Intercalation of Cocamide Diethanolamine into Swellable Clay by Solid-State Process

Abstract

Coconut fatty acid diethanolamine (cocamide-DEA (CDEA)), CH₃(CH₂)₁₆CON(CH₂CH₂OH)₂ was intercalated into montmorillonite using both solution and solid-state reaction methods. In a typical solution process, the CDEA aqueous solution was mixed with a montmorillonite (Kunipia-F) aqueous suspension, which resulted in a CDEA–montmorillonite layer complex with a basal spacing of 13.8 Å. A CDEA–montmorillonite complex was also easily prepared by the solid–solid reaction method. A mixture of CDEA and Na–montmorillonite was ground at ambient temperature. The basal spacing of the mixture increased to approximately 44 Å after grinding for 30 min. Upon washing, the basal spacing decreased to approximately 10 Å, which was close to that of pristine Na–montmorillonite owing to the deintercalation of the CDEA molecules. The basal spacing of the CDEA–montmorillonite composite starting from protonated montmorillonite decreased to 13.5 Å upon washing, indicating the parallel monolayer arrangement of CDEA molecules between the silicate layers. This finding strongly suggests that acid–base intralayer complexation is responsible for the solid-state intercalation reaction.

1. Introduction

Layered inorganic compounds, such as layered aluminosilicates and layered double hydroxides, have received much attention from researchers because of their cationic and anionic exchange properties and biocompatibility. Owing to their diverse intercalation properties, they have been widely applied to catalysts and catalyst adsorbents, inorganic functional additives, and layered nanoparticles in polymeric nanocomposites [1,2,3,4]. Recently, these intercalative layered compounds with functional biomolecules between the layers have been applied in drug or active ingredient delivery systems and in fields in which stabilization, solubilization, and slow-release functions are necessary [5,6,7,8,9].

So-called solid–solid reactions, which occur between powders in the solid state, have attracted much attention not only because of their reaction kinetics but also because of their industrial applications [10,11,12]. In addition, solid–solid reactions can be used to prepare compounds that are not accessible from solutions [13]. Nevertheless, very little is known about the mechanisms and factors controlling solid–solid reactions. Therefore, further studies on solid-state ion-exchange reactions are of great importance from both theoretical and practical viewpoints. In this paper, we report novel solid-state intercalation reactions between coconut fatty acid diethanolamine (cocamide-DEA (CDEA)), which acts as an environmentally friendly biomodifier, and homo-ionic montmorillonite. The aim of this study was to achieve the intercalation of CDEA into montmorillonite via a solid-state reaction without using a solvent, which could be a general and clean process for obtaining organic–inorganic hybrids.

2. Materials and Methods

To prepare the CDEA–montmorillonite hybrid, we used both a conventional intercalation reaction method and a simple solid-state reaction method. The natural Na⁺–montmorillonite (Na⁺-M) was Kunipia-F, which has the structural formula Na_0.35K_0.01Ca_0.02(Si_3.89Al_0.11)(Al_1.60Mg_0.32Fe_0.08)O₁₀(OH)₂·nH₂O and a cationic exchange capacity (CEC) of 119 meq/100 g. Industrial-grade CDEA (CH₃(CH₂)_nCON(CH₂CH₂OH)₂, Taidong Chem. Co. Korea), which has an average molecular weight of 280–290 g/mol, was used as the guest molecule. First, CDEA–montmorillonite (CDEA-M) was prepared using a conventional solution intercalation reaction. CDEA with three times the CEC of Kunipia-F was added to a 1 wt.% colloidal Kunipia-F aqueous suspension under stirring at room temperature. After intercalation for 2 h, the CDEA-intercalated Kunipia-F was centrifuged, washed with deionized water to remove excess guest species, and dried at 80 °C for 10 h.

The CDEA-M hybrids were also prepared using a simple solid-state reaction. In this study, 0.25, 0.5, 1.0, and 2.0 molar ratios of CDEA to the CEC of clay were mixed, ground in an agate mortar for 30 min. To examine the effects of thermal treatment and washing, the sample containing 2:1 molar ratio was heated at 60 °C for 2 h and washed with distilled water. To compare the intercalation reactivity as a function of the interlayer acidity, protonated (H⁺) montmorillonite was also used as the host clay.

