Self-Assembly of Solid Polyelectrolyte-Silicon-Surfactant Complexes Andreas F. Thu ¨ nemann* and Kai Helmut Lochhaas Max Planck Institute of Colloids & Interfaces, Kantstrasse 55, D-14513 Teltow-Seehof, Germany Received February 24, 1998. In Final Form: July 20, 1998 Two solid complexes of surfactants with a pendant trimethylsilyl moiety and a cationic polyelectrolyte are prepared. These complexes form smectic A-like lamellar mesophases and low-energy surfaces (20 and 36 mN/m). The lamellar structures are characterized in detail by small-angle X-ray scattering using the interface distribution function concept. Both complexes are fabricated as highly transparent, flexible films with tensile moduli of 12 and 30 MPa and low glass transition temperatures (-10 and -56 °C). The application of the complexes as new coating materials is discussed. 1. Introduction Many silicones have outstanding surface-modifying properties, e.g., the ability to reduce surface energy, which are responsible for many of their applications. The origins of the unusual and useful surface properties are closely related to the silicones’ unique chemistry. 1 Silicon- containing surfactants have been successfully used as auxiliaries in the manufacturing and processing of paints and coatings. 2,3 They are also valuable as surface-active ingredients in the textile and fiber industries, acting as emulsifiers, softeners, etc. 4 The aggregation behavior of silicon-containing surfactants has been investigated in detail by small-angle neutron and static light scattering, where, for example, hexagonal and lamellar mesophases were found in concentrated aqueous solution. 5 Nothing has yet been reported on the formation of ordered structures, on a nanometer scale, in solid complexes of polyelectrolytes and silicon surfactants. Comparable complexes of fluorine-containing surfactants and poly- electrolytes, which form ultralow-energy surfaces with a surface tension clearly below 20 mN/m, have already been described. 6 Such complexes are very promising as coating materials, but the use of fluorine complexes is limited due to the extremely high prices of fluorine surfactants. Therefore, for water-repellent applications, it is desirable to substitute the fluorine surfactants with silicon sur- factants. In this work we report the preparation and properties of complexes 3a and 3b, formed using a cationic poly- electrolyte, poly(dimethyldiallylammonium chloride) (1), and the anionic surfactants isopropylammonium 6-(tri- methylsilyl)-n-hexylsulfate (2a) and isopropylammonium [3-(1,1,3,3,5,5,5-heptamethyltrisiloxan-1-yl)hexyl-1-sul- fate] (2b). The complex formation is shown schematically in Figure 1. Polyelectrolyte 1 was chosen because it has already been used successfully for many different solid complexes, e.g., with lipids, 7 amphipilic drugs, 8 and fluorinated surfactants. The silicon surfactant 2, with only one trimethylsilyl moiety, was chosen for different reasons: First, salts of sulfatic esters have excellent surfactant properties combined with good hydrolysis stability, 9 and second, it is known that the chemical and physical behavior of silicon compounds, e.g., their high flexibility, is very different from that of their carbon analogues. It must be expected that our experience with non-silyl-containing complexes can only be transferred to silicon-containing surfactants if the influence of silicon is not predominate. Additionally, only atoms at, or nearby, the film/air interface are responsible for the surface energy. A perfect alignment of (CH 3 ) 3 Si groups at the film surface should result in a surface energy significantly lower than that found for high molecular weight poly(dimethylsi- loxane) (24 mN/m). 1 In principle, one trimethylsilyl moiety at the tail of the surfactant may be enough to form a low- energy surface material. For example, sodium methyl- siliconate (CH 3 Si(OH) 2 ONa) is used successfully in the hydrophobation of walls while maintaining permeability for moisture. 10 The trisiloxanylhexamethylsulfate 2b was (1) Owen, M. J. Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 97-103. (2) Fink, H. F. Tenside, Surfactants, Deterg. 1991, 28, 306-312. (3) Dams, R. Tenside, Surfactants, Deterg. 1993, 30, 326-327. (4) Schmidt, G. Tenside, Surfactants, Deterg. 1990, 27, 324-328. (5) Gradzielski, M.; Hoffmann, H.; Robisch, P.; Ulbricht, W.; Gru ¨ ning, B. Tenside, Surfactants, Deterg. 1990, 27, 366-379. (6) Antonietti, M.; Henke, S.; Th u ¨ nemann, A. Adv. Mater. 1996, 8, 41-45. (7) Antonietti, M.; Kaul, A.; Thu ¨ nemann, A. Langmuir 1995, 11, 2633-2638. (8) Thu ¨ nemann, A. Langmuir 1997, 13, 6040-6046. (9) Klein, K. D.; Schaefer, D.; Lersch, P. Tenside, Surfactants, Deterg. 1994, 31, 115-119. Figure 1. Sketch of complex formation: (1) poly(diallyldi- methylammonium chloride); (2) isopropylammonium 6-(tri- methylsilyl)-n-hexylsulfate (p ) 0, 2a) isopropylammonium [3-(1,1,3,3,5,5,5-heptamethyltrisiloxan-1-yl) and hexyl-1-sul- fate] (p ) 2, 2b); (3) stoichiometric polyelectrolyte-surfactant complex. 6220 Langmuir 1998, 14, 6220-6225 S0743-7463(98)00229-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/15/1998