AMPS

Journal of Colloid and Interface Science 319 (2008) 2–11 www.elsevier.com/locate/jcis

Adsorption of 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) and related compounds onto montmorillonite clay Nagi Greesh, Patrice C. Hartmann ∗ , Valeska Cloete, Ronald D. Sanderson UNESCO Associated Centre for Macromolecules, Department of Chemistry and Polymer Science, University of Stellenbosch, Private Bag X1, 7602 Matieland, South Africa Received 18 June 2007; accepted 12 October 2007 Available online 18 October 2007

Abstract Sodium montmorillonite clay (Na-MMT) was modified using 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS). The objective of this study was to determine which chemical group is the ‘driving force’ leading to the adsorption of AMPS inside the clay galleries. AMPS has been reported to be a good candidate as a clay modifier for the preparation of polymer–clay nanocomposites by in situ free radical polymerization in emulsion. However, the way in which AMPS interacts with the surface of MMT has not yet been studied. The type of interaction between organic modifiers and clay plays a determining role in the successful preparation of polymer–clay nanocomposite materials. The adsorption ability of three other organic compounds similar to AMPS, namely sodium 1-allyloxy-2-hydroxypropyl sulfonate (Cops), N -isopropylacrylamide (NIPA) and methacryloyloxyundecan-1-yl sulfate (MET), was also evaluated. These selected compounds also have functional groups potentially able to interact with the clay surface (i.e., a sulfonate group, an amido group, or a sulfate group, respectively). Results of FT-IR, TGA and SAXS analyses showed that AMPS, NIPA, Cops and MET all interacted with clay, but to various extents. © 2007 Elsevier Inc. All rights reserved. Keywords: AMPS; Organic modifier; Clay; Montmorillonite; Adsorption

1. Introduction Organoclays were first introduced by Jordan [1]. Their applications involve the rheological behavior of organoclays in various solvent systems, in areas such as oil well drilling fluids, paint, grease, cosmetics and personal care products [2]. Since 1990, the use of organoclays in polymer–clay nanocomposites has become the field of interest of many researchers. The most common clay used in polymer–clay preparation of nanocomposites is montmorillonite (MMT) [3,4], which has two silica–oxygen tetrahedral sheets sandwiching an aluminum or magnesium octahedral sheet. In the specific case of MMT, due to partial substitution of some of the silicon atoms by aluminum atoms in the tetrahedral layers, and/or substitution of aluminum atoms by magnesium atoms, layers are negatively charged. These negative charges are counterbalanced by cations such as sodium, potassium, and calcium [4,5], which * Corresponding author. Fax: +27 21 808 4967.

E-mail address: hartmann@sun.ac.za (P.C. Hartmann). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.10.019

makes platelets of pristine MMT hydrophilic [6]. However, the counter-cations can be exchanged by various cationic species, including organic cationic molecules [3,7]. Organoclays can be prepared by surface modification with organic compounds via at least one of the following methods: ion-exchange with organic cations, adsorption of organic compounds, and reaction with organic acids [7]. The ion-exchange method is a popular and relatively easy method for modifying the clay surface and making it more compatible with an organic matrix [7]. In the ion-exchange reaction the metallic counter-cations of the surface are exchanged by cationic molecules [8]. Cationic surfactants (e.g., alkyl ammonium and alkyl phosphonium) are commonly used to render the clay surface hydrophobic prior to it being used for the preparation of polymer–clay nanocomposites [8,9]. Organic molecules that contain a chemical group able to form ion–dipole interactions with the exchangeable cations present in the interlayer of clay can also be used as clay modifiers. This phenomenon was first studied with different types of glycols [2]. Polar organic molecules can be adsorbed by clay

