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Ultrasonic velocities in aqueous gellan solutions
Food Hydrocolloids Vol.7 nO.5 pp.407-415, 1993
Ultrasonic velocities in aqueous gellan solutions Y.Tanaka, M.Sakurai and K.J\iakamura Department of Polymer Science, Faculty of Science, Hokkaido University, Sapporo 060, Japan Abstract. The velocity of ultrasonic waves propagating through aqueous gellan solution and gel with and without sodium chloride was measured over a temperature range from 15 to Rl°e. On the whole. the difference in velocities between aqueous gellan solution and solvent, t1V. increases with cooling in a manner similar to most aqueous systems. On cooling. however, tl.V values do not increase. hut decrease in some cases. at a certain temperature (Tvl. Thc gelling temperature of the solution was determined by a falling ball method. Tv is in the vicinity of the gelling temperature and almost independent of the gellan concentration. While the gelling temperature increases with gellan concentration, on heating t1V values decrease steadily. A hysteresis loop can be observed in measuring the velocity in aqueous gcllan solution with falling and rising temperatures. The behavior of velocities was tentatively interpreted in terms of random coil-to-helix conformational change of the gel Ian molecule followed by dehydration of the water molecule. Similar results were obtained for gellan solutions containing sodium chloride.
Introduction
Gellan gum is an extracellular polysaccharide secreted by Pseudomonas elodea, one of the microbes. It consists of tetrasaccharide repeating sequence of 0glucose, n-glucuronic acid, o-glucosc, and L-rhamnose (1). Since o-glucuronic acid has a carboxyl group, gellan is a polyelectrolyte containing one carboxyl group per repeating unit. Gellan gum has potential industrial applications as a high-viscosity biogum, a suspending agent and a gelling agent. Transparent solutions can be obtained by heating the mixture of gellan and water in moderate concentration above 90°C. It forms a gel on cooling to room temperature, Gelation of gellan is essentially caused by thermal treatment. In the presence of a moderate concentration of cations, like Na -, gellan solutions form a more rigid gel. There have so far been acoustic studies on polymer solutions in connection with gelation. For instance, Dela and Rassing have measured the velocity of ultrasonic wave propagating through aqueous gelatin solutions, and determined the melting temperature on the basis of the change in ultrasonic velocities (2). Choi et al. have measured the change in ultrasonic velocities accompanying the gelation of egg white (3). Both results have indicated that ultrasonic velocity decreases with gelation. Choi et al. have proposed an interpretation of decrease in the velocity in terms of the degree of network. According to these papers, ultrasonic velocity measurement seems to be a useful tool for investigating properties of gelation. However detailed discussion about the ultrasonic velocity in solution during the gelation process has so far been limited. The present paper reports that, at first, a hysteresis loop appears in measuring the velocity in aqueous gellan solution with falling and rising temperatures. Then we describe the relationship between the behavior of ultrasonic velocity and 407
Yv'Tanaka. :vI.Sakurai and K.Nakamura
gelling temperature, determined by the falling ball method. The effect of addition of sodium chloride on gellan solutions is also shown.
Materials and methods The powdered gellan used was a deacetylated form supplied by San-ci Chemical Industries Co. Ltd. :'\iaCI, special reagent grade, was dried before use. The solutions were prepared by heating the mixture of powdered gellan and the solvent above 90°C, and pouring it into the cell maintained at a given temperature. Then. ultrasonic velocities were measured at various temperatures while cooling to 15°C. At the same time the gelling temperature was determined by the falling ball method. After cooling ultrasonic velocities were also measured while heating up to 81°C. The falling ball experiment was carried out to observe the melting of gellan gel.
Apparatus Ultrasonic velocities in solutions and in gels were measured at a frequency of 5 MHz by using a sing-around velocimeter (4) constructed in our laboratory. The schematic view of the instrument is shown in Figure I. L in Figure 1 is the effective path length and obtained by direct calibration based on sound velocities in water at various temperatures reported by Del Grosso and Mader (5). T is the measured value of the pulse repetition period (PRP). If we designate velocity and PRP in water by V w- T w respectively, and similarly in sample solution, Vs ' 1~, the following relationships hold.
