Adsorption of Polyetheramines on Montmorillonite at High pH

Oct 21, 2010 - The steep decrease of mobility at very low Ceq is in agreement with the adsorption ... Solid lines indicate power laws: I ∝ q−2 and...
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Adsorption of Polyetheramines on Montmorillonite at High pH Yannan Cui and Jeroen S. van Duijneveldt* School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom Received August 17, 2010. Revised Manuscript Received October 5, 2010 Adsorption of a series of polyetheramines on montmorillonite in aqueous suspension was investigated by a range of methods: elemental analysis, atomic absorption spectroscopy, measurement of pH, conductivity and electrophoretic mobility, and small-angle X-ray scattering. Adsorption proceeds through an ion exchange mechanism. The maximum surface coverage attained is equivalent to about 40% of the cationic exchange capacity of the clay. Adsorption of the poly(oxypropylene) block adjacent to the amine group onto the clay surface may contribute to this. Surprisingly the adsorption takes place at pH conditions well above the pKa of the amine surfactants, where they are not protonated in the bulk solution. The surface coverage as a function of molar mass broadly agrees with predictions assuming adsorbed polymers adopt a densely packed mushroom configuration at the clay surface.

Introduction There is increasing interest in the preparation and properties of suspensions of nonspherical colloidal particles.1,2 An important class of platelike particles are clays which have many applications such as drilling fluids, paints and paper,3 and pharmaceutical and personal care products.4 Among the variety of clay minerals, montmorillonite (MMT) is a widely used smectite clay, with a large surface area, negative surface charge, and ability to swell in water.5 Interactions among clay particles are crucial to the dispersion stability. A gel tends to form at very low concentration in many aqueous clay suspensions, and the structure has been under debate for decades: a “house of cards” model caused by electrostatic attractions among platelets6 or a network formed by the double layer repulsions.7 Polymers and surfactants are usually added to clay dispersions to modify this behavior, to break the gel and improve stability, or conversely to induce flocculation. Due to the existence of silanol groups and negative charges on smectite surfaces, two kinds of polymers could adsorb onto clay surfaces, nonionic polar polymers and cationic polymers, changing the stability and rheological properties of clay suspensions.8-10 The most frequently used nonionic polymers are polar homopolymers and copolymers. It has been shown that poly(ethylene oxide) (PEO) polymers adsorb strongly to clay surfaces by hydrogen bonding with silanol groups on the clay surface.11,12 PEOPPO-PEO copolymers are also found to adsorb to clay surfaces, via hydrophobic interactions between the poly(oxypropylene) *To whom correspondence should be addressed. E-mail: J.S.van-Duijneveldt@ bristol.ac.uk. (1) Davidson, P.; Gabriel, J. C. P. Curr. Opin. Colloid Interface Sci. 2005, 9, 377–383. (2) Glotzer, S. C.; Solomon, M. J. Nat. Mater. 2007, 6, 557–562. (3) Grim, R. E. Clay mineralogy, 2nd ed.; McGraw-Hill: New York, 1968. (4) Cosgrove, T. Colloid science: principles, methods and applications; Blackwell: Oxford, 2005. (5) Velde, B. Introduction to Clay Minerals: Chemistry, origins, uses and environmental significance; Chapman & Hall: London, 1992. (6) Broughton, G.; Squires, L. J. Phys. Chem. 1936, 40, 1041–1053. (7) Hauser, E. A.; Reed, C. E. J. Phys. Chem. 1937, 41, 911–934. (8) Blachier, C.; Michot, L.; Bihannic, I.; Barres, O.; Jacquet, A.; Mosquet, M. J. Colloid Interface Sci. 2009, 336, 599–606. (9) Rossi, S.; Luckham, P. F.; Tadros, T. F. Colloids Surf., A 2002, 201, 85–100. (10) Rossi, S.; Luckham, P. F.; Tadros, T. F. Colloids Surf., A 2003, 215, 1–10. (11) Burchill, S.; Hall, P. L.; Harrison, R.; Hayes, M. H. B.; Langford, J. I.; Livingston, W. R.; Smedley, R. J.; Ross, D. K.; Tuck, J. J. Clay Miner. 1983, 18, 373–397. (12) Parfitt, R. L.; Greenland, D. J. Clay Miner. 1970, 8, 305.

