Lattice structures and phase behavior of amphiphilic monoglycerol monolayers

Advances in Colloid and Interface Science 273 (2019) 102030

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Lattice structures and phase behavior of amphiphilic monoglycerol monolayers D. Vollhardt ⁎, G. Brezesinski Max-Planck Institute of Colloids and Interfaces, D-14424 Potsdam, Golm, Germany

a r t i c l e

i n f o

Article history: 27 August 2019 Available online 29 August 2019 Keywords: Langmuir monolayer Grazing incidence X-ray diffraction (GIXD) Amphiphilic monoglycerols Two-dimensional phase diagrams Chiral discrimination

a b s t r a c t Due to the Angstrom resolution, Grazing incidence X-ray diffraction (GIXD) represents the most important technique for probing the lateral ordering in condensed monolayers at the air/water interface and allows the construction of phase diagrams of amphiphilic monolayers on the basis of two-dimensional lattice structures and tilt directions of the molecules. The high potential of GIXD is demonstrated by the structural characterization of a variety of amphiphilic monoalkanoylglycerol monolayers in Å-scale. The GIXD results have impressively shown that in the racemic 1-monostearoylglycerol monolayer with the appearance of an oblique intermediate phase (Obl) between the nearest neighbor (NN)- and next-nearest neighbor (NNN)-tilted orthorhombic phases symmetry breaking occurs at low temperatures. The generic lateral pressure−temperature phase diagram of racemic monoacylglycerol monolayers constructed on the basis of reliable two-dimensional lattice structures indicates that the new and surprising presence of the oblique phase depends only on the temperature. The significant effect of the substituted polar groups, chemical structure variations at the position of the glycerol backbone and chirality on the lattice structure in Å-scale was highlighted in a systematic overview on the structure and phase behavior of amphiphilic monoglycerol monolayers. The conspicuous effect of the position of the glycerol backbone at which the polar group is substituted is demonstrated. The monolayers of 2-monopalmitoyl-rac-glycerol behave as that of 1-monomyristoyl-rac-glycerol having a two CH2 groups shorter alkyl chain. Further main topics discussed are chiral discrimination and crossover between homo- and heterochiral discrimination supported by quantum chemical calculations. © 2019 Elsevier B.V. All rights reserved.

Contents 1.

2. 3. 4. 5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Brief description of GIXD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2. Substitution in 1-position of the glycerol backbone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3. Effect of head group variation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.1. Monoalkanylglycerol ester monolayers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.2. 1-Monopalmitoyl-rac-glycerol monolayers [43,44] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.3. 1-Monostearoyl-rac-glycerol monolayers [50,51] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.4. Monoalkylglycerol monolayers [53–56] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.5. Mono-glycerolamine monolayers [57,58]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.6. Mono-glycerolamide monolayers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4. Effect of the alkyl chain substitution by an OH group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.1. 1-(12-hydroxy)monostearoyl-rac-glycerol [60] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Substitution in 2-position of the glycerol backbone [61,62] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. 2-Monopalmitoylglycerol monolayers [62] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of chirality and chiral discrimination (3-monostearoyl-sn-glycerol [66], 3-monopalmitoyl-sn-glycerol [67], 1-stearylamine-glycerol [68,69]) Crossover between homo- and heterochiral discrimination. Support by quantum chemical calculations . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⁎ Corresponding author. E-mail address: [email protected] (D. Vollhardt).

https://doi.org/10.1016/j.cis.2019.102030 0001-8686/© 2019 Elsevier B.V. All rights reserved.

