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Glassy Crystalline State and Water Sorption of Alkyl Maltosides Vitaly Kocherbitov* and Olle So¨derman Physical Chemistry 1, Center for Chemistry and Chemical Engineering, P.O. Box 124, Lund University, S-221 00 Lund, Sweden Received August 22, 2003. In Final Form: February 10, 2004 A differential scanning calorimetric and sorption calorimetric study of two alkyl maltosides, C8G2 and C10G2, was performed. In the dry state, C8G2 and C10G2 do not form solid crystals but undergo a glass transition upon temperature change. The glass is partly ordered and has the same lamellar structure as the liquid crystals formed by the two maltosides. To reflect the presence of the glass transition and the structure, the terms “glassy crystals” and “glassy liquid crystals” can be used. A mechanism of the relaxation of the glassy crystals based on the results of small-angle X-ray scattering experiments is proposed. Experiments on water sorption showed that the glassy crystals turn into lyotropic liquid crystals upon sorption of water at constant temperature. This isothermal glass transition can be characterized by water content and change of partial molar enthalpy of mixing of water. A method to calculate the phase diagram liquid crystals-glassy liquid crystals is proposed.
Introduction Alkyl polyglucosides (CxGy) have been known for many decades as being surface active. Although in 1970 Hughes and Lew1 characterized them as biodegradable and “relatively harmless”, alkyl polyglucosides began to draw more attention for their environmentally friendly properties in the 1980s. Since that time, glucose-based surfactants, especially alkyl glucosides CxG1, have been extensively studied. Although most of the research was dedicated to studies of the properties of glucosides in water solutions, there are also several studies of the properties of solid crystalline phases of glucosides.2-6 The properties of dry alkyl maltosides CxG2 (Figure 1) have been studied in less detail. We are not aware of any successful studies of the crystal structures of alkyl maltosides or their solid hydrates. Nevertheless, temperatures of transitions between not very well defined structural forms of maltosides have been reported by several authors. Hughes and Lew1 described a surfactant having a formula C10G2.3 as “glassy amber wax” melting in the range 95-100 °C. Koeltzow and Urfer7 reported a “softening point” of decyl β-maltoside C10G2 at 86 °C and a “flow point” at 156-159 °C. Bonicelly et al.8 reported a pretransition at 78.6 °C, a transition solid-liquid crystals at 96.5 °C, and melting of liquid crystals at 203 °C in C10G2. Boyd et al.9 presented temperatures of solid crystal to smectic liquid crystal * Corresponding author. Current address: Health and Society, Malmo¨ University, SE-205 06 Malmo¨, Sweden. Ph: +4640 6657946. Fax: +4640 6658100. E-mail:
[email protected]. (1) Hughes, F. A.; Lew, B. W. J. Am. Oil Chem. Soc. 1970, 47, 162. (2) van Koningsveld, H.; Jansen, J. C.; Straathof, A. J. J. Acta Crystallogr. Sect. C 1988, 44, 1054-1057. (3) Jeffrey, G. A.; Yeon, Y.; Abola, J. Carbohydr. Res. 1987, 169, 1-11. (4) Kocherbitov, V.; Soderman, O. Phys. Chem. Chem. Phys. 2003, 5, 5262-5270. (5) Kocherbitov, V.; So¨derman, O.; Wadso¨, L. J. Phys. Chem. B 2002, 106, 2910-2917. (6) Dorset, D. L.; Rosenbusch, J. P. Chem. Phys. Lipids 1981, 29, 299-307. (7) Koeltzow, D. E.; Urfer, A. D. J. Am. Oil Chem. Soc. 1984, 61, 1651-1655. (8) Bonicelli, M. G.; Ceccaroni, G. F.; La Mesa, C. Colloid Polym. Sci. 1998, 276, 109-116. (9) Boyd, B. J.; Drummond, C. J.; Krodkiewska, I.; Grieser, F. Langmuir 2000, 16, 7359-7367.
