The sodium-montmorillonite (Na⁺-MMT) was provided by Fenghong Co., Ltd. (Zhejiang, China) with a cation exchange capacity (CEC) of 90 mmol/100 g. A quaternary surfactant, octadecyl trimethyl ammonium chloride (OTAC), was purchased from Tianjin Damao chemical corporation and used without further purification. Chemical grade hydrochloric acid (HCl) was added into MMT suspension, and analytical grade silver nitrate (AgNO₃) was used to detect the residual Cl ions. Deionized water was used throughout this work. Analytical grade toluene was used as the organic dispersion medium.

The Na⁺-MMT was modified according to a standard ion exchange procedure^{[ 18]}. 10.0 g Na⁺-MMT was dispersed in 500 mL of hot water at 70℃ under continuous stirring to obtain a suspension solution. A certain amount of OTAC and concentrated hydrochloric acid were dissolved in 200 mL of hot water at 70℃, and the resultant solution was poured into the MMT suspension solution under vigorous stirring for about 1.5 h. The content of OTAC added in terms of aqueous solution was changed between 0.2 CEC and 2.3 CEC in 0.3 CEC steps. After modification the reaction mixture solution was filtered and washed three times with hot water at 70℃ to remove un-adsorbed surfactant (until no precipitate was detected by 0.1 mol/L AgNO₃), after then washed twice with ethanol. The prepared organo-montmorillonite (OMMT) was dried finally in a vacuum oven at 70℃ for 36 h.

Fourier transform infrared spectroscopy (FTIR) was carried out in a Nicolet-520 spectrometer to characterize the interface of OMMT (the OMMT was crushed and blended with potassium bromide (KBr)). X-ray diffraction (XRD) patterns of MMT and OMMT were collected on a Rigaku D/Max-2550 PC diffractometer using Cu Kα (λ=0.15406 nm) radiation in 2 θ range 2-10° with 0.02°/s. The basal spacing was calculated according to Bragg law^{[ 19]}. The morphology of Na⁺-MMT and OMMT was observed by scanning electron microscopy (SEM, FEI NOVA200 Nanolab). The MMT powders were pressed into a flat sheet, and the contact angle was measured with water drop by a contact angle analyzer (Powereach JC2000C1).

Dispersion of OMMT samples in the purified liquids were prepared in a dry box. Amounts of 5-6 g OMMT were dispersed in toluene and shaken to get a homogeneous dispersion. The sediment volume of dispersions of 0.35 g OMMT in 35 mL toluene was measured in 50 mL graduated test tubes. After intense shaking, the sediment volume was measured after 24 h.

The FTIR spectra of Na⁺-MMT before and after modification are shown in Fig. 1. Before modification (Fig. 1(a)) the band at 3610 cm^-1 is assigned to the stretching vibrations of the -OH groups bonded to the aluminum and/or silicon in the Na⁺-MMT interlayer. The broad band at 3460 cm^-1 is assigned to the hydroxyl stretching vibrations, and the band at 1650 cm^-1 corresponded to H-O-H bending vibrations shows the presence of some moisture in the samples.

Fig. 1

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	Fig. 1 FTIR spectra of Na+-MMT before (a) and after modification ((b) and (c), at COTAC= 0.5 CEC and 2.0 CEC, respectively)

While for the samples after modification (as shown in Fig. 1(b)) the stretching band of the -OH group at 3610 cm^-1 was shifted to 3620 cm^-1, and this slight shift towards the smaller frequencies implies the removal of some structural hydroxyl groups from the Si-OH or Al-OH sites. The broad band at 3460 cm^-1 in Fig. 1(a) was shifted to 3430 cm^-1. The new appearing bands at 2920 and 2850 cm^-1are attributed to asymmetric and symmetric C-H stretching vibrations of OTAC. The weak band at 3267 cm^-1 is attributed to N-H stretching vibrations present in OTAC^{[ 20]} and the peak at 1470 cm^-1is attributed to the CH₃-N⁺ moiety. The adsorption strength difference of the reflections was observed with increasing C_OTAC (in Fig. 1(b) and (c)) implying the different exchange efficiency of the MMT clays.

