© Copyright 2007 by the American Chemical Society
VOLUME 111, NUMBER 15, APRIL 19, 2007
ARTICLES Effect of Guest Hydrophobicity on Water Sorption Behavior of Oligomer/r-Cyclodextrin Inclusion Complexes Marcus A. Hunt,† Alan E. Tonelli,† and C. Maurice Balik*,‡ Fiber and Polymer Science Program, Campus Box 8301, North Carolina State UniVersity, Raleigh, North Carolina 27695-8301, and Department of Materials Science & Engineering, Campus Box 7907, North Carolina State UniVersity, Raleigh, North Carolina 27695-7907 ReceiVed: January 8, 2007; In Final Form: February 21, 2007
Cyclomaltohexaose (R-cyclodextrin, R-CD) can form inclusion complexes (ICs) with polymer molecules in the columnar crystal structure in which R-CD molecules stack to form a molecular tube. Complementary water vapor sorption and wide-angle X-ray diffractomery (WAXD) were performed on oligomer/R-CD ICs to determine their structures and stabilities. To discern the effect of guest molecule hydrophobicity on water adsorption isotherms, polyethylene glycol (PEG, MW ) 600 g/mol) and hexatriacontane (HTC) guests were used. Sorption isotherms for PEG/R-CD IC are similar to those obtained for pure R-CD and PEG, suggesting the presence of dethreaded PEG in the sample. WAXD collected before and after water vapor sorption of PEG/R-CD IC indicated a partial conversion from columnar to cage crystal structure, the thermodynamically preferred structure for pure R-CD, due to dethreading of PEG. This behavior does not occur for HTC/R-CD IC. Sorption isotherms collected at 20, 30, 40, and 50 °C allowed the calculation of the isosteric heats of adsorption and the integral entropies of adsorbed water which are characterized by minima that indicate the monolayer concentration of water in the ICs.
Introduction Cyclodextrins (CDs) are cyclic oligosaccharides capable of forming inclusion complexes (ICs) with small molecules and macromolecules due to their hollow, truncated cone structure. CDs are most commonly composed of 6, 7, or 8 glucopyranose units, which correspond to R-, β-, and γ-CD, respectively. These molecules have a hydrophilic surface and a hydrophobic cavity in which various guest molecules can reside. CDs can exist in three classes of crystal structures called cage, layer, and columnar.1 In the cage and layer structures (Figure 1, parts a and b), the CD cavities are not aligned, whereas they stack on * To whom correspondence should be addressed. Phone: +1-919-5152126. Fax: +1-919-515-7724. E-mail:
[email protected]. † Fiber and Polymer Science Program. ‡ Department of Materials Science & Engineering.
Figure 1. Schematic representation of the packing of CD molecules within their crystal lattices: cage (a), layer (b), and head-to-head columnar (c) structure.
