Viscosity properties of gelatin in solutions of monovalent and divalent Salts

Korea-Australia Rheology Journal, Vol.25, No.4, pp.227-231 (November 2013) DOI: 10.1007/s13367-013-0023-8 www.springer.com/13367 Viscosity properties of gelatin in solutions of monovalent and divalent Salts Congde Qiao 1,2, *, Guangxin Chen 2, Yulong, Li 2 and Tianduo Li 2 1 School of Material Science&Engineering, Shandong Polytechnic University, Jinan 250353, PR China 2 Shandong Provincial Key Laboratory of Fine Chemicals, Shandong Polytechnic University, Jinan 250353, PR China (Received May 2, 2013; final revision received August 17, 2013; accepted August 19, 2013) The viscosity behaviors of gelatin with and without salts were examined in details by a rotational viscometer and a horizontal gravitational capillary viscometer, ranging from extremely dilute to entangled regimes. It was found that gelatin in salt free solution behaviors as neutral polymer in θ solvent. Polyelectrolyte effect can be found in extremely dilute regime. For gelatin/nacl solution, the concentration dependence of specific viscosity showed that gelatin behaviors as neutral polymer in good solvent. The two critical concentration c * and c e of gelatin solutions with and without NaCl moved from 4.0 wt% and 14.0 wt% to 2.0 wt% and 12.0 wt%, respectively. Addition of salts can improve the gelatin viscosity. It is the characteristic of a polyampholyte. The viscosity increased more significantly in CaCl 2 than that in NaCl solutions. Moreover, the effect of Ca 2+ is notable in gelatin solution with high concentrations, especially in entangled solutions. Keywords: gelatin, viscosity, polyampholyte 1. Introduction *Corresponding author: cdqiao@spu.edu.cn Polyelectrolytes are macromolecules bearing ionizable groups, which, in polar solvents, can dissociate into charged polymer chains(polyions) and small counterions. Common polyelectrolytes include proteins, nucleic acids and synthetic systems such as sulfonated polystyrene or polyacrylic acid. Polyelectrolytes play an important role in various fields ranging from materials science to biophysics. These polymers are widely used as food additives, coatings, drilling fluids, drag reducers, water purication membrane, superabsorbers and biomedical implants (Mandel, 1988; Hara, 1993). In many of these applications, polyelectrolytes are used as a viscosity modifier leading to improved performance and properties (Dautzenberg et al., 1994). Since polyelectrolytes are such a special class of polymers, a fundamental understanding of the viscosity behaviors of polyelectrolytes in different solvent conditions is needed. The behavior of polyelectrolytes with added salt is well understood in terms of the screening effect by simple ions of electrostatic interactions among fixed ions and can be described by scaling theory developed for neutral polymer solutions (Mandel, 1993). The addition of counterions screens the electrostatic repulsions between charges on the polymer backbone, which, in turn, allows the chain to collapse and assume a more compact conformation (Trotsenko et al., 2012; Jia and Zhao, 2009; Chun et al., 2009; Chun and Ko, 2012). However, the influence of counterion with various valencies is different (Jia and Zhao, 2009). Generally, in case of monovalent counterions, there is no specific interaction between the counterions and charged groups on the polymer backbone. The addition of counterions leads to an increase in ionic strength simply, which results in a decrease in Debye screening length(r D ). So the electrostatic interactions between charges on the polymer backbone are exponentially screened by the salt ions, resulting in reduction of the chain size with increasing salt concentration. At high salt concentration, the properties of polyelectrolyte solutions will be similar to those of neutral polymers. The situation is different for multivalent counterions. Firstly, a noticeable decrease in the chain dimension was observed at low salt concentration, and it can be due to ion-bridging effect (Pu et al., 2004) or the stronger attraction between the multivalent counterions and the charged chain (Kundagrami and Muthukumar, 2008). Secondly, the polyelectrolyte chain reexpansion when further increase the salt concentration, and it can be explained by overcharging effect (Grosberg, 2002; Hsiao and Luijten, 2006). However, chain re-expansion with monovalent salt has been observed in some polyelectrolyte solutions (Pu et al., 2004), which obviously cannot be understood in this way. One polyelectrolyte of wide application is the biopolymer gelatin. It is a product of the structural and chemical degradation of the insoluble fibrous protein collagen (Kozlov and Burdygina, 1983). The applications of gelatin in the food, pharmaceutical, and photographic industries are directly attributed to its conformation transition. Consequently, study of this conformation transition in gelatin solution has attracted considerable attention for over 50 2013 The Korean Society of Rheology and Springer 227