N. Greesh et al. / Journal of Colloid and Interface Science 319 (2008) 2–11

minerals by the formation of coordination bonds with the exchangeable cations, or by proton transfer from the interlayer water molecules surrounding the inorganic cations to the organic molecules, or by proton transfer from the organic molecules to the interlayer water molecules surrounding the inorganic cations [10]. Some works have focused on studying interactions between clay and compounds with amido groups. Interactions between amides and Na-MMT were investigated by Tahoun and Mortland [11]. According to them, when amides are adsorbed by Na-MMT they can be partially protonated, and the degree of protonation depends on the acid strength of the exchangeable cations and the polarization of the molecules of water solvating the cations. Both the oxygen atom and the nitrogen atom of the amido group can form hydrogen bonds with water molecules. The mode of interaction between urea molecules and clay was studied by Mortland [12]. He showed that molecular urea binds to exchangeable cations via water molecule bridges. The carbonyl groups of urea molecules coordinate the metallic cations. Using FT-IR, Stutzmann and Siffert [13] studied the adsorption mechanism and the fine structure of complexes obtained between Na-MMT and acetamide or polyacrylamide. The adsorption takes place on the external surface of the clay particles. There are two adsorption possibilities: a strong, irreversible adsorption, which corresponds to chemisorption of organic molecules, or a weaker adsorption due to the formation of hydrogen bonds between functional groups of the organic compound and the hydroxyl groups of the clay edges. These two options explain how the surface of clay galleries can be modified using non-cationic organic compounds [10]. The use of a low percentage of 2-acrylamido-2-methyl-1propanesulfonic acid (AMPS) as a specialty monomer seems to play a major role in achieving successful exfoliation of clay in the preparation of polymer–clay nanocomposites by in situ polymerization in emulsion. The main objective of this study was to determine the ‘driving force’ (i.e., the type of interaction) leading to the adsorption of AMPS onto montmorillonite clay (Na-MMT). Xu et al. [14] used AMPS as a clay modifier and found that it had the ability to widen the d-spacing between platelets from 1.17 nm (pristine clay) up to 2.1 nm, depending on the AMPS/clay ratio used. They suggested that AMPS molecules interacted with clay by an ion-exchange reaction. They further stated that in aqueous solution AMPS adopts a zwitterionic form by proton transfer from the sulfonic acid group to the nitrogen of the amido group [14,15]. This hypothesis is however highly questionable as amido groups are very weak bases, with a pKa value of about −0.5. Accordingly, amides do not become positively charged by protonation in water, unless the pH becomes very low (pH 0 or less) [16]. Therefore, since the conclusions drawn by Xu et al. [14] are questionable, AMPS possibly interacts by a mechanism other than ion-exchange. AMPS undoubtedly does interact with a clay surface, even though the interaction mechanism still remains unknown. The present contribution aims to provide insight into the kind of interactions occurring between AMPS and clay. This was done by comparing the adsorption of AMPS with that

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of others similar organic compounds having similar chemical groups. 2. Experimental 2.1. Materials Sodium montmorillonite clay (Na-MMT) “CloisiteNA+” was obtained from Southern Clay Products, Inc. (USA). It is a fine powder with an average particle size of less than 13 µm3 by volume in the dry state, and has a cation exchange capacity of 92.96 meq/100 g clay. 2-Acrylamido-2methyl-1-propanesulfonic acid (AMPS) and N -isopropyl acrylamide (NIPA) were obtained from Aldrich. Sodium 1-allyloxy2-hydroxypropyl sulfonate (Cops) was obtained from Rhodia. Methacryloyloxy-undecan-1-yl sulfonate (MET) was synthesized according to the method described by Unzue et al. [17]. 2.2. Analytical equipment Fourier-transform infrared spectroscopy (FT-IR) was used to qualitatively prove that the organic modifiers had interacted with the clay minerals. FT-IR was carried out using a Nexus FT-IR instrument, by averaging 32 scans, with a wave number resolution of 4 cm−1 . A Perkin–Elmer 1650 transform infrared spectrophotometer was used to detect shifts in vibration bands due to specific interactions between clay and the organic modifiers. Thermogravimetric analysis (TGA) of the organoclays was carried out using a TGA-50 SHIMADZU thermogravimetric instrument, with a TA-50 WSI thermal analyzer. Samples (10– 15 mg) were degraded in a nitrogen atmosphere (flow rate 50 ml/min) at a heating rate of 20 ◦ C/min. Small angle X-ray scattering (SAXS) measurements were performed at 298 K in a transmission configuration. A copper rotating anode X-ray source (functioning at 4 kW) with a multilayer focusing “Osmic” monochromator giving high flux (108 photons/s) and punctual collimation was used. An “image plate” 2D detector was used. Diffraction curves were obtained, giving diffracted intensity as a function of the wave vector q. 2.3. Modification of Na-MMT (without control of the pH of the aqueous medium) The Na-MMT (2 g) was introduced to a 250 ml flask containing 150 ml of deionized water. The suspension was stirred at room temperature until aggregates were no longer observed. Various quantities of AMPS [130, 100, 75, 50 and 25 mol% relative to the cation exchange capacity (CEC, i.e., 92.6 meq/100 g)] were added to the mixture, which was then stirred for an additional 24 h. Organoclays (AMPS–MMT) were collected by centrifugation at 4400 rpm for 30 min. The obtained clay sediment was redispersed in 100 ml of deionized water to remove unadsorbed AMPS remaining in solution. This procedure was repeated five times, until the supernatant water from centrifugation was

