Liquid Crystalline Phases Induced by the Hydroxyl ... - ACS Publications

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Liquid Crystalline Phases Induced by the Hydroxyl Group Stereochemistry of Amphiphilic Carbohydrate Bicatenary Derivatives Hary L. Razafindralambo,* Aurore Richel, Michel Paquot, Laurence Lins, and Christophe Blecker Gembloux Agro-Bio Tech, University of Liege, Passage des déportés 2, B-5030 Gembloux, Belgium ABSTRACT: Liquid crystals (LCs) may exist in different phases depending upon the orientational and positional orders of molecules in the material. Here, we demonstrate that the class of LC state induced by amphiphilic carbohydrate bicatenary derivatives is strictly hydroxyl group stereochemistry-dependent. This statement results from the experimental and theoretical investigations of surface film (2D) and bulk solid (3D) thermal behavior of synthetic stereoisomers ntetradecyl (α-D-n-tetradecyl) galacto- and gluco-pyranosiduronate, with an axial (GalA-C14/14) or equatorial (GlcA-C14/14) hydroxyl group at the fourth carbon, respectively. Surface pressure−area isotherms (283− 310 K), differential scanning calorimetry thermograms (223−573 K), and polarized optical textures (298−363 K) reveal that GlcA-C14/14 organizes as a smectic LC-like phase (positional or lateral order), whereas the analogous stereoisomer GalA-C14/14 behaves as a nematic LC-like phase (orientational order). Thermodynamic investigations and molecular dynamics models computed under similar temperature conditions provide consistent data with physical properties resulting from experimental approaches.

1. INTRODUCTION Carbohydrate-based amphiphilic molecules are today among the most attractive classes of compounds for both fundamental and practical reasons.1,2 From a fundamental point of view, they constitute an ideal prototype class of molecules for investigating the structure−activity-function relationships thanks to a large structural diversity in their molecular stereochemistry, size, and geometry.3 In a series of recent papers, we have reported and elucidated the impacts of different structural attributes, including the ester bond direction,4 the alkyl chain length,5,6 and the OH stereochemistry7 of monosaccharide derivative amphiphilic compounds on their surface activities after a screening approach of basic properties of carbohydrate-based surfactants (CBS).8 By studying the thermal properties of surface film (2D) and bulk solid (3D), we found that two stereoisomers of uronic acid, glucuronic and galacturonic acid bicatenary derivatives, behave as thermotropic liquid crystals (LCs). Those compounds show different expansion degrees and phase transitions at the air−water interface but also distinct solid−fluid transition temperatures, determined by differential scanning calorimetry (DSC) and confirmed by polarized light microscopy images. Liquid crystalline systems are defined as intermediate states of matter, between the isotropic liquid and crystalline solid forms, so-called mesophases. These are identified by changing temperature, solvent, or both, and are classified as thermotropic, lyotropic, or amphitropic phases, respectively.9 Two main classes of LC phase, smectic and nematic (Figure 1), may exist according to the positional and orientational orders of mesogenes, molecules able to form mesophase systems.10,11 © 2012 American Chemical Society

From a practical viewpoint, such functionalities are potentially interesting in various manufactured industrial products in paints, high-tech, cosmetics, food, and pharmaceutical areas, owing mainly to their specific and interesting optoelectronic and mechanical properties.12 Many CBS exhibit LC behavior in 3D states, whether synthetic or natural compounds.12,13 The main approach for predicting the LC phase behavior has been focused on the characterization of different CBS structures by varying the degree of substitution of the head or tail groups and, consequently, the relative cross-sectional areas of hydrophilic−hydrophobic ratios.14 In most cases, monosubstituted derivatives have been found to display smectic phases, whereas those containing more than one lipophilic chain lead to 3D columnar or cubic-like mesophases.15 Despite a large expansion of knowledge on LC compounds and their phase behavior in the last decades,16 no evidence reveals, until now, that the stereochemistry configuration change in their structure may induce only either nematic or smectic LC-like phases. For bicatenary derivatives, in particular, no finding precises that such compounds may display a nematic ordering phase in both 2D and 3D systems. By screening the physical, chemical, and thermal properties of synthetic stereoisomers, galacturonic (GalA) and glucuronic (GlcA) acid bicatenary derivatives, we demonstrate that the LC phase type, under both film (2D) and solid crystalline (3D) states, is strictly 4-OH axial (GalA) or equatorial (GlcA) Received: October 11, 2011 Revised: March 7, 2012 Published: March 8, 2012 3998

dx.doi.org/10.1021/jp209765j | J. Phys. Chem. B 2012, 116, 3998−4005

The Journal of Physical Chemistry B

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Figure 1. Schematic illustration of material phase changes with increasing temperature: (a) solid crystal; (b) smectic LC; (c) nematic LC; (d) isotropic liquid. Arrows indicate the director, a common axe of molecules.