The value of tc denotes the sum of the electrical delay of the velocirneter's cell
Fig. I. Schematic view of sing-around velocimeter. The cell is made of glass. Transducers are made of ceramic and 10 mm in diameter. J. is -5 cm.
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circuit. T he va lue of Vs can he give n rela tive to that in Vw by the fo llowing equatio n .
(1)
In ge neral, th e sing -aro und tec hniq ue is good at measuring th e ve loci ty in sa mple so lutions rel ati ve to that in so lve nts rath er tha n measuring absolute va lues. In t his study we used two ce lls whose characte ris tics are qu ite similar and measured ve locities in ge llan so lutio ns a nd so lve nts sim ulta neous ly. Th e re produc ibi lity o f th e measu re ment was es tima ted to be better th an 5 cm/s ove r the tempe rature ran ge fro m 15 to 70"C. The te mpe rat ure of the the rmost at was maint ain ed to within 0.0 IDC by mean s of th e laboratory-m ad e co ntro lle r usin g a Y-eut 10 MHz q ua rtz as a temperature se nso r. Th e T eflo n ball used for th e falling ball experiment was 4 mm in diameter and 70 mg in gravity. Results T he differences in ultrasonic velocities be tween aqueous gellan so lutions of var ious co nce nt ra tio ns and pure wa ter, ~ V . are plotted in Figure 2 as a fu nction o f tempe ra ture. The short arrows in Figure 2 show th e ge lling temperat ur e for each so lut ion determined du ri ng cooling by fa lling ball method . The melting temper ature of gc llan ge ls co uld no t be determ ined becau se ge l-to -so l t ran sition was no t o bse rved within th is temper ature ran ge except for th e n.5 1% solution which melted at 81°e. The ~ V ve rsus te mperature plo t gave a negati ve slope whic h becam e more stee p as th e co ncentra tio n of th e so lutions incr eased . It is we ll kn own that th e differ en ces in ve loci ties, ~ V, observed for man y aq ueous so lutions incr ease steadily with lo wering tem peratu res (6) . O n the who le , th is was also tru e fo r the ge lla n so lutio ns stud ied . D uring cooling, ho wever , it was ev ide nt that an ano ma lo us ve loci ty cha nge took place a ro und th e gel ling temper ature, that is, ~ V values did not increase with lowerin g temperatures but see me d to change littl e . Further cooling again bro ug ht abo ut the steady incr ease in ~ V. O n th e other hand, the heatin g process from 15 to 8 l oe re sult ed in a steady decrease in ~ V over th e whol e temperature range st udie d, and therefore a di stin ct hysteresis could be observe d . The hysteresis eff ect appe ared more ap precia ble as the concentrat ion incr eased. In orde r to investigate the effec t of added electrolytes o n ge llan so lutio n, aqueous NaCI so lutio ns of gcllan were prep ar ed . and ult rasonic veloci ties were measu red du ring cooling. Th e so lvents used were 20 , 40 a nd 60 rnrnol/dm' NaCI sol utions with a pol ymer co nce nt ratio n of 0.8 1%. The diffe re nces in ultrasoni c ve locities bet ween 0.81% ge llan so lut ions and va rious so lvents, ~ V , ar e plotted in Figure 3 against te mper ature . ~ V va lues in th e 0.8 1°1.. gella n so lutio n in pure wa te r are also show n in Figure 3 by diam ond-shap ed sym bo ls fo r co mpar ison. 409
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The gelling temperatures determined by the falling ball method are 30, 34, 37 and 41°C for 0,20,40 and 60 mmol/drn' respectively. Above 45°C, the plot of ~ V for each solution falls on the same smooth curve which increases steadily with cooling. As the temperature is lowered further, ~V values first decrease and then increase again. It is significant that the temperature at which the ~ V value first decreases is in the vicinity of the gelling temperature. ~ V values in aqueous gellan solutions of various concentrations around the gelling temperature were measured in detail during cooling. The results are shown in Figure 4. Short arrows in Figure 4 designate the gelling temperature. Each gellan solution shows a unique maximum ~ V value at a certain temperature. This temperature, denoted by Tv, has little dependence on 410
