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Controllable Gelation of Methylcellulose by a Salt Mixture Yirong Xu,† Lin Li,*,† Peijie Zheng,‡ Yee Cheong Lam,† and Xiao Hu‡ School of Mechanical & Production Engineering and School of Materials Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 Received January 12, 2004. In Final Form: April 29, 2004 The effects of a salt mixture consisting of a salt-out salt (NaCl) and a salt-in salt (NaI) on the sol-gel transition of methylcellulose (MC) in aqueous solution have been studied by means of micro differential scanning calorimetry and rheometry. The salt mixture was found to have a combined effect from the salt-out and salt-in salts in the mixture, and the salt effect was dependent on the water hydration abilities of the component ions and ion concentration. At a fixed total salt concentration, the sol-gel transition temperature nicely followed a rule of mixing: Tp ) m1Tp1 + m2Tp2 where Tp, Tp1, and Tp2 are the gelation peak temperatures for the MC solutions with a salt mixture, NaCl, and NaI, respectively, and mi is the molar fraction of the salt component i in the salt mixture. The linear rule of mixing proved that the effects of NaCl and NaI on the sol-gel transition of MC are completely independent. In addition, the presence of a single salt or a salt mixture in a MC solution does not change the essential mechanism of MC gelation. Therefore, the sol-gel transition of MC can be simply controlled by a salt mixture consisting of a salt-out salt and a salt-in salt. The rheological results supported the micro thermal results excellently. But the gel strength of MC containing salts was influenced by both salt type and salt concentration.
Introduction Natural cellulose is insoluble in common aqueous solutions due to its tendency to form extensive intra- and intermolecular hydrogen bonds.1 When a certain fraction of hydroxyl groups are substituted by hydrophobic groups such as methyl groups or hydroxypropyl groups, some of the hydrogen bonds are canceled to result in water soluble hydrophobically modified cellulose.2,3 Methylcellulose (MC), a water soluble cellulose derivative with an appropriate degree of substitution between 1.4 and 2.0, can form thermoreversible hydrogels in water upon heating and subsequently dissolve upon cooling.3,4 The gelation of methylcellulose is well-known to be sensitive to temperature3-6 or coexisting solutes including ions.7 It is observed by micro differential scanning calorimetry (micro DSC) and rheology that upon heating a MC aqueous solution absorbs heat to form a gel within a relatively narrow temperature range.5,7 The rheological evidence for the gelation of MC is the appearance of a G′ plateau, which indicates the formation of a threedimensional gel network structure as a result of heating. The salt-containing MC solutions follow the same patterns of gelation as a salt-free MC solution, except that the salts tend to shift the sol-gel and gel-sol transitions to lower or higher temperatures as compared to those of the saltfree MC solution, depending on the salt type.7 We have found that the effects of various salts on the sol-gel transition of MC are excellently consistent with the so-called Hofmeister series,8 an order of ions ranked * To whom correspondence should be addressed. Tel: +65-6790 6285. Fax: +65-6791 1859. E-mail:
[email protected]. † School of Mechanical & Production Engineering. ‡ School of Materials Engineering. (1) Gels Handbook, Vol. 4; Osada, Y., Kanji, K., Eds.; Translated by Hatsuo Ishida; Academic Press: San Diego, CA, 2001. (2) Guenet, J. Thermoreversible Gelation of Polymers and Biopolymers; Academic Press: London, 1992. (3) Kabayashi, K.; Huang, C.; Lodge, T. P. Macromolecules 1999, 32, 7070. (4) (a) Sakar, N. Carbohydr. Polym. 1995, 26, 195. (b) Sarkar, N.; Walker, L. C. Carbohydr. Polym. 1995, 27, 177. (5) Li, L.; Shan, H.; Yue, C. Y.; Lam, Y. C.; Tam, K. C.; Hu, X. Langmuir 2002, 18, 7291. (6) Li, L. Macromolecules 2002, 35, 5990. (7) Xu, Y.; Wang, C.; Tam, K. C.; Li, L. Langmuir 2004, 20, 646.