Figure 1. Structural formulas of octyl β-maltoside (n ) 8) and decyl β-maltoside (n ) 10).
transitions of three alkyl maltosides, as well as their temperatures of transitions to isotropic liquids. Kahl et al.10 reported a transition of the crystalline phase of C10G2 to the liquid crystalline phase at 38.6 °C and melting to isotropic liquid at 102.2 °C. Enthalpies of the transitions reported by different authors are not in agreement with each other and are sometimes characterized as “nonanalyzable”.10 These facts show that despite the large amount of experimental data, the properties of dry maltosides are still not fully understood. Though the solid-state maltosides are commonly assumed to be crystals, this is not the only possible option. Glasses represent another class of solid-state substances. A glass is an amorphous solid, which exhibits a glass transition.11 Unlike crystals, amorphous materials do not possess long-range translational order. Both organic and inorganic substances can form glasses. The most common example of organic glasses is polymers. Normally polymers cannot form crystals because of the size and complexity of their molecules. Instead, when cooled to glass transition temperature Tg, the polymer chains stop moving, keeping the same structure as they had in the liquid state. Glassy states can be found not only in polymers but also in other organic materials, for example, sucrose.12,13 Though amorphous materials are lacking long-range order, there are some examples of materials exhibiting glass transition while being partially ordered: glassy (10) Kahl, H.; Enders, S.; Quitzsch, K. Colloids Surf., A 2001, 183, 661-679. (11) Elliott, S. R. Physics of amorphous materials, 2nd ed.; Longman Group: Harlow, Essex, U.K., 1990. (12) Le Meste, M.; Champion, D.; Roudaut, G.; Blond, G.; Simatos, D. J. Food Sci. 2002, 67, 2444-2458. (13) Blond, G.; Simatos, D.; Catte, M.; Dussap, C. G.; Gros, J. B. Carbohydr. Res. 1997, 298, 139-145.
10.1021/la035553c CCC: $27.50 © 2004 American Chemical Society Published on Web 03/13/2004
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plastic crystals14-17 and glassy liquid crystals.17-20 A typical example of a substance forming glassy plastic crystals is cyclohexanol.14 Above Tg, this substance forms plastic crystals, that is, crystals possessing long-range threedimensional positional order while orientations of molecules exhibit no order. Below Tg (about -120 °C), orientations of molecules of cyclohexanol “freeze in” and dynamic disorder becomes static; a glassy plastic crystal is formed. Another type of glassy structure, glassy liquid crystals, have the same structure as the corresponding liquid crystals (nematic, smectic, or cholesteric), but molecular motion in these structures is “frozen”. In this article, we will show one more example of a partly ordered glass: a lamellar glassy structure formed by a surfactant. Materials and Methods Maltosides. Since maltosides are very hygroscopic, special attention was paid to their drying and handling. All maltosides were dried in a vacuum at room temperature in contact with molecular sieves during 24 h prior to use. Transfer of dried maltosides to calorimetric cells was performed in a glovebag in a dry nitrogen atmosphere. Octyl β-maltoside was purchased from Anatrace (Anatrace Inc., Maumee, OH). In preliminary experiments, decyl β-maltosides from Anatrace, Calbiochem, and Fluka were used. Since C10G2 provided by Anatrace showed the narrowest differential scanning calorimetry (DSC) peak for the transition to isotropic liquid, it was used in all further experiments. For the water sorption experiments, water passed through a Millipore Q purification system was used. DSC. Experiments were made using a differential scanning calorimeter from Seiko Instruments of type DSC220 in different temperature ranges between -100 and 200 °C. We used scan rates from 0.1 to 5 °C/min. Samples with masses from 2 to 7 mg were put into aluminum pans and then sealed. The masses of the pans were controlled before and after experiments. For the preparation of samples containing water, absorption of water vapor on the dry C8G2 was used. Sorption Calorimetry. The method of sorption calorimetry is described in detail elsewhere.4,5,21 A sorption calorimetric cell consisting of a vaporization and a sorption chamber was used. We employed the same procedure as described in ref 5. Here we will give the equations used for calculations of sample composition, enthalpy, and water activity. The mass of evaporated water was calculated using the following equation:
mw )
Figure 2. DSC scans of C8G2 (upper curve) and C10G2 (lower curve). For illustrative purposes, the C10G2 curve is displaced down by 50 µW. where Psorp is the measured thermal power of sorption of water into the sample. The activity of water was calculated in a different way than in previous studies:
pt - (pt - pw0) aw )
(
pt -
)
((Pvap/Pmax))
pt p0w
p0w
(3)
where pt is total pressure, p0w is the vapor pressure of pure water, and Pmax is the maximal thermal power of vaporization of water, reached when the activity of water in the sorption chamber is 0.22 Equation 3 takes into account not only diffusion of water vapor but also the bulk flow of water vapor.22,23 Small-Angle X-ray Scattering (SAXS). Small-angle X-ray scattering experiments were carried out using SEIFERT IsoDebyflex 3000 equipment. Solid powder samples were put into vacuum-tight cells with mica windows. The wavelength was 1.542 Å, and the Q-range was 0.015-0.5 Å-1 with resolution of 8 × 10-4 Å-1. The temperature in the SAXS experiments was maintained with an accuracy of (0.25 °C.