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polyme r co ncen trat ion as indicated by th e das hed line in Figure 4. O n the other hand , the gelling tempe ratu re (1',,) rises noticeably with po lymer conce ntration . It is interest ing to note that Tg. > Tv for higher conce nt ra tions and the reverse was found for lower concentra tions . Th e same tendency can be observed in ultrasonic velocity measure ment during coo ling for gellan solution using a 20 rnrnol/drrr' aqueous ;'\IaCI so lution as a solvent . T he diffe ren ces in velocities between in gellan solutio ns and in solve nts, ~ V, ar e plott ed in Figure 5 as a function of temper ature . Sho rt arrows in Figure 5 also show the ge lling te mperature . Similar ly in the case of using water as solvent , values of Tv show litt le dependence on gellan co ncentration as indicated by the das hed line in Fig ure 5. While the T g va lue rises with concentratio n, both va lues of Tv and T g rise as compared with aq ueo us solvent systems. Discussion
Temp erature dependence of ultrason ic velocities in aqueous gellan solutions The ult rasoni c velocity in gellan solutio ns measur ed dur ing th e coo ling process has a characte ristic temperature depe nden ce around the gelli ng temperature Tg , where va lues of ~ V do not increase wit h decreasi ng temperature but seem to change litt le , or rather that ~ V value s decrease initia lly. We believe that the var iation o r ultrason ic velocity can be attributed to the change of solve nt st ruct ure. Robin son et al. have pro posed th at the gcllan molecule ex ists in solu tion as a disordered coil at high temperature and that it converts reve rsibly to a n ordered helix on cooling (7) . Wada et al. have studied the dependence or ultraso nic ve locities o n pH in aqueo us po lyglutamic acid (PGA) solutio ns in the
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helix-coil transition region (8); the PGA molecule exists in aqueous solution as a random coil at high pl-l. but changes its conformation continuously to the a-helix with decreasing pH. In the case of aqueous PGA solution, the amount of dissociated side chains as well as free C=O and NH groups in the main chain decrease with decreasing pH, and consequently thc number of water molecules associated with PGA molecule decrease. Their results indicate that with decreasing pH the velocity decreases, i.c. adiabatic compressibility of solution increases. They have interpreted an increase in compressibility in terms of the dehydration of water molecules associated with polymer chains, considering that bound water is less compressible than free water. The coil-to-helix transition of gellan solutions might result in the change in solvation behavior, that is, dehydration like the PGA solution. Thus the decrease in LiV around T g can be interpreted as the increase in free water owing to dehydration, which is accompanied by a positive compressibility change. The intramolecular conformational change may be the reason why the 1~ value appears to depend only little on gellan concentration, i.e. the amount of polymer chain, as shown in Figures 4 and 5.
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The results of the falling ball experiments reflect the formation of networks of polymer chains. In an aqueous gellan solution of each concentration a threedimensional network, infinite in extent, is first formed at '1~IJ,' Robinson et al, have proposed that the formation and aggregation of ordered helices arc the mechanisms of gelation of gellan (7). That is, networks come from intermolecular aggregations. Therefore values of I~IJ, depend on the gellan concentration as indicated in Figures 4 and 5. If the formation of ordered helices can be estimated quantitatively, it is expected to elucidate the mechanism of gelation from the point of view of whether formation and aggregation of ordered helices are stepwise or simultaneous. On the whole, during heating ~ V values are lower than those of the cooling process as shown in Figure 2. Such a hysteresis effect must be closely related to the fact that the gelling temperature is much higher than the melting temperature. The ~ V values of heating approach those of cooling gradually as the temperature rises. This behavior of velocities might indicate that during 413
Y.Tanaka, M.Sakurai and K.Nakamura
heating the conformation of gellan molecules converts from ordered helix to disordered coil gradually over a wide temperature range. Consequently it can be considered that partial melting of gellan gels take place. The presence of ;\la-will inhibt mutual repulsions of dissociated carboxyl groups in the polymer chain. As a consequence of this inhibition. the formation of ordered helices, and hence the aggregation of helices might be promoted. Therefore at the temperature at which the initial decrease in ~ V values rises, the gelling temperature also rises along with each solvent's NaCI concentration as shown in Figure 3.
