Deuterium NMR Investigation of the Lyotropic Phases of Alkyl Î²

Article pubs.acs.org/JPCB

Deuterium NMR Investigation of the Lyotropic Phases of Alkyl β‑Glycoside/D2O Systems Omar Misran,† Bakir A. Timimi,‡ Thorsten Heidelberg,† Akihiko Sugimura,§ and Rauzah Hashim†,* †

Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia School of Chemistry, University of Southampton, Highfield, Southampton, SO17 1BJ, United Kingdom § Department of Information Systems Engineering, Osaka Sangyo University, Nakagaito, Daito, Osaka 574, Japan ‡

ABSTRACT: We have investigated the phase behavior of four glycosides (βC8OGlc, βC8SGlc, βC10OGlc, βC8OGal) in water and D2O by optical polarizing microscopy and deuterium NMR. Previously published phase diagrams were evaluated by deuterium NMR, via monitoring D2O spectra, and confirmed the presence of the hexagonal, bicontinuous cubic, and lamellar phases in these glycosides. We have also shown the presence of the gel phase in (βC10OGlc) and observed the extensive supercooling of the lamellar phase to temperatures well below the Kraft line. While the main features of the phase diagrams were confirmed, some phase boundaries were found to be slightly different. Magnetically aligned spectra were also observed for relatively dilute samples for the hexagonal phase (βC8OGlc and βC8OGal) and the lamellar phase (βC8SGlc and βC10OGlc). The average number of bound water molecules per headgroup in the lamellar phase for the glycosides was determined by the systematic measurement of the quadrupolar splitting of D2O over a wide range of values of the (glycoside/water) molar ratio. The number of water molecules bound to the headgroup was found on average to be about 1.6−1.7 water molecules with no significant differences in this value for the different glycosides (and over the temperature range investigated), indicating that the bound water content is predominately influenced by the number of hydroxyl groups of the headgroup only. However, this bound water content of only 1.6−1.7 water molecules per sugar headgroup is surprisingly low, suggesting strong intermolecular interactions of the OH groups of headgroup sugars. The results are in line with computational results reported earlier for the octyl-β-glucoside and β-galactoside, which show the presence of strong intralayer hydrogen bonding.

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INTRODUCTION Alkyl glycosides such as alkyl polyglucosides (APGs) have been applied in many industrial formulations,1 ranging from cosmetics to fertilizers and drilling fluids.2−4 This has drawn a wide range of fundamental research especially in the physics of foams and thin films.5 Therefore, systematically understanding the structure to property relationship of these sugar surfactants is paramount to support the vast commercial interest. Alkyl glycosides with chain lengths at least six carbons show amphitropic behavior exhibiting liquid crystalline behavior both in the absence (thermotropic) and in the presence of a solvent, usually water (lyotropic).6−8 Depending on temperature, simple straight chain alkyl glycosides usually show only a smectic phase in the anhydrous form, but when in contact with water, a variety of liquid crystal phases, (lamellar, hexagonal, and cubic) may be observed.9 Simple packing theory, related to the ratio of the surface area of the headgroup to that of the alkyl chain at the interface has been applied to explain these observations.10 The molecular self-assemblies of amphiphilic glycosides were recently reviewed,6,11 where all kinds of supramolecular organizations from helices and tubes have been characterized with their morphologies connected to molecular structures and detailed stereochemistry of the monomers.11 © 2013 American Chemical Society

Some of the simplest and widely studied sugar surfactants include n-octyl β-D-glucopyranoside (βC8OGlc), n-octyl-β-Dthioglucopyranoside (βC8SGlc) and n-octyl β-D-galactopyranoside (βC8OGal), and n-decyl β-D-glucopyranoside (βC10OGlc). Many of their surface and interfacial properties have been investigated and reviewed5,12 for example, the disjoining pressures as a function of film thickness and sugar stereochemistry have been measured.13,14 In addition, their detailed lyotropic phase behavior (including the biphasic regions) have been studied by many for examples Sakya et al.,15,16 Nilsson et al.,17,18 Kocherbitov et al.19 and Yamashita et al.20 Structurally, βC8OGlc, βC8SGlc, and βC8OGal are very similar, but differ in the linking atom between the sugar unit and the alkyl chain (oxygen or sulfur) and the stereochemical orientation of the hydroxyl group at the fourth carbon position in the sugar ring (C4−OH). This hydroxyl group is equatorial for the glucoside but axial in case of its C4 epimer, the galactoside. However, βC10OGlc differs from βC8OGlc only by the longer chain. Intriguingly however, their lyotropic phase behavior differs Received: February 20, 2013 Revised: May 27, 2013 Published: May 29, 2013 7335

dx.doi.org/10.1021/jp401787b | J. Phys. Chem. B 2013, 117, 7335−7344

The Journal of Physical Chemistry B

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from each other significantly, but is consistent with the molecular packing arguments.10,15 A study of the phase diagrams (see Figure 1) for these compounds indicates similar stabilities for the lamellar phase

Here we report the lyotropic phase behavior of four alkyl glycosides, i.e., βC8OGal in D2O, using deuterium NMR spectroscopy over a very wide range of temperature and composition. The compounds have been chosen to reflect variations of epimers (glucose vs galactose) in the hydrophilic region and the alkyl chain lengths (C8 vs C10) in the hydrophobic region as well as the linkage between them (O vs S). We compare the present experimental results with previously reported simulations, which investigated the effects of different sugar heads in the lamellar phase and the quantifications of different types of hydrogen bonding (intraand interlayer).24 Besides the determination of bound water, we examined the phase boundaries in the binary phase diagram of water and the glycosides and compared them with previous reports.15

