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Adsorption Behavior of Water Molecules onto α-, β-, and γ-Cyclodextrins and Branched α-Cyclodextrins
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
181, 326–330 (1996)
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Adsorption Behavior of Water Molecules onto a-, b-, and g-Cyclodextrins and Branched a-Cyclodextrins SEIKI TANADA, 1 TAKEO NAKAMURA, NAOHITO KAWASAKI, TAK AYUKI KURIHARA,
AND
YUKIHIRO UMEMOTO
Faculty of Pharmaceutical Sciences, Kinki University, Kowakae, Higashi-Osaka, Osaka 577, Japan Received December 27, 1995; accepted March 1, 1996
Moisture adsorption properties of a-, b-, and g-cyclodextrin (CD) and branched a-CDs were investigated to elucidate the basis of the results of adsorption isotherms of water at 10, 20, and 307C, the differential heat of adsorption, the entropy of the adsorbed water, and the heat of immersion in water. It is concluded that the water molecules were adsorbed on the hydroxyl groups of the glucose molecule of a-CD, were first adsorbed on that of b-CD and then were included in the pore structure of b-CD, and were only included in the pore structure of g-CD. Branched a-CDs were developed and produced to increase the interaction with water molecules. However, the interaction between the branched a-CDs and the water was smaller than that between the virgin aCD and the water. The solubility of branched a-CD required a long time because the heat of immersion was larger in the order G2-a-CD and G1-a-CD or a-CD. It is considered that the side chain, glucosyl and maltosyl groups, decreased the interaction between the water molecule and the surface of the CD because of steric hindrance. Therefore, it is concluded that the side chain inhibited the water adsorption of the inner molecule of CD. q 1996 Academic Press, Inc.
Key Words: cyclodextrin; adsorption of water, branched cyclodextrin; adsorption of water, differential heat of adsorption; water molecule, entropy of adsorbed water.
INTRODUCTION
The a-, b-, and g-cyclodextrin (CD), which consist of 6 to 8 glucose units, have a central cavity diameter of 0.6, 0.8, and 1.0 nm, respectively. The three kinds of CDs have a central cavity length of from 0.7 to 0.8 nm. The CDs are linked by an a-1, 4-glucoside bond. The natural CDs ( a-, b-, and g-CD) are hardly soluble in water. There is a limitation of the included cavity of the natural CD. The second generation CDs which are substituted by functional groups have different water solubility and inclusion behavior. In recent years, CDs have been useful in the fields of medical and food industries. CDs are widely used to improve the dissolution of drugs in pharmaceutical science (1, 2) and as 1
To whom correspondence should be addressed.
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MATERIALS AND METHODS
Materials a-Cyclodextrin ( a-CD), b-cyclodextrin ( b-CD), and gcyclodextrin ( g-CD) were commercially obtained from Wako Pure Chemical Co., Ltd., Osaka Prefecture, Japan. 6-O-a-D-glucosyl-a-cyclodextrin (G1-a-CD) and 6-O-a-D-
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substitutes for low-polarity organic chemicals which have caused increasing concern among the public (3). As stable hydrate forms, the water hydrates of 6, 12, and 17 are known for a-, b-, and g-CD, respectively (4). Recently, CD derivatives which have better properties for host molecules have been developed and produced. For example, there are the branched CDs (glucosyl-, maltosyl-, diglucosyl-, and dimaltosyl-CD), the CD derivatives (methyl-, ethyl-, hydroxyethyl-, and hydroxypropyl-CD), and the CD-polymers. We used the branched CDs (glucosyl-a- and maltosyl-a-CD) which have one and two glucose unit molecules, respectively, linked by an a-1, 6-glucoside bond. Because of using them, the number of glucose residues of glucosyl-a-CD (G1a-CD) and maltosyl-a-CD (G2-a-CD) were the same as those of b- and g-CD, respectively. Therefore, the amount of water adsorbed onto the hydroyl groups of G1-a- and G2a-CD was the same as that onto the hydroyl groups of band g-CD, respectively. Evaluation of the water adsorption properties is needed for drug and other uses. Schalchli and Benattar (5) reported the structure of a monomolecular layer of amphiphilic cyclodextrin. The results of analysis by TG, X-ray diffraction, and NMR indicated the condition of the hydrate in the CD molecule. The crystal water of additives exerts an influence on the physicochemical properties and the stability of a drug in a formulation. In this present study, the hydration properties of CDs and branched CDs were investigated on the basis of the adsorption isotherms of water, the differential heat of adsorption, the entropy of adsorbed water, and the heat of immersion in water to elucidate the adsorption mechanism of water molecules and the difference between the CDs and branched CDs with water molecules and the stability of structure.