position-dependent. Thermodynamic investigations and molecular dynamics models computed under similar temperature conditions provide consistent data with physical properties resulting from 2D system experiments. Polarized microscopic images confirm transition temperatures and different LC ordering behavior detected by 3D system thermal characterization using DSC experiments.

Gibbs free energy of compression (ΔG, kJ/mol) was determined by

ΔG = −

Ae

∫A

π dA

c

where Ae and Ac correspond to the molecular areas at the onset and the end of the transition phase, respectively. The surface entropy is determined by

2. EXPERIMENTAL METHODS

ΔSs = (A c − A e) dπ/dT

2.1. Chemical Reagents. All reagents were analytical grades with high purity. Hexane (>99%) and dichloromethane (for GC >99.8), methanol (spectrophotometer quality), and palmitic acid (for GC) were purchased from Sigma-Aldrich (St. Louis, MO, U.S.A.). Dimyristoylphosphatidylcholine (DMPC) was purchased from Avantipolar Lipid. Milli-Q water (resistivity 18 MΩ·cm) was prepared with a Milli-Q 50 apparatus (Millipore Co., Milford, MA). 2.2. Glucuronic and Galacturonic Acid Derivatives Preparation. The two stereoisomers, n-tetradecyl (α-D-ntetradecyl-galactopyranosiduronate) and n-tetradecyl (α-D-ntetradecyl-glucopyranosiduronate), abbreviated GalA-C14/14 and GlcA-C14/14, respectively, have been chemically synthesized, purified, and fully characterized by spectrometric techniques, as described in detail elsewhere.17 2.3. Monolayer Characterization. 2.3.a. Langmuir Film Experiments. Langmuir experiments were performed according to the optimized method previously described in detail.7 Briefly, a small volume of organic sample solution (30 μL), prepared in a dichloromethane−methanol (9:1 v/v) mixture with a typical concentration of 1 mg/mL, was spread dropwise onto the clean Milli-Q water surface. All experiments were carried out with an automatic Langmuir film Waage LFW2 3″5 (Lauda, Königshofen, Germany). The subphase temperature was controlled using a circulating liquid from cryothermostated bath (Lauda, Königshofen, Germany). Surface pressure−area isotherms (π− A) were recorded in a temperature range of 283−310 K. Monolayer was left at least for 15 min prior to starting the compression for ensuring both the spreading solvent evaporation and the equilibrium temperature to be reached. The total surface trough was compressed at a constant barrier speed of 61.8 cm2/min. The reproducibility of experiments was checked by establishing each π−A isotherm at least three times. 2.3.b. Thermodynamic Analyses. Thermodynamic analyses of surface pressure−area isotherm were performed by determining the changes of three fundamental energetic parameters: Gibbs free energy, entropy, and enthalpy.18,19

The surface enthalpy (ΔHs, kJ/mol) can be calculated using the Clayperon equation, which gives equivalent enthalpy values calculated from known values of ΔG and ΔS by ΔH = ΔG + T ΔS

2.4. Differential Scanning Calorimetry Experiments. Differential scanning calorimetry experiments were performed with a DSC 2920 CE (TA Instruments, U.S.A.). DSC thermograms were recorded during heating from −50 to 250 °C in first assays, and then from 0 to 120 °C mainly for calculating enthalpy transitions, at a scan rate of 5 °C/min. A powder sample of known mass with a precision of 0.01 mg was hermetically sealed using aluminum pans. Indium and dodecane were used to calibrate the temperature and enthalpy reading. All these DSC experiments were made with an empty and a sample-containing pan of similar mass. Each experiment was carried out in triplicate. 2.5. Computational Molecular Models. Theoretical models of GalA-C14/14 and GlcA-C14/14 single molecules were established by molecular dynamics simulations. These have been carried out by energy minimization process under NVT (system N, box volume V, and temperature T remained constant) thermodynamic conditions at a given temperature ranging from 283 to 308 K using Culgi version 5.0.1. The number of steps was set to 10 000, and the time step was 1 fs. The velocity was rescaled at a frequency of 20. The force field was dreiding with an internal location. 2.6. Polarized Microscopy Images. The texture of the bicatenary derivative sample was examined at different temperatures using a polarized light microscope, Nikon Eclipse E400 (Linkam Scientific Instruments, England), equipped with a peltier system, Linkam PE60. A drop of organic sample was placed on a glass microscope slide. After solvent evaporation, the solid sample was slightly hydrated and then covered with a coverslip. Sample images were acquired through a PC connected to a camera (Basler) and analyzed by an imageprocessing software (Lucia). 3999



