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Interaction of Gums (Guar, Carboxymethylhydroxypropyl Guar, Diutan, and Xanthan) with Surfactants (DTAB, CTAB, and TX-100) in Aqueous Medium Indrajyoti Mukherjee,† Diptabhas Sarkar,‡ and Satya P. Moulik*,† ‡
† Center for Surface Science, Department of Chemistry, Jadavpur University, Kolkata 700032, India, and Halliburton Energy Services, Halliburton, 10200 Bellaire Boulevard, Houston, Texas 77072, United States
Received July 7, 2010. Revised Manuscript Received October 6, 2010 The interaction of surfactants dodecyltrimethylammonium bromide (DTAB), cetyltrimethylammonium bromide (CTAB), and p-tert-octylphenoxypolyoxyethylene (9.5) ether (TX-100) with guar (Gr), carboxymethylhydroxypropyl guar (CMHPG), diutan (Dn), and xanthan (Xn) gums has been studied employing conductometry, tensiometry, microcalorimetry, viscometry, and atomic force microscopy (AFM) techniques. Both weak and strong interactions were observed. CTAB interacted stronger than DTAB with the gums. The surfactant-gum interaction process was enhanced by the presence of borate ions in the solution; the borate ion itself also manifested interaction with the surfactants comparable with that of water-soluble polymers polyvinyl alcohol, polyoxyethylene, and so forth. Viscometric results supported configurational changes of the gum molecules by interaction with surfactants. The geometry of the pure gums and their CTAB interacted products in the dried states was ascertained from AFM measurements; spherical and prolate shapes were observed for pure gums, and distorted states were observed for their surfactant complexed species. Detailed topological features of these entities were ascertained.
Introduction Many water-soluble polymers are known to interact with surfactants; ionic surfactants favorably contribute to the process.1-6 The interaction essentially produces an early aggregation of surfactants at a concentration lower than the critical micelle concentration (CMC), called the critical aggregation concentration (CAC),6-8 with the formation of small assemblies. At concentrations > CMC, when binding of the monomers and these small assemblies with the polymer are complete, the surfactants in solution self-assemble, producing normal micelles, and this point is termed as the extended CMC or CMCe. Between CAC and CMCe,9 the surfactant-interacted polymer molecules may form a new phase called the “coacervate”,10,11 which by selfassociation may grow into large assemblies (that either remain floating in the medium with nontransparent (opaque) texture or get out of phase in the form of precipitates). Depending on their types and concentration, the grown coacervate phases may disintegrate upon interaction with excess surfactants, forming *To whom correspondence should be addressed. E-Mail: spmcss@yahoo. com. (1) Goddard, E. D. Colloids Surf. 1986, 19, 301–329. (2) Wang, Y.; Han, B.; Yan, H.; Kwak, J. C. T. Langmuir 1997, 13, 3119–3123. (3) Nylander, T.; Samoshina, Y.; Lindman, B. Adv. Colloid Interface Sci. 2006, 123-126, 105–123. (4) Thuresson, K.; Nystrom, B.; Wang, G.; Lindman, B. Langmuir 1995, 11(10), 3730–3736. (5) Kong, L.; Cao, M.; Hai, M. J. Chem. Eng. Data 2007, 52(3), 721–726. (6) Mata, J.; Patel, J.; Jain, N.; Ghosh, G.; Bahadur, P. J. Colloid Interface Sci. 2006, 297(2), 797–804. (7) Goddard, E. D. Colloids Surf. 1986, 19, 255–301. (8) (a) Liu, H.; Hai, M. J. Chem. Eng. Data 2010, 55(1), 354–357. (b) Anghel, D. F.; Saito, S.; Baran, A.; Iovescu, A.; Cornit-escu, M. Colloid Polym. Sci. 2007, 285 (7), 771–779. (9) (a) Dan, A.; Ghosh, S.; Moulik, S. P. Carbohydr. Polym. 2010, 80, 44–52. (b) Mitra, D.; Bhattacharya, S. C.; Moulik, S. P. J. Phys. Chem. B 2008, 112, 6609–6619. (10) (a) Wang, Y.; Kimura, K.; Dubin, L. P.; Jaeger, W. Macromolecules 2000, 33, 3324–3331. (b) Deng, M.; Cao, M.; Wang, Y. J. Phys.Chem. B 2009, 113, 9436– 9440. (11) Menger, M. F.; Sykes, M. B. Langmuir 1998, 14, 4131–4137. (12) Dan, A.; Ghosh, S; Moulik, S. P. J. Phys.Chem. B 2009, 113, 8505–8513.