in terms of how strongly they affect the hydrophobicity of a solute in water.7 According to this series, ions can be classified as either kosmotropes (structure makers) or chaotropes (structure breakers), referring to their abilities to stabilize or weaken the structure of water, respectively. In the typical Hofmeister order SO42- > F- > Cl- > Br> NO3- > I- > SCN-, ions on the left, called kosmotropes, are proved to accelerate the MC sol-gel transition, whereas ions on the right tend to delay the occurrence of MC gelation.7 These phenomena are correspondent to the salt-out and salt-in effects, respectively. Cations were reported to have much weaker effects on the water structuring than anions.7,9-11,16-18 Therefore, salt effects on gelation of MC depend mainly on the anion type.7 Although there have been some reports in the literature that dealt with the salt effects on gelation of MC7,12,13 as well as other gelling systems,9,10,14-18 these studies were all interested in the effects of a single salt on gelation. Since salts are usually present in practical cases (such as biological systems) as a multiple-salted system, however, effects of salts on conformation and activities of a solute should be considered as co-effects from all component salts in solution. So far, no reports have been found on effects of a salt mixture on gelation of hydrogels. In this report, a salt mixture consisting of a salt-out salt and a salt-in salt is added into a MC solution and its effects on the thermodynamic and viscoelastic properties of MC during gelation are investigated. This work is a continuation of our previous study on the salt-assisted and salt-suppressed (8) Hofmeister, F. Arch. Exp. Pathol. Pharmakol. 1888, 24, 247. (9) Byeongmoon, J.; Sung, W. K.; You, H. B. Adv. Drug Delivery Rev. 2002, 54, 37. (10) Starodoubtev, S. G.; Khokhlov, A. R.; Sokolov, E. L.; Chu, B. Macromolecules 1995, 28, 3930. (11) Barbara, H.; Noel, T. S.; Vojko, V.; Ken, A. D. J. Am. Chem. Soc. 2002, 124, 12302. (12) Hirrien, M.; Chevillard, C.; Desbrieres, J.; Axelos, M. A. V.; Rinaudo, M. Polymer 1998, 25, 6251. (13) Kundu, P. P.; Kundu, M. Polymer 2001, 42, 2015. (14) Piculell, L.; Nilsson, S. J. Phys. Chem. 1989, 93, 5596. (15) Kabalnov, A.; Olsson, U.; Wennerstroem, H. J. Phys. Chem. 1995, 99, 6220. (16) Muta, H.; Kawauchi, S.; Satoh, M. J. Mol. Struct. (THEOCHEM) 2003, 620, 65. (17) Muta, H.; Koji, I.; Emi, T.; Satoh, M. Polymer 2002, 43, 103. (18) Collins, K. D. Biophys. J. 1997, 72, 65.