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maltosyl-a-cyclodextrin (G2-a-CD) were purchased from Ensuiko Seitoh Ind. Inc., Kanagawa Prefecture, Japan. These were dried under a vacuum of 1 1 10 02 Torr for 24 h at room temperature. Procedure for Measuring Water Adsorption Isotherm The water adsorption isotherms were determined with a Shibata moisture and surface area apparatus, model P-850 (Shibata Scientific Technology, Ltd., Tokyo, Japan). Deionized water was placed in a 100-ml glass bulb. It was previously confirmed experimentally that a physically adsorbed substance on CDs and branched CDs could be completely removed by degassing under a vacuum of 1 1 10 02 Torr or less in 15 h at room temperature. The CDs and branched CD (approximately 0.1 g) were placed in a silica bucket and degassed at 1 1 10 02 Torr or less for 15 h at room temperature. The temperature of the water jacket was kept constant with circulating water. Water vapor was admitted into the cylindrical bulb (diameter, 5 cm; height, 105 cm), and water adsorption onto the CD and branched CDs occurred. After the adsorption reached equilibrium, the weight of water adsorbed onto CD was determined by observing the stretching of the helical silica spring with a reading microscope, and the equilibrium pressure of water vapor was measured by reading the scale spring of the manometer. The procedures of admitting moisture into the adsorption bulb and measuring the adsorption equilibrium were repeated approximately eight times from low pressure to the saturated vapor pressure of water for the adsorption temperature. The expansion coefficient of the silica spring was obtained from the slope between the weight and the expansion of the spring. It was a constant value, 98.34 mg/mm, under a vacuum of 1 1 10 02 Torr or less at 207C. Differential Heat of Adsorption and Entropy of Adsorbed Water The differential heat of adsorption was calculated by applying the Clausius–Clapeyron equation [1] (6) to the adsorption isotherms of water at 10, 20, and 307C. The equilibrium heat of adsorption was calculated by applying Eqs. [2] and [3], and the entropy of water adsorbed was calculated from Eq. [4] given by Hill et al. (7). qst Å 0 R[d(ln Aw)/d(1/T )]M
[1]
Qeq Å 0 R[d(ln Aw)/d(1/T )]f
[2]
f Å RT e M d(ln Aw)
[3]
DSeq Å 0Qeq /T 0 R ln (Aw)
[4]
qst is the differential heat of adsorption, R is the gas constant, Aw is the relative humidity, T is the absolute temperature, M is the amount adsorbed, Qeq is the equilibrium heat of
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FIG. 1. Adsorption isotherms of water onto a-, b-, and g-CD at 207C. l, a-CD; j, b-CD; m, g-CD.
adsorption, f is the surface energy, and (Aw) of the relative humidity.
is the mean
Heat of Immersion The heat of immersion was determined with a calorimeter, model TIC-22, (Tokyo Riko Co., Ltd., Tokyo, Japan). The conditions for measuring the heat of immersion were as follows: temperature, 257C; weight of sample, approximately 20 mg. RESULTS AND DISCUSSION
The adsorption isotherms of water onto a-, b-, and gcyclodextrin (CD) at 207C are shown in Fig. 1. Our results on adsorption isotherms of water onto a-, b-, and g-CD agreed with those reported by Nakai et al. (4). The adsorption isotherms of water onto a- and b-CD were the Langmuir type (correlation coefficients of the Langmuir plot, 0.995 and 0.992), while that of water onto g-CD was the BET type (correlation coefficient of the BET plot, 0.988). Therefore, it is assumed that the adsorption mechanism of water onto aand b-CD was the monolayer adsorption and that onto gCD was the multilayer adsorption. The Langmuir plot of bCD has a bending point at a relative humidity of 0.35. It is considered that the difference between the type of adsorption isotherms of a- or b-CD and that of g-CD indicated the difference in stability of the microporous structure. The solubilities of CDs in water have been reported to increase in the order b-, a-, and g-CD. Little work on the adsorption properties of water onto CDs has been done so far. Thermodynamic functions are used to obtain information on the interaction between the water molecules and CDs. The differential heat of adsorption and the entropy of adsorbed water were calculated from experimentally determined adsorption isotherms of water at 10, 20, and 307C. The differential heat of adsorption, calculated by applying the Clausius–
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FIG. 2. Differential heat of adsorption onto a-, b-, and g-CD. l, aCD; j, b-CD; m, g-CD.
