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Sorption of Water Vapor, Hydration, and Viscosity of Carboxymethylhydroxypropyl Guar, Diutan, and Xanthan Gums, and Their Molecular Association with and without Salts (NaCl, CaCl2, HCOOK, CH3COONa, (NH4)2SO4 and MgSO4) in Aqueous Solution Paltu Banerjee, Indrajyoti Mukherjee, Subhash Bhattacharya, Sidhhartha Datta, and Satya P. Moulik* Centre for Surface Science, Department of Chemistry, Jadavpur University, Kolkata 700032, India
Diptabhas Sarkar Halliburton, 10200 Bellaire Boulevard, Houston, Texas 77072 Received April 9, 2009. Revised Manuscript Received August 10, 2009 Gums are routinely used in food industry, pharmacy and oil recovery process. In these uses, the hydrocolloids very often encounter interactions with salts at moderate to high temperature. Since they are normally employed in the form of solution and gel, their viscous or fluidity properties need detailed investigation. In the present work, properties such as water vapor adsorption of finely powdered carboxymethylhydroxypropyl derivatized guar (CMHPG) as well as xanthan (Xn) and diutan (Dn) gums, their hydration in solution, their viscosity behaviors, and salt effects on fluidity have been studied. The concentration domains for the existence of free and associated molecules in the studied solutions have been assessed from the viscosity results. The gums have been found to bind a fair amount of water from the vapor phase with them. In solution, they can interact and arrest a large amount of water in their folded configuration. Intrinsic viscosities of the gums in aqueous medium declined in the presence of salts. The activation energies for their viscous flow were moderate and comparable, and were dependent on their concentrations. From the power law relation and viscosity master curve behavior mostly two critical association states of the macromolecular dispersions were envisaged.
Introduction Biopolymers such as gums, carboxymethylcellulose, pectin, and other carbohydrate-based materials are liberally used in the preparation of flow controlling solutions, in pharmacy,1,2 in food industries, in textiles,3 in mineral processing,4 in cementing,5 in oil-field operations,6 and in many other industrial processes.7-11 The gums, derivatized carboxymethyl and hydroxypropyl guar (CMHPG), diutan (Dn), and xanthan (Xn) are used or *Corresponding author. E-mail:
[email protected]. Fax: 91-33-24146266. (1) Gebert, M. S.; Friend, D. R. Pharm. Dev. Technol. 1998, 3, 315–323. (2) Misra, A. N.; Baweja, J. M. Indian Drugs 1997, 34, 216–223. (3) Kokol, V. Carbohydr. Polym. 2002, 50, 237–247. (4) Liu, Q.; Zhang, Y.; Laskowski, J. S. Intl. J. Miner. Proc. 2000, 60, 229–245. Wang, J.; Somasundaran, P.; Nagaraj, D. R. Miner. Eng. 2005, 18, 77–81. Ma, X.; Pawlik, M. J. Colloid Interface Sci. 2006, 298, 609–614. (5) Sakata, N.; Yanai, S.; Yokoziki, K.; Maruyama, K. J. Adv. Concrete Technol. 2003, 1, 37–41. (6) Prud’homme, R. K.; Constien, V.; Knoll, S. Adv. Chem. Ser. 1989, 89, 223– 236. (7) Brode, G. L.; Harris, E. D.; Salensky, G. A. In Cosmetic and Pharmaceutical Applications of Polymers; Gebelein, C. G., Cheng, T. C., Yang, V. C., Eds.; Plenum: New York, 1991; p 117. (8) Tsaur, L. S.; Shen, S.; Jobling, M.; Aronson, M. P.; Lever Brothers Co. U.S. Patent 6,066,613, 2000. (9) Goldstein, A. M.; Alter, E. N. In Industrial Gums, Polysaccharides and their Derivatives; Whistler, R. L., Ed.; Academic Press: New York, 1959; p 321. (10) Goel, N.; Shah, S. N.; Yuan, W. L.; O’Rear, E. A. J. Appl. Polym. Sci. 2001, 82, 2978–2990. (11) Rayment, P.; Ross-Murphy, S. B.; Ellis, P. R. Carbohydr. Polym. 2000, 43, 1–9. (12) Nayak, B. R.; Singh, R. P. Polym. Int. 2001, 50, 875–884.