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Figure 2. Surface pressure vs molecular area: (a) GalA-C14/14 and (b) GlcA-C14/14 spread onto pure water subphase at temperatures ranging from 289 to 311 K. The temperature dependence of the surface pressure at the onset of the transition phase: (c) GalA-C14/14; (d) GlcA-C14/14.

Figure 3. Thermodynamic profiles of 2D transition phase at the air−water interface: (a) GalA-C14/14; (b) GlcA-C14/14. The temperature dependence of the surface enthalpy calculated by the Clayperon equation: (c) GalA-C14/14; (d) GlcA-C14/14.

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Figure 4. Molecular primary structures (a) and models generated by molecular dynamics simulations at different temperatures: 10 (b), 20 (c), and 35 °C (d). Left panel, GalA-C14/14; right panel, GlcA-C14/14.

3. RESULTS 3.1. (π−A) Isotherms versus Temperature. π−A isotherms of GalA-C14/14 and GlcA-C14/14 organic solutions spread at the water subphase maintained at different temperatures, and the variation of surface pressure at the onset of the phase transition (πe), are shown in Figure 2. Both series of π−A curves have the same features and trends when the subphase temperature increases. From a critical temperature, a characteristic plateau of a two-dimensional (2D) phase transition occurs, indicating that two phases may coexist, beyond the three main states (gaseous, expanded, and highly condensed) observed at 293 K, as described in detail in a recent paper.7 Therefore, when such stereoisomeric compounds are spread and compressed at the air−water interface, both behave similarly in forming a transition phase from an expanded state to a highly condensed state nearly incompressible (dA/dπ ∼ 0) like solids; this mesophase behavior is comparable to that of 2D LCs. However, the critical temperature (T0) from which a transition phase appears is different for each stereoisomeric compound. By extrapolating πe to zero from the plot of πe versus temperature, the value of T0 can be determined. Above this, a plateau indicating a transition state appears in the π−A isotherm. This corresponds to 287 K for GlcA-C14/14 and 301 K for GalA-C14/14, whereas the temperature coefficients (dπ/dT) are quite similar (1.3−1.5 mN/m/K). We can assume from these results that the two stereosiomeric compounds are able to form a 2D LCs under different conditions. 3.2. Thermodynamic Analysis. To better understand such behavior, we have performed a complete 2D thermodynamic analysis of data around the phase transition zone of each π−A isotherm by applying fundamental laws. Gibbs free energy, enthalpy, and entropy changes have been evaluated with

increasing the system temperature. For that, we attempted to answer to fundamental questions in both qualitative and quantitative ways. Regarding to the phase transition that depends on the subphase temperature, the molecule arrangement upon compression (more ordered or disordered) and the nature of molecular interactions (van der Waals, H-bonding, hydrophobic interactions) driving the phase transition may be predicted. The thermodynamic profile of the 2D phase transition for each stereoisomeric compound forming film is shown in Figure 3, parts a and b. These plots represent the Gibbs surface free energy, enthalpy, and entropy changes as a function of subphase temperature when the film is in the transition state. First, the variation of Gibbs free energy of compression ΔGc (π dA) at the transition phase is either equal or inferior to zero, and becomes more and more negative when the subphase temperature increases. Qualitatively, a negative value of ΔGc (π dA) means that the 2D phase transition appearance is energetically favorable, and it is strengthened with the increases of temperature. The higher the temperature of the subphase, the easier the formation of the coexisting phase between the expanded and condensed states, since less amount of work is required for reaching one state from the other. The decrease of the Gibbs free energy of compression with temperature (dΔGc/ dT < 0), when the film is in the transition state, also suggests that both compounds behave like “thermotropic” 2D LCs. Second, the enthalpy and entropy surface values are always negative (ΔH < 0 and −TΔS > 0) over the experimental temperature range, indicating that both systems are energetically favorable in terms of enthalpy, and unfavorable regarding the entropy, based on thermodynamic laws. This suggests that 4001