Besides, the bands is in the range of 1300-400 cm^-1 attributed to stretching and bending vibrations of Si-O-Al, Si-O-Si, and Al-O groups of all the samples have no obvious changes, which suggests that the main bones of Na⁺-MMT clay before and after modification are similar and there were no other interactions except the ion exchange during the modification.

The XRD patterns of the Na⁺-MMT and the OMMT are displayed in Fig. 2. After the OTAC modification, the peak at 2 θ = 5.84^o (corresponding to a basal plane spacing ( d₀₀₁) of 1.51 nm for Na⁺-MMT) was shifted to lower 2 θ value. There is a weak reflection at 2 θ of 5.72^o (corresponding to a basal spacing ( d₀₀₁) of 1.61 nm) for OMMT when C_OTACis 0.2 CEC. The OMMT has two characteristic peaks at 2 θ of 4.92^o and 3.38^o in case of C_OTAC = 0.5 CEC, corresponding to d₀₀₁ values of 1.79 nm and 2.61 nm, respectively. Further increase of C_OTAC to 1.1 CEC, the characteristic peak of the OMMT moved to 3.68^o, corresponding to d₀₀₁ = 2.40 nm. When the C_OTAC increases to 1.7 CEC, the OMMT has an intense peak at 2 θ = 2.85^o corresponding to a basal plane spacing of 3.09 nm. The maximum interlayer spacing of OMMT reaches 3.80 nm at C_OTAC = 2.3 CEC. It is thus concluded that the interlayer spacing of the Na⁺-MMT is extended by the intercalation modification. The peak at about 6.94^o (1.27 nm in Fig. 2(f)) is attributed to the aggregation of the surfactant in the OMMT galleries^{[ 21]}.

Fig. 2

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	Fig. 2 XRD patterns of Na+-MMT before (a) and after modification ((b)-(f): COTAC= 0.2 CEC (b); 0.5 CEC (c); 1.1 CEC (d) ; 1.7 CEC (e); 2.3 CEC(f))

Both the results of FTIR and XRD analysis indicate that the surfactants have been successfully intercalated into Na⁺-MMT layers. It is known that the shape of a perfectly straight chain of OTA⁺ looks like a ‘nail’, where the long alkyl chain is the ‘nail-body’ and the chain end holding three methyls is the ‘nail-head’. The length of the nail is about 2.79 nm, consisting of the ‘nail-head’ (0.43 nm) and ‘nail-body’ (2.36 nm). The height of the ‘nail-body’ is about 0.46 nm and that of the ‘nail-head’ is about 0.51 nm, and the heights of the ‘nail-body’ and ‘nail-head’ are 0.41 nm and 0.67 nm, respectively, when the plane of the zigzag arrangement of the carbon atoms of OTA⁺ is parallel to the plane of the MMT layer^{[ 18, 22]}. The thickness of a single MMT’s TOT layer is about 0.96 nm^{[ 22, 23]}. The schematically illustration of five different configurations of OTC⁺ chains in OMMT interlayer was proposed (as shown in Fig. 3) based on the results of the XRD analysis. The values of 1.61 nm and 1.79 nm for the OMMT are consistent with a monolayer arrangement of OTC⁺ chains in the interlayer space (see Fig. 3(b)). The basal spacing of 1.88 nm for the OMMT is in agreement with a bilayer arrangement of OTC⁺ chains in the interlayer space (see Fig. 3(c)). The basal spacings of 2.40 nm to 3.09 nm for the OMMT, corresponds to a pseudo trilayer to paraffin- type monolayer arrangement of intercalated OTC⁺chains (see Fig. 3(d) and (e)). The OTC⁺ chains were thought to radiate away from the MMT layers forming bimolecular arrangement (see Fig. 3(f)).