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have prospects for use in the oil field, particularly as fracturing fluids.12,13 “Fracturing” as related to oil field services, is a process in which a “fluid” laden with “propping agents” is pumped into an underground rock formation at a very high pressure and flow rate. The idea is to fracture the subterranean hydrocarbon bearing formation to create lateral fractures or to enhance existing fractures make recovery of oil and gas faster. The propping agents (compression resistant materials like ground walnut shells, sand, and ceramic beads, etc.) are used to hold the newly formed “fracture” open. The base fluids used are solutions of the biopolymers, which aid in suspending the propping agents; frequently, the solutions are cross-linked. Most of the carbohydrate-based polymers (macromolecules) hydrate and swell in the presence of water forming solutions of increasing viscosity depending on their concentration. A fundamental understanding of the behavior of these biopolymers in solution is essential in optimizing their use in various industrial processes. In addition, the possibility of interaction of these biopolymers with salts has significance with reference to food formulations and fluidity controlling formulations and especially in oil-field applications. Investigations on the interaction of different salts with simple carbohydrates have been done in the past.14 For example, it is (13) Etemadi, O.; Petrisor, I. G.; Kim, D.; Wan, M. W.; Yen, T. F. Soil Sedimentation Contamination 2003, 12, 647–661. (14) Moulik, S. P.; Khan, D. P. Carbohyd. Res. 1974, 36, 147–157. Moulik, S. P.; Mitra, A. K. Carbohyd. Res. 1973, 29, 509–512. Moulik, S. P.; Khan, D. P. Carbohyd. Res. 1975, 41, 93–104.
Published on Web 08/28/2009
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known that the carbohydrates form adducts with alkali and alkaline earth metal salts.14 Interactions of salts with gums and polysaccharides have been also studied by different workers.15-18 In addition, the rheological properties of pure and modified gums and carbohydrate polymers have been explored in details in view of their field applications and flow controlling and pharmaceutical and agricultural uses and applications.19-23 It is thus imperative that there will be physicochemical interactions between the carbohydrate-based polymer (gums) and salts, which constitute a major part of the objective of the present study. Guar gum is a galactomannan, a polysaccharide consisting of (1-4)-linked β-D-mannopyranose backbone with a 1-6linked R-D-galactopyranose as a branch point. It is harvested mainly from the endosperms of the seeds of the legume Cyamopsis tetragonolobus, an annual plant found in arid regions of India. In the derivatized guar (CMHPG) the amount of carboxymethyl and hydroxypropyl groups in the backbone is characterized by a molar substitution (MS) value and a degree of substitution (DS) value, respectively. It is expected that derivatization of gums should produce property changes.24 Thus, gums like carboxymethyl guar, hydroxypropyl guar; CMHPG, etc have evidenced changes in soluibilities and rheological properties without and with salts.25,26 Diutan is a natural high molecular weight gum produced by controlled aerobic fermentation of the bacterialstrain Sphingomonas sp. ATCC 53159. Diutan consists of a repeat unit with L-rhamnose, D-glucose, D-glucuronic acid, D-glucose backbone, and two-sugar L-rhamnose side-chain attached to the (1 f 4) linked glucose residue. Two O-acetyl groups are attached per repeat unit to the 20 and 60 positions