Interaction of PEG with ionic surfactant SDS to form template for mesoporous material

Abstract

In this article a possible mechanism is proposed of the interaction of PEG with an ionic surfactant of sodium dodecylsulphate (SDS) to form composite swelling agent for mesoporous material preparation. With the dynamic light scattering method the sizes of the micelle formed by PEG itself or by PEG and SDS were examined. The result shows that pure 5 wt.% PEG 8000 solution has the micelles with size of 700 nm, whereas after the addition of 0.4 wt.% SDS the micelles decrease to 29.4 nm. The contraction of the micelle size can be illustrated as the interaction between the polymer and the surfactant. And through the adjustment of chain length of the polymer as well as the concentration of polymer and ionic surfactant, tunable micelles can be obtained with diverse sizes, charges and aggregate numbers. When this composite micelle was used for the preparation of porous materials, a uniform pore distribution was obtained similar to those with hydrothermal method. Moreover, a nicely spherical morphology thanks to the chain of polymer was also observed. Therefore, PEG proposes a possible route to synthesize mesoporous materials with excellent micro-mechanism and macrostructure.

Introduction

Currently there is a large interest in the preparation of well-defined mesoporous materials [1], [2] such as M41S for chromatographic separations and catalysts. Recently, the uniform particle size, well-defined pore size and robust framework are the most important fields that interest worldwide researchers. Besides this a novel synthesis route or with new swelling agents instead of CTAB also promotes broad research in this area.

In recent studies some researchers used polymers, for example, PEG and β-cyclodextrin, as the additives to form micelles as templates for the preparation of mesoporous materials and received satisfactory results [3], [4], [5], [6], [7]. However, the exact mechanism of the interaction between the additives and surfactants is not elucidated. In one of our former papers [8], mesoporous materials have been prepared with a novel emulsion method and the polymer PEG was used as the template. When the properties of materials explored, there showed quite a lot of excellent qualities such as average pore size distribution, ease of pore size control, spherical morphology and dispersion. However, the mechanism of the formation of these materials was still uncertain. As known before, polymer cannot serve as the swelling agent of inorganic materials. How can PEG produce uniform pores in the silica framework of this environment? Moreover, an emulsion system usually contains large interfacial areas, when it comes down to so many substances, such as surfactant, polymer, TEOS, catalyst (HCl), oil, water, What results from their organization near interfaces? What are the characteristics of the micelles formed by PEG and a surfactant in an emulsion system? What is the difference between normal surfactant solution and that in the extreme acid environment?

In fact some work has been done on the micelle formation of surfactants under different temperatures and pHs [9], [10], [11], [12], [13]. But none of them is related to the synthesis of particles and the multiphase reaction system. So these questions aroused our interest to further understand the self-organizing behaviors of surfactants in exact and detailed ways, especially in the real reaction media, that is, under the influence of other molecules or polymers and in the acid or basic environment. The research is significant for understanding micelle formation considering the mushrooming development in polymer-templated synthesis of porous particles [14].

The challenge in this work is that of understanding intermolecular actions in systems that contain large interfacial areas. In the projects, we explored new approaches to the measurement and comprehension of intermolecular forces and well as developed a supposition of the mechanism of intermolecular effect in this unfamiliar system.