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transparent and less viscous solution.12 These coacervates can have uses in the pharmaceutical field; they can also be used in the preparation of nanomaterials. Gums are carbohydrate polymers and are used in foods, pharmaceutics, oil recovery, flow controlling fluids, cosmetics, and so forth.13-19 They have plenty of oxygen and hydroxyl centers as well as ionic carboxylate centers in the molecules which may lead to dipolar, ion-dipolar, and hydrogen bonding interactions with other materials in solution. Gums are used in many engineering, technology, and pharmaceutical processes and formulations, individually or in combination with other amphiphilic bodies in solution. Such mixtures exhibit interesting physicochemical features, but their studies are not widely reported in literature. There are reports on the interaction of gums with surfactants. Guar (Gr) and hydroxypropylguar (HPG) have been found to interact with quaternary ammonium surfactants.20 The study has been elaborate wherein HPG and HMHPG (the dodecyl derivative of HPG) interacted with DTAB and its oligomers (dimers and trimers with variable polymethylene spacers) at concentrations below and above their CMCs. Aubry et al.21 studied interaction of HPG with Triton X-100 (TX-100) micelles in aqueous medium. Cationically modified Gr has been reported to interact with anionic surfactants such as sodium decyl-, dodecyl-, and tetradecyl sulfates.22 Fijan et al.23 have conducted (13) Gebert, M. S.; Friend, D. R. Pharm. Dev. Technol. 1998, 3, 315–323. (14) Prud0 homme, R. K.; Constien, V.; Knoll, S. Adv. Chem. Ser. 1989, 89 223–236. (15) Prescott, F. J.; Hahnel, E.; Day, D. Drug Cosmet. Ind. 1963, 93, 443–540. (16) Ali, D.; Bolton, S.; Gaylord, G. J. Appl. Polym. Sci. 1991, 42, 947–956. (17) Zatz, L. J. Ind. Eng. Chem. Prod. Res. Dev. 1984, 23, 12–16. (18) Taugbol, K.; Ly, V. T.; Austad, T. Colloids Surf., A 1995, 103, 83–90. (19) Austad, T.; Ekrann, S.; Fjelde, I.; Taugbol, K. Colloids Surf., A 1997, 127, 69–82. (20) Kastner, U.; Zana, R. J. Colloid Interface Sci. 1999, 218, 468–479. (21) Aubry, T.; Moan, M.; Argillier, F. J.; Audibert, A. Macromolecules 1998, 31, 9072–9074. (22) Anthony, O.; Marques, C. M.; Richetti, P. Langmuir 1998, 14, 6086–6095. (23) Fijan, R.; Sostar-Turk, S.; Lapsin, R Carbohydr. Polym. 2007, 68(4), 708–717.
Published on Web 10/29/2010
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an elaborate study on Gr and its derivatives with several nonionic surfactants. The reports on gum surfactant interactions are relatively more on Gr and modified Gr than on other gums. Nedjhioui and co-workers24 have worked on the Xn-SDS system in the presence of salts. A detailed interfacial and viscosity study has been presented by Ritacco et al.25 on the Xn-DTAB gumsurfactant combination. Glycerol and other polyolic compounds are known to interact with borate ion, producing anionic centers on them.26 Gums are thus expected to be activated by borate ions to favorably interact with cationic surfactants. Power et al.27 studied borate ion cross-linked HPG gels under quiescent conditions as well as at high shear rates. From rheology and atomic force microscopy (AFM), they reported random network structures under quiescent conditions; under high shear, globular structured materials were observed. In this connection, detailed past studies on the probable interaction of borate ions with Gr can be cited.28-31 In a recent paper, Cui et al.32 have reported the interaction probability of DTAB with HPG both in the absence and presence of the borate ion by tensiometry, calorimetry, and small-angle neutron scattering (SANS). They have found that HPG as well as HPG-borate could not promote micellization of DTAB. However, HPGborate could bind, aggregate, and flocculate the existing DTAB micelles. On the other hand, carboxymethylguar (CMG) without borate evidenced strong interaction with DTAB, forming precipitate even below the CMC. Recently, we have studied33a in detail