of the (1 f 3) linked glucose. Xanthan gum is a microbial desiccation-resistant polymer prepared commercially by aerobic submerged fermentation from Xanthomonas campestris. It is an anionic polyelectrolyte with a β-(1f4)-D-glucopyranose glucan (as cellulose) backbone with side chains of (3f1)-R-linked D-mannopyranose-(2 f1)-βD-glucuronic acid-(4f1)-β-D-mannopyranose on alternating residues. The molecular architectures of the three gums used in this study are presented in Scheme 1a-c. Here, we report the water absorption capacity and hydration behavior of the above-mentioned gums, using conductance and isopiestic methods. Their viscosity behaviors and effects of variation in temperature and presence of salts have been investigated in detail. Results have provided information on native configuration and molecular association in solution along with the activation energy for the flow process. It is well-known that the gums show a (15) Whitfield, D. M.; Stojkovski, S.; Sarkar, B. Coord. Chem. Rev. 1993, 122, 171–225. (16) Lynn, J. D.; Nasr-El-Din, H. A. J. Pet. Sci. Eng. 1998, 21, 179–201. (17) Yadira, I.; Cantu, V.; Hauge, R. H.; Norman, L. R.; Powell, R. J.; Billups, W. E. Biomacromolecules 2006, 7, 441–445. (18) Gittings, M. R.; Cipelletti, L.; Trappe, V.; Weitz, D. A.; In, M.; Lal, J. J. Phys. Chem. A 2001, 105, 9310–9315. (19) Mahammad, S.; Comfort, D. A.; Kelly, R. M.; Khan, S. A. Biomacromolecules 2007, 8, 949–956. (20) Kok, M. S.; Hill, S. E.; Mitchell, J. R. Food Hydrocolloids 1999, 13, 535– 542. (21) Lai, L. S.; Chiang, H. F. Food Hydrocolloids 2002, 16, 427–440. (22) (a) Higiro, J.; Herald, T. J.; Alavi, S.; Bean, S. Food Res. Int. 2007, 40, 435– 440. (b) Higiro, J.; Herald, T. J.; Alavi, S. Food Res. Intl. 2006, 39, 165–175. (23) Lai, L. S.; Tung, J.; Lin, P. S. Food Hydrocolloids 2000, 14, 287–294. (24) Rinaudo, M. Biomacromolecules 2004, 5, 1155–1165. (25) (a) Zhang, L.-M.; Zhou, J.-F.; Hui, P. S. J. Sci. Food. Agric. 2005, 85, 2638– 2647. (b) Pasha, Mazhar; Swami, N. G. N. Pak. J. Pharm. Sci. 2008, 21, 40–44. (c) Cheng, Y.; Brown, K. M.; Prud'homme, K. Biomacromolecules 2002, 3, 456–461. (d) Zhang, L.-M.; Jhou, J.-F. Colloids Surf. A: Physicochem. Eng. Aspects 2006, 279, 34–39. (26) Cordova, A. US Patent 6,387,169, May 14, 2002. (27) Robinson, G.; Ross-Murphy, S. B.; Morris, E. R. Carbohyd. Res. 1982, 107, 17–32.
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Banerjee et al. Scheme 1. Molecular Structures of (a) the Gum Carboxymethylhydroxypropylguar, (b) the Gum Diutan, and (c) the Gum Xanthan
threshold concentration above which dimers and multimers are formed.27-29 This aspect was investigated for the three gums through viscometric studies, with particular focus on the effect of salts and temperature. It may be mentioned that of the three gums herein experimented upon, CMHPG has been fairly studied from the standpoint of rheology and interaction with salts, xanthan has been moderately studied, but diutan has been rarely investigated. In addition, the hydration behaviors of all the three gums have been so far remaining unexplored. In the present work, a comparative physicochemical assessment of the solution behaviors of the three gums in the absence and presence of salts has been attempted.
Experimental Section Materials. The gums CMHPG (carboxymethylhydroxypropylguar), diutan and xanthan used in this study were provided by Halliburton Energy Services, Inc. (USA), and were used as (28) de Gennes, P. G. Nature (London) 1979, 282, 367–370. (29) Launay, B.; Cuvelier, G.; Martinez-Reyes, S. Carbohydr. Polym. 1997, 34, 385-390 and references therein.