Congde Qiao, Guangxin Chen, Yulong, Li and Tianduo Li years (Stainsby, 1952; Bohidar, 1998; Wulansari et al., 1998; Li and Cheng, 2006; Olivares et al., 2006). Solution viscometry is one of the simplest methods for investigating the conformational changes of macromolecules in solution (Stainsby, 1952; Li and Cheng, 2006; Olivares et al., 2006). Generally, in salt-free polyelectrolyte solutions the electrostatic interactions between charged groups on the gelatin chains result in a strong chain expansion. The reduced viscosity increased with a decrease in gelatin concentration in vicinity of the crossover from dilute to semidilute solution regime (Ashok and Muthukumar, 2009). This abnormal viscosity behavior has been found in various polyelectrolyte solutions, including flexible (Katchalsky, 1951), semiflexible (Sharma et al., 2010), rigid spherical (Antonietti et al., 1996)and rod-like polyelectrolytes (Brodowski et al., 1996), and the major contribution to the polyelectrolyte effect is caused by intermolecular interactions. The viscosity behavior can be well described by scaling theory proposed by Dobrynin et al, which has been successfully applied in both the flexible polyelectrolytes solutions (Dobrynin et al., 1995) and some rigid polyelectrolyte systems (Zirnsak et al., 1999; Wyatt and Liberatore, 2009; Wyatt and Liberatore, 2010). The viscosity studies of gelatin solution with addition of salt were mainly carried out in the dilute and semidilute concentration regimes. Little experimental work has been done to examine the effect of added salt on gelatin viscosity in the entangled concentration regimes, especially for multivalent salt solutions. Recently, the viscosity behavior of entangled xanthan solutions was systematically investigated by Liberatore group (Wyatt and Liberatore, 2009; Wyatt and Liberatore, 2010; Wyatt et al., 2011). It was found that addition of salt to entangled polylectrolyte solutions resulted in an increase in viscosity, which was attributed to a local charge inversion and subsequent chain expansion between entanglements. It was also found that larger salt counterions produce higher viscosities, and it can be explained by an ion bridging effect. In this paper, the viscosity behaviors of gelatin with and without salts were examined in details ranging from extremely dilute to entangled regimes. The effects of salts with different valencies on gelatin viscosity were investigated. The work aims to shed light on the mechanism of leather tanning, which is directly related to the interaction between collagen and multivalent metal ions. Moreover, the influences of concentration on gelatin viscosity were discussed in details. 2. Experimental 2.1. Materials The gelatin sample (type A, limed-hide hydrolysis), having an approximate M W of 50,000, was purchased from Sinopharm Chemical Reagent Co., Ltd. The isoelectric point (pi) was approximately ph = 8.0 All of the chemicals were of analytical grade. The polyelecrolyte solutions of concentrations ranging from 0.01 wt % to 20 wt % were prepared by dissolving gelatin powder in deionized water. For the polyelectrolytes in monovalent salt solution, NaCl was subsequently added to the dissolved polymer solution. To ensure that the differences in viscosity reported here were caused by the addition of salt rather than slight variations in concentrations or preparation method, one parent polymer solution was made, divided, and salt added to one of the daughter solutions. The end result was two solutions of identical polymer concentration, but differing only in salt concentration (no salt vs. salt). The polymer solutions in deionized water are referred to as salt free solutions since no salt is added. When possible, the solutions were stirred using a magnetic stir bar for approximately 1 h before being allowed to rest for approximately. The ph of all the samples was adjusted to 8.0 with NaOH at 40.0 o C. 2.2. Viscosity measurement The viscosities of gelatin solutions with high concentrations (above 1.0 wt %) were determined by a Rotational viscometer (SNB-1, Shanghai Jingtian Co., Ltd). A rotor with cylinder geometry (of diameter 18.0 mm and length 65.0 mm) was used. The gap of the rotor-cylinder viscometer is 5 mm. A horizontal gravitational capillary viscometer with a capillary radius of around 0.05 cm (Shanghai Liangjing Glass Instrument Factory) was used to determine the viscosities for dilute gelatin solutions with low concentrations (below 1.0 wt %). The shear rates of this horizontal gravitational capillary viscometer are about 50-80 s -1. The method of determining viscosity of dilute gelatin solution can be found in our previous paper (Yuan et al., 2011). 