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Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1. Introduction Monolayers at the air/water interface represent informative model systems at which two phases with completely different dielectric constants are in contact so that the molecularly flat interface is a unique hetero-dielectric medium [1] whose thermodynamic variables temperature and surface pressure can be easily modified. Such monolayer model systems are of interest for better understanding of phenomena concerning the functionality and activity of biological molecules and supramolecular architecture systems with new properties and functions, as well as for the potential application in material science [2–8]. The introduction of new procedures applicable to fluid interfaces, such as X-ray [9,10] and neutron scattering [11,12], Brewster angle [13,14], and fluorescence [15] microscopy, ellipsometry [16], Reflection–Absorption FTIR spectroscopy [17], and nonlinear optical spectroscopy [18] initiated a renaissance of the monolayer research. In particular, the introduction of the highly-sensitive imaging technique realized by Brewster angle microscopy (BAM), has given rise to new knowledges about the mesoscopic topology and ordering of condensed phase domains formed in the two-phase coexistence region of Langmuir monolayers. Information on the structure and phase behavior of condensed Langmuir monolayers in Å-scale advanced rapidly with the development of surface-specific X-ray diffraction techniques and the introduction of high intensity synchrotron X-ray sources. Due to the Angstrom resolution, Grazing incidence X-ray diffraction (GIXD) represents the most important technique for probing the lateral ordering in condensed monolayers at the air/water interface and allows the establishment of phase diagrams of amphiphilic monolayers on the basis of two-dimensional lattice structures and tilt directions of the molecules resulting in a rapid progress in understanding the molecular organization of condensed monolayer phases. The advance of surface-specific X-ray diffraction techniques for the application to Langmuir monolayers was revolutionized with the introduction of well collimated, high intensity synchrotron X-ray sources [19]. In the mid eighties, Als-Nielsen and Kjaer played the pioneering role in the development of the liquid surface X-ray diffractometer at the synchrotron light source HASYLAB at DESY, Hamburg, Germany, for the study of amphiphilic monolayers at aqueous surfaces [20,21]. For the first time, using these procedures the in-plane and out-ofplane structures of such monolayers could be determined with a resolution approaching the atomic level. The homologous series of monoalkanoylglycerols represent amphiphiles frequently studied in the bulk and at the air-water interface. In a recent review a systematic overview on the mesoscopic features of amphiphilic monoglycerol monolayers has been presented which offers an outstanding opportunity to highlight the dramatic effect of chemical structure variations at the position of the glycerol backbone and the substituted polar groups on the basis of the mesoscopic characterization [22]. In the present review, the characterization of amphiphilic monoglycerol monolayers in Å-scale is surveyed with the objective to provide information on the lattice structures and the phase behavior of the amphiphilic monoglycerol monolayers. The review is organized as follows. According to the significance of the 1-substituted monoalkanoylglycerol esters the effect of the chemical structure of the head group is highlighted with selected examples. This gives rise to the construction of the generic lateral pressure−temperature phase diagram of racemic monoacylglycerol ester monolayers. Shape and ordering of the condensed phase domains can be impressively affected by small changes in the polarity by refined variation in the head group structure. Various

types of racemic 1-substituted monoglycerols (amide, ether, and amine) are compared to show the influence of the polar group structure on the two-dimensional lattice structure. Then, the conspicuous effect of the position of the glycerol backbone at which the polar group is substituted is demonstrated. Further main topics to be discussed are the role of chirality and chiral discrimination and the effect of the alkyl chain substitution by a polar group. Finally, the crossover between homo- and heterochiral discrimination, supported by quantum chemical calculations, will be discussed.

1.1. Brief description of GIXD Synchrotron based GIXD technique is used as the most important technique for studying the structural arrangement of amphiphiles in monolayers at the air/water interface on molecular level. However, investigations of the molecular ordering in monolayers at the air/liquid interface are restricted on only a few beamlines at synchrotron sources worldwide. The grazing incidence X-ray diffraction patterns presented in this review were measured using a liquid-surface diffractometer at the undulator beamline BW1 at the HASYLAB (DESY, Hamburg, Germany) [23]. The principle of the GIXD measurements is shown in Fig. 1. A monochromatic X-ray beam (λ = 1.304 Å) is adjusted to strike the water surface at a grazing incidence angle ai = 0.85ac slightly below the critical angle for total reflection (ac = 0.131) to produce an evanescent wave, which propagates with high sensitivity along the surface, illuminates an area of about 100 mm2, has a penetration depth of 8 nm and can be diffracted by the lateral structures in the monolayer. A slow lateral movement of the trough is used to avoid sample damage by the strong X-ray beam. In the experimental set-up at BW1 a Langmuir film balance equipped with a movable barrier and a Wilhelmy plate for monitoring the lateral pressure was placed in a Helium-flushed container with capton windows transparent for X-rays. The diffracted intensity is monitored by a linear position sensitive detector (PSD) as a function of the vertical scattering angle αf and the horizontal scattering angle 2θxy. Generally, the analysis of the diffraction pattern provides information on the organization (unit cell dimensions) and on the orientation of molecules with respect to the interface (tilt and tilt direction). The unit cell dimensions (lattice parameters a, b and c, in-plane area Axy, cross-sectional area Ao and tilt angle t) are obtained from the Bragg peaks by integration of the scattering intensity

Fig. 1. Principle of GIXD measurements. The diffracted beam is detected with a position sensitive detector (PSD).