top of each other in the columnar structure (Figure 1c) to form long cylindrical channels in which various chain-like guest molecules can be complexed.2-5 Relatively little work has been done with regard to absorption of small molecules by CDs. Only water adsorption has been
10.1021/jp070145t CCC: $37.00 © 2007 American Chemical Society Published on Web 03/29/2007
3854 J. Phys. Chem. B, Vol. 111, No. 15, 2007 studied, which is appropriate considering CDs crystallize from water as hydrates. This is made possible by the primary and secondary hydroxyl groups located at the tail and head rims, respectively, of CD molecules. Some hydroxyl groups are involved in intermolecular CD-CD hydrogen bonds, and the frequency of such hydrogen bonding is a function of the crystal structure adopted.6,7 Therefore, the level of CD water sorption is dependent upon the crystal structure as well. Nakai et al.8 characterized as-received R-, β-, and γ-CD in the cage structure by isothermal water vapor absorption experiments at 40 °C as a function of water activity and complementary wide-angle X-ray diffractometry (WAXD) to determine the effect of water content on the CD crystal structure. They confirm the existence of a hydrate containing 6 water molecules per R-CD molecule that forms at water activities of 0.2-1.0. Tanada et al.9 report similar sorption behavior, as well as sorbed water energetics, for cage R-CD. Hunt et al.10 reported the sorption behavior of columnar R-CD containing no guest molecules, in which columnar crystals transformed to the more thermodynamically stable cage structure upon adsorption of water at 40 °C. Water sorption by many polymers has been studied; most notable are the biopolymers used in food applications.11-15 Recently, Perez-Alonzo et al.11 have described the water sorption behavior and subsequent thermodynamic analysis of several pure and blended carbohydrate polymers used extensively in the food industry. They generally observe sorption isotherms that are sigmoidal in shape. Interestingly, some systems exhibit increasing sorption with increasing temperature, while others show the opposite trend. The reasons for such discrepancies are not clear, but are likely due to differing sorption mechanisms. The structures of polymer/CD ICs have been studied in great detail.16-19 However, the effect of water on the structures and stabilities of polymer/CD ICs is a phenomenon that has not been previously studied. This is surprising considering that CDs exist as hydrates regardless of the polymorphs present. Water vapor sorption is a relatively simple and effective means of probing the interactions between water and polymer/CD ICs, as well as the nature of included polymer chains. Here, sorption isotherms were measured for two ICs: hexatriacontane (HTC, n-C36H74)/ R-CD IC and poly(ethylene glycol) (PEG) (600 g/mol)/R-CD IC. These guest molecules were selected to assess the effect of guest hydrophobicity on the water sorption behavior. Both ICs adopt the columnar structure (Figure 1). Subsequent calculations of the isosteric heat of water adsorption and integral entropy change of adsorbed water serve to quantify the interactions of water with these two oligomer/R-CD IC systems. Experimental Section Materials. R-CD was purchased from Cerestar in powder form. HTC and PEG with a molecular weight of 600 g/mol were purchased from Aldrich and used without further purification. Xylene was obtained from Fisher and used without further purification. All water used was deionized with a Millipore Milli-DI system. Preparation of Inclusion Complexes. For the HTC/R-CD IC, approximately 62 mg of HTC was placed in the bottom of a test tube. Then 1 g of R-CD was carefully placed in the test tube above the HTC. Approximately 6 mL of water was poured slowly down the side of the test tube. The test tube was sealed with a rubber septum and placed in an oil bath at 90 °C. The HTC melts and forms a thin layer on top of the water, while the R-CD dissolves in the water. A precipitate begins to form immediately, which is the HTC/R-CD IC. After 3 days at 90
Hunt et al. °C the mixture was slowly cooled to room temperature by turning the bath heater off. The precipitate was then washed with 100 mL of xylenes at 100 °C and water at room temperature and allowed to dry overnight at 50 °C. Differential scanning calorimetry, using a TA DSC Q1000, was performed from 20 to 100 °C with a heating rate of 20 deg/min to confirm the absence of free HTC, which melts at Tm ) 75 °C. Thermogravimetric analysis and elemental analysis were performed to quantify the stoichiometry of the resulting HTC/RCD IC, which is 6 molecules of R-CD per HTC molecule. This corresponds to approximately 3.2 wt % excess R-CD (based on the entire sample) over the stoichiometric ratio of 5.80 R-CDs per HTC calculated from the length of an R-CD molecule versus the length of HTC in the all-trans conformation. The PEG/R-CD IC was prepared by first dissolving approximately 1.45 g of R-CD in 8 mL of water and placing it in an oil bath at 70 °C with stirring. Approximately 130 mg of PEG was dissolved in 3 mL of water and then added to the R-CD solution after all R-CD had dissolved. The resulting solution was kept at 70 °C for 1 h and then slowly cooled to room temperature by turning the bath heater off. The solution