Gelation oj gellan Eldridge and Ferry found a nearly linear relation between the logarithm of the concentration and the inverse of the melting temperature for gelatin gels (9). They regarded the cross-links holding gelatin gel together as a binary association of polymer chains. Using van't Hoff's law, they have proposed a method to determine the heat absorbed on forming a mole of junction zones; ~H
10giliC = 2.303 RT
(2)
+ constant
where C is the polymer concentration, T is the gelling temperature (or the melting temperature) in Kelvin degrees, ~H is the heat of reaction for the process: 2 moles cross-linking loci --,) I mole cross-links and R is the gas constant.
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414
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The relation between the logarithm of concentration and the reciprocal of the gelling temperature determined by the falling ball method for gellan gel with and without NaCI is shown in Figure 6. Since this relation was found to be linear. equation (2) might be effective in calculating the heat of reaction for gellan gel. The heat of the reaction of the formation of gellan gel was -63 kJ/mol and 98 kJI mol for the solution without and with 20 mrnol/drn ' NaCl, respectively. The previously reported value of the heat of gellan gel without salt obtained from the melting temperature is 38 kJ mol (10). This value is smaller than that calculated in the present work. The hysteresis phenomenon appears in the relationship between the gelling and melting temperature. It becomes more appreciable as increase in concentration. That is, in higher concentrations the difference between the gelling and melting temperatures appears more remarkable. This must cause our value, obtained from the gelling temperature, to be apparently larger than the reported value obtained from the melting temperature.
References 1. 2. 3. 4. 5. 6. 7.
Crescenzi.V. and Dcntini.M. (19R7) Carboliydr. Res.. 160,283-302. Dcla.K. and Rassing.J. (1987) Acta Chem. Scand., A32, 925-928., Choi,P.K., Bae.J.R. and Tak agi.K. (19R7) lap. 1. Appl. Phys., 26, (Suppl. 26-1). 32-J4. Grccnspan.M. and Tsehiegg.C. (1957) Re v. Sci. lnstr., 28, 897-901. Del Grosso,V.A. and Mader.C.W. (1972) 1. A COliS. Soc. Amer.: 52,1442-1446. Nornoto.O. and Endo.H. (1970) BIIII. Chern. Soc. Jpn, 43, 2718-2723. Robinson.G.; Manning,C.E. and Morris.E.R. (1991) Food Polym. Gels Coli. Royal Society oj
Chemistry, UK, 22-33. R. Wada,Yoo Sasabe,H. and Tomono.M. (1967) Biopolymers, 5, R87-897. 9. Eldridge.Ll.. and Fcrry.LD. (1954) J. Phys. Chem .. 58. 992-995. 10. Moritaka.II., f'ukuba.lIoo KumenoK.; Nakaharna.N. and l\"ishinari,K. (1991) Food Hydrocoll., 4, 495507.
415
Ultrasonic velocities in aqueous gellan solutions Y.Tanaka, M.Sakurai and K.J\iakamura Department of Polymer Science, Faculty of Science, Hokkaido University, Sapporo 060, Japan Abstract. The velocity of ultrasonic waves propagating through aqueous gellan solution and gel with and without sodium chloride was measured over a temperature range from 15 to Rl°e. On the whole. the difference in velocities between aqueous gellan solution and solvent, t1V. increases with cooling in a manner similar to most aqueous systems. On cooling. however, tl.V values do not increase. hut decrease in some cases. at a certain temperature (Tvl. Thc gelling temperature of the solution was determined by a falling ball method. Tv is in the vicinity of the gelling temperature and almost independent of the gellan concentration. While the gelling temperature increases with gellan concentration, on heating t1V values decrease steadily. A hysteresis loop can be observed in measuring the velocity in aqueous gcllan solution with falling and rising temperatures. The behavior of velocities was tentatively interpreted in terms of random coil-to-helix conformational change of the gel Ian molecule followed by dehydration of the water molecule. Similar results were obtained for gellan solutions containing sodium chloride.
