Electro-Assisted Self-Assembly of Cetyltrimethylammonium-Templated Silica Films in Aqueous Media: Critical Effect of Counteranions on the Morphology and Mesostructure Type

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• 作者：Yann Guillemin ; Jaafar Ghanbaja ; Emmanuel Aubert ; Mathieu Etienne ; Alain Walcarius
• 刊名：Chemistry of Materials
• 出版年：2014
• 出版时间：March 11, 2014
• 年：2014
• 卷：26
• 期：5
• 页码：1848-1858
• 全文大小：669K
• 年卷期：v.26,no.5(March 11, 2014)
• ISSN：1520-5002

文摘

The electro-assisted self-assembly (EASA) of tetraethoxysilane (TEOS) and cetyltrimethylammonium bromide (CTABr) in hydro-alcoholic medium is now recognized to be a versatile method to generate highly ordered mesoporous silica films with unique orientation of mesopore channels normal to the underlying surface. In this work, we have evaluated the possibility to extend the method to aqueous media (i.e., without adding a cosolvent) and to determine the parameters affecting the EASA process and the resulting organization/orientation of the mesoporous framework by using electron microscopies and diffraction techniques. Contrary to water/cosolvent-based sols, the nature of the surfactant and supporting electrolyte counteranions (X^{鈥?/sup>) was found to induce drastic variations on both the morphology and the mesostructural order of the deposits formed by electrochemically induced gelification (by pH increase) of CTAX/NaX-based silica sols. These changes are triggered by different surfactant assemblies arising from lower critical micellar concentration when passing from hydro-alcoholic to aqueous medium, and they are affected by the chaotropic鈥揷osmotropic character of the counteranions. To be brief, cosmotropic anions (such as SO₄2鈥?/sup>) promote the formation of thin films but suffering from poor or no ordering, whereas weakly bonded anions (such as Cl鈥?/sup>) favor the mesostructuration but mainly in the form of particles or aggregates, while chaotropic anions (such as Br鈥?/sup>) lead to rather thick deposits made of poorly organized aggregates. Mixing these anions, to get mixed micelles, enables compromises to be reached between these 鈥渆xtreme鈥?behaviors and mesostructured thin films can be indeed obtained with the CTACl/Na₂SO₄ and CTABr/Na₂SO₄ media, exhibiting, respectively, some vertical or horizontal orientation of mesopore channels. This can be rationalized by taking into account the CTA+, X鈥?/sup> binding strength variations (Cl鈥?/sup> < SO₄2鈥?/sup> < Br鈥?/sup>), thus affecting competitive binding of negatively charged silicate species, and sphere-to-rod transition abilities (SO₄2鈥?/sup> 鈮?Cl鈥?/sup> < Br鈥?/sup>) of the CTA+-based templates. Cyclic voltammetry was also used to characterize mass transport processes through the films. Finally, a preliminary work aiming at getting swelled pores of such electrogenerated films with mesitylene was carried out to evaluate the potential interest of the water-based EASA process for the entrapment of hydrophobic molecules inside the surfactant鈥搒ilica phases.}