10.1021/la049907r CCC: $27.50 © 2004 American Chemical Society Published on Web 06/16/2004
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Table 1. Compositions of MC Aqueous Solutions Containing a NaCl-NaI Salt Mixture molar ratio of NaCl/NaI compositions
0.4 M NaCl
3:1
1:1
1:3
0.4 M NaI
NaCl (M) NaI (M) MC
0.4
0.3 0.1
0.2 0.2 0.03 mM
0.1 0.3
0.4
sol-gel transitions of MC solutions.7 The gelation of MC is expected to be controllable by a salt mixture containing a salt-out salt and a salt-in salt. Experimental Section Materials and Sample Preparation. A cellulose derivative, methylcellulose with a trade name of SM4000, was kindly supplied by Shinetsu Chemical Co. Ltd., Japan. The polymer had an average degree of substitution (DS) of 1.8 and a weightaverage molecular weight of 310 000 determined by light scattering. The viscosity range reported by the manufacturer was 4.54 Pa‚s at 20 °C for a 2 wt % aqueous solution. The material was used as received without further purification. Prior to use, it was vacuum-dried at 55 °C for 24 h and kept in a desiccator at room temperature. NaCl and NaI were purchased from Sino Chemical Co. (Pte) Ltd., Singapore, and used as received. To study the effects of various salts, the MC concentration was fixed at 0.03 mM (approximately 0.93 wt %) within the appropriate concentration range of 0.3-2.5 wt %.5 A pure MC solution of 0.03 mM was first prepared by dispersing the weighed MC powder in deionized water at about 70 °C. After being shaken well, it was then kept in a refrigerator for 24 h. After a homogeneous MC solution was obtained at the low temperature, the weighed salts or salt mixtures were added to prepare the test samples. NaCl and NaI, which are classified as typical salt-out and salt-in salts, respectively,7,12-13 were chosen as components to make the salt mixture. The total salt content in each solution was fixed to be 0.4 M, and the ratio of NaCl to NaI in the salt mixture was varied. This is to ensure that the effects of salts are only due to varying the salt ratio rather than the total salt concentration. Since the effects of single salts have been observed to be a linear function of salt concentration for all the salts used previously,7 it is interesting to know how the effects from the two salts with opposite effects can be simply added up. The compositions of five solutions of MC containing a single salt or a salt mixture are listed in Table 1. Micro Thermal Analysis. A micro differential scanning calorimeter (VP-DSC Microcalorimeter, Microcal Inc.) was used to determine the thermal properties of a salted MC solution during a heating or cooling process. For micro thermal analysis, the sample underwent heating from 10 to 85 °C and subsequently cooling from 85 to 10 °C. A slow heating (or cooling) rate of 1 °C/min was employed to allow an efficient heat transfer between the sample and the heater.5 The respective salt solution (no MC) with the same salt content was used in the reference cell. After each cycle was completed, both sample and reference cells were thoroughly cleaned by a continuous flow of deionized water to confirm a noncontamination condition before the next test. Rheological Measurements. The rheological tests were conducted on a fluid rheometer (ARES 100FRTN1, Rheometric Scientific), which was equipped with two sensitive force transducers for torque measurements ranging from 0.004 to 100 g‚ cm. A parallel plate of 25 cm diameter was used in this work. The dynamic viscoelastic properties such as storage modulus G′ and loss modulus G′′ were measured as a function of temperature. All tests were carried out at an angular frequency of 1 rad/s within the linear viscoelasticity range.
Results and Discussion Effects of Single Salts on Thermal Behavior during Gelation. When one salt is added into a MC aqueous solution, the thermodynamic properties during gelation and degelation will be changed correspondingly. Various salts show significantly different effects depending mostly on the anion type. Some salts (e.g., NaCl) can cause the sol-gel transition to occur at lower temperatures,
Figure 1. Relative thermal capacity Cp as a function of temperature during a heating process at 1 °C/min for 0.03 mM MC solutions in the presence of various concentrations (M) of a salt-out (NaCl) salt and a salt-in (NaI) salt.