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MATERIALS AND METHODS Deuterium NMR of D2O in Lyotropic Systems. Both static and dynamics orientational behaviors of liquid crystals could be studied by the deuterium NMR technique.25−28 One drawback of the method is the requirement of the preparation of specific isotopic labeling on the molecular system under investigation. However, to avoid chemical modification, a deuterium-labeled probe can be used dissolved in the liquid crystal environment.29 In lyotropic liquid crystals, since water forms an integral part of the phase, replacing H2O with D2O also provides a direct means to use deuterium NMR to investigate the ordering and phase behavior of lyotropic systems.23,28,30 The deuterium NMR spectrum of D2O is just a single line in the isotropic and cubic phases. However, when the anisotropic liquid crystal phases are magnetically aligned, this single line will split into a doublet (quadrupolar splitting) because of the interaction of the electric nuclear moment of the deuterium nucleus (nuclear spin I = 1) with the electric field gradient provided by the chemical bond to which the deuterium nucleus is attached. In magnetically unaligned liquid crystal phases (where there is completely a random distribution of the director alignment relative to the magnetic field), a polycrystalline or powder spectrum is obtained. Such a powder spectrum is characterized by the presence of two sets of doublets: the inner doublet, which is usually high in intensity, originates from parts of the sample in which the director is aligned perpendicular to the magnetic field, while the outer doublet, low intensity shoulders, originates from parts of the sample in which the director is parallel to the magnetic field. For a uniaxial phase the quadrupolar splitting of the inner peaks is one-half that from the outer shoulders. Most lyotropic systems are uniaxial and show the powder spectra indicated above. The splitting of the inner sharp peaks is referred to as the quadrupolar splitting Δ and given by the expression.23,31−33

Figure 1. The binary phase diagrams in water of (a) βC8OGlc, (b) βC8SGlc, (c) βC8OGal (reprinted with permission from Sakya et al.15 Copyright (1997) Taylor and Francis), and (d) βC10OGlc (reprinted with permission from Nilsson et al.17,21 Copyright (1998) American Chemical Society).

for βC8SGlc and βC8OGal, with clearing points around 126 °C, while βC8OGlc has a lower clearing temperature of 107 °C.15,17,21 The published phase diagram of βC10OGlc by Nilsson et al. shows the presence of only the lamellar liquid crystal phase and extends to only ∼70 °C and so does not show the limit of the thermal stability of the compound; however, this is not expected to be less than that of the βC8OGlc.17,21 The higher thermal stability and different phase behavior of these compounds are believed to be related to intermolecular interactions within the assembly due to the presence of strong hydrogen bonds in the headgroup region. Figure 1a,b does not show the very narrow biphasic region, which is less than 1 wt % between liquid crystal phases and less than 0.5 wt % between the L1 and liquid crystal phases.16 In the alkyl glycosides/water system, it is reasonable to differentiate between two types of intermolecular hydrogen bonds, i.e., hydrogen bonds between the sugar head groups and hydrogen bonds between the sugar headgroup and water in the interfacial region. Neither the extent of interaction nor the location of interaction sites between the surfactants’ hydroxyl groups and water can be predicted based on the molecular structure. All four hydroxyl groups, as well as the glycosidic and the ring-oxygen, can be cooperatively involved.22 One way of shedding some light on this issue is to experimentally determine the number of water molecules bound to the sugar headgroup using the well-established NMR method based on the free-bound water molecules model.23

Δ=

3 Q SOD 4 OD

(1)

where, QOD is the quadrupole coupling constant of the deuterium nucleus in the OD bond in D2O (220 kHz). SOD is the OD-bond order parameter, which is related to the order parameter S of the water molecule in the lyotropic system given by the following:

S= 7336

1 3 cos2 θ − 1 2

(2)



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the surfactant and water. Hence, a plot of Δ against the molar (surfactant/water) ratio could be used to provide information about the headgroup hydration.23,34−36 From the slope of the linear plot, we could estimate the number of bound water (n) molecules, provided we know or assume a value for the bound water OD-bond order parameter. For a single liquid crystal phase existing over a very wide composition range, such as the lamellar phase, it is found that the splitting usually increases with the mole fraction of the surfactant.23 Furthermore, this increase is linear at the beginning but either levels off, or reaches a maximum and then diminishes in value as the mole fraction of water decreases further. To account for this behavior, the free and bound water model was further refined by assuming that the free water is in equilibrium with the constant number of water molecules n bound to the headgroup, according to the following chemical equilibrium:23

Here the braces ⟨ ⟩ indicate an ensemble or time average, and θ is the angle between the director and the symmetry axis of the water molecule. On the basis of the molecular structure of the surfactants, water can interact with several binding sites in the headgroup. If water molecules exchange rapidly (on the NMR time scale) between different sites, then the observed splitting will be the population weighted average of the splitting of these sites.23,31,34 Therefore eq 1 leads to the following: Δ = Σ PiΔi =