The thermal gravimetric-differential scanning calorimetry (TG-DSC) curves were collected using TGA/DSC 1 (Mettler Toledo, Hong Kong, China) at heating rate of 10 °C/min from 25–1000 °C in a Pt crucible under N₂ atmosphere. A Rigaku Miniflex instrument (Japan) equipped with Cu target (Kα radiation, λ = 1.5406 Å) was used for X-ray diffraction (XRD) patterns. XRD samples were prepared by adding an excess amount of powder on a glass holder and swiping away surplus powder.

3. Results and Discussion

The XRD patterns of the pristine montmorillonite clay (Kunipia-F, M) and CDEA-M hybrids prepared by the solution process are presented in Figure 1. An XRD of CDEA was not measured because it is in a liquid state at room temperature. Upon intercalation of the CDEA molecules, the basal spacing of the pristine aluminosilicate layers (d₀₀₁ = 12.5 Å) increased to approximately 13.8 Å, indicating the successful intercalation of CDEA molecules into the interlayer space of the aluminosilicate layers. Basal spacing is a typical value for a stacking structure with a parallel monolayer arrangement of molecules between the silicate layers. Even though the ion-exchange reaction was repeated three times, the basal spacing remained almost constant, indicating that the intercalation of the CDEA molecules into the silicate layers quickly reached the equilibrium state.

The driving force for solid–solid reactions is the difference between the free energies of the products and reactants. In a previous study on the solid-state intercalation of organic species into montmorillonite, the reactions proceeded rapidly at room temperature [10]. This observation suggests that some organic species have a certain mobility, even in the solid state, and that the diffusion of the organic species is not a dominant factor in solid-state reactions. Additionally, grinding is thought to provide sufficient contact between the reactant particles to accelerate the reactions.

The solid-state reaction between CDEA and montmorillonite caused the d₀₀₁ diffraction peak of CDEA-M to shift to the lower 2θ regions. The variation in the XRD patterns with changes in the host: guest ratio (mole of CDEA:CEC of M) is shown in Figure 2. In the XRD patterns of the products at mixing molar ratios of 0.25:1 (a) and 0.5:1 (b), broad peaks with a basal spacing of 14.2 Å are observed. This basal spacing is owing to the parallel monolayer arrangement between the silicate layers, as observed in the XRD pattern prepared by the solution reaction (Figure 1a). At the mixing ratio of 1.0:1.0 (c), new diffraction peaks with basal spacings of 44.0 Å and 20.1 Å are observed, indicating that the parallel arrangement changed to a perpendicular double-layer arrangement. The interlayer spacing increased to approximately 34 Å, which can be explained by the paraffin-like double-stacking structure of the intercalated CDEA molecules owing to excess guest molecules [14].

Figure 3 shows the XRD patterns upon heating and washing of the CDEA–montmorillonite composites when a guest: host mixing molar ratio of 2.0:1.0 was applied. As previously noted, the fully intercalated compounds exhibit a large interlayer separation of approximately 44 Å, irrespective of the interlayer cations (Na⁺ vs. H⁺). Upon heating the samples at 60 °C for 2 h, no distinct differences are observed in the XRD patterns, implying a fast intercalation reaction of the CDEA molecules even at room temperature. After thoroughly washing the CDEA–clay complexes with distilled water, the basal spacings drastically decreased. In particular, the basal spacing of the Na⁺-type montmorillonite (~10.5 Å) was close to that of the original clay minerals (Figure 3a), indicating the complete removal of the interlayer CDEA molecules during washing. This strongly suggests that the intercalation of CDEA molecules mainly occurred via a weak ion–dipole interactions and not by strong electrostatic attraction. Although the basal spacing of the CDEA and protonated clay complex also decreased significantly upon washing (Figure 3b), it exhibited a distinct diffraction peak at 13.5 Å, which can be assigned to the monolayer CDEA arrangement between the silicate layers, as observed in the solution process. It is clear that the solid-state intercalation reaction between protonated clay and CDEA occurred via intralayer acid–base complexation between the interlayer protons (H⁺) and the basic ethanolamine functional groups. Strong electrostatic interactions acted as the main stabilization energy.