Fig. 3

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	Fig. 3 Schematic illustration of five different arrangements of OTC+ chains in OMMT interlayer(a) Na+-MMT; (b) Lateral monolayers; (c) Lateral bilayers; (d) Pseudotrimolecular layers of chains lying on the surface; (e) Paraffin-type monolayers; (f) Paraffin-type bialayers

The SEM results show the change of the morphology of Na⁺-MMT clay before and after modification with OTAC. Before modification the Na⁺-MMT was initially adhesive and aggregated to form a rather compact structure (shown in Fig. 4(a) and (b)). The irregular particle in different sizes was observed, and the bulk is nearly tens of micrometers. After modification the large particles were split into small ones because of the intercalation reaction with OTAC, and the compact structure got looser with curly margins (shown in Fig. 4(c) and (d)). The intercalation effect was enhanced with the C_OTACincreasing, the agglomeration was broken and the layers were further apart and less compact, displaying a leaf like structure (shown in Fig. 4(e) and (f)). The almost exfoliated layers were achieved at C_OTAC=2.0 CEC (shown in Fig. 4(g) and (h)). The significant variations of the morphologies observed by SEM are consistent with the findings of FTIR and XRD analysis.

Fig. 4

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	Fig. 4 SEM images of Na+-MMT (a and b) and OMMT (c-h): COTAC=0.5 CEC ((c) and (d)), 1.4 CEC ((e) and (f)), 2.0 CEC ((g) and (h))

The contact angle was used to characterize the surface wettability. If the contact angle is less than 90^o, the surface is described as hydrophilic; otherwise the surface is described as organophilic. The values of contact angle of MMT clay before and after modification were summarized in Table 1.

Table 1

Table 1

Table 1 The variations of the contact angle of MMTs to water and the sediment volume of MMTs in toluene after 24 h COTAC (*CEC) Contact angle/(º) Sediment volume /(cm3•g-1) Wettability 0 ― 63.2 Hydrophilic 0.2 25 54.4 Hydrophilic 0.5 38 41.6 Weak Hydrophilic 0.8 49 22.3 Weak Hydrophilic 1.1 75 8.5 Nearly Neutral 1.4 82 3.0 Nearly Neutral 1.7 91 1.5 Organophilic 2.0 97 (gel) Organophilic 2.3 113 (gel) Organophilic

Table 1 The variations of the contact angle of MMTs to water and the sediment volume of MMTs in toluene after 24 h

It is difficult to take an image of the water drop dropped on the Na⁺-MMT before modification, which indicates the surface has strong hydrophilicity before modification. After modification the contact angle value increases with the C_OTACincreasing, which implies that the surface characteristic was converted from hydrophilic to organophilic. According to the principle of polarity compatibility, the dispersion effect will be improved when surface polarity of particles is similar to that of liquid media. Table 1 also displays the sediment volume in toluene after 24 h.

The sediment volume steeply decreases with the C_OTACincreasing. Before modification, the Na⁺-MMT particles has a worse dispersibility and can not swell in toluene, and the supernatant is clear. While the OMMT particles has a better dispersibility and swell in toluene, and the supernatant is cloudy. The dispersion forms gels in case of C_OTAC≥2.0 CEC. The result shows that OMMT has a high affinity to organic medium.

The Na⁺-MMT was successfully intercalation modified by OTAC. Both the structure characteristics and surface properties of the layered clay changed after modification. The modified MMT clays with flake structure would show good compatibility with polymer matrix. The interlayer spacing of OMMT was extended from 1.51 nm to 3.80 nm, and the large particle with compact aggregation was exfoliated. The surface wettability of Na⁺-MMT was converted from hydrophilic to organophilic after modification. Based on the analytical results, it is suggested that OTAC can be used to improve the performance of the MMT nanocomposites.