was then stirred at room temperature for 3 days to ensure maximum precipitation. Solution 1H NMR was performed with deuterated water as the solvent to determine the stoichiometry of the PEG/R-CD IC, which is 2.22 repeat units of PEG per R-CD molecule. This corresponds to approximately 0.91 wt % excess PEG (based on the entire sample) over the stoichiometric ratio of 2 PEG repeat units per R-CD as reported by Harada et al.16 Water Vapor Sorption Isotherms. A custom-built gravimetric balance was used to measure the sorption of water vapor in HTC/R-CD and PEG/R-CD ICs. Samples weighing 0.2-0.3 g were first vacuum-dried in the balance chamber until no further weight loss was recorded, and were then tested at 20, 30, 40, and 50 °C with water activities ranging from a ) 0.1 to 0.9. Using flow meters for regulation, dry air was mixed with air bubbled through a water-containing reservoir to achieve the desired water activity. The humid gas was then passed through the sample chamber at a constant flow rate of 400 cm3/min. Samples were allowed to sorb water until equilibrium weight gain was achieved at each water activity. After reaching a constant weight, the water activity was increased by approximately 0.1 and equilibrium was re-established. This was repeated up to an activity of 0.9. Water Sorption Energetics. To gain understanding of the mechanisms of water adsorption and the nature of adsorbed water, thermodynamic analyses were carried out on the sorption isotherms for both HTC/R-CD and PEG/R-CD ICs. The Clausius-Clapeyron equation relates the vapor pressure of a liquid to temperature and assumes ideal gas behavior.20 It is given by
d lnP ∆Hv ) dT RT2
(1)
where P is the vapor pressure, T is temperature, ∆Hv is the differential heat of vaporization, and R is the gas constant. The isosteric heat of water adsorption, ∆q, can be derived20 from the Clausius-Clapeyron relation to give
∆q )
[ ] d ln(a) d(1/T)
C
) ∆Q + RT - ∆Hv
(2)
where a is water activity (vapor pressure/saturation vapor pressure) and C is concentration of adsorbed water. The isosteric
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heat is related to the differential heat of adsorption of water, ∆Q, as shown in eq 2.20 To calculate the isosteric heat, water sorption isotherms (concentration versus water activity) were first fitted to a smooth line intersecting the origin to enhance the resolution of isosteric heat calculations. Plots of ln(a) versus 1/T at selected constant concentration values were then made and fit to a straight line. The slope of the linear fit, which is proportional to ∆q, was used to calculate isosteric heat as a function of concentration. The calculation of the integral entropy change of sorbed water, ∆S, can also be derived from the Clausius-Clapeyron equation (eq 1) and is given by
∆S ) R
[ ] d ln(a) d ln(T)
(3)
φ
where φ is the surface potential or spreading pressure.21 This is given by
φ ) RT
ln(a ) C{d ln(a)} ∫ln(a ) 2
Figure 2. Adsorption of water vapor in HTC/R-CD IC as a function of temperature and water activity.
(4)
1
The integral entropy is of more interest than the differential entropy, because the integral entropy can be directly associated with sorbed water order/disorder.21 Therefore, it becomes necessary to first calculate spreading pressure, which is done by integrating C with respect to ln(a). Plots of ln(a) versus ln(T) at selected constant φ values were then made and fit to a straight line. The slope of the linear fit, which is proportional to ∆S, was used to calculate integral entropy change of adsorbed water as a function of concentration. Wide-Angle X-ray Diffraction (WAXD). WAXD measurements were performed on a Siemens type-F X-ray diffractometer with a Ni-filtered Cu KR radiation source (λ ) 1.54 Å). Thesupplied voltage and current were 30 kV and 20 mA, respectively. The diffraction intensities were measured every 0.1° from 2θ ) 5° to 30° at a rate of (2θ ) 3°)/min. Results and Discussion Water Vapor Sorption. The linear alkane HTC was chosen for this study because of its hydrophobicity. Water vapor sorption by the HTC/R-CD IC provides a means to separate water adsorption in the R-CD cavity from water adsorption on the R-CD surface. Since HTC is hydrophobic and the cavities are occupied by HTC, water vapor sorption should only occur on the R-CD surfaces. This should amount to four water molecules adsorbed per R-CD at room temperature and a ) 1.0.7 Figure 2 shows water vapor sorption isotherms for HTC/RCD IC as a function of water activity and temperature. The maximum observed sorption of HTC/R-CD IC is slightly less than four water molecules per R-CD, which occurs at approximately a ) 0.9 and 50 °C. The sorption level at 20 °C is significantly lower than the expected value of four water molecules per R-CD, based on the I2‚LiI3/R-CD IC hydrate determined by Noltemeyer and Saenger7 that adopts a columnar structure at room temperature. The discrepancy in sorption levels is likely a result of differences in the unit cell structure of the HTC/R-CD IC and the I2‚LiI3/R-CD IC. Packing differences and the presence of lithium counterions in I2‚LiI3/R-CD IC would partly control the number of water sorption sites. Unfortunately, no other columnar R-CD IC hydrate structure has been reported that can be directly compared with the HTC/ R-CD IC.