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Banerjee et al. received. CMHPG had MS range 0.15-0.35 and DS range 0.05-0.20. Both diutan and xanthan gums had high purity. The gum diutan was isolated from the bacterial broth using enzymes to break down the bacterial cells that were removed by filtration and the biopolymer collected by precipitation. Xanthan was a food grade material collected by drying after removing salts and protein fragments at the lowest possible temperature to preserve the native structure. When not in use, the gums were stored in a desiccator to prevent sorption of moisture. Prepared solutions were not used consecutively for more than 3 days. The salts used were NaCl, CaCl2, HCOOK, CH3COONa, (NH4)2SO4 and MgSO4 obtained from either Merck (India) or SRL (India). They were Pro Analysis grade, and were used without further purification. Water, doubly distilled over alkaline permanganate and exhibiting conductance of 2-4 μS cm-1, was used in the experiments. Methods. Three methods, conductometry, isopiestic, and viscometry, were employed in the study. A brief description of each is presented below. Conductometry. The conductometry method used here was followed by Moulik et al.30 for the determination of hydration of polyvinylpyrrolidone (PVP) system. In brief, measurements were done with a Jenway (UK) (Model No. PCM-3) conductivity bridge. The measurements were taken in a constant temperature water bath with an accuracy of (0.1 °C using a dip type cell of cell constant K = 1.0 cm-1 under constant stirring condition. In the actual experiment, 10 mL of a 0.1% gum solution containing 0.01 M NaCl was taken in a container into which the cell was dipped. The initial conductance of the solution was measured. The 0.01 M NaCl solution was progressively added in small installments into the gum solution. After each addition, it was stirred well and allowed sufficient time to reach temperature equilibrium and then the conductivity of the solution was determined and noted. The procedure was followed until the biopolymer solution was diluted to 0.05%. A control study was performed for an aqueous solution of 0.1% gum in the absence of NaCl, which was diluted by water following the same protocol as with NaCl solution. The conductances of the control solutions were subtracted from the experimented solutions for processing the data as per the physicochemical requirement to be discussed later. Each experiment was duplicated to check reproducibility. Isopiestic Method. In this method, solid dried sample of a powdered gum of definite weight was taken in several accurately weighed sample bottles of special design. The bottles without lid were then kept enclosed in several specially designed desiccators containing concentrated sulfuric acid of varied strengths. The desiccators were then evacuated, and the sulfuric acid solutions were frequently stirred with magnetic stirrers for a week. It was considered that by this time the vapor pressure equilibrium between the hydrated samples present in the bottles and the sulfuric acid solutions in the desiccators was established. The sample bottles were then quickly taken outside the desiccators, closed with the lids, and weighed. From the difference between the final and the initial weights, the masses (mw) of the water vapor associated per gram of the gum in the samples were known. The concentrations of sulfuric acid in the desiccators were determined by the NaOH titration method. The corresponding values of the relative humidities (p/p0) at these acid concentrations were obtained from the standard table. By Raoult’s law, these p/p0 values were the water activities. The measurements were performed at a constant temperature of 303 ( 0.2 K. The standard deviation of the mw values were within 3%. Further details on the isopiestic method can be found elsewhere.30 The plot between mw (30) Bull, H. B. J. Am. Chem. Soc. 1944, 66, 1499–1507. Ghosh, N.; Datta, P.; Mahapatra, P.; Das, K. P.; Chattoraj, D. K. Biophys. Chem. 2001, 89, 201–217. Dan, A.; Ghosh, S.; Moulik, S. P. J. Phys. Chem B. 2008, 112, 3617–3624.
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Article and (p/p0) at a constant temperature then constitutes the sorption isotherm of water with gum. Viscometry. The viscosity measurements were also taken in a water bath with an accuracy of 0.1 K. A three-armed calibrated (with sucrose solution) Ubbelohde viscometer, used earlier by us30 was employed in the study. The flow time of 3 mL water through the bulb was 160 s at 303 K. As per an earlier report of Norman et al.,17 guar gum solutions of concentration e0.3% show Newtonian flow behavior, which is e0.1% according to Ma and Pawlik.31 The gum solutions herein used did satisfy this condition. The measurements were taken in the concentration range 0.01-0.2% at several dilutions, and the time of flow was noted at each concentration after proper temperature equilibration. Averages of three repeat experiments were considered for data analysis.
