. For the system investigated here, Auch et al. have found selfdiffusion constants of