Figure 3. Comparison of water vapor adsorption of R-CD in cage structure, empty columnar structure, and columnar structure occupied with HTC and PEG guests as a function of water activity at 40 °C.
Unlike systems15,22 involving water adsorption to a hydrophilic surface, the amount of water adsorbed by HTC/R-CD IC increases with increasing temperature in the range of 20 to 50 °C. Also, the water sorption kinetics become faster with increasing temperature (data not shown). Neat HTC does not absorb water at all; therefore, all sorption by HTC/R-CD IC must occur on the R-CD surfaces. Although cage and columnar R-CD crystal hydrates contain the same number of water molecules per R-CD,6,7 comparison of water sorption by HTC/R-CD IC to sorption by columnar R-CD would be more appropriate than comparison to pure cage R-CD (Figure 3). Columnar R-CD with no guest molecule can be obtained by rapid precipitation from aqueous solution.23 Unfortunately, such a comparison is complicated by a phase transformation from empty columnar R-CD to cage R-CD, which occurs at high water activities.10 This is illustrated in Figure 3 by the drop in sorption above a ) 0.65 for columar R-CD. Unlike empty columnar R-CD, no such phase transformation occurs upon water adsorption by the HTC/R-CD IC. Interestingly, the sorption of HTC/R-CD IC at a ) 0.1 is comparable to that of columnar R-CD with no guest present (Figure 3). This could mean that the number of sorption sites is similar at this water activity, where the columnar-to-cage conversion does not occur. Figure 4 shows another example of water vapor sorption by an R-CD IC probed under the same conditions as the HTC/RCD IC. In this case the guest molecule selected was PEG (MW
3856 J. Phys. Chem. B, Vol. 111, No. 15, 2007
Figure 4. Adsorption of water vapor in PEG/R-CD IC as a function of temperature and water activity.
) 600 g/mol) because it has a cross-section and all-trans endto-end distance similar to that of HTC. PEG (600 g/mol) is approximately 49 Å long, while HTC is approximately 46 Å in length. Although the sizes of PEG and HTC are similar, the flexibility of PEG is higher than that of HTC due to its C-O backbone bonds. This is inferred based on the melting temperatures, which are 25 °C for PEG with a molecular weight of 600 g/mol and 75 °C for HTC. PEG is also infinitely miscible in water,24 resulting in a system in which water adsorption by both the host and guest is possible and could help to elucidate the water sorption behavior of an R-CD IC in which competitive adsorption between the guest and host can occur. Sorption isotherms for PEG/R-CD IC exhibit two distinct regions. Below a ) 0.65, the temperature dependence of water vapor sorption by PEG/R-CD IC (Figure 4) is similar to that of HTC/R-CD IC (Figure 2) in that sorption increases with increasing temperature. In fact, the sorption levels for the two ICs are very similar. This suggests that water is interacting with the R-CD surfaces only and is independent of the included guest molecule or any free PEG that may be present. The second region of PEG/R-CD IC water sorption begins at a ) 0.65 and is characterized by an increase in sorption with decreasing temperature. Also, the sorption levels in the second region are strongly dependent on water activity, which is similar to the sorption profile of neat PEG.25 A likely cause for such sorption behavior is migration of PEG chains out