3 3 Q Σ PS Q SOD i iOD = 4 OD 4 OD

(3)

where Pi is the fraction of the population in site i. Δi is the splitting associated with site i and SiOD is the OD-bond order parameter of the water molecule at site i and ⟨SOD⟩ is the average OD-bond order parameter over all sites. Depending on the angle θ (between the director and the water symmetry axis), the OD-bond order parameter for different sites in the headgroup could be positive or negative.31 This will lead to a small observed splitting and in some cases to even zero splitting when the positive and negative terms cancel out each other (i.e., 1. From this relation, we can determine the value of n. Hence, a plot of Δ (which is directly proportional to the fraction of bound water) against the molar (surfactant/water) molar ratio should exhibit this maximum, from which the value of n could be determined. This forms the basis of the NMR method we followed in this work to determine the number of water molecules n bound to the glucoside head groups. Experimental Section. With the exception of the galactoside, which required chemical synthesis, commercial alkyl glycosides graded 98% and above (Sigma and Fluka) were applied directly without further purification. The optical examination of thermotropic and lyotropic (in H2O) textures was performed on a polarizing microscope (Olympus BX52) equipped with a heating stage (Linkam THMS 600 with controller TMS 91). A series of alkyl glycoside/D2O mixtures of different compositions ranging from 50% to greater than 92% were prepared in 4 mm diameter NMR tubes of 3−4 cm length containing 30−100 mg sample. The tubes were flame-sealed immediately after preparation, and the mixtures were homogenized by heating to above the melting point with vigorous shaking followed by repeated centrifugation up and down. The samples were left to equilibrate at room temperature for at least one week before NMR measurements were taken. Enough samples were used to ensure the accurate determination of phase boundaries and to examine each phase in sufficient details to obtain a good presentation of the ordering of water molecules in these systems. The deuterium NMR spectra were recorded on a JEOL GSX 270 MHz spectrometer (41.34 MHz for deuterium), with an accuracy of (±0.1 kHz), equipped with a variable temperature probe (±0.5 °C). A standard single pulse sequence was applied. The 90° pulse length is 5 μs, and an interpulse spacing of 0.2 s was used. The number of scans varied between 500 and 4000,

where, Pf refers to the fraction of all the “free” water species, and Pb refers to the fraction of all the “bound” water species. Δf and Δb are the corresponding splittings for “free” and “bound” water, respectively. Furthermore, Δf is assumed to be zero, thus eq 4 reduces to the following: Δ = PbΔb =

(7)

(6)

where n is the number of bound water molecules per headgroup and NS and NW refer to the number of moles of 7337



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and the temperature range covered approximately from 0 to 60 °C in most cases and to higher values in some.

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RESULTS AND DISCUSSION β-Octyl Glucopyranoside (βC8OGlc). We begin by considering the results for βC8OGlc, an alkyl glucoside which is very soluble in water at room temperature. Previously, phase behaviors of βC8OGlc in the dry form and in water have been reported.15,16 Our present investigations confirmed the previous findings15,16 that, on heating the anhydrous solid crystal melts at 68 °C into a SmA phase, which on further heating undergoes a transition to the isotropic phase at 112 °C. The sequence of lyotropic phases exhibited by βC8OGlc is the hexagonal (HI), the cubic (QI) and the lamellar phase (Lα) as the amount of water available in the binary mixture is reduced (see Figure 1). The phase symmetry of the cubic QI has been identified in the phase diagram as the normal gyroid Ia3d by Sakya et al.15,16 Here, we have adopted the nomenclature convention for lyotropic phases commonly used by Seddon et al.37 In H2O, the hexagonal HI phase is stable over the composition range of ∼64−73% glucoside and over the temperature range of 0 °C to ∼31 °C, while the cubic QI phase ranges from ∼73−80% glucoside and up to ∼60 °C. The lamellar Lα phase is stable at all glucoside compositions with 80% or higher and up to a high temperature of ∼120 °C. The equivalent molar ratios converted into w/w percentages in terms of using D2O (as in this work) in place of H2O would be 61.5−71% for the HI phase, 71−78% for the QI phase and more than 78% in D2O for the Lα phase. The phase boundaries from the D2O NMR measurements agree within 1−2% with these values, except perhaps for the HI phase, where we detected its presence at low temperature for a mixture containing 59% βC8OGlc in D2O. Figure 2 shows the D2O NMR spectra for the 59% sample from 0 to 11.5 °C. This would suggest that the boundary between the micellar L1 phase and the HI phase is not the sharp vertical line as shown in Figure 1, but that it is somewhat sloping toward the L1 phase. Our findings agree with that of Karukstis et al. who used the fluorescence technique.38 The spectrum at 0 °C in Figure 2 shows the presence of a single HI phase. The intense center of the spectrum containing two sharp doublets (the inner doublet of lower intensity has a splitting, which is one-half the outer one) is from the D2O molecules within the magnetically aligned sample, while the outer very low intensity broad peaks are due to the four OD groups, which are formed as a result of exchange between the D2O present in the interface and the OH groups of the glucose moiety.39−41 See Figure 3(c) for a better resolved exchange peaks. Another characteristic of the spectra in Figure 2 is the dominance of the outer quadrupole doublet, which indicates that the director of the mesophase is largely aligned parallel to the magnetic field. The highest temperature at which we are able to observe the HI phase is ∼25−26 °C, which is about 5 °C lower than that reported previously (see also Figure 1).15,16 However, our results agree well with that found by Nilsson et al.18 The D2O spectra in the QI cubic phase are the expected single line in common with spectra in the micellar L1 phase. Although D2O NMR is unable, on its own, to differentiate between the cubic and L1 phases, however, by combining it with information from other techniques, one can differentiate between these phases. For example, small-angle X-ray diffraction data show a multitude of diffraction lines in the cubic phases and not in the L1 phases. Also using polarized

Figure 2. Spectra for 59% βC8OGlc in the HI phase at 0, 5, 9, 10, and 11.5 °C.