D. Vollhardt, G. Brezesinski / Advances in Colloid and Interface Science 273 (2019) 102030

(corrected for polarization, effective area, and Lorentz factor) over a certain Qz window, and the Bragg rods by integration of the scattering intensity over a certain Qxy window. The tilt t of the alkyl chains, and the tilt direction ψ*(hk) result from the use of Q(hk) = Q(hk) z xy cos(ψ*(hk)) tan t. Using the Scherrer formula, information about the size of the domains is obtained from the. full-width at half-maximum (FWHM) of the Bragg peak after correction by the instrumental resolution using Lxy ~ 0.9(2π)/FWHM(Qxy) and about the thickness of the diffracting layer from the FWHM of the Bragg rod using 0.9(2π)/FWHM(Qz). The Scherrer formula is also used to determine the length of an extended alkyl chain in all-trans conformation as for most of the studied amphiphiles the alkyl chains are the scattering units. Correspondingly, the thickness of the diffracting layer can be determined from the FWHM of the Bragg rod and should be in agreement with calculated values using L = (1.26·n + 1.54) Å, where n is the number of CH2 groups. Using the tilt angle of the alkyl chains, the cross-sectional area per alkyl chain A0 has been calculated from the in-plane area per alkyl chain Axy: A0 = Axy cos t. Usually only the most intense first-order peaks are observed representing the distances between neighboring alkyl chains. The appearance of two distinct Bragg peaks indicates a rectangular unit cell, whereas three peaks imply an oblique lattice. More details on the principles of GIXD can be found in the literature [10,19,24–26]. 1.2. Substitution in 1-position of the glycerol backbone A recent review highlights the striking influence of the functional groups and their position of the glycerol backbone on the mesoscopic characteristics of amphiphilic monoglycerol monolayers [22]. As the most important technique for probing the lateral ordering in monolayers and due to the Angstrom resolution, GIXD provides information on the lattice structure and tilt direction of the molecules in the condensed phases of monolayers and thus, allows establishing of phase diagrams of amphiphilic monolayers. 1.3. Effect of head group variation Four types (amide, ether, ester and amine) of racemic amphiphilic monoglycerols have been compared to show the influence of small changes of the polar group structure on the monolayer characteristics. This is impressively demonstrated by the mesoscopic topology of condensed phase domains in the recent review [22]. 1.3.1. Monoalkanylglycerol ester monolayers Monoalkanylglycerol esters, occurring in a large diversity of biological systems [27,28], belong to the amphiphiles studied very frequently in the bulk and at the air−water interface. In particular, monopalmitoylglycerol (monopalmitoylglycerol, monopalmitin) and monostearoylglycerol (monostearoylglycerol, monostearin) ubiquitous in nature and technological applications have been used for the large scale production of pharmaceuticals, foods, cosmetics, and agricultural products [29,30]. They are the most applied food emulsifiers [31], for example, in semisolid plastic pastes for food applications [32] are proposed as favorable drug delivery systems for controlling the release of therapeutic agents [29,30]. As advantages for the practical application are discussed the absence of biotoxicity of the carrier, protection of the incorporated compound against chemical degradation, and absence of problems at large scale production [33–39]. Therefore 1-palmitoyl and 1-stearoylglycerol have been the most examined monolayers of the saturated monoacylglycerols [40–42]. 1.3.2. 1-Monopalmitoyl-rac-glycerol monolayers [43,44] 1-Monopalmitoyl-rac-glycerol monolayers develop mesoscopic domains with perfect topology that allowed the geometric analysis of

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the inner segment structure interpreted as dense lattice rows. It was shown that the segment structure is related to the lattice structure [45–47]. The main phase transition of the 1-monopalmitoyl-rac-glycerol monolayers is in an optimal temperature range providing excellent conditions to investigate not only the mesoscopic topology but also the lattice structures in Å-scale and to provide information on the phase behavior. Knowledge of the phase behavior is helpfully complemented by the thermodynamic characteristics for the transition between the fluid and condensed phase obtained from the experimental surface pressuremolecular area (π − A) isotherms [44]. The temperature dependence of the main phase transition pressure (πt), that means, from the kink point in the π − A isotherm at onset of the phase transition, is the basis for the thermodynamic analysis. Using the two-dimensional Clausius−Clapeyron equation, ΔH ¼ ðAc −Ae ÞT

dπt dT

ð1Þ

where the enthalpy change ΔH of the phase transition was calculated by the slope dπt/dT of 0.876 mN/(m·K) obtained by linear fit to the experimental data. The temperature dependence of the entropy change ΔS = ΔH/T for the phase transition shows that the absolute ΔS values increase as the temperature decreases, demonstrating that the ordering of the condensed phase increases at lower temperatures. The critical temperature of 49.2 °C (322.4 K), above which the monolayer cannot exist in the condensed state, results from the linear fit to ΔS and the extrapolation to zero ΔS. The temperature dependence of the main phase transition pressure πt and of the entropy change ΔS at the fluid/condensed phase transition of 1-monopalmitoyl-rac-glycerol monolayers is presented in Fig. 2. Systematic GIXD measurements of 1-monopalmitoyl-racglycerol monolayers were performed over a large pressure interval at 5, 10, and 15 °C to construct the phase diagram on basis of reliable 2D lattice structures [44]. The GIXD results have shown that the phase transition between the two orthorhombic structures with NN and NNN tilted alkyl chains increases linearly with increasing temperature and occurs at 5 °C between 22 and 23 mN/m and at 10 °C between 39 and 40 mN/m. With the further increase in temperature to 15 °C, only NN-tilted orthorhombic lattices are observed in the whole pressure region [44] . As example, Fig. 3 shows the results obtained at 5 °C and 22 mN/m (top) and 23 mN/m (bottom). For the two pressures, the diffracted intensity is plotted as contour plots (Fig. 3 left column), as a function of the in-plane scattering vector component Qxy (Bragg peak, Fig. 3, middle column), and as a function of the out-of-plane scattering vector component Qz (Bragg rod, Fig. 3, right column). Fig. 3 demonstrates clearly that