Results and Discussion
∫
Pvap dt (1)
Hvap w
vap is where Hvap w is the enthalpy of vaporization of pure water, P the measured thermal power of vaporization, and t is time. The partial molar enthalpy of mixing of water, Hmix w , was calculated as follows:
vap sorp Hmix w ) Hw + P
Hvap w Pvap
(2)
(14) Adachi, K.; Suga, H.; Seki, S. Bull. Chem. Soc. Jpn. 1968, 41, 1073. (15) Haida, O.; Suga, H.; Seki, S. J. Chem. Thermodyn. 1977, 9, 11331148. (16) Talon, C.; Ramos, M. A.; Vieira, S. Phys. Rev. B 2002, 66, art. no.-012201. (17) Suga, H.; Seki, S. J. Non-Cryst. Solids 1974, 16, 171-194. (18) Suga, H.; Seki, S. Faraday Discuss. 1980, 221-240. (19) Fan, F. Y.; Mastrangelo, J. C.; Katsis, D.; Chen, S. H.; Blanton, T. N. Liq. Cryst. 2000, 27, 1239-1248. (20) Chen, S. H.; Katsis, D.; Schmid, A. W.; Mastrangelo, J. C.; Tsutsui, T.; Blanton, T. N. Nature 1999, 397, 506-508. (21) Wadso, L.; Markova, N. Rev. Sci. Instrum. 2002, 73, 27432754.
Transitions in Pure Maltosides. DSC studies of two alkyl maltosides were performed. The DSC scans of C8G2 and C10G2 are shown in Figure 2. The curve corresponding to octyl-maltoside consists of a step at 55 °C and a peak at 122.7 °C. The peak corresponds to a transition from a smectic liquid crystalline phase to an isotropic phase.9 The transition temperature reported by Boyd et al. is 2 °C higher than found in the present study. The DSC scan of C10G2 is qualitatively similar to that of C8G2 (Figure 2). Normally, a DSC scan of a well-dried surfactant consists of two peaks: the first (involving a higher heat effect) corresponds to the transition from a solid crystalline phase to a liquid crystalline phase, and the second to the transition to an isotropic phase (see for example refs 4 and 6). The DSC scans presented in Figure 2 have no main peaks (steps are observed instead), while the second peaks are clearly seen. Boyd et al.9 obtained similar results studying C8G2 and C10G2. They explained the absence of the DSC peak by difficulties in obtaining a reliable baseline (22) Kocherbitov, V. Thermochim. Acta, in press. (23) Bird, R. B.; Stewart, W. E.; Lightfoot, E. N. Transport Phenomena; John Wiley & Sons: New York, 1960.
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Kocherbitov and So¨ derman
Table 1. Thermal and Structural Properties of Pure C8G2 and C10G2a surfactant
Tg, °C
∆Cp, J/mol/K
T(lam-iso), °C
∆H(lam-iso), J/mol
d, Å, 25 °C, before heating
d, Å, 60 °C
d, Å, 25 °C, after heating
C8G2 C10G2
54 58
195 227
122.7 205.9
230 1060
33.0 37.8
31.9 35.1
32.1 35.4
a ∆C is the heat capacity change at the glass transition; ∆H p (lam-iso) is the enthalpy change during melting of liquid crystals into the isotropic phase; d is the repeat distance measured by SAXS.