Clapeyron equation, is shown in Fig. 2. The differential heat of adsorption of CDs up to 50 mg/g of adsorbed water decreased with the increasing number of glucose molecules. That of a- and b-CD decreased with an increase in the amount of water adsorbed, while that of g-CD was not changed over a wide range of adsorbed water. The differential heat of adsorption was measured for the bonding force. The result of the differential heat of adsorption indicated that the interaction between the a-CD and the water molecule was the largest of all. The a-CD has a specific adsorption site, for example, the hydroxyl groups of glucose or the smallest central cavity diameter of 0.6 nm. However, the amount of adsorbed water onto a-CD up to a relative humidity of 0.35 was the largest of all because of adsorption on the intermolecule. On the other hand, the differential heat of adsorption of b-CD was larger than that of g-CD because b-CD has a specific adsorption site. Because part of the water molecules were included in b-CD, the amount of adsorbed water onto b-CD at a relative humidity of greater than 0.35 was larger than that onto a-CD. The amounts of adsorbed water onto b- and g-CD up to relative humidities of 0.35 and 0.60, respectively, were smaller than that on aCD, because the pore structure of b- and g-CD was more hydrophobic than that of a-CD. Therefore, it is considered that the water molecule was adsorbed on the hydroxyl groups of the surface of a-CD and/or on the intermolecule of aCD, was first adsorbed on the hydroxyl groups of the surface of b-CD and then was included in the pore structure of bCD, and was only included in the pore structure of g-CD. The Langmuir plot of b-CD has a bending point at a relative humidity of 0.35. These results indicated that the water molecule was adsorbed on the hydroxyl groups of the surface of b-CD at relative humidity of less than 0.35, while that was included in the pore structure of b-CD at relative humidity of more than 0.35.
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The entropy of adsorbed water which was calculated using the equation given by Hill et al. (7) is shown in Fig. 3. The entropy of adsorbed water of a- and g-CD gradually decreased with an increase in the amount of water adsorbed, while that of b-CD increased with the amount of adsorbed water. Generally speaking, the value of the entropy of adsorbed water becomes a guide to the movement of the adsorbed water. Therefore, the movement of the adsorbed water was small in the order a-, g-, and b-CD. It is thought that each kind of CD has a different adsorption site in the CD molecule. The amount adsorbed onto a-CD at a lower relative humidity and the entropy of the adsorbed water were the largest of all. The water did not adsorb in the pore structure of the CD molecule but onto the surface and/or the intermolecule of a-CD. The entropy of the adsorbed water of b-CD was the smallest; thus the water molecule was considered to be introduced onto the inner molecule of b-CD. Nakai et al. (4) reported the adsorption isotherms of water onto a-, b-, and g-CD in the gaseous phase, and the hydration on the crystal form of b-CD in the adsorption and desorption processes was concluded to be different from those of a- and g-CD. Our results on the entropy of adsorbed water agree with those reported by Nakai et al. (4) because the entropy of adsorbed water was the smallest. The branched cyclodextrins, 6-O-a-D-glucosyl-a-cyclodextrin (G1-a-CD) and 6-O-a-D-maltosyl-a-cyclodextrin (G2-a-CD), were developed to increase the water solubility. It is not evident that interaction occurred between the water molecule and the branched cyclodextrin molecules. The adsorption isotherms of water onto a-, G1-a-, and G2-a-CD at 207C are shown in Fig. 4. The adsorption isotherms of water onto a- and G1-a-CD were the Langmuir type (correlation coefficients of the Langmuir plot, 0.995 and 0.991), while that of water onto G2-a-CD was the BET type (correlation coefficient of the BET plot, 0.965). It is thought that the
FIG. 3. Entropy of adsorbed water onto a-, b-, and g-CD. l, a-CD; j, b-CD; m, g-CD.