Fig. 2. Temperature dependence of the main phase transition pressure πt (●) and the entropy change ΔS (▲) at the LE/LC phase transition of 1-monopalmitoyl-rac-glycerol monolayers spread on water.

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Fig. 3. GIXD data of 1-monopalmitoyl-rac-glycerol monolayers on water at 5 °C, measured at 22 mN/m (top) and 23 mN/m (bottom). The diffracted intensities, corrected for polarization, effective area, and Lorentz factor, are plotted as contour lines of equal intensity vs the in-plane component Qxy and the out-of-plane component Qz of the scattering vector (left). The Bragg peak (middle) and the Bragg rod (right) are also presented. Phase transition between 22 mN/m and 23 mN/m from NN to NNN tilted orthorhombic unit cell is clearly seen [44].

the phase transition between the two orthorhombic structures with NN and NNN tilted alkyl chains occurs between 22 and 23 mN/m. The lattice parameters (a, b, c, and α, β, γ; lattice distortion d, polar tilt angle t; molecular in-plane area Axy; cross-sectional area A0 of alkyl chain) of 1-monopalmitoyl-rac-glycerol monolayers at the measured surface pressures π and 5, 10 and 15 °C are presented in Ref. [44]. These results show that the cross-sectional area, A0, is less affected by surface pressure and temperature and amounts to values between 19.7 Å2 and 19.8 Å2, as expected for a rotator phase at the lower limit. The tilt angle t with respect to the surface normal decreases with increasing pressure, and is only slightly affected by the temperature. The transition pressure to untilted alkyl chains, determined by extrapolation of 1/cos(t) to zero tilt angle, is N50 mN/m for all temperatures (Fig. 4,

left), and therefore has not been experimentally determined. The maximum tilt angle was determined by the extrapolation to zero surface pressure and amounts to values between 37° and 39° only with marginal temperature dependence. According to the Landau theory, the tilt contribution to the distortion is proportional to sin2(t) [48,49]. Plotting distortion as a function of sin2(t) and extrapolating to zero tilt, the contribution of the tilt can be separated from other contributions to the distortion, such as backbone ordering (Fig. 4, right). The results of lattice distortion d vs. sin2(t) suggest for 10 °C and 15 °C the tilt of the aliphatic chains as reason for the monolayer lattice distortion whereas at 5 °C the non-zero-tilt-angle intercept d0 could be an indication of preventing the hexagonal packing.

Fig. 4. Dependence of the tilt angle of the alkyl chains (t) represented as 1/cos(t) on the lateral surface pressure (π). Extrapolation of 1/cos (t) to 1 gives the transition surface pressure (πt) at which the tilt angle becomes zero, wheras the extrapolation to π = 0 yields the maximum tilt angle (t0) of the alkyl chains (left) and lattice distortion d versus sin2(t) (right) of 1monopalmitoyl-rac-glycerol monolayers at 15 °C (■), 10 °C (▲ and 5 °C (●) [44].

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The surface-pressure − temperature (π-T) phase diagram of 1monopalmitoyl-rac-glycerol monolayers provides additional information about the temperature effect on the phase behavior in more detail (Fig. 5). As can be seen, the fluid (LE) phase disappears below 17 °C. Only at the lower temperatures of 5 °C and 10 °C, a second order NN → NNN phase transition (L2 → Ov) connected with a jump in the tilt direction takes place.