17210 DOI: 10.1021/la103278v

oxide (PPO) chain and the clay particle.13 Cationic molecules also adsorb strongly to clay surfaces due to the negative surface charge, exchanging interlayer cations from the clay surface. The typical organic cations used in clay dispersions are amine salts or quaternary ammonium salts, with a hydrocarbon chain.14,15 Hexadecyl trimethylammonium,16 dimethyl dioctadecylammonium bromide,17,18 dodecylbenzene sulfonate, and cetylpyridinium bromide19 adsorption on clay surfaces have been investigated and they render the clay particles hydrophobic. They can adsorb to clay surfaces stoichiometrically, and further adsorption giving a bilayer structure can also occur.17 Cationic polymers with a long PEO chain have also been investigated and behave differently from a small cationic molecule with alkyl chains;20,21 instead of adsorbing stiochometrically, a lower coverage is obtained. Polyelectrolytes such as polyethyleneimine22 and polycationic quaternary amine8 are used to flocculate clay suspensions via strong electrostatic interactions. Molecules with a primary amine are known to adsorb to clay surfaces at pH values below their pKa. Laura and Cloos investigated ethylenediamine adsorption on MMT.23 Lin and co-workers found that the intercalation of grafted PPO/PEO diamine or polyamine salt between clay platelets increases the layer spacing dramatically.24-27 A noncovalent bonding mechanism without ionic exchange for the intercalation in poly(oxyalkylene)-amido acids into MMT was also proposed.28 Wang et al. used acidified (13) Nelson, A.; Cosgrove, T. Langmuir 2005, 21, 9176–9182. (14) Van Olphen, H. An introduction to clay colloid chemistry: for clay technologists, geologists, and soil scientists; 2nd ed.; Interscience: New York, 1977. (15) Theng, B. K. G. The chemistry of clay-organic reactions; Wiley: New York, 1974. (16) Xu, S. H.; Boyd, S. A. Langmuir 1995, 11, 2508–2514. (17) Leach, E. S. H.; Hopkinson, A.; Franklin, K.; van Duijneveldt, J. S. Langmuir 2005, 21, 3821–3830. (18) Zhang, Z. X.; van Duijneveldt, J. S. J. Chem. Phys. 2006, 124, 154910. (19) Atia, A. A.; Farag, F. M.; Youssef, A. E. M. Colloids Surf., A 2006, 278, 74–80. (20) Lagaly, G.; Ziesmer, S. Clay Miner. 2007, 42, 255–269. (21) Lagaly, G.; Ziesmer, S. Clay Miner. 2005, 40, 523–536. (22) Alemdar, A.; O. ztekin, N.; G€ung€or, N.; Ece, O. . I.; Erim, F. B. Colloids Surf., A 2005, 252, 95–98. (23) Laura, R. D.; Cloos, P. Clays Clay Miner. 1975, 23, 61–69. (24) Lin, J. J.; Cheng, I. J.; Wang, R. C.; Lee, R. J. Macromolecules 2001, 34, 8832–8834. (25) Lin, J. J.; Hsu, Y. C.; Wei, K. L. Macromolecules 2007, 40, 1579–1584. (26) Chou, C. C.; Shieu, F. S.; Lin, J. J. Macromolecules 2003, 36, 2187–2189. (27) Lin, J. J.; Chen, Y. M.; Yu, M. H. Colloids Surf., A 2007, 302, 162–167. (28) Liao, Y. L.; Chiu, C. W.; Lin, J. J. Ind. Eng. Chem. Res. 2010, 49, 5001– 5005.

Published on Web 10/21/2010

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Article Table 1. Summary of Polymer Properties

polymer Jeffamine M1000 Jeffamine M2070 Jeffamine ED900 PEO 1000 Pluronic L31

Mw (g/mol) amine groups per molecule EO number PO number pKa primary amine content % min water content % max 1041 1975 972 1000 1100

1 1 2 0 0

19 31 12.5 22.7 4

poly(oxypropylene)diamines to render Laponite particles hydrophobic and applied the organo-clays in Pickering emulsions.29 McLauchlin and Thomas also investigated the adsorption of cocamidopropyl betaine on Naþ-MMT as a function of the pH (from 3 to 6) of the system and found an increased layer spacing at pH values between 4 and 6.30 In all previous studies, the amines were acidified to a pH below their pKa to make sure they were protonated before the adsorption. In the current study, it was found that the primary amines studied here adsorb strongly even at a pH values above the pKa, providing a simpler route to modifying clay surfaces with polymer. Adsorption of a series of polyetheramines on MMT was investigated in detail, and the adsorption mechanism was also studied by comparing the adsorption of nonionic and cationic molecules. A wide range of characterization methods were used to elucidate the adsorption mechanism: elemental analysis, atomic absorption spectroscopy, measurement of pH, conductivity and electrophoretic mobility, and small-angle X-ray scattering.

