Two-dimensional phase transition of styrene adsolubilized in cetyltrimethylammonium bromide admicelles on mica

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Abstract

The phase behavior of styrene adsolubilized in cetyltrimethylammonium bromide (C16TAB) aggregates adsorbed on mica was studied in situ using repulsive force atomic force microscopy (AFM) at bulk surfactant concentrations below the critical micelle concentration and without added electrolytes. Two critical phase transitions were observed for the styrene/adsorbed C16TAB system by studying the deflection images and force curves. The first critical phase transition occurred when the phase of the two-component admicelle shifted from a lamellar structure, with no discernable surface structure, to patches of swollen admicelle. Force curve analysis shows that the thickness of the C16TAB admicelle increases as does force required to penetrate the patches. The strong short-range repulsive force (rupture force) is thought to be attributable to the mechanical deformation of the liquid-like droplet within the admicelle by the AFM tip, described by fitting the data to the Hertz and JKR models. The second critical phase transition occurs when the bilayer with swollen patches transforms to a bilayer containing solute droplets with a thickness of 20 nm or more, a transition explainable by existing solubilization theories. Styrene molecules, being slightly polar, solubilize primarily at the admicelle-water interface. At low concentrations the adsolubilization of styrene results in a reduction of the head–head repulsion which causes the lamella to change to short hemi-cylinders and hemi-spherical structures. At high concentrations, styrene molecules begin to preferentially adsolubilize into the hydrocarbon core of the bilayer causing the lamella-droplet transformation.

Introduction

Surfactant adsorption at the solid–liquid interface has been extensively studied. Adsorbed aggregates (admicelles), whether discrete or continuous, possess some properties which are similar to those of micelles including the ability of adsolubilizing sparingly soluble/insoluble compounds in aqueous solutions [1]. Applications of adsolubilization include concentrating and removing selected organic constituents in environmental remediation [2], analyzing mixtures of optical isomers in admicellar chromatography [3], concentration of organic reactants in admicellar catalysis [4] and modification of surfaces by polymerization of monomers within admicelles, a process called admicellar polymerization [5], [6], [7].

In recent years, atomic force microscopy (AFM), operated in the repulsive force region, has captured images of cationic surfactant admicelles [8]. On mica, rod-like cetyltrimethylammonium bromide (C16TAB) admicelles transform to lamella at concentrations above the CMC in the absence of added electrolyte [9]. The measured thickness of the C16TAB admicelle on mica above or near the critical micelle concentration is approximately the length of two extended C16TAB molecules. This thickness, between 3.5 and 4.5 nm, was found by measuring the “sudden jump” in force-separation curves as the tip penetrates the adsorbed layer and reaches the substrate surface [9]. Behrends et al. used a chromatographigraphic method to show that the hydrophilic head of the C16TAB faces the aqueous solution at concentrations as low as 35% of the critical micelle concentration (CMC), supporting the formation of bilayer aggregates below the CMC. They also found that bilayers are more favorable for the adsolubilization of aromatic organic compounds such as benzene and toluene [10].

Several studies have examined the adsolubilization of various aromatic compounds in adsorbed cationic surfactant aggregates. An adsolubilization equilibrium constant or partition coefficient (K) describes the distribution of organic solutes between the aqueous and the admicelle phases [11]:K=XadCbwhere Xad is mole fraction of adsolubilizate in the admicelle and Cb is the equilibrium concentration of solute in the bulk aqueous phase. Several studies have shown that the shape/slope of the graph of the partition coefficient versus mole fraction of solute in the admicelle is due to the locus of adsolubilization within the admicelle (i.e., head groups, palisade, core), which in turn depends upon the polarity of the adsolubilizate [11], [12]. Polar solutes should preferentially partition at polar sites within the admicelle, while non-polar solutes should preferentially adsolubilize in the non-polar admicelle core. According to this theory, the partition coefficient will: increase with increasing solute mole fraction in the admicelle if the solute preferentially partitions in the core; decrease if the solute primary adsolubilizes in the palisade region of the admicelle; and show no change with mole fraction in the admicelle if it adsolubilizes into both the palisade and core regions. The model fits most existing data but does not explain the deviations from a constant partition coefficient value at very low and very high concentrations of naphthalene and 2-napthol adsolubilized in C16TAB and C14TAB on both amorphous silica and powdered mica [12], [13].