1.3.3. 1-Monostearoyl-rac-glycerol monolayers [50,51] Knowledge on the lattice structures and phase behavior of 1monostearoyl-rac-glycerol monolayers is essentially based on systematical GIXD measurements in the accessible range of temperature and surface pressure and is helpfully complemented by the characteristics of the π − A isotherms and the mesoscopic domain topography. The GIXD results obtained over a large pressure interval at 5, 10, 15, and 20 °C indicate impressively that in the racemic 1-monostearoyl-racglycerol monolayer symmetry breaking occurs at low temperatures in a small pressure region where an oblique intermediate phase (Obl) between the nearest neighbor (NN)- and next-nearest neighbor (NNN)tilted orthorhombic phases occurs. As example, Fig. 6 demonstrates these drastic changes in the lattice structures using the GIXD results obtained at 10 °C. At 10 mN/m, two diffraction peaks, characteristic of an orthorhombic unit cell with NNtilted alkyl chains, can be seen (Fig. 6, top). Already at 11 mN/m (Fig. 6, middle) three diffraction peaks indicate the transition to an oblique lattice structure. Finally, at 13 mN/m a second transition to an orthorhombic lattice but with NNN-tilted alkyl chains occurs indicated by two diffraction peaks both with maxima at Qz N 0 (Fig. 6, bottom). At N13 mN/m, the orthorhombic lattice structure with NNN-tilted alkyl chains is found in the whole pressure range. The lattice parameters, obtained at different pressures and 15 °C, are listed in Tables 1–4 of Ref. 51. The cross-sectional area, A0, is affected by surface pressure and temperature and amounts to values between 19.7 and 19.1 Ǻ2. Increasing temperature causes larger A0 values but reduces the effect of pressure, whereas at the lower temperature (e.g., 5 °C) the pressure increase leads to a decrease in A0 indicating considerably reduced rotation of the alkyl chains. The tilt angle with respect to the surface normal, t, decreases with increasing pressure, but obviously influenced by the temperature. The transition pressure to non-tilted alkyl chains, determined by extrapolation of 1/cos(t) to zero tilt angle, is N50 mN/m for all temperatures and may be attainable only at temperatures above 20 °C. The appearance of an oblique phase between the two rectangular phases in a racemic monolayer is a new and surprising phenomenon.

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In the low temperature region an oblique intermediate phase (Obl) has been observed in the racemic monolayer between orthorhombic chain lattices with NN and NNN tilt direction. Therefore, it should be highlighted that also in the racemic monolayer symmetry breaking occurs in a small pressure region, however only at low temperatures. The surface pressure - temperature phase diagram of 1-monostearoylrac-glycerol monolayers shows the temperature influence on the phase behavior in more detail (Fig. 7). It is seen that the existence range of this oblique phase increases with decreasing temperature. Already at 20 °C, the intermediate oblique phase disappears in the racemic1-monostearoyl-rac-glycerol monolayer and a direct NN → NNN transition of the orthorhombic lattices takes place. At higher temperatures, the NN → NNN transition is a first-order transition combined with a jump in the tilt direction. At lower temperatures, this first-order transition is replaced by a pressure range in which the tilt direction changes continuously from NN to NNN, indicating that two second-order transitions are now present. So far, such a behavior is not yet described in the theoretical phase diagram of monolayers [49]. 1.3.3.1. Generic lateral pressure−temperature phase diagram of racemic monoacylglycerol monolayers. The temperature for the phase transition depends on the length of the hydrocarbon chain in single-chain amphiphiles. Therefore, the phase-transition lines can be matched by systematical shifting the temperature axis by a certain value per additional methylene group [52]. In the present case, the transition line describing the first-order phase transition between the liquid and condensed phases can be compared. Correspondingly, the temperature axis is shifted by 16 K (8 K per CH2 group). In the same way, the phase diagram of 1-monopalmitoyl-rac-glycerol monolayers presented in Fig. 5 has been shifted by 16 K and combined with that of 1monostearoyl-rac-glycerol. The generic phase diagram of racemic monoacylglycerol monolayers is shown in Fig. 8. It is seen that the new and surprising presence of the oblique phase between the two rectangular phases depends only on the temperature. For 1-monopalmitoyl-rac-glycerol monolayers, the oblique phase can be expected only close to and below 0 °C. However, it can be expected that the 1-monoarachidoyl-rac-glycerol monolayer does not exhibit the NN tilted L2 phase but rather only the oblique and the NNN tilted orthorhombic phases if the NN → NNN transition does not exhibit a minimum, which is often observed for fatty acid monolayers. In such a normal case, the L2 phase should reappear (reentrant behavior) at lower temperatures. The extension of the intermediate oblique phase cannot be predicted but has to be experimentally determined. On the other hand, it can be expected that for 1-monomyristoyl-rac-glycerol only NN tilted orthorhombic lattices exist at all accessible temperatures. 1.3.4. Monoalkylglycerol monolayers [53–56]

Fig. 5. Lateral pressure - temperature phase diagram of the racemic 1-monopalmitoyl-racglycerol monolayer on water. The LE-L2 transition pressure πt (●) is determined from the kink point in the π–A isotherm at the onset of the first-order phase transition. The tilting transition pressure (Ov-LS and L2-LS) (●) has been determined by extrapolation of 1/ cos(t) to zero tilt angle. The pressure of the second-order phase transition L2-Ov (●) has been estimated from the GIXD data (typical change in the diffraction pattern) [44].