3. Results and Discussion 3.1. Concentration dependence of specific viscosity in salt-free solution η η The specific viscosity η sp = ------------ s (where η and η s are η s the viscosities of the solution and pure solvent, respectively) is plotted against concentration for the gelatin aqueous solutions presented in Fig. 1. It was obvious that the specific viscosity showed a power-law dependence on concentration with the equation as follows: η sp c α (1) Two critical concentration c * (overlap concentration) and c e (entanglement concentration) identified as about 4.0 wt % and 14.0 wt %, respectively, which divided the gelatin solution into three regions. The power law scaling of η sp 228 Korea-Australia Rheology J., Vol. 25, No. 4 (2013)

Viscosity properties of gelatin in solutions of monovalent and divalent Salts Fig. 1. (Color online) Concentration dependence of specific viscosity (η sp ) for gelatin aqueous solutions at 40 o C with critical concentration c ** = 0.1 wt %, overlap concentration c * = 4.0 wt % and entanglement concentration c e = 14.0 wt % indicated by vertical dashed lines. Data points in filled triangle were measured with a horizontal gravitational capillary viscometer, and data points in filled inverted triangle were determined with a rotational viscometer. Fig. 2. (Color online) Concentration dependence of reduced viscosity (η sp /c) for gelatin solutions with(filled inverted triangle) and without(filled square) salt at 40 o C. Fig. 3. (Color online) Concentration dependence of specific viscosity (η sp ) for gelatin/nacl solutions at 40 o C with overlap concentration c * = 2.0 wt % and entanglement concentration c e = 12.0 wt % indicated by vertical dashed lines. Data points in filled triangle were measured with a horizontal gravitational capillary viscometer, and data points in filled inverted triangle were determined with a rotational viscometer. The concentration of NaCl solution is 1 mol/l. for our gelatin sample has an exponent ( α ) of 1.0, 2.0 and 3.5, belonging to the dilute, semidilute unentangled, and semidilute entangled regions, respectively, according to the scaling theory (Colby, 2010). It displays very similar concentration dependence of specific viscosity as the predictions for neutral polymer in θ solvent, which has also been found in gelatin aqueous solutions (Bohidar, 1998). In addition, for gelatin in water the Flory-Huggins interaction parameter χ =0.49 ± 0.01 is close to the θ solvent value of 1/2 (Guo et al., 2003). A third critical concentration c ** (about 0.1 wt %)can be found in Fig. 1, which divided the dilute gelatin solution into two parts, named dilute(0.1 wt % < c < 4.0 wt %) and extremely dilute(c < 0.1 wt %) regimes, respectively. In the extremely dilute regimes, the specific viscosity has a weak dependence on gelatin concentration ( η sp c 0.2 ), which means that the reduced viscosity will increase with decreasing of concentration, and it can be found in Fig. 2. The abnormal viscosity phenomena have been found in many polyelectrolyte solutions (Li and Cheng, 2006; Olivares et al., 2006). It should be pointed out that the capillary inner-surface of the viscometer has been treated hydrophobically to prevent the solution from being adsorbed on the glass surface of the capillary. The reason for this abnormal viscosity behavior should be resulted from the strong electrostatic interactions in extremely dilute solution. Obviously, addition of enough salts will eliminate this abnormal viscosity phenomenon, and this can be found in Fig. 2. The critical concentration at which the abnormal viscosity behavior occurs often falls in vanity of the overlap concentration. For gelatin, the critical concentration (c ** ) is far from the overlap concentration. It may be due to the strong interaction between gelatin and water, which needs further study. 3.2. Concentration dependence of specific viscosity in NaCl solution To test the effect of salt on viscosity of gelatin, chloride salts of monovalent alkali metals (NaCl) were investigated (Fig. 3). Qualitatively, the concentration dependence of specific viscosity in the presence of salts is similar to that Korea-Australia Rheology J., Vol. 25, No. 4 (2013) 229