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FIG. 4. Adsorption isotherms of water onto a-, G1-a-, and G2-a-CD at 207C. l, a-CD; j, G1-a-CD; m, G2-a-CD.
water adsorption mechanisms onto CDs were different between a- or G1-a-CD and G2-a-CD. As a result of using the branched CDs, the number of glucose residues of G1-a- and G2-a-CD were the same as those of b- and g-CD, respectively. Therefore, the amount adsorbed onto the hydroxyl groups of G1-a- and G2-a-CD were the same as that of b- and g-CD, respectively. The amount of water adsorbed onto G1-a- and G2-a-CD up to a relative humidity of 0.4 and 0.8 was smaller than that on aCD, respectively. This result indicated that the side chain, glucosyl or maltosyl groups, decreased the amount of water adsorbed onto cyclodextrin at a lower relative humidity, while increasing the amount adsorbed at a higher relative humidity. The glucosyl and maltosyl groups are hindered as a result of the water adsorbed on the surface of a-CD. The steep rise in the amount adsorbed at higher relative humidity indicated that the structure of G1-a- and G2-a-CD was affected by the water molecules at higher relative humidity. The branched CDs were developed to increase the interaction with water. However, the amount of water adsorbed onto the branched CDs is smaller than that onto the virgin CD and the differential heat of adsorption of the branched CDs is lower than that of the virgin CD. The value of the differential heat of adsorption is a measure of the interaction between the water and the surface of the CDs. Because it is not evident that a relationship exists between the structure of CD and the water molecule, we measured the heat of immersion in water of a-, G1-a-, and G2-a-CD. This result is shown in Fig. 5. The heat of immersion in water was used to ascertain the relationship between excess water and the pore structure of the CDs, because the heat of immersion is the sum of the heat of dissolution, the heat of evaporation of liquid, the differential heat of adsorption of vapor, and so on. The heat of immersion was larger in the order G2-aand G1-a-CD or a-CD (Fig. 6). N. Okada et al. (8) reported the solubility of the branched CDs in water. The solubilities
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FIG. 5. Differential heat of adsorption onto a-, G1-a-, and G2-a-CD. l, a-CD; j, G1-a-CD; m, G2-a-CD.
of the a-, G1-a-, and G2-a-CD in water at 257C were 18, 80, and 24 (mmol/ml 1 10 02 ), respectively. G2-a-CD is particularly apt to crystallize, and moreover, the concentrated solution tends to gel. Therefore, it is considered that the maltosyl groups in the G2-a-CD molecule hinder the introduction of excess water in the core because of the longer branch. The entropy of adsorbed water which was calculated using the equation given by Hill et al. (7) is shown in Fig. 7. The entropy of adsorbed water of G2-a-CD gradually increased with an increase in the amount of water adsorbed, while that of a-CD and G1-a-CD did not change with an increase in adsorbed water. The movement of the adsorbed water for a smaller amount adsorbed was considered to be smaller in the order a-, G2-a-, and G1-a-CD, while that for a larger adsorbed amount was smaller in the order a-, G1-a-, and G2-a-CD. The conclusions are as follows: (i) The adsorption site of water onto a-CD was the surface of the CD and/or the inter molecule of CD, that onto b-CD was the surface of the CD
FIG. 6. Heat of immersion of a-, G1-a-, and G2-a-CD at 257C.
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the CDs and branched CDs did not depend upon the number of hydroxyl groups of the CDs. (iii) In spite of the branched CDs with larger water solubility, the amount of water adsorbed onto the branched CDs was smaller than that onto the virgin CDs. (iv) The side chain, glucosyl or maltosyl groups, inhibited the adsorption of water on the inner molecule of CD. REFERENCES
FIG. 7. Entropy of adsorbed water onto a-, G1-a-, and G2-a-CD at 207C. l, a-CD; j, G1-a-CD; m, G2-a-CD.
and inner molecule of CD, and that onto g-CD was the inner molecule of the CD. (ii) The amount of water adsorbed onto
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1. Pitha, J., Pitha, J., J. Pharm. Sci. 74, 987 (1985). 2. Koizumi, K., Okada, Y., Kubota, Y., and Utamura, T., Chem. Pharm. Bull. 35(8), 3413 (1987). 3. Wang, X., Brusseau, M. L., Environ. Sci. Technol. 29, 2346 (1995). 4. Nakai, Y., Yamamoto, K., Terada, K., Kajiyama, and A., Sasaki, I., Chem. Pharm. Bull. 34(3), 2178 (1986). 5. Schalchli, A., and Benattar, J. J., Langmuir 9, 1968 (1993). 6. Rizvi, S. S. H., and Benado, A. L., Food Technol. 38(3), 83 (1984). 7. Hill, T. L., Emmett, P. H., and Joyner, L. G., J. Am. Chem. Soc. 73, 5102 (1951). 8. Okada, Y., Kubota, Y., Koizumi, Hizukuri, S., Ohfuji, T., and Ogata, K., Chem. Pharm. Bull. 36(6), 2176 (1988).