of the R-CD cavities resulting in water sorption onto free PEG and into the vacated R-CD cavities. Evidence of PEG diffusion out of the R-CD cavities is given in the form of a partial conversion from columnar R-CD crystals to cage R-CD crystals as noted below. The formation of cage R-CD after water sorption can only be explained as a result of the instability of columnar R-CD at high water activities with no guest present.10 As-received R-CD cage structure has salient WAXD reflections occurring at 2θ ) 12.0, 14.4, and 21.7°, while PEG/RCD IC peaks are centered at 2θ ) 10.1, 12.8, and 19.7° as shown in Figure 5. The WAXD of PEG/R-CD IC after water sorption at 40 °C shows the loss of the columnar R-CD peak at 2θ ) 10.1° and reduction in the peak at 2θ ) 12.8°. There is also build-up of the cage R-CD peak at 2θ ) 12.0° indicated by an arrow in Figure 5. The partial phase transformation of PEG/R-CD IC upon water sorption is likely occurring near crystal surfaces, where PEG is capable of diffusing out of the R-CD columns. A similar phase transformation does not occur in the case of HTC/R-CD IC (data not shown). Thermodynamic Calculations. The Clausius-Clapeyron equation was used to quantify the interactions between the ICs
Hunt et al.
Figure 5. WAXD patterns for PEG/R-CD IC (a) before and (b) after water vapor sorption up to a ) 0.9 at 20 °C. The arrow indicates the appearance of the 2θ ) 12° peak associated with the cage structure of R-CD.
Figure 6. Isosteric heat of adsorption (∆q) for HTC/R-CD IC as a function of temperature and adsorbed water concentration.
and adsorbed water. Figure 6 shows the isosteric heat of adsorption calculated from eq 2 for HTC/R-CD IC as a function of both concentration of adsorbed water and temperature. The heats of adsorption are negative, because of the aforementioned trend of increasing sorption with increasing temperature. A minimum in the isosteric heat is observed and the concentration at which the minimum occurs increases with increasing temperature as well. These minima could represent the concentration at which a complete monolayer of water forms on the internal surfaces of the sample. Beyond the minima, water molecules are more likely to undergo water-water interactions than water-R-CD interactions, resulting in a less negative ∆q value. As temperature increases, the number of monolayer adsorption sites apparently increases.11 Figure 7 shows the integral entropy of adsorbed water calculated from eq 3 as a function of both concentration and temperature. The integral entropy change is negative, confirming that the adsorbed water in the crystal with fewer degrees of freedom is more ordered than that in the vapor phase. Also, the values of heat associated with the entropy change upon adsorption (T∆S) are smaller than those for the isosteric heat of adsorption in the range of temperatures and concentrations probed, i.e., ∆q > T∆S. This indicates that the driving force for water adsorption is the heat evolved upon hydrogen bond formation between adsorbed water and R-CD. The entropy curves also exhibit what appear to be minima at concentrations slightly larger than those observed in the isosteric heat curves of Figure 6. The minima in ∆S (Figure 7) are not as well defined
Oligomer/R-Cyclodextrin Inclusion Complexes
Figure 7. Integral entropy of adsorbed water (∆S) for HTC/R-CD IC as a function of temperature and adsorbed water concentration.