Based on the XRD results, we propose a possible structural model for the CDEA–montmorillonite composite, which is depicted in Figure 4. In the intercalation compounds between clays and organic molecules, the interlayer structure is mainly governed by the layer charge of the clays and the interlayer population of the guest species [15,16]. Mixing and grinding the CDEA and powdery clay in an agate mortar resulted in the formation of an intercalative layer nanocomposite, which exhibited a basal spacing as large as 44 Å. In addition, higher-order diffraction profiles were clearly observed, indicating the well-ordered stacking structure of the CDEA molecules between the silicate layers. Assuming that the molecular length of CDEA is approximately 27.5 Å, the interlayer CDEA molecules were arranged in tilted (θ = 53.1°) double-layer paraffin-type stacking, as depicted in the figure. Upon heating the ground mixtures at 60 °C for 2 h in an electric oven, no remarkable changes were observed in either the peak position or intensity, thus confirming that the intercalation of the CDEA molecules occurred quickly, even at room temperature and with grinding alone. This high affinity of CDEA molecules to the silicate surface might be owing to the functional head groups of CDEA, that is, the ethanolamine group. The higher affinity of the ethanolamine moiety led to fast interlayer diffusion of the CDEA molecules, even at room temperature, accompanied by expansion of the silicate layer. Once some part of the silicate surface was covered by CDEA molecules, further incorporation of CDEA molecules exceeding the CEC was accelerated by van der Waals interactions between the CDEA molecules. In the Na⁺-type clay, the main driving force and stabilization energy of the CDEA guest molecules between the silicate layers may be an ion–polar interaction, that is, complexation between the interlayer Na⁺ ions and the ethanolamine moiety in CDEA. Further CDEA sorption beyond stoichiometry is mainly owing to the van der Waals attraction between the hydrophobic alkyl chains. The intercalation of basic CDEA molecules into acidic protonated clay seems to occur via an acid–base complexation, leading to additional stabilization energy.

Figure 5 illustrates the TG-DSC curves for the CDEA-M hybrids prepared by solution and solid-state reactions. Thermal decomposition of the hybrids generally occurred in four steps. In the first temperature region of 25–200 °C, the product prepared by the solution process (Figure 5a) shows a weight loss of 4.5 wt.%, with a broad endothermic differential thermal analysis (DTA) peak, corresponding to the removal of the adsorbed water on the surface of the hybrids and co-intercalated water. The product prepared by the solid-state reaction method (Figure 5b) shows no distinct weight loss in this temperature domain, implying that there was little residual interlayer water owing to its strong hydrophobic nature. The second weight loss in the temperature range of 200–550 °C, with strong exothermic peaks in the DSC curves, can be assigned to the oxidative decomposition of organic molecules. Further weight loss and a weak endothermic reaction at approximately 650 °C are attributed to the dehydroxylation of the clay layers. In the sample prepared by the solid-state reaction method (Figure 5b), additional weight loss and a broad endothermic peak are observed, reflecting the decomposition of residual carbon. Finally, the sharp and small endothermic peak in the DSC curves at approximately 950 °C without a weight change in the TG curves is owing to the recrystallization of amorphous aluminosilicates into crystalline ones.

4. Conclusions

Intercalation compounds comprising cocamide-DEA (CDEA) and a clay mineral were prepared by solution and solid-state intercalation processes. In particular, a simple solid-state route was successfully applied to the preparation of the layered nanocomposites. In the fully intercalated compounds, the CDEA molecules exhibited a tilted, paraffin-like double-layer arrangement between the silicate layers, with a basal spacing as large as ~44 Å. However, upon washing, the interlayer CDEA molecules were completely leached when the interlayer cation was Na⁺. Even though the basal spacing of the CDEA and protonated clay complex also decreased significantly after washing, they exhibited distinct diffraction profiles at 13.5 Å, which can be assigned to a monolayer CDEA arrangement between the silicate layers. It is clear that the solid-state intercalation reaction between the protonated clays and CDEA occurred via intralayer acid–base complexation between the interlayered proton (H⁺) and basic ethanolamine functional groups.