Figure 3. The D2O spectra in the lamellar phase of βC8OGlc from 20 to 60 °C for samples (a) 83.3%; (b) 95%; and (c) is the expanded spectrum of the 95% sample at 60 °C showing the peaks due to the 4 OD bonds (as indicated by the black downward arrows). These are clearly separated from that of the D2O signal indicated by the dotted arrow.

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≈ 1.7 as the number of bound water molecules per βC8OGlc headgroup. We shall discuss the significance of this result later, after we have looked at the hydration of the other three alkyl glycosides. β-Octyl Thio-Glucopyranoside (βC8SGlc). The solid crystal melts at 42 °C into a SmA phase, while the transition from the smectic phase to the isotropic phase takes place at 126.4 °C. These values compare well with those found experimentally which are 41−42 °C and 125−125.7 °C respectively.16 Like the βC8OGlc, its thio-analogue is very soluble in water. The phase diagram (Figure 1), shows the presence of a fluid lamellar, Lα phase as well as a cubic, QI phase.15,16 However, in contrast to the oxygen-analogue, βC8SGlc shows no hexagonal phase. At 0 °C the cubic phase extends over compositions from 71.5% to 90.5% βC8SGlc in water; the equivalent in D2O would be compositions 69.3− 89.6% βC8SGlc. The cubic phase transforms directly on heating into the isotropic L1 phase for compositions up to ∼73% in D2O. The highest temperature at which the cubic phase is stable is ∼65 °C and this occurs at a composition of ∼73% of βC8SGlc in D2O. The boundary between the cubic and lamellar phase steadily shifts toward lower temperatures (Figure 1),15 as the concentration of βC8SGlc increases so that the transition temperature from the cubic to the lamellar phase decreases to 0 °C as the composition increases from 73 to 89% βC8SGlc. This boundary from our NMR measurements agrees within 1−2% with that given by Sakya et al.15,16 Figure 5(a) shows the D2O NMR spectrum for a 79% sample at temperatures from 20 to 60 °C. Below 55 °C, the spectrum is just a single line, which is what is expected from a cubic phase. Above 55 °C, the powder spectrum of a typical liquid crystalline phase, in this case the Lα phase, is gradually evolving; at the same time, the central line reflecting the coexistence of the cubic phase is decreasing. The transition from the Lα to the cubic phase is kinetically hindered. Due to this, we have observed on several occasions that the lamellar phase could be supercooled into the cubic phase region, e.g., see Figure 5(b). As in the case of βC8OGlc, only now, in the lamellar phase, we found that the director could be aligned by the magnetic field for samples of compositions not exceeding ∼80%, especially when the samples are subjected to heating and cooling cycles. However, unlike the oxygen-analogue, the inner quadrupole-splitting would dominate the spectra of βC8SGlc, hence, as expected, the alignment of the director of the Lα phase is perpendicular to the magnetic field corresponding to the diamagnetic susceptibility, Δχ to be negative, i.e., Δχ < 0. In Figure 5(b,c), we give the D2O NMR spectra for two samples of 86 and 92% compositions at temperatures from 20 to 60 °C showing the presence of only the lamellar phase in accord with the phase diagram.16 A feature of the spectra in Figure 5(b,c) is the presence of the small extra splitting in the center of the spectrum at elevated temperatures. These NMR signals, we believe, originate from the -OD in the exocyclic, CH2−OD group, as discussed earlier for the spectra of βC8OGlc. In this case, fast exchange leads to the collapse of the splitting eventually into a single line at 60 °C and above. The other low intensity doublets, with larger splitting than that of D2O, are due to the other -OD groups in the glucose moiety formed by exchange with D2O. The quadrupolar splitting for βC8SGlc in the Lα phase remains practically constant over the temperature range up to 60 °C. This again demonstrates the low influence of temperature on

optical microscopy techniques, one can observe the difference in flow properties between the L1 phase (very fluid) and the cubic phases (very stiff) as well as differences in the shapes of trapped air bubbles;16 also when partially uncrossing the cross polarizers, the phases boundary between the L1 and cubic phases can be observed as a dividing lines between the two isotropic coexisting phases. In Figure 3(a),(b), the D2O spectra in the lamellar phase at 20 to 60 °C are given for samples of compositions 83.3% and 95%, respectively. These show the expected increase of the quadrupolar splitting as the concentration of the βC8OGlc increases. The NMR signals from the exchange between D2O and the four OH groups in the sugar headgroup can also be seen in the spectra [see the expanded spectrum in Figure 3(c)]. Similar exchange signals have been reported for both the α- and βC8OGlc.39,41 Of the four -OD exchange peaks, the largest -OD splitting is most likely associated with that on C4. The prominent doublet found in the center of the spectra (i.e., the smallest splitting) in Figure 3 at 40 °C and above, is due to the deuterium in the exocyclic −CH2−OD group of the glucose moiety formed by exchange with D2O in the interfacial region.34,39 Figure 3(a,b) also shows that, in contrast to the effect of composition, temperature has very little effect on the D2O splitting. This reflects the high thermal stability of the lamellar phase especially as the measurements were taken at temperatures well below 120 °C, which is the transition to the L1. At this point, the ordered liquid crystalline structure is destroyed.15,16 The temperature-insensitive behavior observed in this work supports previous findings of the alkyl-glucosides microemulsions.42 Finally, in Figure 4, we plot the D2O splitting, Δ in the lamellar phase against the molar ratio (βC8OGlc/D2O) for