the solution behavior of the gums carboxymethylhydroxypropyl guar (CMHPG), diutan (Dn), and xanthan (Xn) in aqueous salt solutions with special reference to their viscosity, hydration, molecular association, and so forth. Their interaction study with surfactants has been considered worthwhile. While Gr and CMHPG are neutral, both Dn and Xn have anionic centers on them; thus, interactions of Dn and Xn with the cationic surfactants CTAB and DTAB are expected. The molecular architectures of these gums are presented in Scheme 1. Gr is a galactomannan, a polysaccharide consisting of (1-4)-linked β-D-mannopyranose backbone with a 1-6-linked R-D-galactopyranose as a branch point. It is harvested mainly from the endosperms of the seeds of the legume Cyamopsis tetragonolobus. The derivatized Gr-CMHPG had a molar substitution (MS) range of 0.15-0.35 and a degree of substitution (DS) range of 0.05-0.20. Dn is a natural high molecular weight gum produced by controlled aerobic fermentation of the bacterial strain Sphingomonas sp. ATCC 53159. It consists of a repeat unit with L-rhamnose, D-glucose, D-glucuronic acid, D-glucose backbone, and two-sugar L-rhamnose side chains attached to the (24) (a) Nedjhioui, M.; Moulai-Mostefa, N.; Bensmaili, A.; Morsli, A. Desalination 2005, 185, 543–550. (b) Nedjhioui, M.; Moulai-Mostefa, N.; Canselier, J. P.; Bensmaili, A. J. Dispersion Sci. Technol. 2009, 30(9), 1333–1341. (25) Ritacco, H.; Albouy, P.-A.; Bhattacharyya, A.; Langevin, D. Phys. Chem. Chem. Phys. 2000, 2, 5243–5251. (26) Zhao, Y.; Yan, Y.; Jiang, L.; Huang, J.; Hoffmann, H. Soft Matter 2009, 5, 4250–4255. (27) Power, D.; Larson, I.; Hartley, P.; Dunstan, D.; Boger, D. V. Macromolecules 1998, 31, 8744–8748. (28) Pezron, E.; Ricard, A.; Lafuma, F.; Audebert, R. Macromolecules 1988, 21, 1121–1125. (29) Parris, M. D.; MacKay, B. A.; Rathke, J. W.; Klingler, R. J.; Gerald, R. E. Macromolecules 2008, 41, 8181–8186. (30) Kesavan, S.; Prud0 homme, R. K. Macromolecules 1992, 25, 2026–2032. (31) Tayal, A.; Pai, V. B.; Khan, S. A. Macromolecules 1999, 32, 5567–5574. (32) Cui, Y.; Pelton, R.; Cosgrove, T.; Richardson, R.; DaI, S.; Prescott, S.; Grillo, I.; Ketelson, H.; Meadows, D. Langmuir 2009, 25(24), 13712–13717. (33) (a) Banerjee, P.; Mukherjee, I.; Bhattacharya, S.; Datta, S.; Moulik, S. P.; Sarkar, D. Langmuir 2009, 25(19), 11647–11656. (b) Ma, X.; Pawlik, M. Carbohydr. Polym. 2007, 70, 15–24. (c) Yadira, I.; Cantu, V.; Hauge, R. H.; Norman, L. R.; Powell, R. J.; Billups, W. E. Biomacromolecules 2006, 7, 441–445.
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Article Scheme 1. Molecular Structures of (A) Gum Guar or Modified Guar, (B) Gum Diutan, and (C) Gum Xanthan
(1 f4) linked glucose residue. Two O-acetyl groups are attached per repeat unit to the 20 and 60 positions of the (1f3) linked glucose. Xn gum is a microbial desiccation-resistant polymer prepared commercially by aerobic submerged fermentation from Xanthomonas campestris. It is an anionic polyelectrolyte with a β-(1 f4)-D-glucopyranose glucan (as cellulose) backbone with side chains of -(3f1)-R-linked D-mannopyranose-(2 f1)β-D-glucuronic acid-(4f1)-β-D-mannopyranose on alternating residues. We have found that both DTAB and CTAB strongly interacted with the gums but the anionic surfactant SDS remained nonreactive, whereas the neutral species, TX-100, produced moderate interaction. We have also examined in some detail the effect of borate ion addition on the said interaction process. The methods of tensiometry, conductometry, isothermal titration calorimetry (ITC), viscometry, and AFM were used for probing. The study has provided interesting results with reference to self-aggregation of the surfactants and influence of their interaction on the biopolymer configuration. DOI: 10.1021/la102717v