Experimental Section 1. Clay and Clay Suspensions. A dioctahedral smectite clay, montmorillonite (MMT, Wyoming SWy2), is studied in this study. The composition of SWy2 has been reported as (Si7.94Al0.06)(Al2.88Fe0.5Mg0.62)O20(OH)4Na0.68.31 Montmorillonite is platelike, with particle diameters of 200-2000 nm and an individual platelet thickness of 1 nm. The montmorillonite was purchased from the Clay Minerals Society, source clays repository at Purdue University. MMT clay was dispersed in water for 24 h to make 45 g/L aqueous dispersions. Dispersions were dialyzed in 1 M NaCl (Fisher Scientific, Laboratory grade) for 1 week, changing for a fresh NaCl solution daily. The dispersions were then dialyzed against deionized water (Elga, Ultra Ionic) for 1 week, changing water every day. The dispersions were centrifuged at 2000 rpm to eliminate large particles. A Sorvall RC-5B Plus centrifuge was used to fractionate the dispersions at 7000 rpm for 20 min. The supernatant was collected and kept as stock solution. The solid content was measured by drying at 100 C for at least 5 h until the weight remained constant. 2. Chemicals. Jeffamines (Huntsman, Belgium) contain primary amino groups attached to the end of a polyether backbone. The block copolymer backbone is usually composed of propylene oxide (PO), ethylene oxide (EO), or a PO/EO mixture. The product information was obtained from Huntsman. Jeffamine M1000 and M2070 are hydrophilic polyether monoamines. The typical chemical formula of M1000 is CH3[OCH2CH2]x[OCH2(CH3)CH]yNH2, in which x = 19, y = 3. The formula of M2070 is CH3[OCH2CH2]x[OCH2(R)CH]yNH2, in which R = H or CH3, x = 6, y = 35, and in the whole molecule the (PO/EO) mol ratio =10/31. Jeffamine ED-900 is a polyetherdiamine with the formula of H 2 N(CH 3 )CHCH 2 [OCH 2 (CH3)CH]x[OCH2CH2]y[OCH2(CH3)CH]zNH2, in which y = 12.5, (29) Wang, J.; Liu, G. P.; Wang, L. Y.; Li, C. F.; Xu, J. A.; Sun, D. J. Colloids Surf., A 2010, 353, 117–124. (30) McLauchlin, A. R.; Thomas, N. L. J. Colloid Interface Sci. 2008, 321, 39–43. (31) Michot, L. J.; Bihannic, I.; Porsch, K.; Maddi, S.; Baravian, C.; Mougel, J.; Levitz, P. Langmuir 2004, 20, 10829–10837.

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3 10 6 0 16

9.4 9.6 9.5

90 95 95

0.25 0.25 0.35

and x þ z = 6. Two nonionic polymers are also used for comparison, PEO and Pluronic L31. The formula of Pluronic L31 is H[OCH2CH2]2[OCH2(CH3)CH]16[OCH2CH2]2H. Potassium hydrogen phthalate (KHP, Fisher Scientific) was used as received. Tetrabutylammonium hydroxide (TAH, Acros Organics) comes as a 40 wt % solution in water. See Table 1 for a summary of polymer properties. 3. Adsorption Isotherms. Aqueous polymer stock solutions were prepared and then mixed with MMT stock suspension and deionized water to prepare 0.2 wt % MMT suspensions with different polymer concentrations. The samples were sealed in glass vials and rotated end-over-end for 24 h to allow the adsorption to reach equilibrium. The samples were then centrifuged at 10 000 rpm for 30 min, and the sediment was collected and redispersed into deionized water. This washing procedure was repeated twice. The final sediment was dried at 100 C for 5 h until the weight remained constant. The carbon, nitrogen, and hydrogen content in the dried clay-polymer composite was analyzed using a Euro EA3000 elemental analyzer. The adsorbed amount was calculated according to the carbon content in the clay-polymer composite and in pure polymer. Data were corrected for the carbon content obtained for the untreated clay (0.07%). The cationic exchange capacity (CEC) of the clay was measured by the adsorption of cationic surfactant TAH on MMT.14 The theoretical wet surface area of MMT was calculated according to the density (2.7 g/mL) and thickness (1 nm) of MMT platelets as 760 m2/g. 4. Titration of Jeffamines. The Jeffamines are commercial products and contain a small amount of water. To determine the exact primary amine content in the Jeffamines and the pKa of the amine groups, 50 mL of 0.01 M M1000 solution was titrated against 0.01 M potassium hydrogen phthalate (KHP) solution, which is normally used in standardizing sodium hydroxide solutions. M2070 (50 mL, 0.01 M) and (50 mL, 0.01 M ED900) were also titrated against 0.01 M KHP. A Mettler Toledo FE20/EL20 pH meter was used to measure the pH values. 5. Released Ion Measurement. The adsorption process can involve the release of ions. Identical samples as for the adsorption isotherm determination were prepared, and half of each sample was centrifuged at 10 000 rpm for 30 min to sediment all particles and the supernatant was collected and kept. The pH of the original suspension and supernatant was measured. The released hydroxyl concentration was calculated considering the hydroxyl ion concentration contribution from pure clay (pH 7.26 in 0.2 wt % clay suspension) and pure surfactant (pKa around 9.5). The hydroxyl contribution from clay particles is negligible for the hydroxyl concentration in samples with pH above 10. Using the pKa values of pure surfactants, the hydroxyl contribution from free amine could be calculated at a given pH value of the system. The hydroxyl concentration contributed by the adsorption process was calculated by subtracting the contribution of pure free surfactant from the measured total hydroxyl concentration. The sodium ion content in the supernatant was measured using a UNICAM 919 atomic absorption spectrometer (AAS). Due to possible residual small particles in the supernatant, the sodium ion concentration in pure clay supernatant was also measured and subtracted from the measured values. The conductivity of the supernatant was determined by using a Jenway 4510 conductivity meter. The conductivity in the supernatant is attributed to the DOI: 10.1021/la103278v

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Figure 1. Experimental and theoretical titration curves of Jeffamines against KHP: (b) M1000, (2) M2070, and (9) ED900. Lines are the theoretical titration curves.