Introduction
Gellan gum is an extracellular polysaccharide secreted by Pseudomonas elodea, one of the microbes. It consists of tetrasaccharide repeating sequence of 0glucose, n-glucuronic acid, o-glucosc, and L-rhamnose (1). Since o-glucuronic acid has a carboxyl group, gellan is a polyelectrolyte containing one carboxyl group per repeating unit. Gellan gum has potential industrial applications as a high-viscosity biogum, a suspending agent and a gelling agent. Transparent solutions can be obtained by heating the mixture of gellan and water in moderate concentration above 90°C. It forms a gel on cooling to room temperature, Gelation of gellan is essentially caused by thermal treatment. In the presence of a moderate concentration of cations, like Na -, gellan solutions form a more rigid gel. There have so far been acoustic studies on polymer solutions in connection with gelation. For instance, Dela and Rassing have measured the velocity of ultrasonic wave propagating through aqueous gelatin solutions, and determined the melting temperature on the basis of the change in ultrasonic velocities (2). Choi et al. have measured the change in ultrasonic velocities accompanying the gelation of egg white (3). Both results have indicated that ultrasonic velocity decreases with gelation. Choi et al. have proposed an interpretation of decrease in the velocity in terms of the degree of network. According to these papers, ultrasonic velocity measurement seems to be a useful tool for investigating properties of gelation. However detailed discussion about the ultrasonic velocity in solution during the gelation process has so far been limited. The present paper reports that, at first, a hysteresis loop appears in measuring the velocity in aqueous gellan solution with falling and rising temperatures. Then we describe the relationship between the behavior of ultrasonic velocity and 407
Yv'Tanaka. :vI.Sakurai and K.Nakamura
gelling temperature, determined by the falling ball method. The effect of addition of sodium chloride on gellan solutions is also shown.
Materials and methods The powdered gellan used was a deacetylated form supplied by San-ci Chemical Industries Co. Ltd. :'\iaCI, special reagent grade, was dried before use. The solutions were prepared by heating the mixture of powdered gellan and the solvent above 90°C, and pouring it into the cell maintained at a given temperature. Then. ultrasonic velocities were measured at various temperatures while cooling to 15°C. At the same time the gelling temperature was determined by the falling ball method. After cooling ultrasonic velocities were also measured while heating up to 81°C. The falling ball experiment was carried out to observe the melting of gellan gel.
Apparatus Ultrasonic velocities in solutions and in gels were measured at a frequency of 5 MHz by using a sing-around velocimeter (4) constructed in our laboratory. The schematic view of the instrument is shown in Figure I. L in Figure 1 is the effective path length and obtained by direct calibration based on sound velocities in water at various temperatures reported by Del Grosso and Mader (5). T is the measured value of the pulse repetition period (PRP). If we designate velocity and PRP in water by V w- T w respectively, and similarly in sample solution, Vs ' 1~, the following relationships hold.
The value of tc denotes the sum of the electrical delay of the velocirneter's cell
Fig. I. Schematic view of sing-around velocimeter. The cell is made of glass. Transducers are made of ceramic and 10 mm in diameter. J. is -5 cm.
408
Ultrasonic velocities in aq ueous gellan solution s
circuit. T he va lue of Vs can he give n rela tive to that in Vw by the fo llowing equatio n .