1.3.4.1. 1-O-hexadecyl-rac-glycerol monolayers. The change from an ester linkage of 1-monopalmitoyl-rac-glycerol to an ether linkage of 1-Ohexadecyl-racglycerol does not affect the π-A isotherm significantly, so the π-A isotherms of the two compounds are closely similar [53]. However, comparison of the mesoscopic condensed phase domains has shown that their topological characteristics are complete different [53]. Special interest found the anomalous temperature dependence observed in the shape of the condensed phase domains in the Langmuir monolayer of 1-hexadecyl-rac-glycerol. The domain shapes are elongated at a lower temperature in contrast to the known effect of line tension and dipolar repulsion. Quantum mechanical calculations of molecular dipole moments and the analysis on the basis of GIXD data demonstrate that the enhancement of the regular arrangement of in-plane dipole moment at azimuthal tilt direction at lower temperature is the origin of the anomaly [54]. Systematic GIXD measurements of 1-O-long chain alkyl-rac-glycerol monolayers which would allow a comparison with the corresponding 1-monoalkanoyl-rac-glycerol monolayers in Å-scale are not available.

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Fig. 6. GIXD data of 1-monostearoyl-rac-glycerol monolayers on water at 10 °C, measured at 10 mN/m (top), 11 mN/m (middle), and 13 mN/m (bottom). The diffracted intensities are plotted as contour lines of equal intensity vs. the in-plane component Qxy and the out-of-plane component Qz of the scattering vector (left). The Bragg peak (middle) and the Bragg rod (right) are also presented. Phase transitions between 10 mN/m and 11 mN/m from NN tilted orthorhombic to oblique unit cell and between 11 mN/m and 13 mN/m from oblique to NNN tilted orthorhombic unit cell are clearly seen [51]. 10 mN/m. 11 mN/m. 13 mN/m.

Merely 1-O-hexadecyl-rac-glycerol monolayers were investigated at 5 and 20 °C and different surface pressures [55]. The GIXD results have shown that a phase transition between two orthorhombic structures with NN and NNN tilted alkyl chains occurs at 5 °C at 15 mN/m and at 20 °C at 18 mN/m.

Interesting theoretical results about chiral discrimination effects in the monolayer of 1-O-hexadecyl glycerol on the basis of a detailed coarse-grained molecular model are discussed in a later chapter. The study revealed an interesting crossover from heterochiral to homochiral preference [56].

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between molecule chain and headgroup have significant influence on the mesoscopic morphology and the Å-scale lattice structure of the racemic amphiphilic monoglycerols. The racemic 1-palmitylamide-racglycerol monolayers change the nature of the condensed phase into a crystal-like two-dimensional state due to hydrogen bonding of the amide groups. The concept of the relation of domain morphology and lattice structure can also be applied on monolayers with a crystal-like condensed phase. The growth directions of five-, four-, and three-arm dendritic domains can be related to distinct directions of the oblique lattice of the molecules. The growth directions in all domain types are based on the same five lattice directions. This signifies a correlation between micrometer and angstrom length scales. 1.4. Effect of the alkyl chain substitution by an OH group

Fig. 7. Surface pressure - temperature phase diagram of the racemic 1-stearoyl-racglycerol monolayer [51].

1.3.5. Mono-glycerolamine monolayers [57,58] 1.3.5.1. 1-Stearylamine-rac-glycerol monolayer. The two-phase coexistence region of the π-A isotherm of 1-stearylamine-rac-glycerol monolayers at temperatures of ≥30 °C resembles that of 1monostearoyl-rac-glycerol monolayers. The racemic 1-stearylaminerac-glycerol monolayers form orthorhombic lattices characterized by changing the tilt direction of the alkyl chains from NN at low surface pressures of 1 mN/m to NNN at higher pressures of ~35 mN/m. The constant cross-sectional area of the alkyl chains (A0) of 19.9 Å2 is typical for the free-rotator phase of n-alkanes. The tilt angle of the molecule chains decreases noticeably with increasing surface pressure from 43° at 1 mN/m to 18° at 35 mN/m. The low molecular ordering inferred from the characteristics of the mesoscopic topology [57] is also corroborated by the high tilt angles at low surface pressure and the beginning of the pressure increase at comparably large areas per molecule.

1.3.6. Mono-glycerolamide monolayers 1.3.6.1. 1-Palmitylamide-rac-glycerol [59]. The results of BAM and GIXD measurements indicate that small changes of the linkage group