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Adsorption Behavior of Water Molecules onto a-, b-, and g-Cyclodextrins and Branched a-Cyclodextrins SEIKI TANADA, 1 TAKEO NAKAMURA, NAOHITO KAWASAKI, TAK AYUKI KURIHARA,
AND
YUKIHIRO UMEMOTO
Faculty of Pharmaceutical Sciences, Kinki University, Kowakae, Higashi-Osaka, Osaka 577, Japan Received December 27, 1995; accepted March 1, 1996
Moisture adsorption properties of a-, b-, and g-cyclodextrin (CD) and branched a-CDs were investigated to elucidate the basis of the results of adsorption isotherms of water at 10, 20, and 307C, the differential heat of adsorption, the entropy of the adsorbed water, and the heat of immersion in water. It is concluded that the water molecules were adsorbed on the hydroxyl groups of the glucose molecule of a-CD, were first adsorbed on that of b-CD and then were included in the pore structure of b-CD, and were only included in the pore structure of g-CD. Branched a-CDs were developed and produced to increase the interaction with water molecules. However, the interaction between the branched a-CDs and the water was smaller than that between the virgin aCD and the water. The solubility of branched a-CD required a long time because the heat of immersion was larger in the order G2-a-CD and G1-a-CD or a-CD. It is considered that the side chain, glucosyl and maltosyl groups, decreased the interaction between the water molecule and the surface of the CD because of steric hindrance. Therefore, it is concluded that the side chain inhibited the water adsorption of the inner molecule of CD. q 1996 Academic Press, Inc.
Key Words: cyclodextrin; adsorption of water, branched cyclodextrin; adsorption of water, differential heat of adsorption; water molecule, entropy of adsorbed water.
INTRODUCTION
The a-, b-, and g-cyclodextrin (CD), which consist of 6 to 8 glucose units, have a central cavity diameter of 0.6, 0.8, and 1.0 nm, respectively. The three kinds of CDs have a central cavity length of from 0.7 to 0.8 nm. The CDs are linked by an a-1, 4-glucoside bond. The natural CDs ( a-, b-, and g-CD) are hardly soluble in water. There is a limitation of the included cavity of the natural CD. The second generation CDs which are substituted by functional groups have different water solubility and inclusion behavior. In recent years, CDs have been useful in the fields of medical and food industries. CDs are widely used to improve the dissolution of drugs in pharmaceutical science (1, 2) and as 1
To whom correspondence should be addressed.
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MATERIALS AND METHODS
Materials a-Cyclodextrin ( a-CD), b-cyclodextrin ( b-CD), and gcyclodextrin ( g-CD) were commercially obtained from Wako Pure Chemical Co., Ltd., Osaka Prefecture, Japan. 6-O-a-D-glucosyl-a-cyclodextrin (G1-a-CD) and 6-O-a-D-
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substitutes for low-polarity organic chemicals which have caused increasing concern among the public (3). As stable hydrate forms, the water hydrates of 6, 12, and 17 are known for a-, b-, and g-CD, respectively (4). Recently, CD derivatives which have better properties for host molecules have been developed and produced. For example, there are the branched CDs (glucosyl-, maltosyl-, diglucosyl-, and dimaltosyl-CD), the CD derivatives (methyl-, ethyl-, hydroxyethyl-, and hydroxypropyl-CD), and the CD-polymers. We used the branched CDs (glucosyl-a- and maltosyl-a-CD) which have one and two glucose unit molecules, respectively, linked by an a-1, 6-glucoside bond. Because of using them, the number of glucose residues of glucosyl-a-CD (G1a-CD) and maltosyl-a-CD (G2-a-CD) were the same as those of b- and g-CD, respectively. Therefore, the amount of water adsorbed onto the hydroyl groups of G1-a- and G2a-CD was the same as that onto the hydroyl groups of band g-CD, respectively. Evaluation of the water adsorption properties is needed for drug and other uses. Schalchli and Benattar (5) reported the structure of a monomolecular layer of amphiphilic cyclodextrin. The results of analysis by TG, X-ray diffraction, and NMR indicated the condition of the hydrate in the CD molecule. The crystal water of additives exerts an influence on the physicochemical properties and the stability of a drug in a formulation. In this present study, the hydration properties of CDs and branched CDs were investigated on the basis of the adsorption isotherms of water, the differential heat of adsorption, the entropy of adsorbed water, and the heat of immersion in water to elucidate the adsorption mechanism of water molecules and the difference between the CDs and branched CDs with water molecules and the stability of structure.