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Experimental Section Materials. The gums (Gr, CMHPG, Dn, and Xn) used in this study were the same materials as used earlier.33a The surfactants, sodium dodecyl sulfate (SDS), dodecyltrimethylammonium bromide (DTAB), cetyltrimethylammonium bromide (CTAB), and p-tert-octylphenoxypolyoxyethylene (9.5) ether (Triton X-100; TX-100) were of same origin as documented earlier.34-37 Borax and salts (NaCl, Al2(SO4)3) used were AR grade materials of BDH and SRL, India, respectively. Measurements. Conductometry. Conductance was measured with a Jenway (U.K.) (model no. PCM-3) conductometer. The measurements were taken in a constant temperature water bath (accuracy of (0.1 K) using a dip type cell (cell constant of 1.0 cm-1) under constant stirring conditions. The experimental protocol involved dipping the conductivity cell in either 10 mL of water or 10 mL of gum solution of known concentration in a beaker placed in the constant temperature water bath, and under constant stirring. Conductance was measured as a function of incremental addition of surfactant solution to the abovementioned beaker with the help of a Hamilton microsyringe. The experiments were repeated two to three times to check reproducibility.38-40 The averages of these readings were documented and discussed. Tensiometry. Tensiometric measurements were taken with a calibrated du No€ uy tensiometer (Kr€ uss, Germany) following the ring detachment technique, described earlier.41-43 A total of 10 mL of aqueous gum solution of desired concentration was taken in a thermostatted (accuracy within (0.1 K) double walled jacketed glass container at 303 K. A stock aqueous surfactant solution of desired concentration (10-40 times CMC) was stepwise added in the gum solution with a Hamilton microsyringe, stirred well using a magnetic stirrer at each addition, followed by measurement of surface tension after equilibration. Duplicate measurements were taken to check reproducibility. Ritacco et al.25 reported earlier the requirement of a long time for stabilization of the surface tension of the Xn solution after addition of DTAB in a system under nonstirred conditions. In our study, the system was stirred well for 5 min after addition of the surfactant portion in the gum solution, and it was then allowed to stabilize for 10 min more. Thereafter, the attainment of a steady value of surface tension was monitored at intervals of 5 min. The constancy of γ (surface tension) was found to be attained nearly within 30 min after the initial addition of the surfactant portion in the solution. Therefore, the reported results are all 30 min elapsed time values Viscometry. A two-armed calibrated (with sucrose solution) Ubbelohde viscometer (flow time of 68 s for 10 mL of water at 303 K) was employed as reported earlier.44,45 The concentration of the gum solution used was 0.05 g % which was well below 0.3 g %, the limit for Newtonian flow.33a-c A concentrated solution of surfactant (well above CMC) was added to the gum solution in the (34) Majhi, P. R.; Moulik, S. P. Langmuir 1998, 14, 3986–3990. (35) Chatterjee, A.; Majhi, P. R.; Sanyal, S. K.; Moulik, S. P. Biophys. Chem. 2002, 98, 313–327. (36) Mukherjee, S.; Mitra, D.; Bhattacharya, S. C.; Panda, A. K.; Moulik, S. P. Colloid J. 2009, 71(5), 677–686. (37) Majhi, P. R.; Moulik, S. P.; Burke, S. E.; Rodgers, M.; Palepu, R. J. Colloid Interface Sci. 2001, 235(2), 227–234. (38) Chakraborty, T.; Chakraborty, I; Ghosh, S. Langmuir 2006, 22, 9905–9913. (39) Chakraborty, T.; Chakraborty, I.; Moulik, S. P.; Ghosh, S. J. Phys. Chem. B 2007, 111, 2736–2746. (40) Mitra, D.; Chakraborty, I.; Bhattacharya, S. C.; Moulik, S. P. Langmuir 2007, 23, 3049–3061. (41) Dan, A.; Chakraborty, I.; Ghosh, S.; Moulik, S. P. Langmuir 2007, 23, 7531–7538. (42) Mukherjee, I.; Halder, D.; Ghosh, S.; Moulik, S. P. J. Dispersion Sci. Technol. 2009, 30, 1430–1441. (43) Basu Ray, G.; Chakraborty, I.; Ghosh, S.; Holgate, C.; Glen, K.; Palepu, R. M.; Moulik, S. P. J. Phys. Chem. B 2007, 111, 9828–9837. (44) Basu Ray, G.; Chakraborty, I.; Ghosh, S.; Moulik, S. P.; Palepu, R. M. Langmuir 2005, 21, 10958–10967. (45) Naskar, B.; Dan, A.; Ghosh, S.; Moulik, S. P. Carbohydr. Polym. 2010, 81, 700–707.