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found to be free of AMPS. This was monitored by measuring the pH of the washing water, which should be similar to that of the dispersed clay without AMPS, i.e., pH 10.5. Fig. 1 illustrates the change in pH of the centrifuged water as a function of the number of washing cycles, showing that four to five washing/centrifugation cycles are necessary to achieve total removal of any unadsorbed AMPS. The obtained silicate cakes were then dried in a vacuum oven at 50 ◦ C for 3 days prior to analysis. Modifications of Na-MMT by Cops, MET and NIPA were performed under similar conditions. 2.4. Modification of Na-MMT by AMPS at different pH values Different samples of AMPS-modified MMT were prepared with the same amount of clay and same amount of AMPS, but at different pH values. A quantity of Na-MMT (2 g) was dispersed in 150 ml deionized water and the dispersion was vigorously stirred overnight. The AMPS (25 mol% relative to CEC of clay) was dissolved in 50 ml deionized water and the resulting solution was slowly added to the clay suspension. The pH of the resulting suspension was 2.8. The pH was adjusted to the desired value by adding an aqueous solution of either HCl (0.1 M) or NaOH (0.1 M) and the reaction mixture stirred for 24 h at room temperature. The obtained modified clay was then filtered, thoroughly washed with water (acidified water for low pH samples), and dried for 3 days at 55 ◦ C in a vacuum oven.

erals are thermally more stable than organic molecules. Clays begin loosing structural hydroxyl groups only above 600 ◦ C but will maintain their layered structure up to 800 ◦ C [18]. The decomposition behavior of pure AMPS is also shown in Fig. 2. The AMPS starts to decompose at about 185 ◦ C and does not yield any residue at 600 ◦ C. The weight loss of pure clay between 20 and 100 ◦ C corresponds to the removal of water coordinated with Na+ from the interlayer [19]. It has been reported that physically adsorbed water gives a sharp decreases in weight loss at low temperature [20]. The weight loss of Na-MMT in the temperature range 100–600 ◦ C is about 2.6%. This can be attributed to the loss of hydrogen-bonded water molecules and of some of the hydroxyl groups from the octahedral sheets of the clay [19,21]. Table 1 summarizes the amounts of AMPS used in the modification process, and the true amounts of AMPS found inside the clay galleries as experimentally determined by TGA from the weight loss at 600 ◦ C. If all the AMPS molecules were successfully adsorbed then the mass of AMPS found inside the clay galleries should be equal to the initial quantity of AMPS added. This was not verified, as the experimental amounts of AMPS inside the clay galleries, determined by TGA, were found to be lower than expected. For example, in the case of 25% CEC the

3. Results and discussion 3.1. Determination of the amount of AMPS adsorbed in the clay galleries Thermogravimetric analysis (TGA) is commonly used to determine the amount of organic modifier present in clay galleries [9,10,14]. Various amounts of AMPS were used to modify the clay surface (25 to 130% relative to CEC of the clay). The amount of AMPS loaded in the clay was determined from the difference between the residual weight difference between clay with AMPS and pristine clay at 600 ◦ C (cf. Fig. 2). Clay min-

Fig. 2. Thermal gravimetric curves of (a) pristine Na-MMT, and Na-MMT with different AMPS concentrations: (b) 25% CEC, (c) 50% CEC, (d) 75% CEC, (e) 100% CEC, (f) 130% CEC. Table 1 Initial AMPS concentrations (feed) and amount of AMPS found inside clay galleries (determined experimentally) Initial concentration of AMPS

Fig. 1. pH values of centrifuged water vs number of washings when 100 mol% AMPS relative to CEC was used to modify the clay surface.

Rel. to CEC of claya (%)

Feed mass (g)

0 25 50 75 100 130

0.000 0.095 0.191 0.287 0.383 0.495

Weight loss at 600 ◦ C (%)

Quantities of AMPS in clay galleries Massb (%)

Exp. mass (g)

4.02 6.15 6.32 8.42 8.60 8.81

0.00 2.13 2.30 4.41 4.60 4.84

0.000 0.002 0.004 0.014 0.019 0.023

a Relative to cation exchange capacity of clay. b Percentage of AMPS adsorbed by clay, relative to the cation exchange ca-

pacity.