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both systems are relatively in ordered states at the air−water interface under the range of temperature conditions. By extrapolating the enthalpy value to zero (Figure 3, parts c and d), the transition temperature (Tm) can be assessed.20 In 2D systems, GalA-C14/14 has a Tm value (354 K) superior to that of GlcA-C14/14 (330 K). When we examine in a quantitative way the changes of each energetic parameter as a function of temperature, the enthalpy variation (dΔH/dT > 0) is unfavorable with increasing the temperature, whereas that of the entropy (−T dΔS/dT < 0) is favorable to GlcA-C14/14. Thermodynamically, this means a loss in enthalpy and a gain in entropy of the mesophase system when the subphase temperature increases. In contrast, the thermodynamic profile is quite different for GalA-C14/14. In this case, the ΔH and −T dΔS values are practically unchanged, i.e., there is no loss, neither in enthalpy nor in entropy, while their values remain negative and positive, respectively, in the range of subphase temperature investigated. 3.3. Molecular Models. A computational approach performed in the same temperature range than that of experimental investigations generates various molecular models. In Figure 4, the change of stereoisomeric configurations with temperatures inferior, equal, and superior to 293 K is shown. At a temperature of 283 K (inferior to T0), both stereoisomers adopt condensed configurations. Around 293 K, GalA-C14/14 is still in a compacted state, whereas GlcA-C14/14 is in a “V-like” configuration, a more open structure, regarding to the two alkyl chains. For higher temperature (308 K), the alkyl chains of the GalA-C14/14 model are completely opened, and even more extended, compared to those of GlcA-C14/14. These models are in agreement with the degree of expansion measured by the initial molecular area (A0) determined experimentally by surface pressure isotherms at various temperatures. 3.4. Transition Characteristics of the Solid State. Transition characteristics of each stereoisomer have been screened by DSC in heating and cooling for a wide range of temperatures from −50 to 250 °C at 5 °C/min. Four distinct transition temperatures, corresponding to the maximum signal value of heat for each peak, are identified (Figure 5). The lowest values are closed for both stereoisomers, T1 ∼ 323 K, whereas the intermediate transition temperatures T2 = 336 K or T3 = 353 K, and the highest values T4, ranging from 370 to 424 K, are totally different. The intermediate transition temperatures T2 and T3 correspond to the main peaks, which are not present for each compound. GlcA-C14/14 shows rather a main transition temperature T2 at 336 K, whereas GalA-C14/14 exhibits a T3 at 353 K; the corresponding transition enthalpies (ΔHm) for the main transition states are 78 and 91 kJ/mol, respectively. All these transition temperatures are lower than the temperature of the onset of the solid sample weight loss (Tl) for each compound, according to thermogravimetric analyses, confirming that all Tm values correspond to real solid−fluid transition temperatures of stereoisomeric compounds. T1 and T4 are naturally attributed to the transition temperature from solid to a liquid−solid state and the clearance temperature corresponding to the maximum transition value observed, respectively. Here again, each component exhibits different Tm values, demonstrating that each stereoisomer is in a different preferential mesophase state. Figure 6 illustrates the optical texture of GalA-C14/14 and GlcA-C14/14 systems spread onto a glass surface at different temperatures in the range of 298−358 K, which are inferior to clearance points.

Figure 5. DSC heating thermograms by scanning, from −50 to 250 °C by 5 °C/min, a solid sample: (a) GalA-C14/14; (b) GlcA-C14/14.

Their appearance is totally different. With GalA-C14/14, spherical- and marbled-like textures are observed for temperatures where no main transition occurs. At the transition (353 K), GalA-C14/14 molecules arrange in the same tilted direction. Beyond the main transition temperature, a threaded-like texture in parallel to the surface is observed. In contrast, a “bâtonnetlike” texture is observed at room temperature for GlcA-C14/14. At higher temperatures close to the transitions, its texture is in a lamellar form, which becomes more pronounced beyond the main transition temperature.