as the minima in ∆q (Figure 6), which could account for the slight differences in their location. In the case of nitrogen gas sorption by graphite,21 a minimum in entropy was assumed to be indicative of the nitrogen concentration required to completely form a monolayer. Although R-CD ICs do not present surfaces in the same sense as graphite, the minima in Figure 7 could represent the preferred hydrate composition at a given temperature. If this analysis is correct, the number of sorption sites increases with temperature from approximately 1.8 mol of water per mol of R-CD at 25 °C to approximately 3 mol of water per mol of R-CD at 45 °C for HTC/R-CD IC. An increase in sorption sites with temperature could be an indication that the columnar stacks in the IC crystals are expanding due to additional thermal energy. This behavior is similar to the observation made by Steiner et al.,26 who demonstrated via molecular modeling that R-CD in its cage structure is too dense to allow transport of water through the crystal without any local distortions in packing. At higher temperatures the hydrogen bonds between R-CD molecules that serve to stabilize the crystals are disrupted enough to allow water/R-CD hydrogen bonds to form instead while maintaining hydrophobic interactions between R-CD molecules in the columnar stacks. This explanation can only be possible if the hydrogen bond strength between R-CDs is less than the strength of water/R-CD hydrogen bonds. Such a scenario is probable since water is a very strong hydrogen bond former. Interestingly, a similar thermodynamic analysis for water sorption by pure cage R-CD does not exhibit entropy minima in the temperature range between 10 and 30 °C.9 This difference is probably due to the fact that cage R-CD is incapable of adsorbing more than one monolayer of water, which would prevent ∆S from increasing at concentrations beyond the monolayer value. Similar thermodynamic calculations were also carried out for PEG/R-CD IC. Figure 8 shows the isosteric heat of adsorption as a function of both temperature and water concentration. The isosteric heat of PEG/R-CD IC is negative up to a concentration of approximately 3 mol of water per mol of R-CD. Just as in the case of HTC/R-CD IC, minima in the heat of adsorption are observed. However, the concentration of water at the minimum changes less with temperature, ranging from 2 to 2.2 mol of water per mol of R-CD at 25 and 45 °C, respectively. The minimum at 25 °C is much larger than the minima at 35 and 45 °C, because the difference in sorption levels (Figure 4) at 20 and 30 °C below a ) 0.65 is larger than the differences between 30 and 40 °C and between 40 and 50 °C. Above a concentration of 3 mol of water per mol of R-CD, the heat of
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Figure 8. Isosteric heat of adsorption (∆q) for PEG/R-CD IC as a function of temperature and adsorbed water concentration.
Figure 9. Integral entropy of adsorbed water (∆S) for PEG/R-CD IC as a function of temperature and adsorbed water concentration.
adsorption becomes positive, apparently because the mechanism of sorption changes. Here, sorption increases with decreasing temperature, causing sorption to become enthalpically unfavorable. This behavior is due to sorption by free PEG and vacated R-CD cavities as a result of PEG diffusing out of the R-CD crystal as explained earlier. PEG prefers the hydrophilic water vapor environment over the hydrophobic R-CD cavity. Positive values for the isosteric heat of sorption of water by pure cage R-CD were also reported by Tanada et al.9 Figure 9 shows the integral entropy change for water sorption by PEG/R-CD IC. The location of the minima in ∆q (Figure 8) and ∆S (Figure 9) are closer and better defined than