Figure 2. Adsorption isotherm of TAH on MMT.

released Naþ and OH- ions. The molar ionic conductivities of Naþ and OH- are 5.00 and 19.91 mS m2 mol-1, respectively.32 6. Electrophoresis. The electrophoretic mobilities of 0.02 wt % clay suspensions with different polymer concentrations were measured using a Malvern Instruments Zetasizer Nano Z. 7. Small Angle X-ray Scattering (SAXS). SAXS was used to measure the interparticle distance in both MMT suspensions and dry powders. MMT (1 wt %) aqueous suspensions were submitted to synchrotron X-ray scattering at Diamond Light Source, beamline I22, with scattering vectors ranging from 0.004 to 0.4 A˚-1. MMT (0.2 wt %) suspensions with different M1000 loadings were centrifuged, and the sediment was dried. The powder obtained was analyzed using home-built SAXS facilities in the Physics Department, University of Bristol, using a sealedtube Cu KR source with a wavelength of 1.54 A˚. The scattering pattern covered scattering vectors ranging from 0.01 to 0.5 A˚ -1.

Results and Discussion

Figure 3. Adsorption isotherms of polymers.

concentration Cb, hydrogen concentration [Hþ], and water dissociation constant Kw. Similarly, the concentration of the anion from the weak acid can be calculated from Ka, acid concentration Ca, and [Hþ]. The charge balance equation is used to relate the two expressions, and the final form of the equation is33

1. Titration of Jeffamines. According to the experimental titration data of Jeffamines, the stoichiometric points in the titration experiments of 50 mL of 0.01 M solutions of M1000, M2070 and ED900, are at 48, 45, and 98 mL of 0.01 M KHP solution, respectively. The pKa value of a weak base is equal to the pH after 0.5 mol equiv of monobasic acid has been added. pKa values of the primary amines are 9.6, 9.6, and 9.5 for M1000, M2070, and ED900, respectively. The measured primary amine contents in the Jeffamines are 0.96 mol for 1041 g of M1000, 0.90 mol for 1975 g of M2070, and 1.96 mol for 972 g of ED900. The primary amine content in M1000 and ED900 is very close to the theoretical value, but in M2070 it appears that either 10% of the molecules do not have amine end groups or the Mw of M2070 is 11% higher than that reported by the supplier. In the following calculations, the latter was assumed. No further experiments were carried out to determine the exact molecular weight of the Jeffamines. The samples have pH values ranging from 8.5 to 11; Jeffamines are only partially protonated in this range. KHP is a weak acid and has a pKa of 5.4 at 20 C. The theoretical titration curves of KHP titrating Jeffamines were calculated considering the disassociation equilibrium of weak acid and weak base. The concentration of the cation dissociated from the weak alkali can be expressed in the form of Kb, base

To simplify the calculation, the concentration of the acid and base can be expressed as a function of the volume of the added acid. Equation 1 can then be rearranged to express the volume of added acid as a function of pH, which can be used to plot the theoretical titration curves. The experimental titration curves were then overlaid with theoretical titration curves, as shown in Figure 1. The experimental values are consistent with the calculated theoretical curves. 2. Cationic Exchange Capacity. The isotherm was plotted as the adsorbed amount, Γ, against equilibrium concentration Ceq, which is the concentration of the free TAH after adsorption, calculated by subtracting the adsorbed amount of TAH from the total added amount (Figure 2). TAH has one positive charge per molecule and adsorbs to clay stoichiometrically and hence is used

(32) Wright, M. R. An introduction to aqueous electrolyte solutions; Wiley: Chichester, 2007.

(33) Kealey, D.; Haines, P. J. Analytical chemistry: BIOS Scientific Publisher Limited: Oxford, 2002.

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Ca Ka Ka Kb Cb ½Hþ  þ þ ¼ þ ½Hþ  þ ½H  þ Ka ½H  Kw þ Kw ½Hþ 

ð1Þ

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Figure 4. Released ions upon polymer adsorption on MMT. (a) Released sodium ions. (b) Released hydroxyl ions; solid symbols stand for the OH- released from adsorption process, and open symbols for the total OH- in the suspension. (c) Conductance of supernatant; solid symbols stand for the experimental data, and open symbols for the data calculated from sodium and hydroxyl concentrations. (d) pH of samples at different polymer loadings. For graphs from (a) to (d), the symbols are the same as in Figure 3. (e) Released ion concentrations against adsorbed amount for M1000 adsorption.

to determine the cationic exchange capacity (CEC) of clays.17 The adsorbed amount of TAH is 84 mequiv/100 g. This agrees reasonably well with the previously calculated value of 91 mequiv/ 100 g for the same clay.14 In this study, each sample has 0.2 wt % MMT and the concentration of ions can be normalized to the CEC, with 1 CEC being equivalent to 1.68 mmol/L at this clay concentration. Langmuir 2010, 26(22), 17210–17217

3. Adsorption Process. Adsorption isotherms are shown in Figure 3. The adsorbed amount Γ is expressed as mmol polymer per gram of clay. All the adsorption isotherms show a high affinity for the clay surface, with a steep increase at very low Ceq and a gradual increase at high Ceq. Graphs of released sodium and hydroxyl ion concentrations after adsorption of polymers onto MMT are shown in Figure 4a and b. The released ion concentration DOI: 10.1021/la103278v