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viscometer with a microsyringe, followed by mixing and equilibration. The time of fall of the solution in the viscometer after each addition was noted. All viscosity measurements were performed in a water bath (accuracy of (0.1 K). The density of the solution was measured in a calibrated pycnometer. Duplicate measurements were taken, and the mean values are used in this report. Microcalorimetry. An Omega ITC microcalorimeter from Microcal Inc. (Northampton, MA) was used for the microcalorimetric measurements. Concentrated surfactant solution (about 10-20 times its CMC) was added in installments (e.g., 5 μL at each step) after equal time intervals (210 s) into 1.325 mL of water or gum solution taken in the calorimeter cell under constant stirring (300 rpm) conditions; the reference cell contained 1.65 mL of either water or gum solution. The heat of dilution of the surfactant solution was recorded at each step. A Neslab RTE100 water bath was used to circulate constant-temperature water at several degrees lower than that in the calorimetric cell surroundings wherein the internal device maintained the experimental temperature with an accuracy of (0.01 K. The enthalpy per mole of surfactant addition was calculated with the help of Microcal Origin ITC software. The experiments were repeated to check reproducibility. The measurement details and data analysis can be found in our previous reports.35,38-41 AFM. AFM images of dried aqueous gum solutions were obtained using a multimode atomic force microscope (Vecco Metrology, Autoprobe CP-II, model no. AP 0100) at ambient temperature using silicone probes (RTESPA-M,Vecco, Santa Barbara, CA) in noncontact mode. Long tip (aspect ratio 4:1) cantilevers with spring constants ranging from 20 to 80 N/m and resonance frequencies of 245-287 kHz, thickness ∼3.54.5 μm, and length 115-135 μm were used to image the surface morphology Initially the scanning area was 50 μm 50 μm and subsequently reduced to isolate the data points. Offline analysis of each image was performed to obtain information on the sample morphology and height. Proscan Image Processing Programme software provided by the manufacturer was used to measure the roughness of the samples. Surface roughness (Rrms) was measured using the following equation:
Rrms
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ffi P zi - zavg ¼ Np
where zi is the elevation of the tip at the ith data point, zavg is the average elevation of the tip within the given area, and Np is the number of points within the given area.
Results and Discussion Evaluation of Interaction by Tensiometry, Conductometry, and Calorimetry. Of the four gums used in this study, Dn and Xn are anionic polyelectrolytes while Gr and CMHPG are neutral. Interaction with cationic surfactants DTAB and CTAB by way of electrostatic and also by hydrophobic interactions was envisaged. Hydrophobic and ion-dipole interaction of Gr and CMHPG with the surfactants was considered possible. Our observations are described below. Tensiometry. In this study, none of the gums used evidenced interaction with SDS in the studied aqueous medium, although there were reports of interaction of Gr and its derivative with SDS in aqueous salt solution.24 Tensiometric profiles of dilution of concentrated solution of SDS in salt-free aqueous medium without and with gum showed virtually superimposable features. Similar results of tensiometry were reported for both CMGDTAB and HPG-DTAB systems by Cui et al.32 However, visible interaction of CTAB with Dn, Xn, and even CMHPG was Langmuir 2010, 26(23), 17906–17912
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Article Table 1. CAC, Cm, and CMCe Values for Gum-CTAB Systems at 303 Ka tensiometry gum
conductometry
microcalorimetry
CAC Cm CMCe CAC Cm CMCe CAC
Cm
CMCe
Dn 0.18 0.64 1.64 0.89 1.64 0.26 1.06 2.5 Xn 0.17 1.26 2.6 1.54 2.57 0.36 1.87 2.7 Gr 0.76 1.7 0.74 1.21 0.20 1.77 2.85 CMHPG 0.04 0.63 2.2 1.0 1.90 0.36 1.86 3.2 a The results are in mM units. [gum] = 0.1 g % except Dn (0.08 g %) in microcalorimetry. The CAC, Cm, and CMCe values of CTAB for 0.1 g % Gr in 0.1 M borate solution were 0.07, 0.72, and 4.5 mM, respectively (cf. Figure 1C, main diagram).
Table 2. CAC, Cm, and CMCe Values for Different Gum-DTAB Systems at 303 Ka DTAB tensiometry
Figure 1. (A) Surface tension isotherms of aqueous gum solutions (0.1 g %) with CTAB: (4) Dn, (O) CMHPG, (0) Xn, (g) Gr, and (]) CTAB. (B) Surface tension isotherms of aqueous gum solutions (0.1 g %) with DTAB. (C) Main plot: Surface tension isotherms of Gr-gum solutions (0.1 g %) in borax (0.1 M) medium with CTAB and DTAB. Inset: Surface tension isotherms (coordinates same as in main plot) of (O) CTAB and (g) DTAB solutions in borax (0.1 M) medium. (D) Specific conductance versus surfactant concentration of aqueous gum solutions (0.1 g %) with CTAB.