Figure 4. The plot of the splitting against the molar ratio of βC8OGlc/ D2O the Lα phase at 20, 30, 40, 50, and 60 °C. The first three points (larger dots) at low molar ration are from the hexagonal phase at 20 °C. The plot also shows the data for (βC10OGlc/D2O) at 20 °C (crosses).

temperatures 20 to 60 °C (as well also as 3 points at 20 °C from the hexagonal phase) in order to determine the number of water molecules bound to the glucoside headgroup, n. As the molar ratio increases, the splitting increases roughly linearly at the beginning, the rate of increase then slows down and the splitting reaches a maximum, after which it starts to decrease. This form of the change in splitting with molar ratio is expected from the constant water binding model.34 The splitting reaches a maximum or levels off at a molar ratio of ∼1.5 and this gives n 7339



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Figure 6. The plot of the splitting against the molar (βC8SGlc/D2O) ratio for βC8SGlc in D2O at 20 °C.

compound is readily soluble in water at room temperature to give only the micellar L1 and lamellar phases. No hexagonal phase was observed in agreement with the literature (see Figure 1).17 One intriguing aspect observed in the micellar region is the presence of a two-phase “isotropic solutions” in that region.17,21 We have, however, not investigated this part of the phase diagram. The change in phase behavior on going from βC8OGlc to βC10OGlc is similar to that observed in the C8, C10, and C12 α-mannopyranoside15 series, where the increase in chain length led to the progressive destabilization of the hexagonal and the cubic phases. This was interpreted on the basis of the idea of decreasing interfacial curvature as the chain length increases.15 Figure 7(a,b) shows some typical D2O

Figure 5. D2O spectra of βC8SGlc/D2O at different temperatures and compositions (a) 79%, (b) 86%, and (c) 92% of βC8SGlc in D2O. For (a) 60 °C, the spectrum is for the Lα phase, while at 20 and 40 °C, the spectra are of the cubic phase (Q). For (b) and (c), the spectra are of the Lα phase. Figure 7. The spectra of D2O in βC10OGlc. (a,b) are at 20 °C for different compositions.

the phase stability compared to the influence of the composition. Figure 6 shows the plot of the D2O splitting for βC8SGlc against the molar (βC8SGlc/D2O) ratio at 20 °C. The splitting achieves a maximum at a molar ratio of ∼1.8. This gives the number of bound water, n, as ∼1.6 water molecules per βC8SGlc headgroup for temperatures up to at least 60 °C (the highest we investigated). β-Decyl Glucopyranoside (βC10OGlc). On the polarizing microscope, the crystalline βC10OGlc transforms into the SmA at 76.5 °C and turns into the isotropic phase at 132.5 °C. The clearing temperature is slightly lower than that reported by Goodby (133.5 °C),9 but the melting temperature exceeds Goodby’s observation (70.3 °C) and matches more than that reported by Noller and Rockwell (75 °C).43 Both transition temperatures are higher than those observed for the βC8OGlc (by 8 and 20 °C, respectively), which is as expected due to the increase in the alkyl chain length by two methylene groups. The

spectra at 20 °C for compositions from 64.5% to 94%. The spectrum for the 64.5% is mostly of the lamellar phase with a small isotropic component. So the phase boundary for the Lα phase could be taken at 65% βC10OGlc/D2O at 20 °C, which is in good agreement with the phase diagram of Nilsson et al.17 (see Figure 1). On raising the temperature to above 30 °C, the spectrum of the 64.5% sample becomes completely that of the lamellar phase (not shown here). This is in accordance with the phase diagram given by Nilsson et al.17 in Figure 1, which shows the lamellar phase boundary tilted toward the temperature axis. We have also observed that magnetically aligned spectra could be obtained by thermal cycling of the lowest composition samples. The spectra (not shown here) of D2O in βC10OGlc, in common with other alkyl glucosides, show splittings, which are 7340



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139 °C. Both temperatures are higher than that reported previously of 96 and 127 °C (see also Figure 1).15 While the melting point matches well with Vill et al., the clearing temperature is still higher.45 In contact with water, it forms three lyotropic phases: hexagonal, cubic, and lamellar phases as the amount of water is reduced. This is very similar to the phase behavior exhibited by βC8OGlc except for two features: the first is the higher stability region of the hexagonal phase, which extends from 63−83% in H2O, and the second is the higher Kraft line, which starts at 42 °C (for the hexagonal and cubic phases) and rises steadily to 70 °C for the lamellar phase at 5% hydration (which is very close to the lowest water content sample we have investigated).15 We have ensured that all our samples are heated to temperatures above the Kraft line in order to obtain D2O NMR spectra for the hexagonal and lamellar phases. We were also able to obtain spectra below the Kraft line because of the strong tendency of this system to supercool even to room temperature (∼20 °C).15 The hexagonal phase was observed over the composition range (60−81% in D2O), the cubic phase range (81−86%) and the lamellar phase at compositions >86% βC8OGlc. Figure 9 shows some typical D2O spectra in the hexagonal phase for a 72% galactoside sample at different temperatures;