4. DISCUSSION Recently, we have demonstrated that GalA-C14/14 (4-OH in axial position) and GlcA-C14/14 (4-OH in equatorial position) show different conformations and behavior at the air−water interface by physical and optical investigations at a temperature around 293 K, using different complementary techniques.7 Experimental results have been supported by molecular models in single and associated systems. Briefly, GlcA-C14/14 exhibits a 2D phase transition but not GalA-C14/14, which has been attributed to the difference in intra-H bonding within the polar headgroup. In the present paper, the thermal behavior and conformations of these stereoisomeric compounds at the air− water interface (2D) and in a bulk solid state (3D), investigated in experimental and theoretical ways, are presented and discussed. Either at the air−water interface, or in a bulk solid state, all results demonstrate that both stereoisomers show a transition phase, at least, at two distinct temperatures. The main transition temperature (Tm) is always lower for the GlcAC14/14 sample (T2) compared to GalA-C14/14 (T3). In 2D systems, the appearance of a transition phase is an indication of expanded and condensed state coexistence 4002



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phase transition is favorable with the temperature increase. That suggests a thermotropic behavior of the 2D LC states for both stereoisomeric compounds. Second, GlcA-C14/14 and GalA-C14/14 molecules in the 2D transition systems are relatively in an ordered state based on the surface entropy changes ΔSs (measuring the disorder degree), which are always negative (−TΔS > 0) in the range of the subphase temperature investigated. However, a sharp decrease trend of −TΔS, that is, an increase of entropy, is observed for the GlcA-C14/14 system, whereas it remains stable for the GalA-C14/14 system when the subphase temperature rises, indicating that GlcA-C 14/14 molecules become earlier in a disordered state than those of GalA-C14/14. Such fundamental behavior is the consequence of the lower transition temperature, evaluated in the 2D system, for GlcA-C14/14 compared to that of GalA-C14/14 (Table 1). Table 1. Thermal Properties of the Surface Film and Bulk Solid of Bicatenary Derivativesa 2D

GalAC14/14 GlcAC14/14

3D

dπc/dT [mN·m−1·K−1]

T0 [K]

Tm [K]

Tm [K]

ΔHm [kJ/mol]

Tl [K]

1.54

302

354

353

91

428

1.32

287

330

336

78

395

a

T0, the lowest temperature for observing 2D transition phase; Tm, the main transition temperature; Tl, the onset temperature of significant solid sample weight loss.

This thermodynamic analysis leads us to assume the smectic and nematic phase-like behavior of GlcA-C14/14 and GalA-C14/14 LCs, respectively. Basically, molecules arranged in smectic phases (S) are close to those of crystalline solids (Cr) with a transition temperature (TCrS) much lower than those organized in nematic phase (N). This has rather a molecular arrangement close to the isotropic liquid state, characterized by higher transition temperatures (TCrN). Third, by comparing the enthalpy−entropy changes as a function of subphase temperature (dΔH and dΔS vs temperature) we attempt to identify the nature of predominant forces driving the molecular interactions involved in the 2D phase transition.28 For the GlcA-C14/14 system, both ΔS and ΔH become more and more positive with increasing subphase temperature. This situation corresponds to a gain of entropy and a loss of enthalpy, i.e., the mesophase is an order-to-disorder transition. In other words, the GlcA-C14/14 2D system is entropy-driven, suggesting that hydrophobic interactions, which are well-known as entropydepending forces, would be strengthened to the detriment of van der Waals and H-bonding molecular interactions, which are favored by the enthalpy contributions.29 For the GalA-C14/14 system, the molecule order degree is more stable in maintaining the surface entropy for the range of subphase temperatures studied. This situation may be related to the stronger intra-Hbonding interactions within GalA-C14/14 molecules as recently published.7 For 3D systems, DSC investigations, a useful methodology for studying and detecting LC phase transitions,21 show differences in the transition temperatures of each stereoisomer. The same arguments as in 2D systems can be evoked about the smectic or nematic mesophases of the two stereoisomers. Moreover, the behavior of GalA-C14/14 and GlcA-C14/14 is comparable to that of conventional LCs of alkoxy-cyanobiphenyl (ROCBs) series in 3D systems, i.e., from the solid-

Figure 6. Examples of polarized microscopy images (binocular ×40) taken at room temperature: (a) GalA-C14/14; (b) GlcA-C14/14.