observed by us (Figure 1A); the unmodified Gr evidenced mild interaction. DTAB also evidenced mild to moderate interaction with CMHPG, Dn, and Xn, and no interaction with Gr (Figure 1B). The interaction of the modified Gr, CMHPG, was in line with Kastner and Zana20 who reported interaction of quaternary ammonium surfactant, DTAB, and its oligomers (dimers and trimers) with HPG and hydrophobically modified guar, HMHPG. The present results show both a CAC and CMCe (Table1). In the plots, the maximum between CAC and CMCe meant the completion of binding of small micelles with the gum and denoted as Cm. DTAB evidenced existence of CAC, Cm, and CMCe with some exceptions (cf. Figure 1B and Table 2). The neutral gums Gr and CMHPG produced CAC at much higher concentration than the anionic Xn. The Cm points were not clearly visible for the first two; the post CMC transition points corroborated with the CMCe of other gums. In the past, CAC formation of DTAB with Xn was reported.25 A comparison of results of interaction of neutral Gr with both DTAB and CTAB in the presence of borate ion with reference to that of Cui et al.32 of CMG and HPG with DTAB by tensiometry would not be out of place here. The expected borate ion induced ionization of -OH groups in the carbohydrate polymers (CMG and HPG) like glycerol were not observed by the authors, since no visible interaction of DTAB with the gums arose. Our results with Gr in 100 mM aqueous borax solution at pH = 9.3 using both DTAB and CTAB are displayed in Figure 1C. It was found that, in borate medium, the cationic surfactants produced unusual tensiometric profiles presented in the main diagram (CTAB and DTAB with Gr) and in the inset (CTAB and DTAB without Gr) of Figure 1C. Water-soluble polymer-surfactant interaction type tensiometric features46-48 were observed. In the presence of Gr, such features were enhanced. Thus, sodium metaborate did not (46) Cosgrove, T.; Mears, S. J.; Obey, T.; Thompson, L.; Wesley, R. D. Colloids Surf., A 1999, 149, 329–338. (47) Dai, S.; Tam, K. C. J. Phys. Chem. B 2001, 105, 10759–10763. (48) Khanal, A.; Li, Y.; Takisawa, N.; Kawasaki, N.; Oishi, Y.; Nakashima, K. Langmuir 2004, 20, 4809–4812.
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gum
CAC
Cm
CMCe
Dn 4.6 16.5 Xn 1.35 3.4 17.5 Gr 9.4 16.0 CMHPG 6.6 17.8 a The results are in mM units. [gum] = 0.1 g %. The CAC, Cm, and CMCe values of DTAB for 0.1 g % in 0.1 M borate solution were 2.3, 12.7, and 71 mM, respectively (cf. Figure 1C, main diagram).
perform like normal electrolytes to reduce the CMC of an ionic surfactant.49,50 It enhanced the CMC of both CTAB and DTAB from 1 and 14.5 mM to 2.7 and 68 mM, respectively. The borate ions interacted with Gr, making them anionic to interact with CTAB and DTAB. The results herein presented were the average of two repeat experiments that agreed within 5% (see footnotes of Tables 1 and 2). Further study with other gums would shed more light on this phenomenon. Conductometry. The anionic surfactant SDS was found not to interact with the gums. The results with and without gums were equivalent. The conductometric results of CTAB interaction with Gr, CMHPG, Dn, and Xn evidenced two breaks nearly matching to the Cm and CMCe, observed by tensiometry (Figure 1D and Table 1). Indications for CAC formation were not found in the plots, which are frequently found with other systems. Microcalorimetry. Similar to tensiometry and conductometry, calorimetry also evidenced no interaction of SDS with the gums. The enthalpies of dilution of SDS without and with gums in solution were equivalent. The ITC results of interaction of the gums with CTAB are presented in Figure 2, where the enthalpy of dilution results without and with gums (Figure 2A) revealed interaction. Gr, CMHPG, and Xn at 0.1 g % and Dn at 0.08 g % produced enthalpograms that were one way or another different from those of CTAB dilution in water. Dn produced marked differences compared to the others. The comparative features were better revealed in the resultant enthalpograms (relative to CTAB dilution in water) presented in Figure 2B. The first endothermic peaks for Gr, CMHPG, and Xn indicated CAC formation, the second endothermic regions up to the peaks stood for complete binding of small micelles with the polymers, and the subsequent exothermic regions represented the association of the free monomers in solution to form CMCe (marked in the graph for Xn only). The exothermic peaks for Dn were distinct deviations from the courses shown by the other gums. The (49) Martins, R. M.; Da Silva, C. A.; Becker, C. M.; Samios, D.; Christoff, M.; Bica, C. I. D. J. Braz. Chem. Soc. 2006, 17(5), 944–953. (50) Kogej, K.; Skerjanc, J. Langmuir 1999, 15(12), 4251–4258.