initial quantity of AMPS was 0.095 g. However, only 0.002 g entered inside the clay galleries, meaning that about 0.093 g of AMPS was lost during the washing process. In the case of 100% CEC, the initial quantity of AMPS was 0.383 g while the amount of AMPS adsorbed inside the clay galleries was found to be only 0.019 g, given that unadsorbed AMPS molecules were removed during the washing process. The amount of AMPS introduced was quantified relative to the CEC value of the clay, even though AMPS was not believed to be able to ion-exchange with clay surface cations. This was done in order to be able to make a comparison between the extent to which AMPS can to adsorb onto the surface of clay and the extent to which more conventional clay modifiers, such as cationic organic species, e.g., quaternary ammonium surfactants, can adsorb. The AMPS was expected to be fully adsorbed inside the clay galleries when small quantities were used (e.g., 25% CEC). However, TGA results indicated that this was not the case, as only a fraction of AMPS was found to be still adsorbed after washing, irrespective of the initial concentration of AMPS used (25 to 130% CEC). This is most probably due to the chemical nature of AMPS; AMPS is uncharged, unlike cationic surfactants. It is therefore proposed that AMPS does not interact with clay according to the ion-exchange model, because ionexchange only occurs with positively charged molecules [10, 22,23]. This is not in accordance with the mode of interaction of AMPS with clay as proposed by Choi et al. [15], who suggest that AMPS has the ability to ion-exchange onto clay by adopting a zwitterionic form, occurring by proton transfer from its sulfate group to the nitrogen atom of the amido group. Typically, true interaction of positively charged organic molecules via the ion-exchange mechanism occurs quantitatively, as reported by Biasci et al. [24], for example, with cetyltrimethylammonium bromide (CTAB). Since AMPS is not supposed to be able to interact with clay by ion-exchange it is believed that AMPS rather adsorbs onto clay by either formation of hydrogen bonding between its amido groups and the hydroxyl groups present on the edges of clay platelets [25] or by formation of complexes between its sulfonate groups and water molecules solvating the sodium counter-cations present in the interlayer galleries [10,25]. Sulfonate groups can also interact directly with interlayer exchangeable cations by ion dipole force, according to Beall and Goss [2]. They suggest that the main requirement for a molecule to ion–dipole bond to cations is that the molecules contain groups that carry a partial negative charge. As these bonds are relatively weak (i.e., 1–5 kcal/mol) and reversible [25], desorption of a fraction of the AMPS molecules may have possibly occurred during the washing process. The extent to which desorption of the clay modifier takes place during the washing process could have been determined by microcalorimetry, although this was not included in this specific study. 3.2. The organization of AMPS in the clay galleries Changes in the interlayer distance caused by ion-exchange of cationic surfactant, as determined by SAXS measurements,

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Fig. 3. Interlayer distances (d-spacing) of AMPS–MMT as a function of AMPS concentration.

have been reported [26]. Variation of the d-spacing has been previously reported as a function of the initial surfactant concentration [5]. The d-spacing is calculated according to Bragg’s law, namely d = 2π/q (where q is a wave vector and its value corresponds to the associated Bragg peak position) [9]. Fig. 3 shows the d-spacing calculated from the peaks obtained by SAXS of Na-MMT and AMPS–MMT at different AMPS concentrations. The basal spacing increased as a function of the AMPS concentration, and then reached a maximum of 1.82 nm in the case of AMPS concentrations of 75% CEC and greater. This is in accordance with SAXS results reported by Xu et al. [14], namely that AMPS successfully enlarged the d-spacing of clay, even though a slightly higher maximum interlayer dspacing of 2.1 nm was reached with a higher AMPS concentration limit of 100% CEC. This is most likely because they did not wash the modified clay until the washing water reached the original pH of the pristine clay suspension, so as to ensure no residual unbound AMPS was trapped in the galleries. The increase in d-spacing of AMPS–MMT relative to pure clay confirms the insertion of AMPS between clay platelets, and not only the presence of AMPS on the external surface of clay. Molecules of AMPS can interact with clay via both its amido and its sulfate groups. Simultaneously, amido groups can form hydrogen bonds with water molecules surrounding the inorganic exchangeable cations. This is in agreement with the findings of Nasser et al. [27] after they investigated the adsorption of alachlor [2-chloro-2,6diethyl-N -(methoxymethyl)acetanilide]. Their results showed that alachlor interacts with clay by formation of hydrogen bonds between its amido groups and the interlayer water molecules. Mortland [12] showed that urea molecules can bind to exchangeable cations via water molecule bridges, via hydrogen bonds. On the other hand, the sulfate groups of AMPS can form ion–dipole interactions directly with the interlayer exchangeable cations. This is in accordance with the findings of Beall and Goss [2], who suggest that organic molecules that contain groups that possess partial negative charges can ion–dipole bond to the interlayer exchangeable cations. Surprisingly, the interlayer distance was found to be larger for 10% CEC than for 25% CEC. This kind of behavior has also been reported for clay modification using ion-exchangeable or-