whereas others (e.g., NaI) may shift the transition to higher temperatures from a salt-free MC solution. These phenomena are termed the salt-out (or salt-assisted) and saltin (or salt-suppressed) effects, respectively.7 The typical micro DSC heating curves for 0.03 mM MC aqueous solutions in the presence of single salts (NaCl or NaI) with increasing salt concentration are shown in Figure 1. The curve with the cross symbols (×) in the center represents the salt-free 0.03 mM MC solution, the curves on the left stand for the MC solutions mixed with various concentrations of NaCl, and the curves on the right are those with various concentrations of NaI. The addition of NaCl causes the peak to shift to the lower temperature, and at the same time the peak height is enhanced. The higher the NaCl concentration, the more the peak shifts and is enhanced. NaI, in contrast, tends to increase the peak temperature. With the same concentration of salt, the absolute shift of the peak temperature caused by NaI is much smaller than that by NaCl. The peak temperature (Tm) in the heating curve is defined as the gelation peak temperature of MC, and the integrated peak area is considered as the enthalpy change (∆H) or the energy required for the gelation.5,6 The continuity and patterns of these two groups (i.e., the salt-out and salt-in) of curves indicate that the thermal behaviors involved in all salt-containing MC solutions are similar. In other words, addition of the salts to a MC solution does not change the mechanism of gelation, but the salt-out salt (e.g., NaCl) accelerates or assists the gelation and the salt-in salt delays or suppresses the sol-gel transition. The gelation temperature in a single-salted MC solution is observed to be a linear function of salt concentration as can be found in Figure 2. The up-going line of NaI stands for the salt-in effect, whereas the down-going line of NaCl with a negative slope represents the salt-out effect. The absolute value of the slope for the latter (NaCl) is greater than that of the former (NaI), implying that NaCl causes stronger effects on gelation of MC than NaI at the same salt concentration. We have been studying the salt-out and salt-in effects on the thermal behavior of MC gelation and proposed the
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Figure 2. Peak temperatures during gelation for 0.03 mM MC solutions with single salts (NaCl or NaI) as a function of salt concentration (M).
mechanisms for the effects of anions on the hydrophobicity of MC in water.7 It is widely accepted that the good solubility of MC in water at low temperatures is due to the formation of cagelike water structures or hydrogen bonds around MC chains,3,4,19 and these structures will be destroyed upon heating, causing the formation of hydrophobic aggregates and further the formation of a gel network structure. According to the Hofmeister theory,8 the addition of a salt will affect the water structure through the interactions between ions and water molecules. Cl-, which is categorized as a salt-out ion, exhibits strong interactions with water molecules. Therefore, Clions tend to compete with MC chains for water molecules, and they can attract more water molecules to surround them due to their strong hydration abilities, leading to a poorer solubility of MC in water. As a result, at the same temperature, there are more hydrophobic MC aggregates in a salted MC solution than a salt-free one. Thus upon heating it will then be easier to meet the requirement for the critical number of hydrophobic aggregates for a gel to be formed, so that the gelation occurs at a lower temperature in the presence of a salt-out salt. I-, a typical salt-in ion, in contrast, shows weak interactions with water molecules, which do not destroy the cagelike structures surrounding MC chains. Instead, these big single-charged ions, acting like hydrophobic molecules, tend to disperse the MC chains from interconnection or association. Thus, upon heating it is more difficult for MC chains to form hydrophobic aggregates, leading to more energy or a higher transition temperature for gelation. The above mechanism makes it clear that salts affect the gelation or degelation of MC in solution through a water-restructuring process rather than direct interactions with MC chains. Therefore, the tendency and degree of salt effects depend on the salt-out (or salt-in) strength of respective ions and their concentration. When a saltout salt and a salt-in salt coexist in a MC solution, if there are no interactions between the salt-out salt and the saltin salt to affect the water structuring, the coexisting ions in the system are supposed to deliver their influences independently, and the salt effects will be observed as a combination of contributions from component ions. This hypothesis will be examined below. Effects of a Salt Mixture on Gelation. The DSC heating curves for the MC solutions with NaCl-NaI salt mixtures of different salt ratios are presented in Figure 3. The total salt concentration was 0.4 M for every solution sample. (19) Haque, A.; Morris, E. R. Carbohydr. Polym. 1993, 22, 161.
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Figure 3. Relative thermal capacity Cp as a function of temperature during a heating process at 1 °C/min for 0.03 mM MC solutions in the presence of salt mixtures with various NaCl versus NaI ratios. The curve with stars (*) represents the salt-free 0.03 mM MC solution.