Congde Qiao, Guangxin Chen, Yulong, Li and Tianduo Li Fig. 4. (Color online) The effect of salt concentration on viscosity of gelatin with different concentrations (NaCl in open symbols, and CaCl 2 in filled symbols) at 40 o C. The dot lines represent the salt free viscosity of gelatin solutions. of the salt free case. However, the power law scaling of η sp has a different exponent (α) of 1.0, 1.5 and 3.7, belonging to the dilute, semidilute unentangled, and semidilute entangled regions, respectively. Good agreements with the scaling predictions for a neutral polymer in a good solvent were observed in the dilute, semidilute unentangled, and semidilute entangled concentration regimes, respectively. It is concluded that the gelatin in sodium chloride solution behaviors as neutral polymer in good solvent, and the Coulombic interactions which the polymer experiences in salt-free aqueous solution are fully screened. Moreover, absence of the critical concentration (c ** ) validates the shielding of the electrostatic interactions among gelatin molecules in sodium chloride solution. Interestingly, from Fig. 3 it can be found that the two critical concentrations c * and c e of gelatin in sodium chloride solution moved from 4.0 wt % and 14.0 wt % to 2.0 wt % and 12.0 wt%, respectively. The reason can be explained as follows. As gelatin are known as a polyampholyte, and the polymer chains collapse at its isoelectric point. Addition of salt made the gelatin chains expand due to the screening of electrostatic interactions. Thus the two critical concentrations decreased comparing to the saltfree condition. In addition, the intrinsic viscosity of gelatin can be obtained to be about 41 ml/g by a safe extrapolation (Fig. 2). Then the critical overlap concentration c * can be estimated by c 1 [ η] 2.5 wt %, which agrees well with the concentration indicated in Fig. 3. 3.3. Effect of salts with different ion valency on gelatin viscosity Fig. 4 shows the effect of salts with different valency on the gelatin viscosity. For the three gelatin solutions (with concentration of 1.0, 8.0 and 18 wt %, respectively) the effects of the two salts (NaCl and CaCl 2 ) are similar. The viscosities of gelatin increase with an increase in salt concentrations, due to the shielding of electrostatic interactions with addition of salts. However, for the same polymer solution the extent of increasing of viscosity is different for different salts. The viscosity increased more significantly in CaCl 2 than that in NaCl solutions. Because the ionic charge of Ca 2+ is higher than that of Na +, Ca 2+ can screen the electrostatic interactions more efficiently. Moreover, the effect of Ca 2+ is notable in gelatin solutions with high concentration, especially in entangled solutions. It was probably due to the forming of new crosslinks in gelatin/cacl 2 solution, which can be explained using the so-called overcharging effect (Grosberg et al., 2002). It needs study in further. 4. Conclusions The viscosity behaviors of gelatin with and without salts were examined in details ranging from extremely dilute to entangled regimes. For gelatin aqueous solutions, the concentration dependence of specific viscosity indicates that gelatin behaviors as neutral polymer in θ solvent. However, the polyelectrolyte effect can be found in extremely dilute regime. For gelatin/nacl solution, it is concluded that gelatin behaviors as neutral polymer in good solvent. The two critical concentration c * and c e of gelatin in sodium chloride solution moved from 4.0 wt % and 14.0 wt % to 2.0 wt % and 12.0 wt %, respectively. The viscosities of gelatin increase with an increase in salt concentrations. The viscosity increased more significantly in CaCl 2 than that in NaCl solutions. Moreover, the effect of Ca 2+ is notable in gelatin solution with high concentrations, especially in entangled solutions. Acknowledgments The work was financially supported by the Project of Shandong Province Higher Educational Science and Technology Program (No. J11LB13). References Antonietti, M., A. Briel and S. Förster, 1996, Intrinsic viscosity of small spherical polyelectrolytes: Proof for the intermolecular origin of the polyelectrolyte effect, J. Chem. Phys. 105, 7795-7807. Ashok, B. and M. Muthukumar, 2009, Crossover behavior of the viscosity of dilute and semidilute polyelectrolyte solutions, J. Phys. Chem. B. 113, 5736-5745. Bohidar, H.B., 1998, Hydrodynamic properties of gelatin in dilute solutions, Int. J. Biol. Macromol. 23, 1-6. Brodowski, G., A. Horvath, M. Ballauff and M. Rehahn, 1996, Synthesis and Intrinsic Viscosity in Salt-Free Solution of a 230 Korea-Australia Rheology J., Vol. 25, No. 4 (2013)

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