insensitive to temperature changes. Moreover, these spectra also show the presence of a smaller central splitting, which appears as the temperature is raised. This small central splitting is due, as we have explained for the other alkyl glucosides, to the signal from the deuterium in the OD of the exocyclic −CH2OD group. The other low intensity doublets are due to the other -OD groups (within the sugar head) formed by exchange with D2O and will not be discussed further as they do not overlap with the D2O NMR signal. Furthermore, we observed on a number of occasions that for samples of composition ≥83% (that have been left at room temperature or in a refrigerator for a week or more) a sudden jump in the D2O quadrupole splitting from a smaller splitting at 20 to 30 °C to a much larger splitting on heating above 30 °C. This jump is due to the transition from Lβ (gel phase) to Lα. However, the larger splitting is retained on cooling such samples from the lamellar phase. These observations are indicative of the formation of an Lβ or a gel phase (with a smaller splitting), which for the 95% sample melts above 30 °C to the fluid lamellar phase (Lα). From the spectra observed on heating and cooling, see Figure 8(a,b), it is apparent that the fluid lamellar phase supercools into the region, where the gel phase is expected.

Figure 8. (a) The D2O spectra: in (I) a biphasic region of possibly an Lc phase (the single peak) plus a gel phase at 19.8 °C of an 89% sample of βC10OGlc; in (II) most likely of the gel phase at 25 °C of the 89% sample of βC10OGlc; in (III) the same sample as in (II) but after it was cooled from 30 to 25 °C. (b) The spectra for the gel phase at 22 °C for 85.6% and 87% samples of βC10OGlc.

The bound water content per headgroup for βC10OGlc can be inferred from the plot given in Figure 4, which displays the quadrupole splitting in the lamellar phase against the molar ratio (βC10OGlc/D2O) at 20 °C. Very similar plots were obtained for temperatures up to 60 °C and above. The plot in Figure 4 shows that βC10OGlc behaves identically to βC8OGlc and so the bound water content, n is about ∼1.7 as for βC8OGlc β-Octyl Galactopyranoside (βC8OGal). The compound was prepared using method described previously.44 The pure compound melts at 98 °C into a smectic A phase that clears at

Figure 9. D2O spectra in the hexagonal phase for a 72% galactoside sample in D2O at 20, 30, 40, 50, and 60 °C. 7341



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lamellar phase for βC8OGlc, βC8SGlc, and βC10OGlc. However, they plotted the values of Δ against the mole fraction of the sugar surfactant XS instead of against the molar ratio of (sugar surfactant/water), i.e., (NS/NW) (see eqs 5 and 6).49 The quadrupolar splitting in our investigation levels off at much higher values of the molar glycoside/water ratio than in the data given by Bonicelli et al.49 For example, for βC8OGlc, our Δ value levels off at a molar ratio of ∼1.4 with a splitting of about 6.7 kHz, compare this with that of Bonicelli et al. who found a maximum at a molar ratio of ∼0.45 with a D2O splitting of ∼4 kHz.49 Furthermore, the average initial slope of the plot for Δ against the molar ratio of NS/NW should pass through zero (eqs 5 and 6 and Figures 4, 6, and 10). When using their method49 and taking ≈ 0.01 the number of bound water for βC8OGlc and βC10OGlc was about ∼5.7; while for the βC8SGlc and βC8OGal, n was 4. However, the method proposed by Rendall et al.23 is more advantageous since by extending the measurement to very high amphiphile to water molar ratio (where there is insufficient water to hydrate all headgroup sites), a maximum can occur in the plot of Δ versus this molar ratio and n may be determined without assuming the value of the order parameter. In addition, the slow rise in the value of Δ with the increase in the molar ratio, as in the present case (see also Figure 9(a,b) in Rendall et al.23) contrasts sharply with the rapid rise in the plot for large values of n (see for examples Figure 9(c) in Rendall et al.23 for n = 8 and Figure 4 in Ockelford et al.46 for n = 11). By using a value of n ≈ 2, we estimate the average OD bond order parameter to be ∼0.033 in the case of βC8OGlc and βC10OGlc and ∼0.025 in the case of βC8SGlc and βC8OGal. Table 1 summarizes bound water values for some surfactants (ionic and nonionic).

the spectra below 42 °C are in the supercooled state. At 60 °C, the hexagonal phase passes into the isotropic phase in accordance with the phase diagram.15 The major splitting is due to D2O, and the much smaller peaks present are due to the OD deuterons in the galactose ring formed by exchange with the D2O present in the interface, as we have explained for the other glycosides. On many occasions, as the D2O NMR spectra showed, the hexagonal and lamellar liquid crystal phases were observed below the Kraft line in the supercooled state. Also, magnetically aligned spectra were obtained for samples in the hexagonal phase over the composition range 60−66% βC8OGal in D2O. For samples in the lamellar phase that were left at room temperature for a long time, no significant quadrupole splittings could be observed. This reflects the existence of the crystal lamellar phase Lc at various degrees of hydration. The D2O NMR spectra in the lamellar phase (not shown here) were found to be insensitive to temperature, and so the splitting for any of the samples is practically constant over the temperature range of our measurements. A plot of the quadrupole splitting against the molar (βC8OGal/D2O) ratio covering both the hexagonal and the lamellar phases and extending to a molar ratio of ∼1.65 is given in Figure 10. The splitting at molar ratio of ∼1.65 is slightly