caused by the hydrophilic nature of maltosides. Below we will present an alternative explanation of the observed phenomenon. The hydrophilic nature of a surfactant can indeed affect calorimetric results by sorption or desorption of water vapor during experiments. In the present study, we excluded effects associated with water sorption by preparing samples in a dry nitrogen atmosphere and carrying out DSC experiments in hermetically sealed aluminum cells. The masses of the cells were controlled before and after experiments. In addition, we performed a study of water sorption on n-octyl-maltoside (see section below). The result of this study (sorption isotherm) shows that the used procedure excludes effects of water sorption on the DSC results. When studying chemically stable pure substances, three kinds of transitions can take place upon changing temperature: first-order phase transitions, second-order phase transitions (sometimes referred as continuous phase transitions), and glass transitions. On the temperatureheat capacity curves, they are seen as sharp peaks, steps, and distortedly shaped steps, respectively. While firstand second-order phase transitions always occur at a certain temperature, glass transition temperatures Tg can vary in a range of several °C. An accurate value of Tg and the shape of the step on the Cp curve may depend on conditions of the experiment, thermal history, and way of preparation of the sample. The shape of the step on the heat capacity curves (Figure 2), especially the ill-reproducible small peak after the step, which is typical for glass transitions,24 clearly indicates the presence of the glass transition. A glass transition is a transition (upon heating) from a glassy state to a liquid state. Glassy and liquid states have the same structure, but in the glass the molecular motion is frozen. To determine the structure of the phases below and above the glass transition, a SAXS study was performed. Repeat distances d found at 25 and 60 °C in experiments with two maltosides are shown in Table 1. Data for 25 °C correspond to the samples stored for a long time at room or lower (-20 °C) temperature prior to experiments. After measurements at 25 °C, the samples were heated to 60 °C and then cooled. SAXS measurements repeated at 25 °C showed that the value of d was closer to the value measured at 60 °C than at 25 °C. This fact means that the maltosides in the glassy state retain the structure they had above the glass transition temperature, and a very slow relaxation of the glass structure takes place. In the SAXS measurements, only one main peak was observed, but we concluded that C8G2 and C10G2 have lamellar structures below and above the glass transition temperature, based on the following facts: (I) Above the glass transition, the structures are liquid crystalline, because at higher temperatures (see Table 1) transitions to isotropic phases are registered in DSC experiments. (24) Richardson, M. J. The glass transition region. In Calorimetry and Thermal Analysis of Polymers; Mathot, V. B. F., Ed.; Hanser: Munich, 1994; pp 169-188.
(II) The geometry of the surfactant molecule makes the formation of other (than lamellar) liquid crystalline structures at low water contents very unlikely. (III) Alkyl glucosides that have similar structures and properties form only lamellar phases at low water contents.4,5 (IV) Calculation of the area per headgroup from the repeat distance measured by SAXS gives a reasonable value (about 39 Å2) if a lamellar structure is assumed. Glass transitions are observed in amorphous solids;11 therefore the studied maltosides are both amorphous and glassy below the glass transition temperature. Amorphous solids are solids lacking long-range translational crystalline order. In the considered case of two maltosides, the glassy phase is not completely disordered but has a lamellar structure, the same structure as in the liquid crystalline phase at higher temperatures. Since this glassy phase combines the properties of a glass and liquid crystals, it may be called “glassy liquid crystals”. Strictly speaking, the name “glassy liquid crystals” reflects not the properties of the system (there is no liquid in the glassy liquid crystals) but its origin (liquid crystals). Perhaps a better name for this state of matter is “glassy crystals”, since it reflects both amorphous nature and presence of order. The latter term is normally used for glassy states formed by quenching plastic crystals.14-16 The term “glassy plastic crystals” is also used.15 We propose to use the term “glassy crystals” to denote glassy states obtained by cooling of liquid crystals as well as of plastic crystals. The expressions “glassy liquid crystals” and “glassy plastic crystals” can be used as well, especially when it is necessary to show the difference between these two states of matter. The glass transition and the glassy state were not observed in the studies of R- and β-octyl glucosides (RC8G1 and β-C8G1), which in many respects have properties similar to those of β-C8G2. Since these three surfactants have the same hydrocarbon tail but different headgroups, this would mean that the reason for the formation of the glassy state is in interactions of headgroups. The headgroup of a maltoside consists of two glucose rings and therefore is twice as big as the glucoside headgroup. The bulky maltose groups are gathered together in the headgroup layer of the lamellar phase and therefore interact mostly with each other but not with the hydrocarbon part. We suggest that these interactions are the main reason for the formation of the glassy state. Glass transition (for example upon cooling) occurs when molecules cannot move freely with respect to each other but due to intermolecular interactions are stuck in the position they had at the moment of the transition. A complicated structure of the molecules hampers the molecular motion and therefore can make the process of glass formation easier. In the case of maltosides, the bulky headgroups interact with each other stronger than glucoside headgroups do and therefore can more easily form the glassy state. The rotation of the two glucose rings in the maltose group gives one more degree of freedom, which makes the structure even more complicated. When the two rings of the maltose group lie in nonparallel planes, the movement
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Figure 3. Schematic structures of the lamellar phases formed by alkyl maltosides: relaxed glassy crystals (left); liquid crystals and freshly formed glassy crystals (right).