1.4.1. 1-(12-hydroxy)monostearoyl-rac-glycerol [60] The results obtained in ref. [60] demonstrate that the bipolar character of amphiphiles OH substituted in the mid-position of the alkyl chain is obviously responsible for the special monolayer characteristics deviating from those of typical amphiphilic monolayers. This can be convincingly concluded from the comparison of the main characteristics of 1-(12-hydroxy)stearoyl-rac-glycerol monolayers with those of the unsubstituted 1-stearoyl-rac-glycerol and 12-hydroxystearic acid monolayers. The contour plots of the GIXD measurements reveal considerable differences between the nonsubstituted and the 12OHsubstituted monostearoyl-rac-glycerol (Fig. 9). According to the two reflexes of 1-monostearoyl-rac-glycerol (Qz = 0 and Qz N 0), the alkyl chain pack in an orthorhombic lattice with alkyl chains tilted toward the NN direction. On the other hand, the three reflexes (Qz N 0) of 1(2-hydroxy)monostearoyl-rac-glycerol indicate that the alkyl chains form an oblique lattice. The contour plots of the 12-hydroxystearic acid monolayers, showing also three reflexes (Qz N 0) characteristic of the oblique lattice structure, suggest the dominant effect of 12OH substitution of the alkyl chain. These conclusions are supported by the π − A isotherms and the BAM results. 2. Substitution in 2-position of the glycerol backbone [61,62] For systematic characterization of amphiphilic monoglycerol monolayers it is important to know the role of the position of the aliphatic chain at the glycerol backbone. However, corresponding literature was not available, obviously caused by the fact that the GIXD studies of 2monopalmitoyl-rac-glycerol obtained at ≥13 °C indicated an illdefined packing of the alkyl chains caused by a misfit of headgroups and alkyl chains which cannot be compensated by the tilt of the alkyl chains [61]. A correlation between the disordered packing of the alkyl chains and the absence of long-range tilt orientation order is obvious which can be reduced by low temperatures. 2.1. 2-Monopalmitoylglycerol monolayers [62]

Fig. 8. Generic lateral pressure - temperature phase diagram of the racemic monoacylglycerol monolayers on water. The temperature axis of the phase diagram of 1-monopalmitoyl-rac-glycerol has been shifted by 16 K to match the phase diagram of 1-monostearoyl-rac-glycerol. The temperature shift of 16 K has been determined by comparing the πt vs. T curves (●) of the used monoacylglycerols [44].

The extension of the comprehensive characterization of 2monopalmitoyl-rac-glycerol monolayers to the low-temperature range of ≤10 °C made it possible to highlight the effect of the position of the aliphatic chain at the glycerol backbone. Large differences between the π−A isotherms of 1- and 2monopalmitoylglycerol monolayers and their thermodynamic analysis indicate that the change of the substitution from position 1 to position 2 of glycerol backbone is consistent with a shortening of the alkyl chain by roughly two CH2 groups. The thermodynamic results have been corroborated by packing calculations and quantum chemical calculations of the molecular structure [63,64]. The two-dimensional packages were considered on the basis of packing calculations by the program Hardpack [65] to characterize the influence of the palmitoyl chain substitution from the position 1 to position 2 of glycerol backbone. Fig. 10 compares the alkyl chain length in

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1.55

10 mN/m

1.50

1.60

12 mN/m

Qxy (Å-1)

1.55

Qxy (Å-1)

1.45

1.50

1.40

1.45

1.35

1.40

1.30

1.35

1.25

1.30

0.0

0.2

0.4

0.6

Qz (Å-1)

0.8

1.0

0.0

0.2

0.4

0.6

Qz (Å-1)

0.8

1.0

Fig. 9. Contour plots of 1-monostearoyl-rac-glycerol at 30 °C (top) and 1-(12-hydroxystearoyl)-glycerol at 16 °C (middle) and 12-hydroxystearic acid at 10 °C (bottom) monolayers [60].

the 1-monoalkanoyl-racglycerols ((a) 1-monomyristoyl- (24.6 Å), (c) 1-monopalmitoyl- (27.2 Å), (d) 1-monstearoyl-rac-glycerol (29.7 Å) with that of (b) 2-monopalmitoylglycerol (25.7 Å). Thus, the change of the palmitoyl chain substitution from the position 1 to position 2 of glycerol backbone results in a shortening of the molecule length of 1.5 Å in reasonable confirmation of the somewhat larger length difference for 2 CH2 groups of about 2.55 Å in the 1-monoacyl-rac-glycerols.

Previous GIXD studies with 2-monopalmitoylglycerol monolayers verify the disorder in the packing behavior at higher temperatures (≥13 °C) [61]. For obtaining reliable structure information the GIXD experiments were extended to the low temperature range (10 °C and 1 °C) [62]. Two clear diffraction signals indicate generally well-defined lattice structures at all surface pressures. At 10 °C, orthorhombic structures with NN tilted alkyl chains exist over the whole pressure region, and also at 1 °C orthorhombic structures with NN tilted alkyl chains occur over a large pressure range up to 35 mN/m. Then, a phase transition between the two orthorhombic structures with NN and NNN tilted alkyl chains occurs between 35 and 40 mN/m (Fig. 11). Based on the generic π − T phase diagram of racemic monoalkanoyl glycerol monolayers (see Fig. 8), it was concluded that 2-monopalmitoylrac-glycerol behaves as 1-monomyristoyl-rac-glycerol having a two CH2 groups shorter alkyl chain [62]. 3. Effect of chirality and chiral discrimination (3-monostearoyl-snglycerol [66], 3-monopalmitoyl-sn-glycerol [67], 1-stearylamineglycerol [68,69])

Fig. 10. Quantum chemical calculation of molecule lengths assuming flexibility of the headgroups: (a) 1-monomyristoyl-rac-glycerol, (b) 2-monopalmitoylglycerol, (c) 1monopalmitoyl-rac-glycerol, (d) 1-monostearoyl-rac-glycerol [62].