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maltosyl-a-cyclodextrin (G2-a-CD) were purchased from Ensuiko Seitoh Ind. Inc., Kanagawa Prefecture, Japan. These were dried under a vacuum of 1 1 10 02 Torr for 24 h at room temperature. Procedure for Measuring Water Adsorption Isotherm The water adsorption isotherms were determined with a Shibata moisture and surface area apparatus, model P-850 (Shibata Scientific Technology, Ltd., Tokyo, Japan). Deionized water was placed in a 100-ml glass bulb. It was previously confirmed experimentally that a physically adsorbed substance on CDs and branched CDs could be completely removed by degassing under a vacuum of 1 1 10 02 Torr or less in 15 h at room temperature. The CDs and branched CD (approximately 0.1 g) were placed in a silica bucket and degassed at 1 1 10 02 Torr or less for 15 h at room temperature. The temperature of the water jacket was kept constant with circulating water. Water vapor was admitted into the cylindrical bulb (diameter, 5 cm; height, 105 cm), and water adsorption onto the CD and branched CDs occurred. After the adsorption reached equilibrium, the weight of water adsorbed onto CD was determined by observing the stretching of the helical silica spring with a reading microscope, and the equilibrium pressure of water vapor was measured by reading the scale spring of the manometer. The procedures of admitting moisture into the adsorption bulb and measuring the adsorption equilibrium were repeated approximately eight times from low pressure to the saturated vapor pressure of water for the adsorption temperature. The expansion coefficient of the silica spring was obtained from the slope between the weight and the expansion of the spring. It was a constant value, 98.34 mg/mm, under a vacuum of 1 1 10 02 Torr or less at 207C. Differential Heat of Adsorption and Entropy of Adsorbed Water The differential heat of adsorption was calculated by applying the Clausius–Clapeyron equation [1] (6) to the adsorption isotherms of water at 10, 20, and 307C. The equilibrium heat of adsorption was calculated by applying Eqs. [2] and [3], and the entropy of water adsorbed was calculated from Eq. [4] given by Hill et al. (7). qst Å 0 R[d(ln Aw)/d(1/T )]M
[1]
Qeq Å 0 R[d(ln Aw)/d(1/T )]f
[2]
f Å RT e M d(ln Aw)
[3]
DSeq Å 0Qeq /T 0 R ln (Aw)
[4]
qst is the differential heat of adsorption, R is the gas constant, Aw is the relative humidity, T is the absolute temperature, M is the amount adsorbed, Qeq is the equilibrium heat of
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FIG. 1. Adsorption isotherms of water onto a-, b-, and g-CD at 207C. l, a-CD; j, b-CD; m, g-CD.
adsorption, f is the surface energy, and (Aw) of the relative humidity.
is the mean
Heat of Immersion The heat of immersion was determined with a calorimeter, model TIC-22, (Tokyo Riko Co., Ltd., Tokyo, Japan). The conditions for measuring the heat of immersion were as follows: temperature, 257C; weight of sample, approximately 20 mg. RESULTS AND DISCUSSION
The adsorption isotherms of water onto a-, b-, and gcyclodextrin (CD) at 207C are shown in Fig. 1. Our results on adsorption isotherms of water onto a-, b-, and g-CD agreed with those reported by Nakai et al. (4). The adsorption isotherms of water onto a- and b-CD were the Langmuir type (correlation coefficients of the Langmuir plot, 0.995 and 0.992), while that of water onto g-CD was the BET type (correlation coefficient of the BET plot, 0.988). Therefore, it is assumed that the adsorption mechanism of water onto aand b-CD was the monolayer adsorption and that onto gCD was the multilayer adsorption. The Langmuir plot of bCD has a bending point at a relative humidity of 0.35. It is considered that the difference between the type of adsorption isotherms of a- or b-CD and that of g-CD indicated the difference in stability of the microporous structure. The solubilities of CDs in water have been reported to increase in the order b-, a-, and g-CD. Little work on the adsorption properties of water onto CDs has been done so far. Thermodynamic functions are used to obtain information on the interaction between the water molecules and CDs. The differential heat of adsorption and the entropy of adsorbed water were calculated from experimentally determined adsorption isotherms of water at 10, 20, and 307C. The differential heat of adsorption, calculated by applying the Clausius–
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FIG. 2. Differential heat of adsorption onto a-, b-, and g-CD. l, aCD; j, b-CD; m, g-CD.