of two maltose groups with respect to each other is even more difficult and molecules are stuck in their current positions when the temperature is decreased and thermal motion is getting weak. When the motion of the headgroup layer is hampered and therefore the layer is in the glassy state, the hydrocarbon layer can be in the liquid state or in the glassy state. In the former case, one more glass transition, corresponding to the formation of a glassy structure in the hydrocarbon layer, can be observed. To find it, we have carried out a DSC study of pure C8G2 at low temperatures (down to -100 °C). Since all observed events on the DSC scans at the low temperatures were not reproducible and probably caused by kinetic reasons, we concluded that the second glass transition in the system does not exist. This means that the hydrocarbon part becomes glassy in the same transition as the headgroup part. Nevertheless, since the transition to the glassy state of the whole system is caused by the decrease of mobility of the headgroup layer, the interactions between the hydrocarbon chains play a minor role in this particular glass transition. On the contrary, the hydrocarbon part cannot keep the liquid structure when the surrounding headgroup layers making up most of the mass of the system stop moving. As mentioned above, the repeat distance in both studied maltosides measured by SAXS decreases when a sample is heated from 25 to 60 °C. After cooling to 25 °C, it does not return to the initial value but increases only slightly (see Table 1). To explain these SAXS data, we propose the following mechanism. During long-time storage at room or lower temperatures, maltoside molecules slowly rearrange in a way that provides the lowest level of free energy within the lamellar structure. This rearrangement leads to a certain two-dimensional order in the headgroup layer (Figure 3). When the sample is heated, it turns from the glassy to liquid crystalline state; therefore the molecules become more mobile, and the headgroups lose the order and become distributed in a more random way (right part of Figure 3). When the two-dimensional order is lacking in the headgroup layer, the area per headgroup is higher than in the case of more ordered structures. When the area per headgroup is higher, the hydrocarbon chain has more space to occupy in the horizontal plane and, if it is incompressible, occupies less space in the vertical direction. Therefore the repeat distance is lower at 60 °C than at 25 °C. During cooling, the liquid crystals undergo a glass transition and therefore the newly formed glassy crystals keep almost the same structure as in the liquid crystals. A slow relaxation to the more ordered state (the same state as it had before heating) occurs, but since it takes a long time, the structure of the newly formed glassy crystals is closer to the structure of liquid crystals than that of glassy crystals formed a long time ago. The proposed
Figure 4. The activity of water as a function of water content in C8G2 (lower curve) and in C8G1 (upper curve, from ref 5) at 25 °C.