Results and Discussion Water Content. The gum samples used in this study were initially desiccated for removing the sorbed water in them. Desiccation to constant weight of definite amounts of the gums has shown the presence of water to the extents of 0.0786, 0.0994, and 0.0708 g/g for CMHPG, diutan, and xanthan, respectively. All through the study, the gum samples were kept in desiccating condition, and were only taken out during solution preparation. Sorption of Water Vapor by the Gums. The water vapor sorption property of the gums was studied by the isopiestic method as described above. The results are presented in Figure 1.The g/g adsorption of water vapor are plotted against water activity or p/p0 where p is the equilibrium vapor pressure, and p0 is the aqueous tension at 303 K. Although the curves have a Bruner-Emmet-Teller (BET, type III) isotherm like look, the results did not fit the BET plots. Exponential growth fitting procedure has yielded the maximum water adsorption extents of CMHPG, diutan, and xanthan as 0.826, 0.673, and 0.816 g/g, respectively, at p/p0 = 1. It may be mentioned here that isopiestic method applied to the water vapor sorption on the polymer polyvinylpyrrolidone (PVP) has evidenced formation of type III multilayer adsorption isotherm which has responded to BET treatment producing monolayer capacity of 0.40 g/g, and formation of trilayer on the PVP binding centers,30 at p/p0 = 1. The results have suggested that water molecules from the vapor phase formed three stacked layers on the binding centers (N centers) on the PVP at the maximum binding state. The total capacity of binding was thus 1.2 g/g of PVP. In the case of gums, there were plenty of oxygen centers in the molecules where water molecules from the vapor can get attached by hydrogen bonding. Formation of stacks (as in the case of PVP) was doubtful since the multilayer rationale of BET did not fit to the data. Calculation of water molecules bound per oxygen center on the gum could not be performed for want of accurate knowledge on the molar masses of the gums. It may be mentioned that water vapor sorption studies with carbohydrate polymers were also reported in the past.30 Hydration of Gums. In aqueous solution, the gums slowly get hydrated. Their dilute solutions look transparent. We have herein used low concentration (∼0.05-0.1 g dL-1) of the gums to study their hydration characteristics by the method of conductance. The eq 1 to be used to process the data has been found to obey in low concentration of the obstructant (here the gums) to ion conductance. The procedure has been described in the Method section. (31) Ma, X.; Pawlik, M. Carbohydr. Polym. 2007, 70, 15–24.
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Figure 1. Representations of Isopiestic plots for water vapor adsorption by gums. Here n represents moles of water/kilograms of gum, and a is water activity.
The following equation (a rearranged form of the equation used in earlier references)30,32,33 was used to monitor the hydration properties of the gums k ¼ 1þ1:93V h c k0
ð1Þ 0
where k is the specific conductance of 0.01 M NaCl, k is the conductance of the same electrolyte solution and c is the concentration of the gum expressed in g mL-1 at a constant temperature, and Vh is its hydrated specific volume in solution. The measurements were taken at different concentrations, and the Vh was determined from the slope of the least-squares linear plot between k/k0 and c. The nature of the observed plots is illustrated in Figure 2, where fairly good linear dependences were observed. The anhydrous specific volume of the gum (reciprocal of the anhydrous density) was subtracted from the Vh to evaluate mg, the g/g hydration of the gum. The slope, Vh, and the mg values for the three studied gums are presented in Table 1. The isopiestic and conductivity methods have produced data, which are quite interesting. The former method has yielded results ∼20-30 fold lower than the latter. This type of nonequivalence was not observed34 for the interaction of PVP with water molecules in the vapor form as well as in the associated form in solution when experimented following the same procedure herein followed. This difference can be reasoned out in the following way. In the isopiestic method, water vapor became adsorbed on the gum particles, penetration into the interior was minor. The extent of hydration was thus much lower than that in solution, where the individual gum molecules got the full share of hydration since all the hydrophilic centers were exposed to water. The Vh values were thus greater. The trapping of water molecules into the segmental folds/overlaps of the biopolymers increased Vh, and hence the hydration (Scheme 2). In the PVP molecule, the nitrogen centers were only solvated by the water molecules; there were no other potential water interacting centers in the molecule. Hence, at equal concentrations, PVP produced less interaction than the gums. It is accepted that on the average in solution 2-3 water molecules get bound to one oxygen center.35 The carbohydrate polymer (gums) have such centers in plenty, and hence, it is not unreal that they would show large hydration when measured using the method of conductance in solution where the biopolymer molecules have free access to organize their mole(32) Moulik, S. P. Electrochim. Acta 1972, 17, 1491 - 1496; 1973, 18, 981 - 986. (33) Mandal, A. B.; Biswas, A. M.; Ray, S.; Moulik, S. P. J. Phys. Chem. 1980, 84, 856–859. (34) Dan, A.; Ghosh, S.; Moulik, S. P. Langmuir 2007, 23, 7531–7538. (35) Moulik, S. P.; Gupta, S.; Das, A. R. Can. J. Chem. 1989, 67, 356–364.