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Fig. 4. SAXS patterns of AMPS–MMT samples with different AMPS concentrations: (a) 0% (unmodified Na-MMT), (b) 10%, (c) 25%, (d) 50%, (e) 75%, (f) 100%, (g) 130% AMPS, relative to CEC.

ganic species [5], where partial surface modification decreases the affinity of clay for water, subsequently reducing the quantity of water hydrating the internal surface of the galleries, even in a dry state. The interlayer distance was found to increase with increasing AMPS concentrations up to 100% CEC, whereas no further increase in d-spacing was observed for AMPS loadings higher than 100% CEC. This is in good agreement with the TGA results, which indicated that there is a limit in terms of the amount of AMPS remaining adsorbed on the clay surface after washing, i.e., the clay surface reached a maximum coverage by AMPS when the concentration of AMPS was around 75–100% CEC. The change in interlayer distance as a function of AMPS concentration could result from a change in the conformation of AMPS molecules inside the clay galleries, as has been reported previously for other organic modifiers, and it depends on the molecules’ chemical structure, chain length, concentration and temperature [9,28]. The chains of the organic modifiers are thought to lie either parallel to the host layers, forming monoor bi-layers, or radiate away from the surface, forming extended (paraffin-type) mono- or bi-molecular arrangements [4,9]. As mentioned, the objective of the present study is to determine what is the driving force leading to the adsorption of AMPS into clay, rather than the way it organizes on the surface of clay. Although beyond the scope of the present paper, it is worth mentioning that AMPS molecules probably also organize in different ways in the clay galleries, depending on their concentrations. The molecules of AMPS probably arrange as a monolayer at low concentrations. A bilayer arrangement with AMPS molecules aligned parallel to the clay surface could be formed in the case of intermediate concentrations, while AMPS most likely arranges as a paraffin-type monolayer or bilayer in the case of high concentrations [28]. The increase in d-spacing from 75 to 100% CEC is probably due to a transition from a flat bilayer-type organization to a paraffin-type organization. This is further supported by the changes in the shape of SAXS diffraction peaks with changing AMPS concentrations (cf. Fig. 4). It is known that on XRD diffractograms a narrow peak indicates that clay platelets are regularly packed while a broad peak show heterogeneity of packing, with a relatively undefined d-spacing [29]. In other words, this means that the relatively

Fig. 5. FT-IR spectra of (i) pristine Na-MMT, (ii) neat AMPS, and (iii) AMPS– MMT.

narrow peaks observed in Fig. 4, for 25, 50 and 130% CEC indicate a rather regular packing of AMPS on the clay surface, namely a flat mono-layer (c), a flat bi-layer (d) and a paraffintype mono or bilayer (h), respectively. The broad peaks in Fig. 4 for intermediate concentrations of AMPS (f and g) show a relative disorder existing during the transition between the flatand paraffin-types of packing. These assumptions are, however, only based on basal spacings observed by SAXS, and on reports of findings of previous studies about the organization of organic molecules on clay surfaces. Further correlation with geometric models is beyond the scope of the present contribution, although it is necessary to confirm SAXS observations. 3.3. Characterization of AMPS–MMT by FT-IR FT-IR spectra of neat AMPS, neat Na-MMT and Na-MMT modified with AMPS were recorded and are compared in Fig. 5. The FT-IR spectrum of Na-MMT shows absorption bands at 3432, 3630 and 1640 cm−1 (see Table 2), corresponding to the OH stretching of the clay silicate layers [14]. The narrow band at 1043 cm−1 is related to the stretching of Si–O bonds of the silicate layer [14,30]. The band at 620 cm−1 is


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Table 2 FT-IR wave number of main bands for pristine Na-MMT, neat AMPS, and AMPS–MMT Assigned groups

Na-MMT wavelength (cm−1 )

Al–O Si–O N–H C–H S=O O–H

620 526, 463, 1043

AMPS wavelength (cm−1 )

3235, 1542 2950, 2848 1372 1640, 3432, 3630

AMPS–MMT wavelength (cm−1 ) 620 520, 460, 1040 1543, 3250 2850, 2947 1372 3434, 3631, 1650

Fig. 7. X-ray traces for AMPS–MMT prepared under different pH conditions.