mechanism explains the difference between the repeat distance in the glassy liquid crystals measured before and after heating. It also explains the presence of the small peak sometimes present on the heat capacity curve after the glass transition (Figure 2). The entropy effect associated with the peak reflects the decrease of order in the lamellar phase during the transition to the liquid crystalline structure. The poor reproducibility of the peak is due to the different history of the studied samples: if a sample is studied after a long-time storage at low temperature, then there is enough time for the relaxation processes to introduce two-dimensional order and a larger peak is observed. In the opposite situation, if the sample was heated to a temperature higher than Tg shortly before the experiment, the peak is small. Sorption of Water. Above we discussed thermal and structural properties of pure alkyl maltosides. This section deals with sorption of water; therefore the properties of the binary system maltoside-water will be discussed. The sorption calorimetric technique is used to study not only the process of sorption itself but also the properties of the binary system obtained during sorption. We have performed a sorption calorimetric study of n-octyl β-maltoside/water at 25 and 40 °C. The sorption isotherm (activity-composition curve) and partial enthalpy of mixing of water in the binary system at 25 °C are shown in Figures 4 and 5, respectively. Hydration of C8G2 starts with an exothermic regime at both 25 and 40 °C. At higher water contents, the partial molar enthalpy of mixing of water has a value close to zero. The initial hydration of octyl glucosides4,5 involves a high endothermic heat effect corresponding to the transition from solid crystals to liquid crystals (melting). The absence of the endothermic effect during the sorption experiments for the case of maltosides confirms the absence of solid crystals in the studied samples of C8G2 and C10G2. The presence of the endothermic heat effect of mixing during hydration of glucosides indicates the presence of a first-order phase transition involving an endothermic heat effect. This effect is observed both during addition of water and during heating. In both cases, the transition from solid crystals to liquid crystals occurs. During the hydration of octyl maltoside, an exothermic effect is observed (Figure 5), while during heating of the dry substance in DSC experiments no significant enthalpy change (no peak on the heat capacity curve) is registered
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Figure 5. Partial molar enthalpy of mixing of water in the C8G2-water system as a function of mole fraction of water at 25 °C.
(Figure 2). A glass transition should not involve an enthalpy change; therefore the observed exothermic effect of mixing needs an explanation. Below we shall give an explanation based on the properties of phases involved in the transitions. The glass transition in a pure substance observed upon heating involves phases (glass and supercooled liquid) that basically have the same structures; the main difference between them is not in structure but in the mobility of the structural units. Since the structures of the two phases are the same, the entropy (determined by the number of microstates available for a structure) does not change during transition. The enthalpy change (directly related to the entropy change in a one-component system) is also zero. In the case of mixing of water and the surfactant in the sorption calorimetric cell, the process involves two different phases with two different structures: glassy crystals and pure liquid water. (Since the mixing properties are recalculated for the liquid water, we will refer to the liquid, although technically water in the vapor state is mixed with surfactant in the sorption cell.) The mixing of water involves a transfer of water molecules from one phase to another and therefore a finite change in the properties of water accompanied by a heat effect. The properties of the structural changes upon transfer of water molecules into the surfactant phase are the following: (I) water molecules lose their mobility; (II) water molecules become incorporated into the lamellar structure, and therefore a lamellar order lacking in liquid water is introduced; (III) water molecules can be bound to some specific sites in headgroup layers, especially in the relaxed (more ordered) glassy phase. These changes lead to an exothermic heat effect of mixing. The value of the heat effect is of the same order of magnitude as the value of the enthalpy of crystallization of pure water into ice. The composition range of the exothermic heat effect (i.e., the composition range of the existence of the glass) is narrower at 40 °C than at 25 °C. In DSC experiments, the temperature of the glass transition of C8G2 also depends on composition (data not shown). The dependence water content-glass transition temperature (Figure 6) determines the field of existence of the glassy state on the binary phase diagram surfactant-water. Since both sorption and DSC data for the glass transition in C8G2 are available, one can compare them by making
Kocherbitov and So¨ derman
Figure 6. Composition dependence of the glass transition temperature in the system octyl maltoside-water. Circles denote experimental data (DSC for the pure maltoside and sorption data for the mixture), while the solid line is calculated from eq 6 using the parameters ∆Cp ) 195 J/mol/K and ∆Hm w ) 10.5 kJ/mol.