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Figure 2. Conductivity plots for the determination of hydration of gums. Table 1. Slope (S), Vh and Hydration Values (mg)a,b of the Gums at 303K gum
S
Vh, mL . g -1
mg, g g-1
CMHPG 38.50 19.9 19.2 diutan 35.64 18.5 17.9 xanthan 46.14 23.9 23.3 a The measured anhydrous densities of the gums were 1.376, 1.500, and 1.596 for CMHPG, diutan, and xanthan, respectively. Their reciprocals, 0.727, 0.667, and 0.627 were their respective anhydrous specific volumes. At 303K, the density of water was taken to be unity to evaluate mg (g/g hydration of the gums). b Errors in mg were within (7%.
Scheme 2. Schematic Configurations of Gum in Aqueous and Salt Environments
cular configurations to maximize their interaction with water molecules. Viscosity of Gum Solutions. The relative viscosities of the aqueous gum solutions at different concentrations were determined at different temperatures, and the specific viscosity (ηsp) values were obtained from the relation ηsp = ηr - 1. The ηsp per unit concentration i.e., ηsp/C or reduced viscosity was then plotted against C to obtain the intrinsic viscosity according to the Huggins equation, ηsp ¼ ½ηþk½η2 C C
ð2Þ
where, [η] is the intrinsic viscosity, C is the gum concentration expressed in g dL -1, and k is the Huggins constant. A representative plot for the gums is exemplified in Figure 3. The derived [η] and k values are presented in Table 2. The plots in Figure 3 are fairly linear with good correlations except for diutan at 338 K. It is generally considered that 1/[η] is the highest polymer concentration to be used in the Huggins equation to evaluate [η]. Thus, in the studied range of temperature Langmuir 2009, 25(19), 11647–11656
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Figure 3. Trends of reduced viscosities in different concentrations of gum solutions (aqueous) in different temperatures: 1, CMHPG, 318 K; 2, CMHPG, 328 K; 3, Dn, 303 K; 4, Dn, 338 K; 5, Xn, 298 K; 6, Xn, 303 K. Table 2. Intrinsic Viscosity Values and Huggins Constants for Gums at Different Temperatures temp, K 298 303 308 318 328 338
[η]CMHPG, dL g-1
kCMHPG
68.40 53.60 48.29 45.93 36.15
0.15 0.34 0.39 0.56 1.04
[η]Dn, dL g-1
kDn
[η]Xn, dL g-1
kXn
41.65 40.27 34.74 29.90 22.04 14.44
0.13 0.20 0.38 0.55 1.33 3.74
55.16 50.33 46.73 37.66 31.13 27.20
0.19 0.37 0.66 0.76 1.05 1.29
(Table 2), the upper limits of concentration of the gums required to be used should range between 0.02-0.10 g dL -1. We have used C = 0.05 g dL -1 uniformly at all the temperatures used in the study. We considered that on the overall basis, the uniform range of experimented concentration, 0.05 g dL -1 would not incur serious errors in the reported [η]. We may mention that higher polymer concentrations than the required upper limit according to the above rationale have been used by others.22,23 Ma and Pawlik31 have shown a nice linear course in the concentration range of 0-0.08 g dL -1 for guar of [η] = 22 dL.g -1 in saturated LiCl solution The Huggins equation is meant for neutral polymers. The gums herein studied were polyelectrolyte type with weakly acidic (-COOH) groups. We have assumed that the Huggins equation was applicable to these weak polyelectrolytes. In the salt environment manifestation of their charge effect was restricted by way of electrostatic screening. In the studied concentration range, their ηr values were within 1.5 in salt solution so that Kraemer’s equation (subsequently described) was applicable. In the aqueous medium, the [η] of all the three gums has decreased with increasing temperature. For a 30 K increase, the percent decreases were 50, 53, and 65% for CMHPG, xanthan, and diutan, respectively. On a comparative basis, [η] followed the order CMHPG > xanthan > diutan; this was the effective sequence of the gums to obstruct the laminar flow of the solvent medium (water) in their solution. The decline in the reported [η] with temperature followed overall smooth trends. The [η] values obtained by the linear extrapolation method (Table 2) were thus reasonable. The configuration of the polymers in solution can also be assessed,36 on an overall basis, from the Huggins constant values. For flexible polymers in a good solvent, the values range between 0.20-0.40, the value is greater in poor solvent; in Θ solvent (36) (a) Tanford, C. Physical Chemistry of Macromolecules; John Wiley: New York. 1961; Chapter 6, p 392 ; (b) Pamies, R.; Cifre, J. G. H.; Martinez, M. d. C. L.; de la Torre, J. G. Colloid Polym. Sci. 2008, 286, 1223–1231.