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is expressed as normalized to the CEC. The plots of released ion concentration against Ceq for Jeffamine adsorption have a similar shape as the adsorption isotherms. There is a steep increase of ion concentration at very low Ceq and a slow increase at Ceq > 0.2 mmol/L. The released ion concentration upon PEO and L31 adsorption is very low compared to that following Jeffamine adsorption. The expected conductance was calculated from the measured sodium and hydroxyl ion concentrations and compared to the experimental value measured by conductivity meter in Figure 4c. For all samples, the experimental conductance corresponds very well to the calculated value, proving the main ions released in the adsorption process are sodium and hydroxyl. Cationic surfactant TAH adsorbs to the clay surface stoichiometically, and the plateau adsorbed amount at Ceq=1.0 mmol/L is 84 mequiv/100 g, referred to as 1 CEC. Jeffamine adsorption is different from that of TAH in that the equilibrium adsorbed amount of Jeffamine is much lower than that of TAH. The adsorption isotherms are shown in Figure 3. The adsorbed amount of M1000 at Ceq = 1.5 mmol/L is 0.34 mmol/g clay, which can be expressed as 0.40 CEC. This adsorbed amount is less than half of the stoichiometric adsorbed amount based on ion exchange. Therefore, each M1000 molecule occupies 2.5 ion exchange sites at equilibrium. In the meantime, according to Figure 4e, each M1000 molecule only releases 0.7 sodium ions. This suggests that the adsorption of M1000 involves more than just an ion exchange mechanism alone. The possibilities include (1) hydrogen bonding between EO and silanols occupying the rest of exchangeable sites, or (2) hydrophobic interaction between PO and clay attaching to clay surface. Lagaly and Ziesmer investigated the adsorption of cationic end-capped PEO polymer on a MMT (CEC 1.09 mmol/g) surface, and the adsorbed amounts of the low Mw polymers were 0.52 mmol/g (0.47 CEC) and 0.47 mmol/g (0.43 CEC) for Mw’s of 1575 and 1923 g/mol,20 respectively, similar to M1000 adsorption (0.40 CEC). Also, the adsorbed amount (in mmol/g) decreases with increasing PEO chain length,20 which is probably a result of the steric hindrance between long PEO chains of the adsorbed molecules. This is discussed in detail below. 3.1. Comparison of M1000 and Nonionic Surfactants. The released sodium and hydroxyl ion concentrations are a significant fraction of the adsorbed amount for Jeffamine M1000 adsorption. On the contrary, very few ions were released upon adsorption of PEO or Pluronic. Adsorption of PEO is known to occur via hydrogen bonding.11,12 The present results indeed show very few ions are released in this adsorption process and ion exchange does not happen. It has been suggested that Pluronic adsorption proceeds via hydrophobic interactions between propylene oxide chains and clay.13 The results here show no ions are exchanged or released from this process either. The very small amount of sodium ions seen might be due to residual clay particles in the supernatant solution for AAS. Therefore, the released ions are from the adsorption of the amine end group onto the clay surface rather than from PEO or PPO block adsorption. The ion exchange mechanism can be expressed as MMT - - Naþ þ R- NH2 þ H2 O f MMT - - NH3 þ - R þ Naþ þ OH 3.2. Effect of PO Chain Length. Adsorption of Jeffamines M1000 and M2070 is examined; both of them have a similar structure and one amine group in each molecule. The Mw and PO number of M2070 are both twice those of M1000. The release of 17214 DOI: 10.1021/la103278v