between gaseous, liquid, and solid at the surface. This situation may be considered as 2D LC systems. In the past, several investigations have been reported concerning 2D phase transitions.22−25 As shown in a recent paper, 2D LCs can also adopt either nematic or smectic phases like in 3D LC systems.26 According to their respective T0 and Tm values determined experimentally from the surface pressure−area isotherms, GlcA-C14/14 spread and compressed at the air−water interface shows a lower transition temperature (Tm = 330 K) and, consequently, less organized than GalA-C14/14 (Tm = 354 K). Fundamentally, the transition temperature of crystalline solid to smectic phases is lower than that of solid crystalline to nematic phases.16 On the other hand, GlcA-C14/14 occupies a larger surface area and adopts a more expanded configuration than GalA-C14/14 at a large temperature range between 287 and 301 K. This is consistent to the fact that, basically, smectic phases organize in layers or planes owing to long-range translational and orientational orders, in contrast to nematic phases, for which molecules are arranged with parallel but no lateral order (see Figure 1). This arises from a long-range orientational without a long-range translational (or positional) order.27 Molecular models generated by the dynamics simulation method at different temperatures support such configurations, notably for the smectic phase of GlcA-C14/14, which adopts a “V-shaped” structure at 293 K.26 By thermodynamic approaches of 2D phase transitions in the range of the subphase temperature (283−310 K), three fundamental features can be deduced. First, negative values of Gibbs surface free energy indicate that the formation of the 4003



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systems. On the basis of all experimental and theoretical investigations, it can be assumed and concluded that GlcAC14/14 molecules appear to organize in smectic-like phases, whereas those of GalA-C14/C14 seem to arrange in nematic-like mesophases. Besides scientific interests, we expect such a specificity in functionalities of stereoisomeric carbohydratebased surfactants contributing to a net progress for potential development and use of LC materials for their optoelectronic and mechanical properties but also for stabilizing colloidal systems like emulsions.

sample thermal investigation, based on the similarity in their primary structures. In the case of ROCBs LC compounds, the homologues with R ≤ 7 do not show a smectic phase, whereas those with R ≥ 10 do not exhibit a nematic phase; the homologues with the R = 8 and R = 9 showing both smectic-A (TSN ∼ 333−353 K) and nematic (TNI ∼ 353−363 K) phases.21,30 From DSC heating thermograms, the two stereoisomers under investigation display a unique main transition temperature, which could be compared to a smectic-to-nematiclike LC phase transition (∼336 K) for GlcA-C14/14 and a nematic-to-isotropic-like LC phase transition (∼353 K) for GalA-C14/14. In addition, the investigation of thermal properties in 2D and 3D provides equivalent Tm values (Table 1) and shows the same trend. Molecular interactions within GlcAC14/14 molecules are weaker than those within GalA-C14/14 at the air−water interface, as clearly indicated by the slope of the transition pressure (dπe/dT) and the critical temperature T0, below which no transition phase occurs. A greater enthalpy with the GalA-C14/14 system indicates that this stereoisomeric compound is more stable than its analogous GlcA-C14/14 owing to both stronger intermolecular and intramolecular H-bonding forces within the polar headgroup.7 The polarized light microscopy images provide qualitative information on the LC phase texture. Since LC materials are birefringent, they possess two refractive indexes that are direction-dependent, based on the orientation of the LC relative to the plane of polarization light. The spherical- and marbled-like textures detected with GalA-C14/14 are comparable to those of nematic droplets, either in bipolar or in radial texture, whereas a nonspherical aspect like a batônnet, observed with GlcA-C14/14 molecules, is rather a characteristic of a smectic phase.9 The GlcA-C14/14 texture appears more continuous with a lamellar-like structure at the transition temperature T2. Thus, this situation could be assumed to a transition of smectic phase-to-isotropic phase. The oriented texture in a certain uniaxial angle observed for the GalA-C14/14 system at 353 K close to T3 suggests a similarity with nematiclike mesophases. Beyond the main transition temperatures T2 and T3, at 358 K for instance, both samples have a quite similar fluid-like planar texture, which approaches the isotropic-like phase. Besides the thermotropic behavior that was evidenced by 2D and 3D thermal characterization experiments, the lyotropic character of both compounds may be evoked based on the appearance of the 2D phase transition for the specific range of surface concentrations at a given subphase temperature. Therefore, these sugar disubstituted derivatives exhibit amphitropic (both thermotropic and lyotropic) behavior similar to those of lecithin, a bicatenary amphiphilic compound of the phospholipid class.9