Fig. 6. Thermal gravimetric curves of (a) Na-MMT, and AMPS–MMT at different pH values: (b) 0.63, (c) 1.81, (d) 2.8, (e) 3.5 and (f) 6.0.

related to the Al–O of the silicate layers. The appearance of new bands in the FT-IR spectrum of clay modified by AMPS indicates the presence of AMPS in the clay. The bands at 2947 and 2850 cm−1 are related to C–H stretching of AMPS molecules. Other new bands in the FT-IR spectrum of AMPS–MMT can be seen at: 1540 cm−1 , corresponding to NH bending, and 1372 cm−1 , related to S=O of AMPS. The FT-IR spectrum of AMPS–MMT was found to be similar to spectra reported in the literature [14,31]. It is difficult to determine the type of interaction occurring between clay and AMPS by FT-IR. Only a small amount of AMPS is present in the clay galleries [14,32] since the excess was removed by washing. Furthermore, Na-MMT itself has a complicated FT-IR spectrum. Consequently, overlapping between bands originating from both clay and AMPS makes it very difficult to detect characteristic bands of AMPS in the presence of clay [10]. However, FT-IR at least confirms the presence of AMPS in clay after washing AMPS–MMT four to five times. 3.4. pH dependence of Na-MMT surface modification using AMPS The treatment of the clay surface using a fixed amount of AMPS was also carried out at different pH, namely 0.63, 1.18, 2.8, 3.5 and 6.0. According to Avena and De Pauli [33], at low pH and low ionic strength Na-MMT ion-exchanges its surface cations by protons and is converted into H-MMT. However, the objective

here was to determine the ability of AMPS to ion-exchange with interlayer cations, irrespective of what the interlayer cation is, i.e., H+ or Na+ . The amount of AMPS successfully inserted inside the clay galleries was determined using TGA (cf. Fig. 6). The weight loss for neat clay observed at 105 ◦ C refers to the loss of the water present inside the galleries [14]. The difference in the weight loss found at 600 ◦ C for organically modified clays relative to the pristine clay was due to the amount of AMPS successfully adsorbed inside the clay galleries. Clearly, the amount of AMPS inserted into clay galleries was the same, irrespective of the pH conditions. This was confirmed by SAXS measurements (cf. Fig. 7). Hence no change in the basal spacing was observed by changing the pH at which AMPS–MMT was prepared. Once again, this demonstrates that AMPS does not interact with clay via the ion-exchange mechanism via a questionable zwitterionic form, as claimed by Xu et al. [14]. Deng et al. [34] studied the adsorption of anionic, non-ionic and cationic polyacrylamides on smectite clay as a function of pH and polyacrylamide concentration. The amido groups on nonionic polyacrylamide units can hydrolyze at high pH and form carboxylate groups. This generates negative charges which decrease the affinity of polyacrylamide for the negative charges of the clay surface. 3.5. The interaction between AMPS and clay via adsorption In order to understand which chemical group(s) of AMPS contribute actively to its interaction with the surface of clay, AMPS and selected compounds with similar chemical groups were compared with regards to their respective degrees of adsorption with the surface of Na-MMT clay. The compounds selected were Cops, NIPA and MET (cf. Scheme 1 for detailed structures). MET and Cops have a sulfate and a sulfonate group, respectively, but no amido group. This makes them good candidates for evaluating the ability of the SO3 H group to contribute to the interaction between AMPS and clay. On the other hand, NIPA has an amido group, but no sulfonate group, which makes it

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Scheme 1. Chemical structures of Cops, AMPS, NIPA and MET.