Dickson found that the partition coefficient showed inflection points at certain critical concentrations, which he hypothesized might be due to changes in the structure of the adsorbed layer [14]. Kovacs et al. suggested, based on AFM images, that a phase transition of the admicelle is responsible for a decrease in the partition coefficient with increasing concentration of an aromatic compound [13]. Cylindrical C14TAB admicelles transform to a featureless lamellar structure at solute concentrations above which the graph of the partition coefficient undergoes an inflection [13]. Their system, studied at twice the CMC of C14TAB, could not provide information about low solute concentrations as the presence of micelles necessitated high solute levels to fill both admicelles and micelles [14].

Some have suggested that the phase changes in the adsorbed layer may be due to changes in the amount of adsorbed surfactant, analogous to changes in the aggregation number of micelles in the presence of added solute [15]. However, Esumi and Kovacs showed that surfactant adsorption is virtually constant in the added solute concentration ranges that are typically examined [13], [16].

Adsolubilization has been utilized in the formation of polymer thin films. The structure of the formed polymer thin film has been examined using ellipsometry [5], scanning electron microscopy [17], and tapping mode atomic force microscope at ambient condition [18], [19]. However, without an understanding of the behavior of the multicomponent adsorbed layer, the structure and properties of the formed polymer film cannot be predicted or effectively controlled [20]. It has been shown that the level of adsolubilization impacts the physical and cure properties of admicellar polymerization modified silica-reinforced rubber compounds [7], [21], [22], [23]. More recently the impact of the levels of adsorbed surfactant and adsolubilized monomer on the polymer formed during admicellar polymerization has been examined on flat and amorphous silica surfaces [18], [19]. Results on both surfaces show that the distribution, thickness, and morphology of the formed polymer are very strongly determined by the amount of added monomer, and somewhat determined by the amount of adsorbed surfactant [18], [19]. Although other effects such as the molecular structure of surfactant and monomer, counterion concentration and type, initiator type, washing solvent and temperature may also influence the morphology of the polymer film, the systematic analysis of those effects on the polymer film has yet to be studied. The objective of this work is to examine in situ the impact of the concentration of the monomer on admicelle phase behavior during the adsolubilization process. The phase behavior of the admicelle has been examined within the first 2 h of contact and at 4 days time over a wide range of styrene concentrations. The results provide insight into the interactions between adsolubilizates and the admicelle at the solid–liquid interface.

Section snippets

Materials

Mica disks purchased from Digital Instruments Inc. (Santa Barbara, CA) were freshly cleaved in air and immediately immersed in solution before imaging. Cetyltrimethylammonium bromide cationic surfactant (C16TAB), and styrene, both 99% purity, were obtained from Sigma (St. Louis, MO). All materials were used as received. Deionized water was obtained by a Barnstead E-pure water system with a 0.2 μm fiber filter (Barnstead DS3750) and a resistivity of 18.3  cm.

Sample preparation

C16TAB solutions at 80% CMC (0.72 mM)

Adsorption of C16TAB on mica

Deflection images of C16TAB adsorbed on mica at 80% of CMC without added electrolytes were identical to those observed by others at concentrations above the CMC, that is, a continuous bilayer having a hemicylindrical surface structure [8], [9]. A force curve ‘jump-in’ distance of 3.2–3.5 nm was obtained. Assuming bilayer coverage on the entire mica surface, the effective area per C16TAB head group at the mica/water interface would be 0.32 nm2, which is larger than the values obtained from AFM

Discussion

The phase transitions observed with increasing styrene concentration are summarized in Table 1. These transitions are not due to changes in the amount of adsorbed surfactant, but to the incorporation of the hydrocarbon inside the admicelle, much as those seen in the phase transitions for micelles during solubilization. Earlier studies show that the addition of aliphatic hydrocarbons into micellar ionic surfactant solutions causes a single shape transformation, that is, spherical micelles to

Conclusion

The adsolubilization of styrene by C16TAB admicelle at the mica/water interface has been imaged in situ by AFM operated in the repulsive force region. Deflection images of the initial state have been captured and compared with the structures seen after 4 days time at varying styrene feed concentrations. The AFM images suggest that most of the adsolubilization process is completed within the first 3 h followed by the slow re-organization of the surfactant molecules at the interface. When the feed

Acknowledgements

Funding for this project came from the National Science Foundation, The University of Mississippi Loyalty Foundation, The University of Mississippi Associates and Partners Program, and the Department of Chemical Engineering.

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