As shown in Figure 3, the curve of the pure salt-out sample (i.e., 0.4 M NaCl) appears on the leftmost side with the lowest transition temperature. With increasing NaI content in the salt mixture, the curve shifts smoothly to higher temperature with a reduced peak height, until ending at the pure salt-in one (i.e., 0.4 M NaI). The greatest salt-out effects appear in the MC solution with 100% NaCl. The salt-out strength of the salt mixture then decreases gradually with reducing NaCl and increasing NaI by following the sequence of 3:1, 1:1, and 1:3. The peak temperature of the NaCl/NaI ) 1:3 sample is 61.5 °C, which is very close to that (61.1 °C) of the salt-free MC solution, indicating that the salt mixture made up of 1 mol of NaCl and 3 mol of NaI has nearly no effect on the gelation temperature. This is like an acid-base neutralization process where at the equivalence point the acid has been completely neutralized by the base. However, although the transition peak temperatures are similar between the salt-free solution and the 1:3 salted solution, the different peak heights indicate the difference in the total endothermic heat. This is attributed to the fact that in the heating process the extra energy has to be used to destroy the interactions between the salt ions and water molecules. After the DSC curve passes over the salt-free MC solution one, it starts to show the salt-in effect. All these DSC curves show a similar shape or pattern, and they shift to left or right along the temperature axis depending on the salt type and concentration. These results strongly suggest that the gelation mechanisms involved in all MC solutions are similar. As the MC concentration and the total salt concentration in all the solutions are the same (i.e., 0.03 mM of MC and 0.4 M of salts), the differences in the transition temperature and enthalpy change are only due to the different salt compositions. Therefore, the gelation of MC should be adjustable by a designed salt mixture. According to the mechanism proposed, in a MC solution mixed with NaCl and NaI, Cl- in the solution tends to destroy the cagelike structures between MC chains and water molecules, increasing the hydrophobicity of MC in water. I-, on the other hand, tends to disperse the MC molecules or aggregates, preventing MC chains from interconnection, so as to increase the solubility as well as the stability of MC in water. Since both Cl- and I- affect the gelation of MC through the different mechanisms, their influences on the gelation of MC are expected to be independent of each other. Thus, the solubility of MC in water will be modified by a combined effect of salt-out and salt-in after a new balance between the salt-out and salt-
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Table 2. Peak Temperatures in the Micro DSC Heating Curves and Transition Temperatures for the Sharp Increase in G′ upon Heating for a 0.03 mM MC Solution Containing a NaCl-NaI Salt Mixturea peak temp in DSC, °C theoretical peak temp, °C transition temp in G′, °C a
pure NaCl
3:1
1:1
1:3
pure NaI
50.1 50.1 52.9
54.4 53.8 56.7
56.9 57.5 59.1
61.5 61.1 64.5
64.8 64.8 67.8
The total salt concentration in every sample was 0.4 M. The theoretical peak temperatures were calculated using eq 1.
Figure 4. Peak temperatures during gelation of 0.03 mM MC solutions containing NaCl-NaI salt mixtures as a function of NaI fraction. The total salt concentration in each sample was 0.4 M. The solid line was calculated using eq 1.
in effects is established, leading to a new gelation temperature. The peak temperatures in the heating process for the MC solutions with the salt mixtures are given in Table 2, and these temperatures are plotted in Figure 4 against the molar fraction of NaI in the salt mixture. From the results, a linear relationship exists between the peak temperature and the NaI fraction. A linear rule of mixing is then simply obtained:
Tp ) m1Tp1 + m2Tp2
(1)
where Tp, Tp1, and Tp2 are the peak temperatures for the MC solutions with a salt mixture, NaCl, and NaI, respectively, and mi represents the molar fraction of each salt component in the salt mixture. At the total salt concentration of 0.4 M (i.e., Figure 4), Tp1 and Tp2 are 50.1 °C (NaCl) and 64.8 °C (NaI), respectively. When 50.1 and 64.8 °C are used as Tp1 and Tp2, respectively, to calculate Tp using eq 1, the theoretical values of Tp (as shown in Table 2) are obtained, and they are plotted as the solid straight line in Figure 4. The excellent consistence between the experimental data and the solid line indicates that a linear rule of mixing is valid for the MC solution mixed with the salt mixture. This linear rule of mixing also proves our hypothesis that the effects of NaCl and NaI on the sol-gel transition of MC are independent. Any gelation temperature can be exactly predicted using eq 1 without doing real experiments. Within the concentration limit above which the salt or salt mixture may cause precipitation of MC, we can reasonably consider that the linear rule of mixing (i.e., eq 1) will be valid at any other total salt concentrations. Thus, the gelation of MC becomes controllable by using a tailored salt mixture of NaCl and NaI. Similarly, when other salts are used as components in a salt mixture and added into a MC solution, the modified gelation temperature of MC would depend on the total water hydration abilities of the component ions and the individual ion concentration. In other words, each component of the salt
Figure 5. Storage modulus G′ as a function of temperature in a heating process at about 1 °C/min for a 0.03 mM MC solution in the presence of various NaCl concentrations (M).