Clapeyron equation, is shown in Fig. 2. The differential heat of adsorption of CDs up to 50 mg/g of adsorbed water decreased with the increasing number of glucose molecules. That of a- and b-CD decreased with an increase in the amount of water adsorbed, while that of g-CD was not changed over a wide range of adsorbed water. The differential heat of adsorption was measured for the bonding force. The result of the differential heat of adsorption indicated that the interaction between the a-CD and the water molecule was the largest of all. The a-CD has a specific adsorption site, for example, the hydroxyl groups of glucose or the smallest central cavity diameter of 0.6 nm. However, the amount of adsorbed water onto a-CD up to a relative humidity of 0.35 was the largest of all because of adsorption on the intermolecule. On the other hand, the differential heat of adsorption of b-CD was larger than that of g-CD because b-CD has a specific adsorption site. Because part of the water molecules were included in b-CD, the amount of adsorbed water onto b-CD at a relative humidity of greater than 0.35 was larger than that onto a-CD. The amounts of adsorbed water onto b- and g-CD up to relative humidities of 0.35 and 0.60, respectively, were smaller than that on aCD, because the pore structure of b- and g-CD was more hydrophobic than that of a-CD. Therefore, it is considered that the water molecule was adsorbed on the hydroxyl groups of the surface of a-CD and/or on the intermolecule of aCD, was first adsorbed on the hydroxyl groups of the surface of b-CD and then was included in the pore structure of bCD, and was only included in the pore structure of g-CD. The Langmuir plot of b-CD has a bending point at a relative humidity of 0.35. These results indicated that the water molecule was adsorbed on the hydroxyl groups of the surface of b-CD at relative humidity of less than 0.35, while that was included in the pore structure of b-CD at relative humidity of more than 0.35.
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The entropy of adsorbed water which was calculated using the equation given by Hill et al. (7) is shown in Fig. 3. The entropy of adsorbed water of a- and g-CD gradually decreased with an increase in the amount of water adsorbed, while that of b-CD increased with the amount of adsorbed water. Generally speaking, the value of the entropy of adsorbed water becomes a guide to the movement of the adsorbed water. Therefore, the movement of the adsorbed water was small in the order a-, g-, and b-CD. It is thought that each kind of CD has a different adsorption site in the CD molecule. The amount adsorbed onto a-CD at a lower relative humidity and the entropy of the adsorbed water were the largest of all. The water did not adsorb in the pore structure of the CD molecule but onto the surface and/or the intermolecule of a-CD. The entropy of the adsorbed water of b-CD was the smallest; thus the water molecule was considered to be introduced onto the inner molecule of b-CD. Nakai et al. (4) reported the adsorption isotherms of water onto a-, b-, and g-CD in the gaseous phase, and the hydration on the crystal form of b-CD in the adsorption and desorption processes was concluded to be different from those of a- and g-CD. Our results on the entropy of adsorbed water agree with those reported by Nakai et al. (4) because the entropy of adsorbed water was the smallest. The branched cyclodextrins, 6-O-a-D-glucosyl-a-cyclodextrin (G1-a-CD) and 6-O-a-D-maltosyl-a-cyclodextrin (G2-a-CD), were developed to increase the water solubility. It is not evident that interaction occurred between the water molecule and the branched cyclodextrin molecules. The adsorption isotherms of water onto a-, G1-a-, and G2-a-CD at 207C are shown in Fig. 4. The adsorption isotherms of water onto a- and G1-a-CD were the Langmuir type (correlation coefficients of the Langmuir plot, 0.995 and 0.991), while that of water onto G2-a-CD was the BET type (correlation coefficient of the BET plot, 0.965). It is thought that the
FIG. 3. Entropy of adsorbed water onto a-, b-, and g-CD. l, a-CD; j, b-CD; m, g-CD.