both sodium and hydroxyl ions is similar to the adsorbed amount, indicating an ion exchange mechanism in the adsorption process. At high Ceq, 1.5 mmol/L, for example, the adsorbed amount of M1000 (0.34 mmol/g), is a little higher than that of M2070 (0.29 mmol/g), so each M2070 molecule occupies more surface area than the M1000 molecule does. The specific area of clay in this study is 760 m2/g, so the area per M1000 molecule is 2.0 nm2 and the area per M2070 molecule is 2.35 nm2. In this study, the average area of each charge is equivalent to 0.83 nm2, which is less than half of the actual area occupied by each adsorbed Jeffamine molecule. M1000 has three PO units connected to the NH2 head, and the PPO block is more hydrophobic compared to the PEO block, so it is possible that the PPO block adsorbs as well. 3.3. Effect of Amine Group Numbers. Adsorption isotherms of Jeffamines M1000 and ED900 are shown in Figure 3. Both polymers have a similar Mw, but the latter has two amine groups in each molecule. The adsorption isotherms are similar at Ceq < 0.3 mmol/L. At higher Ceq, 1.5 mmol/L, for example, the adsorbed amount of M1000 (0.34 mmol/g), is higher than that of ED900 (0.30 mmol/g). Each adsorbed M1000 molecule can adsorb only to one ion exchange site, but each ED900 could adsorb on up to two ion exchange sites via an ion exchange mechanism. Theoretically, ED900 can adsorb at both ends, and if it does, the adsorbed amount should be close to half the adsorbed amount of M1000. In practice the coverage is 0.88 times the adsorbed amount of M1000. The released sodium ion concentration at Ceq = 1.5 mmol/L for ED900 adsorption (0.36 CEC) is 20% more than that for M1000 adsorption (0.30 CEC). Both observations are consistent with a small fraction (around 20%) of ED900 molecules adsorbing onto the clay surface with both amine end groups. The MMT-M1000 samples settled out after standing overnight and could be redispersed while the MMT-ED900 samples aggregate, possibly due to a bridging effect between clay particles as a result of the amine groups adsorbing to two different particles. According to the ion exchange mechanism, one sodium ion and one hydroxyl ion were released stiochiometrically upon adsorption of each amine group. However, the hydroxyl concentration in ED900 at Ceq =1.5 mmol/L is about 0.59 CEC, which is almost twice the released sodium ion concentration. This might be explained by the fact that each ED900 molecule has two amine groups and it is difficult to determine whether both of them adsorb or only one of them adsorbs. If both amine groups adsorb, the released sodium and hydroxyl concentrations should be similar, because the pKa value of ED900 is 9.5 and free ED900 molecules are not highly protonated at pH 11 so very few hydroxyl ions are from free ED900 protonation. However, if only one amine group adsorbs, the MMT-ED900 composite might change the pKa of the other amine group, which thus might be protonated even at high pH. 3.4. Relation of Released Ion Concentration and Adsorbed Amount. Figure 4e shows the released ion concentrations plotted against the adsorbed amount for M1000 adsorption, both normalized to CEC. If each adsorbed M1000 molecule exchanges one sodium ion from the clay surface, the plot would have a gradient of 1. In practice, the gradient is 0.7 in this study, which suggests 70% of the adsorbed M1000 adsorbs via an ion exchange mechanism. The rest of the adsorbed M1000 molecules could adsorb in other ways as mentioned before, via adsorption of PEO or PPO blocks. At adsorbed amounts higher than 0.38 CEC, the plot reaches a plateau and no further ions are released while the adsorbed amount increases to 0.43 CEC. 3.5. Effect of Temperature and Salt Concentration. To investigate the effect of solvent quality on adsorption, temperature Langmuir 2010, 26(22), 17210–17217

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Figure 5. Variation of electrophoretic mobility with polymer adsorption. Symbols are as in Figure 3.

and salt concentration were varied. This approach was chosen following Hariharan and Russel,34 who investigated the adsorption of poly((dimethylamino)ethylmethacrylate)/poly(n-butylmethacrylate) onto silica particles and found the adsorbed amount is higher in θ solvent than in good solvent. Experiments at room temperature and 90 C show similar adsorbed amounts and released ion concentrations. The salt concentration was varied from 0.001 to 0.01 M, which did not affect the adsorption either. We could not demonstrate such an effect of solvent quality on adsorption. However, we need to bear in mind that the Mw of the polymer in this study is quite low, so the change of temperature or salt concentration may have only a limited effect on these polymers. 4. Electrophoretic Mobility. The electrophoretic mobility of samples was determined. At a NaCl concentration of 0.001 and 0.01 M, no obvious change in the mobility could be detected for any of the samples and the value of the mobility fluctuated between -2.9 and -3.1  10-8 m2/(V s). However, at 0.1 M NaCl, the mobility became less negative with increasing polymer adsorption. To understand this, it is instructive to compare the thickness of the adsorbed layer and the double layer thickness (Debye length). The thickness of the adsorbed polymer is likely to be similar to the radius of gyration RG ≈ 1 nm, which can be estimated from scaling theory.4 The Debye length decreases with increasing salt concentration and is about 1 nm at 0.1 M NaCl. At this point, the mobility starts to be sensitive to the adsorbed amount. The results are shown in Figure 5. All the mobility plots have a steep increase in the Ceq range of 0-0.08 mmol/L. At Ceq > 0.1 mmol/L, the mobility of all samples increases slowly. At the highest Ceq, the mobility of the clay is still negative. With regard to the absolute value of the mobility, at high Ceq, 2 mmol/L, for example, Jeffamine M2070 samples have the lowest mobility of about -2  10-8 m2/(V s), M1000 and ED900 have a similar mobility of about -2.3  10-8 m2/(V s), Pluronic L31 has a higher mobility of -2.65  10-8 m2/(V s), and PEO has the highest at -2.85  10-8 m2/(V s). As shown in Figure 5, the shape of all the mobility versus Ceq plots is similar to the shape of the adsorption isotherms in Figure 3, indicating the progress of adsorption. The steep decrease of mobility at very low Ceq is in agreement with the adsorption isotherms, indicating a sudden increase of adsorbed amount. At higher Ceq, the mobility decreases slowly, indicating adsorption equilibrium. (34) Hariharan, R.; Russel, W. B. Langmuir 1998, 14, 7104–7111.

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Figure 6. X-ray scattering results of M1000 samples. (a) Intensity against scattering vector: (O) 1 wt % MMT aqueous suspension and (4) 1 wt % MMT suspension with 1 CEC M1000 loading. To make it easy to read, this plot is shifted up by multiplying all data by 10. (b) Intensity against scattering vector of powder samples. The inset lists the M1000 coverage on the particles expressed as a multiple of the CEC. Successive curves are shifted by 1 order of magnitude for clarity. Solid lines indicate power laws: I  q-2 and I  q-3. (c) d-spacing against M1000 loading of MMT powder samples.