mixture would contribute its own part in affecting the sol-gel transition, and each contribution would be independent of the other. However, the above hypothesis for other salt pairs (not the pair of NaCl and NaI) would not be valid if there are strong interactions between the saltout salt and the salt-in salt used. Rheological Evidence. The calorimetric results showing the salt-out and salt-in effects on the gelation of MC can be further supported by the corresponding rheological measurements at the same rate of heating in a similar range of temperature. Figures 5-7 illustrate the temperature dependence of storage modulus G′ during heating for a MC solution (0.03 mM) with NaCl, NaI, and a NaClNaI mixture, respectively. The salt-out or salt-in effects are observed. Each curve of G′ follows a similar pattern. That is, for G′ there is a gradual increase at low temperatures and then a sharp increase within a narrow temperature range until a near plateau is eventually reached. The G′ plateau proves that a gel network structure is formed through the heating process.23 The temperature at which a sharp increase of G′ occurs is defined as the sol-gel transition temperature in the heating process. As shown in Figure 5, in the presence of NaCl, the G′ curve shifts to lower temperatures with increasing NaCl, and this is consistent with the DSC results shown in Figure 1. Different from the salt-out effect of NaCl, NaI causes the salt-in effect as shown in Figure 6, which causes the G′ curve for a MC solution to shift to higher temperatures with increasing concentration of NaI. Although a G′ plateau does not appear in the curves because a complete three-dimensional gel network structure has not been formed yet due to the temperature limit by the rheometer, the salt-in phenomenon is still significantly observed. Similarly to the results observed in Figures 1 and 2, the shift caused by NaI is (20) Clark, A. H. Properties of Biopolymer Gels. In Industrial Water Soluble Polymers; Finch, C. A., Ed.; Royal Society of Chemistry: Cambridge, 1996. (21) Carnali, J. O.; Maser, M. S. Colloid Polym. Sci. 1992, 270, 183. (22) Kim, J. Y.; Song, J. Y.; Lee, E. J.; Park, S. K. Colloid Polym. Sci. 2003, 281, 614. (23) Ferry, J. D. Viscoelastic Properties of Polymers, 3rd ed.; John Wiley & Sons: New York, 1980.
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Figure 6. Storage modulus G′ as a function of temperature in a heating process at about 1 °C/min for a 0.03 mM MC solution in the presence of various NaI concentrations (M).
Figure 7. Temperature dependence of storage modulus G′ in the heating process at about 1 °C/min for 0.03 mM MC solutions in the presence of salt mixtures with various NaCl versus NaI ratios.