valuable to evaluate the contribution of the amido group with regards to interaction with clay. A comparison of the adsorption behavior of AMPS with that of Cops, MET and NIPA could contribute to clarifying whether the adsorption of AMPS is due to its sulfonate group, to its amido group, or to both groups. 3.6. FT-IR characterization of Cops–MMT, MET–MMT and NIPA–MMT The FT-IR spectra of Na-MET, MET, and MET–MMT are shown in Fig. 8A. In the spectrum of MET–MMT new strong bands appeared at 2930 and 2848 cm−1 , relative to the spectrum of pristine Na-MMT. These bands are related to the CH bonds of MET. Another new band appeared at 1714 cm−1 , due to the C=O group of MET. This provided proof that MET was incorporated inside the clay galleries. Furthermore, the band due to the O–H bond stretching in the silicate lattices was found to be shifted from its original wave number, i.e., it appeared at 3438 cm−1 for neat clay and 3425 cm−1 for MET-modified clay. This may be due to interactions between the hydroxyl groups of the clay octahedral sheets and the sulfate groups of MET, possibly through hydrogen bonding. However, as the hydroxyl groups of the octahedral sheets are inaccessible from the internal surface of the galleries, such hydrogen bonding is likely to occur only at the edges of the clay platelets. The FT-IR spectrum of Cops–MMT showed additional bands with respect to the FT-IR spectrum of neat Na-MMT. The band at 2920 cm−1 corresponds to the C–H bonds of Cops (cf. Fig. 8B). When the clay was modified by Cops the O–H band of silicate at 3436 cm−1 became broader, and shifted to 3424 cm−1 . This indicated that Cops molecules adsorbed onto the edges of the silicate layers probably in a similar manner to the way in which MET did. This is in agreement with the findings of Choi and Chung [35], who suggest that the adsorption of potassium persulfate (KPS) onto clay takes place via a similar type of interaction. The FT-IR spectrum of NIPA–MMT exhibited several new bands compared to neat Na-MMT. The strong band at 2920 cm−1 is due to the vibration of the C–H bonds of NIPA molecules (cf. Fig. 8C). The strong NH band of NIPA at 3300 cm−1 was shifted toward a lower wave number (i.e., 3237 cm−1 ) and became weaker and broader when NIPA was adsorbed on the clay. Similarly, the NH band at 3235 cm−1 observed with neat AMPS was found to weakened and broadened (cf. Fig. 5) on the spectrum of AMPS–MMT. This is due to the formation of hydrogen

Fig. 8. FT-IR spectra of the clay modified with different clay modifiers. (A) i–iii represents: Na-MMT, MET, and MET–MMT, (B) i–iii represents: Na-MMT, Cops and Cops–MMT, (C) i–iii represents: Na-MMT, NIPA, and NIPA–MMT, respectively.


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Fig. 9. TGA thermograms of (A) Na-MMT, (B) Cops–MMT, (C) NIPA–MMT, (D) MET–MMT, and (E) AMPS–MMT. The insert shows the thermal decomposition of dried (I) AMPS, (II) MET and (III) NIPA.

bonds between the NH of the amido groups of AMPS and NIPA with the hydroxyl groups at the clay edges. This is in accordance with the literature, where the adsorption of acetamide onto clay is reported to occur via hydrogen bonding between the NH of the amido group and the OH of the silicate, leading to a decrease in the NH stretching frequency relative to that of “free” amido groups [10,25]. The adsorption of polyacrylamide was investigated by Deng et al. [36]. They found that polyacrylamide adsorbed onto clay via its amido group by the formation of hydrogen bonds between the amido group and water molecules surrounding the interlayer cations, leading to shifts of FT-IR bands to lower wave numbers. Not all of the expected distinctive bands of Cops, MET and NIPA were found in the FT-IR spectra of the respective modified clays. However, the appearance of some bands specific to the organic modifier in the FT-IR spectra of the modified clays indicates that Cops, MET and NIPA do indeed interact with the clay. 3.7. The amounts of organic modifiers inside the clay galleries TGA was used to determine the quantities of the organic modifiers that successfully entered the clay galleries. Thermograms of unmodified clay and clay modified with the different organic modifiers are shown in Fig. 9, and values are summarized in Table 3. The quantities of organic modifiers relative to clay were calculated from the difference in the weight loss determined between unmodified clay and modified clay at 600 ◦ C. This was done in order to take into consideration the loss of adsorbed water, as was determined for the unmodified clay. The relative weight losses were found to vary depending on the type of organic modifier used. This variation could be due to the difference in molecular weight of the various organic modifiers, given that they were used in similar molar ratios (i.e., 100% CEC). As did the results obtained by FT-IR, TGA showed that Cops, MET and NIPA could also interact with clay surfaces,

Table 3 Adsorption of various organic modifiers on Na-MMT clay Organic modifiers

Na-MMT Cops–MMT AMPS–MMT MET–MMT NIPA–MMT

Molecular weight of organic modifier (g/mol)

d-spacing (nm)

N/A 223 207 342 113

1.12 1.45 1.82 1.64 1.56

Residual mass at 600 ◦ C (%)

Quantities of modifiers in clay galleries

4.03 5.86 8.60 7.95 7.00

0 1.8 4.4 3.8 2.9

Rel. to claya (wt%)