(1)
In ge neral, th e sing -aro und tec hniq ue is good at measuring th e ve loci ty in sa mple so lutions rel ati ve to that in so lve nts rath er tha n measuring absolute va lues. In t his study we used two ce lls whose characte ris tics are qu ite similar and measured ve locities in ge llan so lutio ns a nd so lve nts sim ulta neous ly. Th e re produc ibi lity o f th e measu re ment was es tima ted to be better th an 5 cm/s ove r the tempe rature ran ge fro m 15 to 70"C. The te mpe rat ure of the the rmost at was maint ain ed to within 0.0 IDC by mean s of th e laboratory-m ad e co ntro lle r usin g a Y-eut 10 MHz q ua rtz as a temperature se nso r. Th e T eflo n ball used for th e falling ball experiment was 4 mm in diameter and 70 mg in gravity. Results T he differences in ultrasonic velocities be tween aqueous gellan so lutions of var ious co nce nt ra tio ns and pure wa ter, ~ V . are plotted in Figure 2 as a fu nction o f tempe ra ture. The short arrows in Figure 2 show th e ge lling temperat ur e for each so lut ion determined du ri ng cooling by fa lling ball method . The melting temper ature of gc llan ge ls co uld no t be determ ined becau se ge l-to -so l t ran sition was no t o bse rved within th is temper ature ran ge except for th e n.5 1% solution which melted at 81°e. The ~ V ve rsus te mperature plo t gave a negati ve slope whic h becam e more stee p as th e co ncentra tio n of th e so lutions incr eased . It is we ll kn own that th e differ en ces in ve loci ties, ~ V, observed for man y aq ueous so lutions incr ease steadily with lo wering tem peratu res (6) . O n the who le , th is was also tru e fo r the ge lla n so lutio ns stud ied . D uring cooling, ho wever , it was ev ide nt that an ano ma lo us ve loci ty cha nge took place a ro und th e gel ling temper ature, that is, ~ V values did not increase with lowerin g temperatures but see me d to change littl e . Further cooling again bro ug ht abo ut the steady incr ease in ~ V. O n th e other hand, the heatin g process from 15 to 8 l oe re sult ed in a steady decrease in ~ V over th e whol e temperature range st udie d, and therefore a di stin ct hysteresis could be observe d . The hysteresis eff ect appe ared more ap precia ble as the concentrat ion incr eased. In orde r to investigate the effec t of added electrolytes o n ge llan so lutio n, aqueous NaCI so lutio ns of gcllan were prep ar ed . and ult rasonic veloci ties were measu red du ring cooling. Th e so lvents used were 20 , 40 a nd 60 rnrnol/dm' NaCI sol utions with a pol ymer co nce nt ratio n of 0.8 1%. The diffe re nces in ultrasoni c ve locities bet ween 0.81% ge llan so lut ions and va rious so lvents, ~ V , ar e plotted in Figure 3 against te mper ature . ~ V va lues in th e 0.8 1°1.. gella n so lutio n in pure wa te r are also show n in Figure 3 by diam ond-shap ed sym bo ls fo r co mpar ison. 409
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The gelling temperatures determined by the falling ball method are 30, 34, 37 and 41°C for 0,20,40 and 60 mmol/drn' respectively. Above 45°C, the plot of ~ V for each solution falls on the same smooth curve which increases steadily with cooling. As the temperature is lowered further, ~V values first decrease and then increase again. It is significant that the temperature at which the ~ V value first decreases is in the vicinity of the gelling temperature. ~ V values in aqueous gellan solutions of various concentrations around the gelling temperature were measured in detail during cooling. The results are shown in Figure 4. Short arrows in Figure 4 designate the gelling temperature. Each gellan solution shows a unique maximum ~ V value at a certain temperature. This temperature, denoted by Tv, has little dependence on 410
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Temperature / °C Fig. 3. Effect of ad d it io n of sod ium chlo ride o n a q ue o us gell an so lu tio n . tl.V valu e is th e diffe re nce in ult raso nic ve locit ies between ( U~I % ge llan solu tions a nd various so lvents . So lven ts co nta in :-.IaCI in the concentration of 0 mmolldm ' (0 ). 20 mrnol/drrr' ( 1\ ), 40 mm ol/drn' (:J ) and 60 mm o l/dm' (0 ).