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FIG. 4. Adsorption isotherms of water onto a-, G1-a-, and G2-a-CD at 207C. l, a-CD; j, G1-a-CD; m, G2-a-CD.
water adsorption mechanisms onto CDs were different between a- or G1-a-CD and G2-a-CD. As a result of using the branched CDs, the number of glucose residues of G1-a- and G2-a-CD were the same as those of b- and g-CD, respectively. Therefore, the amount adsorbed onto the hydroxyl groups of G1-a- and G2-a-CD were the same as that of b- and g-CD, respectively. The amount of water adsorbed onto G1-a- and G2-a-CD up to a relative humidity of 0.4 and 0.8 was smaller than that on aCD, respectively. This result indicated that the side chain, glucosyl or maltosyl groups, decreased the amount of water adsorbed onto cyclodextrin at a lower relative humidity, while increasing the amount adsorbed at a higher relative humidity. The glucosyl and maltosyl groups are hindered as a result of the water adsorbed on the surface of a-CD. The steep rise in the amount adsorbed at higher relative humidity indicated that the structure of G1-a- and G2-a-CD was affected by the water molecules at higher relative humidity. The branched CDs were developed to increase the interaction with water. However, the amount of water adsorbed onto the branched CDs is smaller than that onto the virgin CD and the differential heat of adsorption of the branched CDs is lower than that of the virgin CD. The value of the differential heat of adsorption is a measure of the interaction between the water and the surface of the CDs. Because it is not evident that a relationship exists between the structure of CD and the water molecule, we measured the heat of immersion in water of a-, G1-a-, and G2-a-CD. This result is shown in Fig. 5. The heat of immersion in water was used to ascertain the relationship between excess water and the pore structure of the CDs, because the heat of immersion is the sum of the heat of dissolution, the heat of evaporation of liquid, the differential heat of adsorption of vapor, and so on. The heat of immersion was larger in the order G2-aand G1-a-CD or a-CD (Fig. 6). N. Okada et al. (8) reported the solubility of the branched CDs in water. The solubilities
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FIG. 5. Differential heat of adsorption onto a-, G1-a-, and G2-a-CD. l, a-CD; j, G1-a-CD; m, G2-a-CD.
of the a-, G1-a-, and G2-a-CD in water at 257C were 18, 80, and 24 (mmol/ml 1 10 02 ), respectively. G2-a-CD is particularly apt to crystallize, and moreover, the concentrated solution tends to gel. Therefore, it is considered that the maltosyl groups in the G2-a-CD molecule hinder the introduction of excess water in the core because of the longer branch. The entropy of adsorbed water which was calculated using the equation given by Hill et al. (7) is shown in Fig. 7. The entropy of adsorbed water of G2-a-CD gradually increased with an increase in the amount of water adsorbed, while that of a-CD and G1-a-CD did not change with an increase in adsorbed water. The movement of the adsorbed water for a smaller amount adsorbed was considered to be smaller in the order a-, G2-a-, and G1-a-CD, while that for a larger adsorbed amount was smaller in the order a-, G1-a-, and G2-a-CD. The conclusions are as follows: (i) The adsorption site of water onto a-CD was the surface of the CD and/or the inter molecule of CD, that onto b-CD was the surface of the CD
FIG. 6. Heat of immersion of a-, G1-a-, and G2-a-CD at 257C.
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the CDs and branched CDs did not depend upon the number of hydroxyl groups of the CDs. (iii) In spite of the branched CDs with larger water solubility, the amount of water adsorbed onto the branched CDs was smaller than that onto the virgin CDs. (iv) The side chain, glucosyl or maltosyl groups, inhibited the adsorption of water on the inner molecule of CD. REFERENCES
FIG. 7. Entropy of adsorbed water onto a-, G1-a-, and G2-a-CD at 207C. l, a-CD; j, G1-a-CD; m, G2-a-CD.
and inner molecule of CD, and that onto g-CD was the inner molecule of the CD. (ii) The amount of water adsorbed onto
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1. Pitha, J., Pitha, J., J. Pharm. Sci. 74, 987 (1985). 2. Koizumi, K., Okada, Y., Kubota, Y., and Utamura, T., Chem. Pharm. Bull. 35(8), 3413 (1987). 3. Wang, X., Brusseau, M. L., Environ. Sci. Technol. 29, 2346 (1995). 4. Nakai, Y., Yamamoto, K., Terada, K., Kajiyama, and A., Sasaki, I., Chem. Pharm. Bull. 34(3), 2178 (1986). 5. Schalchli, A., and Benattar, J. J., Langmuir 9, 1968 (1993). 6. Rizvi, S. S. H., and Benado, A. L., Food Technol. 38(3), 83 (1984). 7. Hill, T. L., Emmett, P. H., and Joyner, L. G., J. Am. Chem. Soc. 73, 5102 (1951). 8. Okada, Y., Kubota, Y., Koizumi, Hizukuri, S., Ohfuji, T., and Ogata, K., Chem. Pharm. Bull. 36(6), 2176 (1988).
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