Rel. to CECb (mol%) 0 9 25 13 29

a Weigh percentage relative to clay. b Mole percentage relative to cation exchange capacity of clay.

as AMPS does. In the case of Cops and MET the interaction could occur via hydrogen bonding between hydroxyl groups of the clay edges and the SO3 Na group of Cops and MET. The SO3 Na group of Cops and MET could also interact with water molecules surrounding the exchangeable cations present on the surface of the clay galleries, or could also directly interact to interlayer exchangeable cations via ion–dipole interaction. Chisholm et al. [37] studied the effect of the sodium sulfonate functionalization of poly(butylene terephthalate) on the properties of nanocomposites. They found that the mechanical properties were improved at higher SO3 Na content, because a higher SO3 Na content lead to an increase in the number of interactions between clay and polymer via electrostatic interaction involving SO3 Na groups. Similarly to Cops and MET, NIPA might also interact with the clay in two ways: hydrogen bonds may form between the amido groups and hydroxyl groups of the clay edges, and/or the amido groups could form hydrogen bonds with water molecules coordinated to the interlayer cations [25]. The mol% (relative to CEC) of modifier adsorbed on the surface of clay galleries was found to be the highest for the amidogroup-containing organic modifiers (i.e., AMPS and NIPA). On the other hand, organic modifiers with sulfonate and sulfate groups (i.e., Cops and MET, respectively) were found to be less

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Fig. 10. SAXS patterns of (A) AMPS–MMT, (B) MET–MMT, (C) Cops–MMT, (D) Na-MMT, and (E) NIPA–MMT.

adsorbed, on a molar basis. This shows that although AMPS does undoubtedly interact with the clay surface via both its sulfonate group and its amido group, the amido group seems to contribute to AMPS adsorption more actively than the sulfonate group. Surprisingly, on a molar basis, NIPA was found to be slightly more adsorbed than AMPS. This could be due to the more hydrophobic character of NIPA relative to AMPS, allowing NIPA to better withstand the washing process. 3.8. Determination of d-spacing of organoclays by SAXS SAXS measurements were used to compare the basal spacing of four types of organoclays. Fig. 10 shows an overlay of the SAXS patterns of unmodified clay and clays modified with AMPS, MET, Cops and NIPA, respectively. As reported elsewhere [38], the interlayer distance was found to be affected by the chemical structure of the organic clay modifier. The significant increases in the d-spacing of modified clay, relative to that of pure clay, confirmed that the four different clay modifiers did indeed insert inside the clay galleries instead of only being adsorbed on the external surface [18]. The d-spacing depends on the organic modifier’s chemical structure, its packing and the amount readily adsorbed inside the clay galleries. Since none of the clay modifiers was adsorbed to the same extent, relative to each other, no conclusions could be drawn in terms of their respective types of packing or interaction within the clay galleries. However, there does appear to be a simple correlation between the relative masses of modifier adsorbed and an increase of the d-spacing, as shown in Fig. 11. 4. Conclusions The mechanism of the interaction between AMPS and clay was studied in depth. Results of this study do not support the ion-exchange mechanism suggested in the literature. The interaction of AMPS with clay occurs by adsorption on the edge of the clay platelets, by formation of hydrogen bonds between sulfates and amido groups of AMPS with hydroxyl groups from the clay octahedral sheet. This interaction has no effect on the basal spacing.

Fig. 11. Correlation between mass of organic modifier adsorbed and d-spacing observed.

AMPS can also adsorb onto the surface of the clay galleries in two ways: first, by formation of hydrogen bonds between the amido groups and water molecules surrounding the exchangeable cations and, second, by formation of ion–dipole interactions between the sulfonate groups and the interlayer exchangeable cations. These interactions lead to an increase in the basal spacing relative to pristine Na-MMT. According to TGA results the amido group of AMPS seems to contribute to AMPS adsorption more actively than the sulfonate group. The ability of these interactions to promote adsorption of AMPS onto the clay surface was confirmed by modifying Na-MMT using alternative organic modifiers having chemical groups similar to those of AMPS. Among these, Cops contained a sulfonate group, MET contained a sulfate group, and NIPA contained an amido group. These three compounds were readily adsorbed onto the surface of the clay galleries. Results showed how AMPS interacts favorably onto the surface of the clay galleries via both its amido group and its sulfonate group. Acknowledgments The authors are grateful to the Macromolecules and Materials Science Institute of Tripoli, Libya, for financial support, and to Mondi Packaging South Africa (MPSA) and the University of Stellenbosch for the use of their research facilities. References [1] [2] [3] [4] [5] [6]

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