Synthesis of silica spheres with neutral and ionic amphiphiles and their interaction with photosensitive spiropyrans José M. Domínguez, Raúl Rosas, Jorge Aburto, Eduardo Terrés, Alfonso López, Rafael Martínez-Palou * Instituto Mexicano del Petróleo, Department of Ingenieria Molecular, Eje Central Lázaro Cárdenas No. 152, San Bartolo Atepehuacan, 07730 México, D.F., Mexico article info Article history: Received 30 April 2008 Received in revised form 11 August 2008 Accepted 13 August 2008 Available online 29 August 2008 Keywords: Neutral amphiphiles Mesoporous silica spheres Imidazoline Microwave-assisted synthesis Spiropyrans Single-particle analysis abstract A study was made on the influence of ionic and non-ionic amphiphiles, i.e., CTAB and 1-(2-aminoethyl)- 2-heptadecyl-2-imidazoline (1), on the pore hierarchy and morphology of siliceous particles synthesized with. The amphiphile (1) leads to formation of spherical silica particles of about 250 nm diameter, while the ionic surfactant (CTAB) led to porous silica spheres of about 750 nm diameter. A single-particle anal- ysis using High Resolution Electron Microscopy (HREM) and Optical Density Transforms (ODT) show the inner pore structure of the particles synthesized with CTAB and low molecular weight alcohols co-surfac- tants (ethanol and propanol) is oriented along the sphere radius, while silica particles synthesized with (1) have a random-like pore structure. Both systems were used as supports for adsorption of a photosen- sitive spiropyran (i.e., Spiro-6) type compound (1’,3’,3’-trimethyl-6-nitrospiro-(2H-1-benzopyran- 2,2’indoline)) and their interaction with the silica surface was characterized by IR spectroscopy, showing a weak interaction with respect to other silicates (lamellar clays). These effects are potentially interesting for applications of biomarkers, stable photosensitive materials, solid-phase organic synthesis and dyes. The UV–vis reflectance spectra of the materials series impregnated with Spiro-6 indicated that siliceous IMP-type materials (i.e., prepared with CTAB and low molecular weight alcohols: Pr-1, Pr-2, Pr-3 and Et-1, Et-2) contained a small shoulder at 460 nm of the less conjugated protonated merocyanine form, together with a broad band at 517 nm, which corresponds to the merocyanine form. The incorporation of alumi- num into the silica framework results in a single broad band at 435 nm corresponding to the protonated merocyanine form. Thus, the acidic sites created by aluminum incorporation into the silica spheres seem to favor the formation of protonated merocyanine instead of the basic merocyanine molecule. Ó 2008 Elsevier Inc. All rights reserved. 1. Introduction The lyotropic properties of organic amphiphiles have been used for the synthesis of precursor mesophases in aqueous solutions, which interact with inorganic species, i.e., Si(OH) 4 , to produce a variety of mesoporous materials of technological interest [1–3]. The basic mechanism behind the formation of those mesoporous materials lays in the interaction of ionic (i.e., C n TAB. (8 < n < 18), SDS) and non-ionic (i.e., L64, P123, Triton X, etc.) amphiphiles with the silica inorganic species, which are anionic entities (I  ) derived from hydrolysis of metal salts [4–10]. Thus, the inherent properties of the organic structure directing agents (SDA) and the chemical potential of the inorganic aggregates may influence the way these phases organize in solution. Among the most important interaction parameters are the molecular charge, hydrocarbon chain length and chain rigidity. In this way, mesoporous silica structures are ob- tained with potential properties as catalysts [11], adsorbents [12], quantum dots (matrix) [13], label biomarkers [14], column pack- ing’s for high resolution chromatographic and electrokinetic sepa- rations [15], chemical sensors [16], biomarkers [17], adsorbents for hydrogen storage [18], ‘‘hosts” of chemical species [19] and cata- lyst supports [20] among others. In this work a newer non-ionic surfactant (1: 1-(2-aminoethyl)-2-heptadecyl-2-imidazoline) was used as SDA for synthesizing silica mesoporous materials and their structural and textural properties were compared with silica parti- cles prepared with the CTAB surfactant. Although the synthesis conditions of both materials required slight differences with re- spect to the temperature and time of aging, the rest of the condi- tions were similar. In both cases low molecular weight alcohol co-solvents (i.e., EtOH, PrOH) were used to facilitate the primary interactions of surfactants and silica moieties at the molecular le- vel [10]. With this purpose in mind a single-particle analysis was performed for each case using High Resolution Transmission Elec- tron Microscopy (HRTEM) and Fourier Transform Analysis of the optical density distribution from the TEM micrographs. Finally, the siliceous materials obtained with both surfactants types were tested as hosts of the photochromic spiropyran (1 0 ,3 0 ,3 0 -trimethyl-6-nitrospiro-(2H-1-benzopyran-2,2 0 indoline). These compounds switch from uncolored (i.e., spirobenzopyran) 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.08.040 * Corresponding author. Tel.: +52 55 91757846; fax: +52 55 91756380. E-mail address: rpalou@imp.mx (R. Martínez-Palou). Microporous and Mesoporous Materials 118 (2009) 121–133 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso