J. Phys. Chem. B 2009, 113, 3423–3430
3423
Self-Assembly of Silica Particles in a Nonionic Surfactant Hexagonal Mesophase Kamendra P. Sharma,† Guruswamy Kumaraswamy,*,† Isabelle Ly,‡ and Olivier Mondain-Monval‡,§ Complex Fluids and Polymer Engineering, National Chemical Laboratory (NCL), Pune 411008, India, and UniVersity of Bordeaux, Centre de Recherche Paul Pascal (CRPP)sCNRS, AVenue A. Schweitzer, Pessac, 33600, France ReceiVed: December 8, 2008; ReVised Manuscript ReceiVed: January 21, 2009
We investigate the process of self-assembly, and the resultant structures in composites of silica particles with a hexagonal mesophase of a nonionic surfactant and water. We report a systematic transition in behavior when the particle size is increased relative to the characteristic mesophase spacing. Water dispersible cagelike silsesquioxanes that are molecular analogues of silica particles and are smaller than the mesophase spacing swell the space between the surfactant cylinders. Silica particles comparable to the characteristic hexagonal spacing partition into the hexagonal phase and into strandlike particulate aggregates. Even larger particles phase separate from the hexagonal phase to form particulate strands that organize with a mesh size comparable to the wavelength of visible light. This self-assembly is reversible and the particles disperse by breaking up the aggregates on heating the composite into the isotropic phase. On cooling from the isotropic phase into the hexagonal, the particles are expelled from the growing hexagonal domains and finally impinge to form strandlike aggregates. Unusually, the isotropization temperature is increased in the composites as the particles nucleate the formation of the hexagonal phase. Introduction When colloids are dispersed in an ordered matrix, they selfassemble, driven by their interactions with the matrix, and by matrix-mediated particle-particle interactions. Materials with self-assembled particulate phases are found in many applications such as cosmetics, food, pharmacy, or catalysis. Increasingly, it has been realized that control over spatial localization and assembly of particles in ordered block-copolymers1,2 or in liquid crystals has important implications for novel functional materials, such as, negative refractive index “metamaterials”,3 etc. In orientationally ordered nematics, dispersion of colloids results in long-range, anisotropic interparticle interactions originating from the elasticity of the liquid crystalline matrix.4-6 These interparticle interactions result in self-assembly of the particles in the liquid crystalline matrix. For example, chainlike linear arrays of colloidal silicone drops with dipolar interactions, mediated by a nematic matrix, have been reported;7 as well as two-dimensional crystals of colloids with quadrupolar interactions.8 Other morphologies have also been reported: in cholesteric nematic9 and lamellar smectic phases,10,11 where large colloidal inclusions phase separate to defect nodes and stabilize a disclination network, resulting in increased solid moduli. Further, kinetically determined cellular, striped or “rootlike” structures have been shown to form by particles excluded from the nematic phase.12-14 Purely geometrically driven localization of particles in one of the contiguous fluid regions in sponge phases has also been reported.15,16 Most of the literature on interparticle interactions in liquid crystals focus on colloidal entities with typical size scales, R∼O (100 nm-1 µm), dispersed in a thermotropic nematic such as * To whom correspondence should be addressed. E-mail: g.kumaraswamy@ ncl.res.in. Tel.: +91-20-2590-2182. Fax: +91-20-2590-2618. † NCL, Pune, India. ‡ CRPP, Pessac, France. § University of Bordeaux.
5-cyanobiphenyl (5CB). In such nematics, the orientational elasticity is characterized by three elastic constants K1 (splay), K2 (twist), and K3 (bend). In most cases, it is assumed that K1 ≈ K2 ≈ K3 ≈ K ≈ O (10 pN). Thus, the free energy cost of accommodating the colloidal particles in the liquid crystal, KR is several hundred to several thousand times the thermal energy kBT, and thermal motion is strongly reduced. Inclusion of colloids in an orientationally ordered fluid results in topological defects that satisfy boundary conditions for director orientation at the surface of and far from the colloid. Thus, the nature of interparticle interactions are determined by the ratio between WR2 (where W is the anchoring energy at the surface) and KR. Here, we focus on the organization of particles in lyotropic hexagonal surfactant-water systems. It has been reported that addition of nanoparticles to even relatively simple surfactant micellar systems results in rich phase behavior since preferential adsorption of the surfactant onto the particles could open up a ternary region in the surfactant-water-particle phase diagram.17 In hexagonal phases (H1), cylindrical micelles of surfactants are packed in a hexagonal array in water. For such phases, typical repeat spacings are between 5 and 10 nm, about 10 times larger than the mesogen size in thermotropic nematics, and the elastic constants are estimated to be K ≈ O (1 pN),18 that is, about a tenth of those for thermotropic nematics. Thus, for particles with a size, R, comparable with the hexagonal phase repeat distance, KR is on the order of a few kBT. Thus, as particle size changes in this range, one would imagine a qualitative change in particle organization. It has been reported that nanoparticles smaller than the characteristic size of the hexagonal mesophase are included in and therefore, template the mesophase,19,20 while larger particles are expelled from the cylindrical micelles.19,21 A mean field description22 of large particles (viz., much larger than the characteristic mesophase repeat spacing) in a hexagonal columnar phase indicates that the particles align at some finite angle, θ (