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Figure 2. (A) Enthalpy of dilution versus surfactant concentration of aqueous gum solutions (0.08 g % Dn and 0.1 g % Xn, Gr, and CMHPG) with CTAB and pure aqueous CTAB solution. (B) Difference plot of the enthalpy of dilution versus surfactant concentration of the different gums with respect to the CTAB. (C) Enthalpy of dilution versus surfactant concentration of aqueous Gr gum (0.1 g%) with and without borax solution (0.1 M) with CTAB. (D) Difference plot of the enthalpy of dilution versus surfactant concentration of Gr gum in the presence and absence of borax solution 0.1 M with respect to the CTAB solutions.
resultant enthalpogram plots (Figure 2B) are quite revealing, and such a treatment of ITC results is seldom found in literature.51 The procedure leads to better understanding of the polymersurfactant interaction process. The results are presented in Table 1. It is to be noted that the method overestimated the parameters compared to tensiometry and conductometry. It may be mentioned that Cui et al.32 did not observe interaction of DTAB with both CMG and HPG from ITC measurements. In Figure 2C, the enthalpies of dilution of CTAB without Gr, with Gr, and Gr with borax are presented. There are differences, but the extents and nature are more revealing again in the difference plots shown in Figure 2D. Thus, the interaction possibility found by the methods was supported by ITC. Evaluation of Interaction by Viscometry. It is known that aggregation behavior in colloidal and polymer solution can be directly understood from SANS, light scattering, and viscosity measurements. Kastner and Zana20 reported aggregation behavior of HPG-DTAB and HMHPG-DTAB systems at higher gum concentrations. We observed turbidity with formation of a trace amount of precipitate in the system at [gum] ≈ 0.1 g % in the presence of both DTAB and CTAB. For CMHPG, the turbidity disappeared at higher surfactant concentration. In the case of Dn-DTAB, the product formed lumps that could be transformed into small fragments (without dissolution) by the addition of Al2(SO4)3 in the solution. The fiberlike particles formed in the Xn-CTAB system could be dissolved by the addition of NaCl. To obviate the above effect, we used gum solutions of concentration 0.05 g % in our viscosity study. The concentration was low enough to make the gum solutions Newtonian both in the presence and absence of surfactants. As indicated earlier,33b for Newtonian flow, a gum concentration of 10 CMC (region III), resolubilization of the precipitate of HMHPG-surfactant complex and solution-to-gel transition occurred. At [surfactant] > 100 CMC, all solutions became clear and formed hydrogels. In the present study, an initial decrease and then an increase in viscosity with a minimum was observed. For CTAB, except Gr, the decrease started at 0.02 CMC, the formation of minimum occurred at 1.7 CMC, and then ηr rose, which was studied up to 10 CMC. Nearly similar features with varied values (0.07, 1.5, and 3.8 CMC, respectively) were observed for DTAB. These features are marked in Figure 3A-C. For TX-100, the decline started at 0.7 CMC and went down up to 14.5 CMC, beyond which measurements were not taken (Figure 3D). The above viscosity behaviors (with minor variations) were observed for all the herein studied gums.. The gum concentration we used Scheme 2
was 0.05 g %, which was well below the concentration (1.0 g %) used by Kastner and Zana,20 so formation of gels was not observed by us. The intrinsic viscosity behavior of HPG in TX100 solution of Aubry et al.21 apparently has more relevance to the current work for the results in both the studies with reference to low concentrations. These authors found a minimum in [η] of HPG at [TX-100] = CMC; thereafter, [η] sharply increased. In this respect, our results are contradictory to theirs; the viscosity minimum did not manifest for the gums even up to 14 CMC, and only the decline in ηr became sharp. The behavior was independent of the types of gum used. The initial decline and rise afterward (excepting TX-100) of ηr can be rationalized in the following way. In the beginning, amphiphile monomers alone as well as in the form of assemblies such as reverse micelles became attached to the specific hydrophilic (polar and/or ionic) centers (regions) of the polymer forming hydrophobic (water repellant) centers. As a consequence, the configuration of the complexed gum shrunk. In this process, the [η] of the solution decreased. On completion of the process, the added surfactant molecules then adhered to the hydrophobic regions of the polymer in the form of assemblies (such as normal micelles), causing expansion of the chain by way of repulsion among such (polar/ionic) neighboring assemblies. This process produced an increase in [η]. At a surfactant concentration much higher than the CMC, in concentrated gum solutions (used by Kastner and Zana20), water molecules became entrapped in the network structures formed by the complexed polymer and the free normal micelles to form hydrogels. In the present study, this stage was absent for the concentration of gum used which was much smaller (0.05 g%). The abovedescribed features are depicted in Scheme 2. This fluidity variation property of the gum solutions by the presence of amphiphilic
Figure 4. (A-D) Topological 2D Images of Dn, Xn, Gr, and CMHPG, respectively. (E-H) 3D images of gums Dn, Xn, Gr, and CMHPG, respectively. (I-L) Histograms of Dn, Xn, Gr, and CMHPG, respectively. Langmuir 2010, 26(23), 17906–17912
DOI: 10.1021/la102717v
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Article
Mukherjee et al.