,
polyme r co ncen trat ion as indicated by th e das hed line in Figure 4. O n the other hand , the gelling tempe ratu re (1',,) rises noticeably with po lymer conce ntration . It is interest ing to note that Tg. > Tv for higher conce nt ra tions and the reverse was found for lower concentra tions . Th e same tendency can be observed in ultrasonic velocity measure ment during coo ling for gellan solution using a 20 rnrnol/drrr' aqueous ;'\IaCI so lution as a solvent . T he diffe ren ces in velocities between in gellan solutio ns and in solve nts, ~ V, ar e plott ed in Figure 5 as a function of temper ature . Sho rt arrows in Figure 5 also show the ge lling te mperature . Similar ly in the case of using water as solvent , values of Tv show litt le dependence on gellan co ncentration as indicated by the das hed line in Fig ure 5. While the T g va lue rises with concentratio n, both va lues of Tv and T g rise as compared with aq ueo us solvent systems. Discussion
Temp erature dependence of ultrason ic velocities in aqueous gellan solutions The ult rasoni c velocity in gellan solutio ns measur ed dur ing th e coo ling process has a characte ristic temperature depe nden ce around the gelli ng temperature Tg , where va lues of ~ V do not increase wit h decreasi ng temperature but seem to change litt le , or rather that ~ V value s decrease initia lly. We believe that the var iation o r ultrason ic velocity can be attributed to the change of solve nt st ruct ure. Robin son et al. have pro posed th at the gcllan molecule ex ists in solu tion as a disordered coil at high temperature and that it converts reve rsibly to a n ordered helix on cooling (7) . Wada et al. have studied the dependence or ultraso nic ve locities o n pH in aqueo us po lyglutamic acid (PGA) solutio ns in the
411
Y.Tanaka, M.Sakurai and K.:'Iakamura
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50 Fi~. 4. Temperature dependence of ~ V, the difference in ultrasonic velocities between aqueous gcllan solutions of various conccntrations and watcr. The gelling temperatures are: 24°C (0.50°;',), noe (0.()7%). 29CC ((Ull%). :WC (J.()()%), WC (1.34%).
helix-coil transition region (8); the PGA molecule exists in aqueous solution as a random coil at high pl-l. but changes its conformation continuously to the a-helix with decreasing pH. In the case of aqueous PGA solution, the amount of dissociated side chains as well as free C=O and NH groups in the main chain decrease with decreasing pH, and consequently thc number of water molecules associated with PGA molecule decrease. Their results indicate that with decreasing pH the velocity decreases, i.c. adiabatic compressibility of solution increases. They have interpreted an increase in compressibility in terms of the dehydration of water molecules associated with polymer chains, considering that bound water is less compressible than free water. The coil-to-helix transition of gellan solutions might result in the change in solvation behavior, that is, dehydration like the PGA solution. Thus the decrease in LiV around T g can be interpreted as the increase in free water owing to dehydration, which is accompanied by a positive compressibility change. The intramolecular conformational change may be the reason why the 1~ value appears to depend only little on gellan concentration, i.e. the amount of polymer chain, as shown in Figures 4 and 5.
412
Ultrasonic velocities in aqueous gellan solutions
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Temperature / °C Fig. 5. Temperature dependence of c. V, the difference in ultrasonic velocities between aqueous gcllan solutions of various concentrations containing 20 mrnol/drn' :-laCI and its solvent. The gelling temperatures are: 33°C (0.50%), :WC (0.67°/,,), 36°C (0.81%), 38°C (1.0%). 41 cC (1.34%).