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Figure 4. Plot of log[η] vs T-1 for the estimation of A and Ea of the gums.
k =0.50-0.70. For uncharged solid spheres, the k value should be ∼1.0, and 2.0 in good and poor solvent, the constant for flexible coils is greater than 0.50. In a Θ solvent, k = 0.50-0.70 The constant k is also sensitive to molecular aggregation. At 298 K CMHPG, diutan and xanthan have shown k values of 0.15, 0.13, and 0.19 respectively. The k values have increased with temperature. Solvation of the biopolymers has lessened with increasing temperature-causing increase in k, which has even exceeded the value of 2.0 (expected for sphere) for diutan at 338 K. At 318 K, the criterion of the Θ solvent was applicable to the aqueous solution of the gums for the values of k were 0.56, 0.55, and 0.76 for CMHPG, diutan, and xanthan, respectively. The [η] can be related through the shape factor of the solute species in solution and the partial specific volumes of the solute and the solvent,33,37 in the following way ½η ¼ υðvg þδvw Þ
ð3Þ
where vg and vw are the partial specific volumes (for dilute solutions, partial specific volumes have been considered specific volumes)of the solute (gum) and the solvent (water), respectively, δ is the g/g hydration of the solute, and ν is the shape factor of the solute in solution. For spherical geometry of the solute, ν = 2.5; for spheroids (prolates and oblates), ν > 2.5, and it depends on the axial ratio of the entity in solution. Considering the specific volumes of the gums equal to the reciprocal of their densities (see footnote a, Table 1), and the knowledge of their hydration in solution (determined by the conductance method and given in the table), their shape factors, ν have been estimated. The magnitudes obtained at 298 K were 3.3, 2.2, and 2.3 for CMHPG, diutan and xanthan, respectively. The shape factors were close to that of spheres (2.5) except CMHPG, which has shown spheroidal characteristic at 298 K. This has been an interesting observation found from the estimated overall hydration of the gums and the use of eq 3. Similar analysis on gums was not done in the past. According to reports27-29 polysaccharides are semiflexible random coils in solution. Their overall geometry can thus be modeled as sphere-like in line with the depiction of Robinson et al.27 In the illustration (Scheme 2), we have pictorially shown the possible states of water (free, trapped, and hydrated) associated with the random coil biopolymer entity. Similar experimental results at higher temperatures would add more insight into the molecular geometry of the gums which is contemplated to be examined in a future study. The temperature dependence of the [η] for the gums studied in the temperature range 298-338 K were processed in terms of (37) Tanford, C. Physical Chemistry of Macromolecules; John Wiley: New York, 1961; Chapter 6, p 391.
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Table 3. The Arrhenius Constant A and Ea Values of the Studied Gums gum
A, dL g-1
Table 4. Concentration Dependent Activation Energy (Ea) Values of the Gums
Ea, kJ mol-1
Ea, kJ mol-1 -1
CMHPG diutan xanthan
14.8 15.8 11.8
[gum], g dL
15.0 13.8 15.3
Arrhenius type equation (eq 4 shown below) to obtain the activation energy and the pre exponent coefficient A for the viscous flow according to the plots shown in Figure 4. The results are presented in Table 3. Thus, ½η ¼ AeðEa =RTÞ
ð4Þ
where A is a constant and Ea is the activation energy for the viscous flow of the gum solution expressed in kilojoules per mole (the average molar mass of the entire chain). The gums are fairly polydisperse but that does not bar them to be physicochemically characterized in terms of molar mass, Ea, etc. The comparison of their physical properties in terms of their structural/compositional variations is not a simple proposition, and such studies are scarce in literature. This has not been the main focus of the present study. It has been observed that the Ea values for CMHPG and xanthan were comparable but the value for diutan was smaller. The barrier for the viscous flow of diutan was less than that for CMHPG and xanthan. Zhang et al.25 have reported Ea for CMHPG to be 22.1 and 30.1 kJ mol -1; the results were fairly higher than the present findings. The reports on Guar and xanthan by Launay et al.29 were 13.2 and 17.2, respectively, which fairly agreed with this work. The past workers seldom used [η] in the evaluation of Ea (which conceptually accounts for the flow activation process of nonassociated gum molecules). The Ea values derived