Figure 5. (A-C) Topological 2D images of Dn, Xn, and CMHPG, respectively, after interaction with CTAB. (D-F) 3D images of the above systems.
Table 3. Detail Region Analysis of AFM Results of (0.05 g %) Gums Br (nm) gum
Rq (nm)
Ra (nm)
Ht (nm)
Hm (nm)
@20.0%
@80.0%
Rp (nm)
Rv (nm)
Dn Xn Gr CMHPG
39.0 17.9 1.71 1.11
28.9 11.5 1.28 0.84
90.1 46.3 4.07 4.58
81.2 43.7 3.75 4.52
116 51.7 5.17 5.29
61.7 34.6 2.71 3.62
177 88.7 7.30 4.85
-90.1 -46.3 -4.07 -4.58
bodies has potential for uses as stabilizing and flow controlling media in pharmacy, food, enhanced oil recovery, and so forth. Morphology of Solvent Removed Pure and Surfactant Interacted Gums by AFM. In our earlier paper,33a the configurations of the gums in aqueous and in salt solutions were reported to be globular. Herein, we present AFM results of pure and CTAB interacted gums in Figures 4 and 5 (where 4A-D and 5A-C are 2D illustrations and 4E-H and 5D-F are 3D illustrations). The pure gums appeared globular (sphere and prolate), whereas the CTAB interacted products were nonglobular, where the native configurations were destroyed. This was direct proof of our previous prediction of globular shapes of the gums from solution property studies.33a Further, the studied gums were globular also in dilute surfactant solutions, and irregular nonglobular structures resulted in higher surfactant concentrations. The morphological information on the gums was obtained from the 3D AFM images. Different characteristic parameters such as Rp (peak height), Rv (depth of valley), Rq (root-mean-square roughness), Ra (average roughness), Ht (mean height), Hm (median height), and Br (bearing ratio) are presented in Table 3. The parameters followed the order Dn > Xn . Gr > CMHPG. The first two had fairly rough surfaces. The histograms of the total AFM images are presented in Figure 4I-L. Both Gr and CMHPG evidenced Gaussian distribution of the globule sizes with spikes in regular intervals in the Gr sample. Dn produced a distribution with a broad top, whereas Xn produced a relatively less broad distribution with a narrow peak. The peak sizes followed the order Xn > Gr > Dn ≈ CMHPG. Interaction of the gums with CTAB produced large distortion in the shapes for Dn and Xn. The effect was moderate on CMHPG (Figure 5A-C). The modes of interaction between the gums and CTAB made the difference. In Figure 5A and B, reaction between gum and CTAB was performed by direct addition of surfactant solution in gum solutions on the glass
17912 DOI: 10.1021/la102717v
slides. In the preparation of the sample slide (Figure 5C), the components were allowed to react in solution, a drop of which was dried on the glass slide for AFM measurements.
Conclusions Both DTAB and CTAB interacted with the gums Gr, CMHPG, Dn, and Xn in aqueous medium; the interaction with Gr was mild. In comparison, DTAB was weaker interacting than CTAB. For both the surfactants, the strength of the interaction followed the order Gr < CMHPG < Xn < Dn. Similar to many water-soluble polymers, the formation of CAC and CMCe in the gum solutions was observed. The gums manifested configurational contraction in the early stage of surfactant addition followed by expansion and aggregation in the later stage.. The nonionic surfactant TX-100 also manifested nearly similar effects. Both DTAB and CTAB were observed to produce uncommon interaction with the borate ion: kind of CAC and CMCe forming behavior was observed. In the presence of the borate ion, Gr evidenced enhancement of interaction with both DTAB and CTAB; this unique behavior is expected to prevail with other neutral gums, namely, HPG, CMHPG, and so forth. The AFM study of the solvent removed gum samples showed the formation of distinct globules (spheres and prolates). Their surfactant complexed species were of irregular and deformed configurations. Molecular architecture related hydrophilic-hydrophobic properties of the gums along with their polyelectrolyte nature were the determinant factors for their interactions with the cationic alkyltrimethylammonium bromide analogues and nonionic TX-100. Acknowledgment. I.M. thanks Halliburton Energy services for financial support, and S.P.M. thanks Indian National Science Academy for an Honorary Scientist position. We are thankful to Mr. Abhijit Dan for his help in the ITC measurements.
Langmuir 2010, 26(23), 17906–17912