The results of the falling ball experiments reflect the formation of networks of polymer chains. In an aqueous gellan solution of each concentration a threedimensional network, infinite in extent, is first formed at '1~IJ,' Robinson et al, have proposed that the formation and aggregation of ordered helices arc the mechanisms of gelation of gellan (7). That is, networks come from intermolecular aggregations. Therefore values of I~IJ, depend on the gellan concentration as indicated in Figures 4 and 5. If the formation of ordered helices can be estimated quantitatively, it is expected to elucidate the mechanism of gelation from the point of view of whether formation and aggregation of ordered helices are stepwise or simultaneous. On the whole, during heating ~ V values are lower than those of the cooling process as shown in Figure 2. Such a hysteresis effect must be closely related to the fact that the gelling temperature is much higher than the melting temperature. The ~ V values of heating approach those of cooling gradually as the temperature rises. This behavior of velocities might indicate that during 413
Y.Tanaka, M.Sakurai and K.Nakamura
heating the conformation of gellan molecules converts from ordered helix to disordered coil gradually over a wide temperature range. Consequently it can be considered that partial melting of gellan gels take place. The presence of ;\la-will inhibt mutual repulsions of dissociated carboxyl groups in the polymer chain. As a consequence of this inhibition. the formation of ordered helices, and hence the aggregation of helices might be promoted. Therefore at the temperature at which the initial decrease in ~ V values rises, the gelling temperature also rises along with each solvent's NaCI concentration as shown in Figure 3.
Gelation oj gellan Eldridge and Ferry found a nearly linear relation between the logarithm of the concentration and the inverse of the melting temperature for gelatin gels (9). They regarded the cross-links holding gelatin gel together as a binary association of polymer chains. Using van't Hoff's law, they have proposed a method to determine the heat absorbed on forming a mole of junction zones; ~H
10giliC = 2.303 RT
(2)
+ constant
where C is the polymer concentration, T is the gelling temperature (or the melting temperature) in Kelvin degrees, ~H is the heat of reaction for the process: 2 moles cross-linking loci --,) I mole cross-links and R is the gas constant.
0.2 0.1
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1000 IT Fig. 6. The logarithm of the concentration of gellan gel versus the reciprocal of the gclling temperature. O. without NaCI; 6. with 20 rnrnol/drrr' NaCI.
414
Ultrasonic velocities in aqueous ~cllan solutions
The relation between the logarithm of concentration and the reciprocal of the gelling temperature determined by the falling ball method for gellan gel with and without NaCI is shown in Figure 6. Since this relation was found to be linear. equation (2) might be effective in calculating the heat of reaction for gellan gel. The heat of the reaction of the formation of gellan gel was -63 kJ/mol and 98 kJI mol for the solution without and with 20 mrnol/drn ' NaCl, respectively. The previously reported value of the heat of gellan gel without salt obtained from the melting temperature is 38 kJ mol (10). This value is smaller than that calculated in the present work. The hysteresis phenomenon appears in the relationship between the gelling and melting temperature. It becomes more appreciable as increase in concentration. That is, in higher concentrations the difference between the gelling and melting temperatures appears more remarkable. This must cause our value, obtained from the gelling temperature, to be apparently larger than the reported value obtained from the melting temperature.
References 1. 2. 3. 4. 5. 6. 7.
Crescenzi.V. and Dcntini.M. (19R7) Carboliydr. Res.. 160,283-302. Dcla.K. and Rassing.J. (1987) Acta Chem. Scand., A32, 925-928., Choi,P.K., Bae.J.R. and Tak agi.K. (19R7) lap. 1. Appl. Phys., 26, (Suppl. 26-1). 32-J4. Grccnspan.M. and Tsehiegg.C. (1957) Re v. Sci. lnstr., 28, 897-901. Del Grosso,V.A. and Mader.C.W. (1972) 1. A COliS. Soc. Amer.: 52,1442-1446. Nornoto.O. and Endo.H. (1970) BIIII. Chern. Soc. Jpn, 43, 2718-2723. Robinson.G.; Manning,C.E. and Morris.E.R. (1991) Food Polym. Gels Coli. Royal Society oj
Chemistry, UK, 22-33. R. Wada,Yoo Sasabe,H. and Tomono.M. (1967) Biopolymers, 5, R87-897. 9. Eldridge.Ll.. and Fcrry.LD. (1954) J. Phys. Chem .. 58. 992-995. 10. Moritaka.II., f'ukuba.lIoo KumenoK.; Nakaharna.N. and l\"ishinari,K. (1991) Food Hydrocoll., 4, 495507.
415