less obvious than that of NaCl at the same salt concentration, implying the relatively weak salt effects of NaI on the sol-gel transition of MC. For example, the addition of 0.4 M NaCl to a 0.03 mM MC solution causes a temperature shift of about 11.2 °C, while the addition of 0.4 NaI causes a shift of less than 4 °C. The reason for the weak salt-in effect would be attributed to the hightemperature effect. As a salt-in salt enhances the sol-gel transition temperature with the mechanism of changing the interaction between MC chains and water molecules, the higher the temperature, the less salt-in effect is expected. When the temperature approaches the boiling temperature of water, the water structures are mainly destroyed by heat so that the effects of salts will be negligible. The second important feature observed in Figure 5 is that the G′ plateau increases with the salt concentration, indicating the strength (represented by G′ plateau) of the gels is affected by addition of salts and the gel becomes stronger with increasing salt concentration. This feature can be explained by the fact that NaCl tends to enhance the hydrophobicity of MC to result in a stronger hydrophobic association, and further a stronger gel network. In contrast, the gel strength is not much affected by the saltin salt (NaI) as shown in Figure 6. Consequently, a MC solution with a salt-in salt is expected to form a weaker gel network than a salt-free MC solution at the same concentration of MC. The rheological results for the MC solutions with the salt mixtures are presented in Figure 7. All the G′ curves follow a pattern similar to that of the salt-free solution or the single-salt-containing samples as shown in Figures 5
Xu et al.
and 6. From the 0.4 M NaCl-containing MC solution, the G′ curve shifts nearly in parallel to the right side with decreasing NaCl and increasing NaI, until the 0.4 M NaI one. The abrupt rising of G′ appears within a similar narrow temperature range, which is corresponding to the sharp endothermic peak of DSC (see Figure 3). As compared in Table 2, the peak temperatures determined by DSC are very close to those estimated from the sharp increase of G′, demonstrating the consistence between the thermal behavior and the viscoelastic one during the gelation of MC in a similar heating process. However, the transition temperature in G′ is always slightly higher than the peak temperature in DSC. Although this difference would be considered to be due to the different sensitivities in methodology between DSC and rheology, we explain it here in a different way. As discussed earlier, a MC solution absorbs heat to destroy the “cagelike” structures among MC chains and then to form hydrophobic aggregates, followed by the formation of a network structure. In such a procedure, DSC measures the endothermic energy used to destroy the cagelike structures for the formation of hydrophobic aggregates of MC, while the abrupt increase in G′ reflects the connection of the hydrophobic aggregates into a gel network. Therefore, the endothermic peak emerges first, followed by the abrupt increase of G′ in the subsequent step. As a conclusion, the difference between the peak temperature in DSC and the transition temperature observed by rheology is reasonable and inevitable. The MC solution containing 0.4 M NaCl shows the highest G′ plateau among all the curves in Figure 7. The G′ plateau is slightly reduced with decreasing NaCl and increasing NaI. In other words, for the formation of a threedimensional gel network at a fixed total concentration of salt(s), NaCl results in the strongest gel, followed by NaCl-NaI mixtures and eventually by NaI. As discussed in the early part of this report, the difference in the gel strength is attributed to the hydrophobicity of MC caused by the salt ions in water. The continuous change of the gel strength from NaCl to NaI through the NaCl-NaI mixture, as shown in Figure 7, strongly suggests that the salt-out and salt-in components in the salt mixture deliver their influences independently, and the gel strength of MC with a salt mixture depends on the composition of the salt mixture and the total salt concentration, which can be excellently correlated with the thermal results in Figure 3. Conclusions A salt mixture consisting of a salt-out salt (NaCl) and a salt-in salt (NaI) was added into a 0.03 mM MC aqueous solution, and the effects on the gelation of MC were investigated using micro DSC and rheology. The salt mixture did not change the pattern or mechanism of gelation of MC, but the gelation temperature was shifted to lower or higher temperatures according to the composition of the salt mixture. When the total salt concentration was fixed, the shift in the gelation temperature by the salt mixture was observed to be a linear function of salt ratio. The effects of salts in the mixture were independent of each other, and a linear rule of mixing was found for the effect of the salt mixture. The thermal behavior observed by micro DSC was nicely supported by the rheological properties. The gel strength indicated by the G′ plateau was affected by the salt type and salt concentration. When the salt concentration was the same, the gel strength depended on the water restructuring abilities of the composition salts in the salt mixture. In conclusion, the gelation of MC is controllable by a salt mixture consisting of a salt-out salt and a salt-in salt. LA049907R