The electrophoretic mobility reflects the mobility of ions at the shear plane, which can be affected by distortion of the ion atmosphere and polymer adsorption.35 In the case of nonionic polymers (35) Mpandou, A.; Siffert, B. Colloids Surf. 1987, 24, 159–172.

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Cui and van Duijneveldt Table 2. Summary of Adsorbed Amount of Different Polyetheramines on Smectite Clay Surfaces adsorbed amount

ref

CEC of clay (mmol/g)

polymer

present work

0.84

M1000 M2070 ED900 PEO 1000 Pluronic L31

21

1.09

TMA-PEO1500 TMA-PEO4000 TMA-PEO20000 TMA-PEO35000

29

0.75

13

0.75

Mw (g/mol)

amine no.

EO no.

1041 1975 972 1000 1100

1 1 2 0 0

19 31 12.5 22.7 4

1575 4414 17985 37113

2 2 2 2

30 95 400 838

D230 D400

430 230

2 2

0 0

F108 F104 P103 P85 P84

14600 5900 3465 2300 4200

0 0 0 0 0

248 54 34 52 38

L31 and PEO, the mobility is affected by the neutral chain adsorption and does not involve exchanging sodium ions from the clay surface. The Jeffamines change the mobility more dramatically than nonionic polymers; this is probably caused by the ion exchange in the adsorption. Jeffamine adsorption involves exchanging 30-40% of sodium ions on the clay surface by protonated Jeffamine amine groups; this decreases the sodium ion atmosphere density in the shear plane, and also the shear plane shifts due to polymer adsorption. The combined effect of the low surface sodium ion concentration and the shear plane shift leads to the more dramatic drop of the mobility in Jeffamine adsorption. 5. Interparticle d-Spacings. The plots of synchrotron X-ray scattering intensity against scattering vector of 1 wt % MMT suspensions with and without M1000 are shown in Figure 6a. The I-q plot of the 1 wt % MMT aqueous suspension shows no peak in the whole range of detection, with q ranging from 0.004 to 0.4 A-1, indicating well dispersed MMT platelets. The plot of MMT with 1 CEC (equivalent to 0.84 mmol/g clay) M1000 looks similar to that of pure MMT. The curve deviates somewhat from the theoretical slope of -2 expected for flat particles, indicating weak aggregation.36 The fact that the scattering of 1 wt % MMT with 1 CEC M1000 is very similar indicates the addition of M1000 does not induce any aggregation and the platelets are still well dispersed. Therefore, no spacing could be determined in aqueous samples. In dried MMT samples, the slope of the scattering is about -3, which was also found by van Duijneveldt et al. and is indicative of the existence of aggregates of single disks or stacks.36 Also, a layered structure is found with a well-defined interparticle distance. This increases with the M1000 loading, as shown in Figure 6c. Air-dried MMT has a spacing of 13.5 nm in this study, corresponding to a monolayer of interlayer water. Previous studies8,23,37,38 show a basal spacing of pure MMT ranging from 10 to 13 nm, which is due to different origins of clay and different drying methods. The thickness of MMT without interlayer water is 0.96 nm, and the van der Waals diameter of a methyl group is about 0.4 nm.39 The spacing at low M1000 loading is 1.37 nm, which indicates a monolayer of PEO segments. At high M1000 (36) van Duijneveldt, J. S.; Klein, S.; Leach, E.; Pizzey, C.; Richardson, R. M. J. Phys.: Condens. Matter 2005, 17, 2255–2267. (37) Lagaly, G.; Ziesmer, S. Colloid Polym. Sci. 2006, 284, 947–956. (38) Theng, B. K. G. Clays Clay Miner. 1982, 30, 1–10. (39) Jordan, J. W. J. Phys. Colloid Chem. 1949, 53, 294–306.

17216 DOI: 10.1021/la103278v

PO no.

mmol/g

g/g

CEC

3 10 6 0 16

0.36 0.29 0.31 0.34 0.26

0.37 0.57 0.30 0.34 0.29

0.43 0.34 0.37 0.40 0.31

0 0 0 0

0.52 0.19 0.04 0.02

0.82 0.86 0.69 0.80

0.48 0.17 0.04 0.02

6.1 2.5

0.58 1.17

0.25 0.27

0.77 1.56

0.042 0.069 0.087 0.17 0.09

0.62 0.41 0.30 0.39 0.38

56 61 60 40 43

Figure 7. Adsorbed amount against Mw in different studies.

