Hygroscopic Growth of Self-Assembled Layered Surfactant Molecules at the Interface between Air and Organic salts
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Hygroscopic Growth of Self-Assembled Layered Surfactant Molecules at the Interface between Air and Organic salts Yongsoon Shin*, Li-Qiong Wang, Glen E. Fryxell, Greg J. Exarhos Pacific Northwest National Laboratory, Richalnd, WA 99352 ABSTRACT In this paper, we report a self-assembly of surfactant molecules at the interface of air/hygroscopic quaternary ammonium salts such as tetrabutylammonium acetate (TBAAc), tetrabutylammonium bromide (TBAB), and tetrabutylammonium nitrate (TBAN), where they show different hygroscopicity, TBAAc > TBAB ≥ TBAN. Homogeneously dissolved surfactants rearrange themselves when they contact air due to high moisture adsorption behavior of such organic salts. Highly ordered lamellar phases with different lattice spacings have been observed when surfactants with long alkyl chains were used. Alkylammonium halides form monolayers, while neutral alkylamines forms bilayers based upon basal spacings of their X-ray diffraction patterns. The change in basal spacings in lamellar patterns, the alkyl chain conformation of surfactants, and Hbonding property of neutral amine surfactants are discussed in detail. INTRODUCTION Self-organization of surfactant-type organic molecules into ordered structures has been widely investigated due to their useful applications in chemical sensors, [1] artificial membranes, [2] host structures for polymer intercalations, [3] and mediums for the synthesis of macrocycles [4] and mesoporous silicates [5]. Intercalation of organic molecules into inorganic layered crystalline materials provides unique inorganic-organic nanostructured composites [6]. The precise control of the interactions between hosts and surfactant molecules through interlayer modification and the functionalities of surfactants are crucial for the understanding and the construction of the nanocomposite materials. Electrostatic interaction [7] and hydrogen bonding [8] between organic surfactants and inorganic precursors in their miceller solution have been reported. The self-assembly of alkylsiloxanes with long alkyl chains also provided interstratified inorganic-organic composites [9,10]. Organic salts are very frequently used as phase-transfer catalysts, supporting electrolytes, and surfactants [11]. Their versatile solvent properties have been widely investigated because they have virtually no vapor pressure [12]. The location and partitioning of organic salts as additives at different regions of the micelles played important roles in controlling the miceller structure and its growth. Organic salts with short chains have partitioned between surfactants as co-ions, [13] or intercalated between the head groups of layered structures as spacers [14]. In this communication, we report the first observation of the moisture-induced growth of self-ordered layered structures of surfactants at the interface of air/organic salts. The layered structures show subtle differences in their basal spacings when surfactants with different alkyl chain lengths are used, and two different types of layers are formed
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