those for HTC/R-CD IC. In fact, the trends in ∆S (Figure 9) are similar to those observed for ∆q (Figure 8). The primary difference is that the integral entropy change is negative at all concentrations, which means that the adsorbed water molecules are more ordered in the IC than in the vapor phase. Conclusions Complementary water vapor adsorption and WAXD are useful techniques for probing the structures and stabilities of R-CD ICs. The effect of guest hydrophobicity is great, as dethreading of PEG is observed in PEG/R-CD IC upon water adsorption and is not observed in HTC/R-CD IC. Moreover, these ICs have different sorption behavior than both cage and columnar R-CDs with no guest molecules. Thermodynamic calculations for the isosteric heat and integral entropy of adsorption versus water concentration show minima that likely
3858 J. Phys. Chem. B, Vol. 111, No. 15, 2007 correspond to the preferred hydrate composition at a given temperature. These calculations also show that the driving force for water adsorption is the heat evolved upon hydrogen bond formation between adsorbed water and R-CD. Acknowledgment. The authors thank North Carolina State University and the National Textile Center (Grant no. M06NS02) for funding. Birgit Anderson assisted with elemental analysis. References and Notes (1) Harata, K. In Crystallographic Studies; Atwood, J., Davies, J., MacNicol, D., Vogtle, F., Lehn, J., Eds.; Comprehensive Supramolecular Chemistry; Pergamon: Oxford, UK, 1996; Vol. 3, pp 279-303. (2) Szejtli, J. Pure Appl. Chem. 2004, 76, 1825-1845. (3) Nepogodiev, S. A.; Stoddart, J. F. Chem. ReV. 1998, 98, 19591976. (4) Harata, K. Chem. ReV. 1998, 98, 1803-1827. (5) Rusa, C. C.; Rusa, M.; Peet, J.; Uyar, T.; Fox, J.; Hunt, M. A.; Wang, X.; Balik, C. M.; Tonelli, A. E. J. Incl. Phenom. Macrocycl. Chem. 2006, 55, 185-192. (6) Manor, P. C.; Saenger, W. J. Am. Chem. Soc. 1974, 96, 36303639. (7) Noltemeyer, M.; Saenger, W. J. Am. Chem. Soc. 1980, 102, 27102722. (8) Nakai, Y.; Yamamoto, K.; Terada, K.; Kajiyama, A.; Sasaki, I. Chem. Pharm. Bull. 1986, 34, 2178-2182. (9) Tanada, S.; Nakamura, T.; Kawasaki, N.; Kurihara, T.; Umemoto, Y. J. Colloid Interface Sci. 1996, 181, 326-330. (10) Hunt, M. A.; Rusa, C. C.; Tonelli, A. E.; Balik, C. M. Carbohydr. Res. 2004, 339, 2805-2810.
Hunt et al. (11) Perez-Alonso, C.; Beristain, C. I.; Lobato-Calleros, C.; RodriguezHuezo, M. E.; Vernon-Carter, E. J. J. Food Eng. 2006, 77, 753-760. (12) McMinn, W. A. M.; Al-Muhtaseb, A. H.; Magee, T. R. A. Food Res. Int. 2005, 38, 505-510. (13) Fasina, O. O. J. Food Eng. 2006, 75, 149-155. (14) Ariahu, C. C.; Kaze, S. A.; Achem, C. D. J. Food Eng. 2006, 75, 355-363. (15) Basu, S.; Shivhare, U. S.; Mujumdar, A. S. Drying Technol. 2006, 24, 917-930. (16) Harada, A.; Li, J.; Kamachi, M. Macromolecules 1993, 26, 56985703. (17) Li, J.; Harada, A.; Kamachi, M. Bull. Chem. Soc. Jpn. 1994, 67, 2808-2818. (18) Porbeni, F. E.; Shin, I. D.; Shuai, X. T.; Wang, X. W.; White, J. L.; Jia, X.; Tonelli, A. E. J. Polym. Sci., Part B: Polym. Phys. 2005, 43, 2086-2096. (19) Girardeau, T. E.; Zhao, T. J.; Leisen, J.; Beckham, H. W.; Bucknall, D. G. Macromolecules 2005, 38, 2261-2270. (20) Rizvi, S. S. H.; Benado, A. L. Food Technol. 1984, 38, 83-92. (21) Hill, T. L.; Emmett, P. H.; Joyner, L. G. J. Am. Chem. Soc. 1951, 73, 5102-5107. (22) Mihoubi, D.; Bellagi, A. J. Chem. Thermodyn. 2006, 38, 11051110. (23) Rusa, C. C.; Bullions, T. A.; Fox, J.; Porbeni, F. E.; Wang, X. W.; Tonelli, A. E. Langmuir 2002, 18, 10016-10023. (24) Piculell, L.; Lindman, B. AdV. Colloid Interface Sci. 1992, 41, 149178. (25) Metz, S. J.; van der Vegt, N. F. A.; Mulder, M. H. V.; Wessling, M. J. Phys. Chem. B 2003, 107, 13629-13635. (26) Steiner, T.; Koellner, G. J. Am. Chem. Soc. 1994, 116, 51225128.