loading, the basal spacing reaches a plateau of 1.75 nm and bilayers of M1000 are formed between MMT particles. Similar values of spacings are also obtained in PEO adsorption, cationicend-capped PEO,12,21 and alkylammonium salt adsorption on MMT.39 Jordan mentioned the carbon number in the alkyl chain of alkylammonium salt affects the basal spacing of amine MMT compounds.39 Similarly, in this study, the molecules occupy less than half of the surface area at low M1000 loadings. In the process of drying, the molecules adsorbed on a clay particle can fit into the gap between adsorbed molecules on the adjacent particle, and the resulting basal spacing is the sum of thickness of dehydrated MMT and a single layer of molecule, 13.7 nm. At higher M1000 loadings, more than half of the MMT surface area is covered by PEO chains and a double layer of molecules is formed on approaching each other because they cannot fit into the gap. The basal spacing is the sum of thickness of MMT and two layers of molecules, which is 17.5 nm. 6. Mechanism of Jeffamine Adsorption. Adsorption of Jeffamines involves an ion exchange between the protonated amine group and a sodium ion and at the maximum adsorbed amount, about 40% of the exchangeable sites are occupied by amine groups, around 70% of which are protonated. At pH 10, only 31% of Jeffamine M1000 (pKa 9.5) is protonated in solution. However, in adsorption on MMT, almost Langmuir 2010, 26(22), 17210–17217

Cui and van Duijneveldt

Article

100% M1000 molecules were adsorbed at lower than 0.4 CEC polymer loading and almost every adsorbed M1000 molecule releases a sodium ion and a hydroxyl ion. However, ion exchange only happens to about 40% of all the available exchangeable sites. The reason for this might be the steric interaction between polymer chains, which stops other molecules from approaching the clay surface. It is also possible that PO segments adsorb onto the clay surface via hydrophobic interaction. To compare our data with previous studies, the adsorbed amount of various polyetheramine surfactant on smectite clay surfaces is listed in Table 2. In this study, the adsorbed amount of the Jeffamine polymers is about 0.30-0.60 mmol/g clay at high Ceq (2 mmol/L) and the average area for each molecule is 2.2-1.8 nm2 per molecule. Wang et al. studied the adsorption of two poly(oxypropylene) diamines on Laponite and at Ceq 0.2%, the adsorbed amount was 0.58 mmol/g clay (Mw 430) and 1.17 mmol/g clay (Mw 230), and the area per molecule was 2.23 and 1.11 nm2 per molecule.29 Lagaly and Ziesmer investigated the adsorption of cationic endcapped poly(ethylene oxide) on NaþMMT, for the low Mw polymer (Mw 1575), and the equilibrium adsorbed amount was 0.52 mmol/g clay, equivalent to 2.49 nm2 per molecule.20 Nelson and Cosgrove studied the adsorption of Pluronic polymers on Laponite, which yielded an adsorbed amount of 0.17 mmol/g for P85 (Mw 2300).13 The adsorbed amount of polymers with various structures and Mw values is difficult to compare when considered in units of g/g or mmol/g. In order to make the comparison easier, the adsorbed amount is expressed as a fraction of the CEC of the corresponding clay. Figure 7 illustrates the relation between Mw and adsorbed amount in CEC. The data points of all three studies related to amine adsorption on smectite clay surface follow a power law with an exponent of -0.83. The radius of gyration of a polymer chain scales as4 ÆRG æ  Mw

υ

ð2Þ

where υ = 0.5 in θ conditions and υ = 0.59 in good solvent. An M1000 molecule is 80% PEO, so we used the parameters for PEO homopolymer4 to estimate the RG of M1000, which is about 1 nm in θ solvent. The cross section of the polymer coil is thus about

Langmuir 2010, 26(22), 17210–17217

3 nm2. The average area per adsorbed M1000 molecule is 2 nm2, which is smaller than the cross section of a polymer coil. This indicates the adsorbed polymer might be arranged on the clay surface as polymer coils touching each other and interpenetrating slightly. In a dense brush, the area per molecule would be (much) smaller. By assuming such a “touching mushroom” regime, the scaling observed in Figure 7 can be understood. The distance between neighboring coils is of order RG. The surface coverage Γ expressed in CEC (or, equivalently, in mol/m2) thus follows as ΓðCECÞ 

1 s2

ð3Þ

Comparing eqs 2 and 3, the relation between Γ and Mw is ΓðCECÞ  Mw - 2υ

ð4Þ

The exponent -0.8 is slightly higher than the value -1 expected for θ conditions.

Conclusion The adsorption mechanism of Jeffamine polyetheramines on NaþMMT was investigated by a range of methods. The following mechanism is suggested: about 40% of the stoichiometric amount of Jeffamine adsorbs to the clay surface, largely by an ion exchange mechanism, even at high pH where the amine is mostly uncharged in solution. The adsorbed molecules are arranged like touching mushrooms rather than a more densely packed brush. The maximum adsorbed amount is likely to be governed by steric repulsion between adsorbed polymer coils, and possibly also by adsorption of the PPO block adjacent to the amine. Acknowledgment. Y.C. is funded by an AkzoNobel EPSRCDHPA studentship. Jeffamines were kindly provided by Huntsman and Pluronic L31 by Ms. Shirin Alexander. We thank Prof. Robert Richardson (access to SAXS in Physics, Bristol), Dr. Claire Pizzey (Diamond beamline), Mr. Desmond Davis (elemental analysis and atom absorption spectroscopy), and Bristol ChemLabS (zetasizer and conductivity meter).

DOI: 10.1021/la103278v

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