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AUTHOR INFORMATION

Corresponding Author

*Phone: +32 81 62 21 48. Fax: +32 81 601767. E-mail: h. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS L.L. is a Research Associate at the National Funds for Scientific Research (FNRS) of Belgium. This work, carried out in the framework of the “TECHNOSE” excellence program, was supported by the Belgian Walloon region. The authors thank Mr. Alexandre Schandeler and Ms. Lynn Doran for their technical assistance. We thank Dr. Jan-Willem Handgraaf and Professor Johannes Fraaije from CULGI company for their granted software license during two months.

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REFERENCES

(1) Stubenrauch, C. Curr. Opin. Colloid Interface Sci. 2001, 6 (2), 160−170. (2) Ruiz, C. C. Sugar-Based Surfactants Fundamentals and Applications; CRC Press: New York, 2009; Vol. 143, p 639. (3) Razafindralambo, H.; Blecker, C.; Paquot, M. Carbohydratebased surfactants: structure−activity relationships. In Advances in Chemical Engineering; Nawaz, Z., Navieed, S., Eds.; InTech: New York, 2012; Chapter 8, pp 215−228. (4) Razafindralambo, H.; Blecker, C.; Mezdour, S.; Deroanne, C.; Crowet, J.; Brasseur, R.; Lins, L.; Paquot, M. J. Phys. Chem. B 2009, 113, 8872−8877. (5) Laurent, P.; Razafindralambo, H.; Wathelet, B.; Blecker, C.; Wathelet, J.; Paquot, M. J. Surfactants Deterg. 2011, 14, 51−63. (6) Blecker, C.; Piccicuto, S.; Lognay, G.; Deroanne, C.; Marlier, M.; Paquot, M. J. Colloid Interface Sci. 2002, 247 (2), 424−428. (7) Razafindralambo, H.; Richel, A.; Wathelet, B.; Blecker, C.; Wathelet, J.; Brasseur, R.; Lins, L.; Miñones, J.; Paquot, M. Phys. Chem. Chem. Phys. 2011, 13, 15291−15298. (8) Razafindralambo, H.; Blecker, C.; Paquot, M. Screening of basic properties of amphiphilic molecular structures for colloidal system formation and stability. In Amphiphiles: Molecular Assembly and Applications; Nagarajan, R., Ed.; American Chemical Society: Washington, DC, 2011; Vol. 1070, pp 53−66. (9) Barón, M.; Noël, C.; Shibaev, V. P.; Hess, M.; Jenkins, A. D.; Jin, J. I.; Sirigu, A.; Stepto, R. F. T.; Work, W. J.; Luckhurst, G. R.; Chandrasekhar, S.; Demus, D.; Goodby, J. W.; Gray, G. W.; Lagerwall, S. T.; Lavrentovich, O. D.; Schadt, M. Pure Appl. Chem. 2001, 73 (5), 845−895. (10) de Gennes, P. G.; Prost, J. The Physics of Liquid Crystals; Claredon: Oxford, U.K., 1993. (11) Chandrasekhar, S. Liquid Crystals; Cambridge University Press: Cambridge, U.K., 1992. (12) Goodby, J.; Görtz, V.; Cowling, S.; Mackenzie, G.; Martin, P.; Plusquellec, D.; Benvegnu, T.; Boullanger, P.; Lafont, D.; Queneau, Y. Chem. Soc. Rev. 2007, 36 (12), 1971−2032. (13) Singh, M. K.; Jayaraman, N. J. Indian Inst. Sci. 2009, 89 (2), 113−135.

5. CONCLUSION The experimental approaches based on thermal investigations of two stereoisomeric CBS at the air−water interface and in bulk solid states demonstrate that the OH group in equatorial (GlcA) or axial (GalA) position on the fourth carbon of the polar headgroup is a key factor in the determination of the type of LC ordering in both 2D and 3D systems. Theoretical approaches through thermodynamic investigations of the surface pressure−area isotherms and molecular modeling support the experimental data and reveal the possible configuration, organization, and the nature of molecular interactions involved for each compound forming 2D LC 4004



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