from processing [η] dependence on T are herein reported. They, we consider, have correctly represented the activation process. In the overall consideration, the activation process was moderate for the gum solutions. Dilute solutions of small carbohydrate and polyhydroxy compounds viz., glucose, sucrose, glycerol, mannitol, sorbitol, etc. have been reported38 to produce activation energies or enthalpies in the range 15-17 kJ mol -1. In comparison, the flow of dilute solutions of (high molecular weight) gum was energetically not appreciably different from the small carbohydrate molecules. The activation enthalpy for water has been reported to be 16.4 kJ mol -1 The comparable order of magnitude of the activation energy for the viscous flow of water with both small polyhdroxy molecules and the very large molecules of gums in dilute solutions has suggested that the activation for the flow of water molecules controlled the transport energetics of the studied biopolymer solutions determined from their [η] values. How the concentration of the solutes in solution could affect the energetic parameter should be an important and interesting aspect of analysis. For glucose, sucrose, and polyhydroxy compounds, the activation parameters for the viscous flow have been found to increase with concentration, initially linearly up to a concentration of 0.5 M and nonlinearly thereafter.38 We have, therefore, estimated the Ea values for the gums, at several concentrations: the results are presented in Table 4. Instead of ln[η] vs T-1, a plot of ln [ηsp] vs T-1 was used in the calculation of Ea. The concentration dependence of Ea has been found to be comparable among the studied gums. However, the activation energy for each gum has been found to decrease with concentration in contrast to what has been observed for solu(38) Moulik, S. P.; Khan, D. P. Indian J. Chem. 1978, 16A, 16–19.
11652 DOI: 10.1021/la901259e
0.010 0.020 0.035 0.050
CMHPG
Dn
Xn
10.2 6.88 4.12 0.97
14.4 9.35 5.36 1.72
15.0 7.87 5.69 3.98
tions of small carbohydrate molecules discussed above. The flow mechanisms for solutions of small molecules and large (polymer) molecules in moderate concentrations have been considered to be different. With increasing concentration, the biopolymers have formed associated entities with voids through which the solvent molecules have flown relatively easily and the requirement for activation was lower. It is thus recommended that, for a macromolecular solution, Ea should be measured from the temperature dependence of [η] and compared among different systems for a better understanding. It will be shown subsequently that, at C ∼ 0.02 g dL-1 (Table 6 and 7 below), the gum molecules start forming molecular association. The results in Table 4 have revealed that the Ea values started declining fairly also at C g 0.02 g dL -1. This aspect of viscosity dependence on simple carbohydrates, and macromolecules of carbohydrate origin require a detailed investigation. The intrinsic viscosity, Huggins constant, and the activation energy values are vital information on the nature of the gums in solution, which will be further discussed in a subsequent section. We have also studied the viscosity behaviors of the gums in salt solutions. It has been found that Huggins equation (eq 2) does not hold in salt solutions. Therefore, as done by others,22,39 we tried to use Kraemer’s equation,40 in the following form (eq 5) for the determination of the intrinsic viscosity. but without success. ln ηr ¼ ½ηþk 00 ½η2 C C
ð5Þ
k00 is Kraemer’s constant Of the other forms of equation tried, the following ln ηr - C form41 has been found to obey the present results on the studied gums in electrolyte solutions. ln ηr ¼ ½ηC
ð6Þ
Equation 6 is equivalent to the Kraemer equation for dilute solution where the second term on the right side is neglected. Figure 5 is a representative plot that shows the validity of eq 6. The intrinsic viscosities realized in the salt solutions of the gums are presented in Table 5. The results have evidenced that in the salt environments, the [η] of all the gums have followed similar trends as in aqueous solution but its magnitudes were significantly lower (cf. Scheme 2). However, (NH4)2SO4 strikingly reduced the viscosity of diutan at both the temperatures. The [η] values for each gum depended on the types of salts used. The monovalent electrolytes produced comparable viscosity reduction. The bivalent salts reduced the viscosity more but their effects were nearly comparable. Ma and Pawlik31 reported [η] of guar to be independent of the salt type and concentration