Table 1. Number of Bound Water for Some Ionic and NonIonic Surfactants lipid

number of bound water

ionic surfactant βC8OGlc

Figure 10. Plot of the D2O quadrupole splitting against the molar ratio for the βC8OGal in D2O system; the first point in the lamellar phase at a molar ratio of ∼0.4 is in a biphasic (Q1+ lamellar).

βC10OGlc βC8SGlc βC8OGal

larger than that at a molar ratio of 1.1. At the molar (βC8OGal/ D2O) ratio of 1.65, we get the number of bound water molecules per βC8OGal headgroup as n ≈ 1.6 molecules, as the splitting is insensitive to temperature in the range covered in our experiments and so n is the same for temperatures 30, 40 to 60 °C and higher. Bound Water. Despite the differences in stereochemistry, linkage and alkyl chain length, the bound water content for all investigated alkyl glycosides is practically the same, n ≈ 1.6− 1.7. Compared to other previous measurements for ionic surfactants, where the number of bound water can be as high as 11, (see Figure 4 in Ockelford et al.46), the amount of n ≈ 1.6− 1.7 water molecules per surfactant is remarkably low, given the presence of four hydroxyl groups and two additional oxygen (the anomeric and the ring oxygen). The reason for this observation may be due to the fact that glycolipid system has the ability to participate in hydrogen bonding as both donor and acceptors unlike phosphatidylcholines, which can only acts as an acceptor.47,48 Our experimental results are in apparent disagreement with Bonicelli et al.49 who reported bound water contents of 4 in the

alkyl poly(oxyethylene) C12 EO6 monoolein monoglycerides

11 1.7 4 (Lα), 6 (H) 1.5 1.7 1.6 1.6 1.5 6−7 ∼4 3

ref 46 current 49 24 current current current 24 23 50 34

work

work work work

The question now is why is there such a low number of bound water molecules? A clue to understanding this may be provided by computer simulations of glycolipids bilayers.24,51 In one of these simulations, the number of hydrogen bonds, has been measured for both βC8OGlc and βC8OGal systems in the thermotropic and lyotropic states.24 In an idealized situation, with each glucose/galactose group possessing four polar hydrogens able to donate a hydrogen bond and six oxygens able to accept two hydrogen bonds, there should be a total of 16 possible hydrogen bonds per glycolipid. But in the simulations much less total number of hydrogen bonds of 5.9 (galactoside) and 5.7 (glucoside) were obtained and these are divided between inter- and intralayer hydrogen bonds. The intralayer hydrogen bonds are holding the lipid assembly structure together, while the interlayer ones are those able to 7342



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involve in bonding with water. It was shown from this study24 that strong intralayer hydrogen bond interaction in the βC8OGlc exist between adjacent headgroups, which leave the OH-groups on C3 and C4 free for interlayer hydrogen bonding. Thus, there were only about 3.1 hydrogen bonds per lipids involved in bonding with water for the glucoside and this is only 3 for the galactoside. If water is capable of making two hydrogen bonds, then the numbers of bound water for these systems seem to agree with those obtained by the deuterium NMR investigation presented here. The recently reported simulation for the disaccharide glycolipids also gave a similar trend of low interlayer hydrogen bond availability.51 Thus, the ability to bound water originates from the presence of hydroxyl groups in the sugar moieties and makes sugar-based surfactants very interesting in terms of their solid state behavior.52 Moreover, some literatures have reported that detailed stereochemistry of these polyhydroxy compounds generate hydrophobic or apolar surface on the sugar moiety itself giving it an amphoteric character important for molecular recognition.53−55 This unique property of sugar increases the intralayer hydrogen-bond interaction, self-assembly stability, and makes the phase boundary to be temperature independent. At the same time, it reduces the availability of these hydroxyl groups to bond with water, hence the low bound water number.

ACKNOWLEDGMENTS The grants from MOSTI (13-02-03-3021) and UM.C/HIR/ MOHE/SC/11 are gratefully acknowledged. Authors also acknowledge NMR facility, JEOL GSX 270, at IPPP University of Malaya.