a thermochemical circle. The heat released or absorbed by transferring a system from one state to another should not depend on the way the transfer is done but should depend only on the initial and final states of the system; therefore,
∫14 Hmw dr ) ∫12 Cp dT + ∫23 Hmw dr + ∫34 Cp dT
(4)
where r is molar ratio, and numbers 1-4 denote points on the phase diagram as shown in Figure 6. Assuming that the heat capacity of the liquid crystalline phase changes little with composition and the partial enthalpy of mixing of water in the same phase changes only slightly with temperature, one can rewrite eq 4:
∫TT
g
(g) (C(l) p - Cp ) dT )
∫0r
g
(Hm(l) - Hm(g) w w ) dr
(5)
where indices l and g refer to the liquid crystalline and glassy phase, respectively, or
∆Cp(Tg - T) ) ∆Hm w rg
(6)
where ∆Cp is the change of the heat capacity at the glass transition in the dry substance in the DSC experiment and ∆Hm w is the change of the partial molar enthalpy of mixing in a sorption experiment at a glass transition at composition rg and temperature T. In the case of the presence of a peak after the heat capacity step, the corresponding heat effect should be subtracted from the left part of eq 5. Equation 6 is not strictly valid because it does not take into account the temperature dependence of ∆Cp and ∆Hm w but can still be used for a qualitative analysis of the composition dependence of glass transition and involved change of partial enthalpy of mixing. One can see, for example, that the change of enthalpy of mixing ∆Hm w at the glass transition should be positive if the glass transition temperature of the mixture is lower than that of the dry substance. In other words, the partial heat effect of mixing of water with the glassy crystals should be negative with respect to the heat effect of mixing of water with liquid crystals. The value of the ∆Hm w calculated
Crystalline State of Alkyl Maltosides
using eq 6 from DSC data is about 10 kJ/mol, which is in agreement with the results of the sorption experiment. In Figure 4, the sorption isotherm of n-octyl β-Dglucoside C8G1 taken from our previous work5 is also shown. In the composition range presented here, both substances have lamellar liquid crystalline structures at higher water contents (>50 mol % of water); therefore one can make a comparison of the properties of the lamellar structures formed by the two substances. The activities of water in the two binary systems differ significantly. The water activity in the C8G2-water system is lower than in the C8G1 system, especially in the beginning of the hydration of the liquid crystalline phase (about 50 mol %). In other words, octyl maltoside is more hydrophilic in the studied composition range than octyl glucoside. To find the reason for this, let us consider some models of the process of hydration. According to the model of onedimensional swelling, all the water molecules are placed between the monolayers of surfactant headgroups, and the structure of the headgroup layers remains intact; therefore the area per surfactant headgroup is constant during swelling. Alternatively, water molecules can not only be placed in the space between headgroup layers but can also be incorporated into the structure of the layers, therefore changing the area per headgroup. Nilsson et al.25 showed that C8G1 keeps almost constant area per headgroup at different water contents, which indicates one-dimensional swelling, while for C8G2 this has not yet been studied. The lamellar phases of C8G1 and C8G2 have similar structures (here we consider only β-anomers). The headgroups consisting of one glucose ring in the case of glucoside and two glucose rings in the case of maltoside are in perpendicular position with respect to the plane of the monolayers. Therefore, in the case of onedimensional swelling, the water molecules should interact with the same hydrophilic groups of the surfactant. In other words, the presence of the second glucose ring between the first ring and the hydrocarbon tail would not affect the surfactant-water interactions in the case of (25) Nilsson, F.; So¨dernan, O.; Johansson, I. Langmuir 1996, 12, 902-908.
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one-dimensional swelling. But since the observed activities of water differ very much for the two surfactants, then the swelling of the lamellar phase of C8G2 is probably not one-dimensional. The headgroup layer of octyl maltoside is more bulky and contains more sites where water molecules can be placed during sorption or mixing. Therefore water is better distributed in the hydrophilic moiety of the surfactant and consequently has lower activity (see Figure 4). The presence or the absence of the glassy state in surfactants is of high importance for their use in practical applications. When a dry substance is in the crystalline form, then significant uptake of water by the substance starts from the water activity level corresponding to the equilibrium between solid crystals and liquid crystals. For β-C8G1, this activity is about 0.6 (Figure 4), which is above the normal relative humidity level indoors. If crystals are not formed, like in the β-C8G2 case, sorption of water starts at much lower relative humidity; therefore the glassy forms of surfactants require much drier storage conditions than crystal ones. Conclusions The results presented in this work can be summarized as the following points: (I) Commercially available octyl and decyl maltosides are in the lamellar glassy crystalline state at room temperature. (II) The glassy crystalline structure turns into the lamellar liquid crystalline structure upon heating or addition of water. (III) Uptake of water by glassy crystals is accompanied by an exothermic heat effect of mixing. (IV) The glass transition occurring at constant temperature upon addition of water can be characterized by water content and change of the partial molar enthalpy of mixing of water. (V) The calorimetric data allow calculation of the binary phase diagram liquid crystals-glassy liquid crystals. LA035553C
