Alcohols solubilization in a nonionic fluorinated surfactant based system: Effect on the mesoporous silica characteristics

Abstract

In this study, we have used hydrogenated alcohols with different chain lengths and one fluorinated alcohol as additives to determine their effect on the characteristics of mesoporous materials prepared from fluorinated micelles.

Introduction

Porous materials have found wide applications in many traditional fields such as catalysis, adsorption, electronics and environmental technology because of their high surface area coupled with many other physical and chemical properties [1], [2], [3], [4], [5]. Recently, there has been a rapid growth in emerging areas such as nanotechnology, photonics and bioengineering, which require porous structures with well-defined structural, interfacial, compositional and morphological properties. Among the various methods for creating pores, the surfactant templating strategy affords a variety of porous networks with a wide range of pore sizes, well-defined morphologies on controllable length scales and various chemical functionalities to match the needs of different applications [6], [7], [8]. As a matter of fact, in water, surfactant molecules can pack together to form either micelles or liquid crystals that can be used as starting block to synthesize ordered mesoporous materials. Two mechanisms can lead to the formation of these ordered mesostructures. The first one is the self-assembly mechanism (CTM) and in this case the building blocks are the micelles; so the CTM occurs at low surfactant concentrations [8], [9], [10], [11], [12]. The second approach to the preparation of ordered mesostructures uses liquid crystal phases and is labeled as the direct liquid crystal templating (LCT) pathway [13], [14], [15]. The inorganic precursors grow around the liquid crystal. After the polymerization and the condensation, the template can be removed, leaving a mesoporous material whose structure, pore size and symmetry are determined by the liquid crystal scaffold. Nevertheless, whatever the synthesis pathways, the characteristics of the recovered materials, such as the structure and the pore diameter, are strongly related to the properties of the surfactant used for their preparation. Therefore, one of the main parameter that should be taken into account for the design of mesoporous materials is the behavior of the surfactant in aqueous solution, which can be affected by the presence of additives. For example using both hydrogenated and fluorinated systems, we have evidenced that the self assembly mechanism is favored if the lower consolute boundary is shifted toward high temperatures and if the phase separation temperature is moved away from the temperature at which the silica source is added to the micellar solution [16], [17]. As a matter of fact, the ${\text{R}}_7^{\text{F}}{\text{(EO)}}_8$–water system presents a cloud point at 34 °C and the addition of NaI shifts the lower consolute boundary towards higher temperatures (salting in effect). While only disordered mesostructured are recovered from the ${\text{R}}_7^{\text{F}}{\text{(EO)}}_8$–water system, well-ordered mesostructures are synthesized from an aqueous solution of sodium iodide. The behavior of the surfactant can also be influenced by the presence of alcohols [18], [19], [20], [21], [22]. For example, it has been shown with the Brij 35–water system that the structural properties of micelles are sensitive to the addition of alcohols [22]. Longer the chain of the alcohol, more it behaves like real oil. It means that the alcohol locates in the core of the micelle, which results in an increase of the micelle size. When the chain of the alcohol is short, the alcohol acts as a co-solvent. At last, for the medium chain, depending on the alcohol chain length, they can behave either as co-solvent or as co-surfactant on the function of the length of the chain. Till now the effects of alcohol in the synthesis of mesoporous silica with ionic and nonionic surfactants as templates have been roundly studied [23], [24], [25], [26], [27], [28], [29], [30], [31]. For mesoporous silica synthesis using nonionic surfactant, alcohol could tune the mesostructure efficiently through modifying surface curvature of the micelles [30]. For instance, by increasing the amount of alcohol such as ethanol, propanol and methanol in the synthesis mixture of mesoporous materials prepared from the CTMABr-surfactant based system Liu et al. [31] have observed the following transition mesophase sequence: hexagonal → cubic → lamellar → radially arranged hexagonal closed packed mesophase. To explain the phase transition the authors consider a variation of the surfactant packing parameter. They claim that at the lower concentration, alcohol molecules penetrate into the surfactant micelles and act as a co-surfactant. However they have not performed a study to understand the effect of alcohol addition on the surfactant phase behavior.

In this work, we have performed such study and we have mainly investigated the effect of the addition of alcohol having various hydrogenated chain length on the characteristics of mesoporous materials prepared from the C₈F₁₇C₂H₄(OC₂H₄)₉OH $[{\text{R}}_8^{\text{F}}(\text{EO}{)}_9]$ fluorinated surfactant based system. Interest focused on this surfactant is its high thermal stability compared to that of hydrogenated surfactants. Indeed, this property results in a material with improved hydrothermal stability [32]. In addition mesoporous materials prepared with ${\text{R}}_8^{\text{F}}(\text{EO}{)}_9$ as surfactant exhibit a higher degrees of organization and higher pore diameters than those obtained with analogous hydrogenated surfactant [C₁₆(EO)₁₀] [33].

We have considered in particular the incorporation of iso-propanol, n-butanol and 1-octanol. To compare to 1-octanol, we have chosen the fluorinated alcohol C₆F₁₃C₂H₄OH which contains two hydrogenated methylene groups due to the synthesis limit. It can be noted as ${\text{R}}_6^{\text{F}}(\text{EO}{)}_0$ by analogy with the label of C₈F₁₇C₂H₄(OC₂H₄)₉OH.