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REFERENCES

(1) von Rybinski, W.; Hill, K. Alkyl PolyglycosidesProperties and Applications of A New Class of Surfactants. Angew. Chem., Int. Ed. 1998, 37, 1328−1345. (2) Nicora, L. F.; McGregor, W. M. In Biodegradable Surfactants for Cosmetics Find Application in Drilling Fluids, IADC/SPE Drilling Conference, Dallas, Texas, 3−6 March 1998; Dallas, Texas, 1998; 723−730. (3) Savic, S.; Tamburic, S.; Savic, M. M. From Conventional Towards NewNatural Surfactants in Drug Delivery Systems Design: Current Status and Perspectives. Exp. Opin. Drug Del. 2010, 7, 353−369. (4) Wu, Y.; Iglauer, S.; Shuler, P.; Tang, Y.; Goddard, W. A. Alkyl Polyglycoside-Sorbitan Ester Formulations for Improved Oil Recovery. Tenside, Surf., Deterg. 2010, 47, 280−287. (5) Claesson, P.; Stubenrauch, C.; Krastev, R.; Johansson, I. Thin Film and Foam Properties of Sugar-Based Surfactants; CRC Press: Boca Raton, 2009. (6) Goodby, J. W.; Gortz, V.; Cowling, S. J.; Mackenzie, G.; Martin, P.; Plusquellec, D.; Benvegnu, T.; Boullanger, P.; Lafont, D.; Queneau, Y.; et al. Thermotropic Liquid Crystalline Glycolipids. Chem. Soc. Rev. 2007, 36, 1971−2032. (7) Hashim, R.; Sugimura, A.; Minamikawa, H.; Heidelberg, T. Nature-Like Synthetic Alkyl Branched-Chain Glycolipids: a Review on Chemical Structure and Self-Assembly Properties. Liq. Cryst. 2012, 39, 1−17. (8) Jeffrey, G. A. Carbohydrate Liquid Crystals. Acc. Chem. Res. 1986, 19, 168−173. (9) Goodby, J. W. Liquid-Crystal Phases Exhibited by Some Monosaccharides. Mol. Cryst. Liq. Cryst. 1984, 110, 205−219. (10) Israelachvili, J. N.; Mitchell, D. J. A Model for the Packing of Lipids in Bilayer Membranes. Biochim. Biophys. Acta 1975, 389, 13−19. (11) Shimizu, T.; Masuda, M.; Minamikawa, H. Supramolecular Nanotube Architectures Based on Amphiphilic Molecules. Chem. Rev. 2005, 105, 1401−1443. (12) Stubenrauch, C. Sugar Surfactants-Aggregation, Interfacial and Adsorption Phenomena. Curr. Opin. Colloid Interface Sci. 2001, 6, 160−170. (13) Bergeron, V.; Waltermo, A.; Claesson, P. M. Disjoining Pressure Measurements for Foam Films Stabilized by a Nonionic Sugar-Based Surfactant. Langmuir 1996, 12, 1336−1342. (14) Waltermo, A.; Claesson, P. M.; Simonsson, S.; Manev, E.; Johansson, I.; Bergeron, V. Foam and Thin-Liquid-Film Studies of Alkyl Glucoside Systems. Langmuir 1996, 12, 5271−5278. (15) Sakya, P.; Seddon, J. M.; Vill, V. Thermotropic and Lyotropic Phase Behaviour of Monoalkyl Glycosides. Liq. Cryst. 1997, 23, 409− 424. (16) Sakya, P.; Seddon, J. M.; Templer, R. H. Lyotropic PhaseBehavior of n-Octyl-1-O-β-D-Glucopyranoside and its Thio Derivative n-Octyl-1-S-β-D-Glucopyranoside. J. Phys. II 1994, 4, 1311−1331. (17) Nilsson, F.; Soderman, O.; Hansson, P.; Johansson, I. PhysicalChemical Properties of C(9)G(1) and C(10)G(1) β-Alkylglucosides. Phase Diagrams and Aggregate Size/Structure. Langmuir 1998, 14, 4050−4058. (18) Nilsson, F.; Söderman, O.; Johansson, I. Physical−Chemical Properties of the n-Octyl β-D-Glucoside/Water System. A Phase Diagram, Self-Diffusion NMR, and SAXS Study. Langmuir 1996, 12, 902−908. (19) Kocherbitov, V.; Soderman, O.; Wadso, L. Phase Diagram and Thermodynamics of the n-Octyl β-D-Glucoside/Water System. J. Phys. Chem. B 2002, 106, 2910−2917. (20) Yamashita, I.; Kawabata, Y.; Kato, T.; Hato, M.; Minamikawa, H. Small Angle X-Ray Scattering From Lamellar Phase for β-3,7-

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CONCLUSIONS All investigated glycosides show a bound water content of about 1.6−1.7 molecules per surfactant headgroup in the lamellar phase. The experimental results are in good agreement with computationally determined hydrogen bonding between monosaccharide based octyl glycosides and water in lamellar assemblies. Apparently neither the linkage, nor the alkyl chain or the sugar configuration significantly affects the average number of bound water. The bound water content for monosaccharide glycosides is surprisingly small, given the fact that more bound water molecules (three) have been reported for monoglycerides,34 despite significantly lower binding sites (only two hydroxyl groups). The lower bound water content for the glycosides may reflect strong intralayer hydrogen bonding interactions in the sugar head groups, in line with the results of a previously reported molecular dynamic study on related systems.24 Our experimental investigations, based on deuterium NMR using D2O as the probe molecule, give phase diagrams for the four β-alkyl glycosides, which are generally in good agreement with the published ones based on X-ray diffraction.15,16 We have examined the hexagonal, cubic, lamellar, as well as gel phases, which are found in these four alkyl glycosides. The high tendency in some members to supercool below the Kraft line allowed us to extend the measurements to room temperature. The small number of water molecules found to bind to these glycoside headgroups is interesting and adds to our quantitative knowledge of the hydration profile, which is useful for the interpretation of many thin film properties, such as the surface adsorption behavior of monolayers and thin films.12

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 7343



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Deuterium NMR Investigation of the Lyotropic Phases of Alkyl Î²

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