Chirality-based phenomena are of permanent interest. Monoacylglycerol esters are suitable candidates for systematic studies of chiral discrimination effects between enantiomeric and racemic structures. In recent papers, studies of the phase and lattice features of the racemic 1-monopalmitoyl-rac-glycerol and 1-monostearoyl-rac-glycerol and the enantiomeric 3-palmitoyl-sn-glycerol and 3-monostearoyl-sn-glycerol monolayers were performed using π–A isotherms, BAM and GIXD. For the two homologues the π–A isotherms of the enantiomeric and racemic monolayers are similar to each other suggesting small energetic differences between the enantiomeric and racemic forms. Correspondingly, similar to the racemic monoacyl-rac-glycerol esters, round and cardioid-like mesoscopic domains are observable in the chiral 3monopalmitoyl-sn-glycerol but their domains show a higher tendency to irregular deviations in the inner texture and shape from the perfect circular domain. GIXD experiments provide insight into the characteristic features of the lattice structure of condensed monolayer phases in Angstrom scale.

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Fig. 11. GIXD contour plots of the corrected diffraction intensities as a function of the in-plane Qxy and out-of-plane Qz components of the scattering vector for 2-monopalmitoylglycerol monolayers at 1 °C and different pressures (top: 5, 20, 30 mN/m; bottom: 35, 40 mN/m; from left to right).

The measurements of the enantiomeric 3-palmitoyl-sn-glycerol and 3monostearoyl-sn-glycerol monolayers were performed over a large range of lateral pressures at four different temperatures (5, 10, 15 and 20 °C) to obtain reliable two-dimensional lattice structures in the accessible temperature and surface pressure ranges. The contour plots with three clearly separated diffraction signals, are shifted to higher lateral pressures with increasing temperature, The comparison of the contour plots of the enantiomeric 3-monostearoyl-sn-glycerol and 3-monopalmitoyl-sn-glycerol monolayers indicates the stronger dominance of the chiral nature on the lattice symmetry in the longer chain homologues showing a larger pressure range with three separated diffraction peaks at all temperatures. For example, Fig. 12 displays the contour plots measured at 10 °C and 2, 10, 20 mN m−1 (top) and 30, 40, 50 mNm−1 (bottom). Here, the three diffraction signals are clearly separated at 10 and 20 mN m−1 and satisfactorily separated at 30 mN m−1. The similarity of the lattice data obtained by fitting the contour plots with 3 or 2 peaks demonstrates the resemblance to orthorhombic structures with NN tilted molecules at low pressures and NNN tilted molecules at high pressures. The systematic studies reveal the dominance of the chiral nature with the increase of lateral pressure and temperature.

The difference in the limiting temperature for observing distortion values at zero-tilt angle, d0, between the C18 and C16 compounds shows the stronger influence of chirality on the lattice distortion for the shorter-chain compound. The similar d0 values of racemate and enantiomer suggest that the lattice distortion is mainly determined by the packing properties and not by the chirality. Striking chiral discrimination effects were also observed in 1stearylamine-glycerol monolayers [68,69]. The absence of differences in the π-A isotherms of the enantiomeric forms and the racemic mixtures indicate their identical thermodynamic and thus, energetic behavior. This is also manifested by similar topological structural elements in all chiral forms, but fluorescence microscopic studies indicate clearly chiral discrimination of the condensed phase domains [68]. Chiral discrimination has also been observable in the lattice structure of the 1-stearylamine monoglycerol monolayers. The enantiomeric monolayers have an oblique lattice where the tilt direction changes continuously at compression from angles nearly toward NN direction to angles nearly toward NNN direction. The condensed phases of the racemic mixtures give rise to orthorhombic structures. Here, a phase transition occurs accompanied by a change in the tilt direction from NN at 1 mN/m to NNN at 5 mN/m. Both the enaniomeric monolayers and their racemic mixtures have strongly tilted molecules at low surface

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Fig. 12. GIXD contour plots of the corrected diffraction intensities as a function of the in-plane Qxy and out-of-plane Qz components of the scattering vector for 3-stearoyl-sn-glycerol monolayers at 10 °C and different pressures (top: 2, 10, 20 mN/m; bottom: 30, 40, 50 mN/m; from left to right).

pressures. This allows the conclusion of low ordering of the alkyl chains in the filigree domain structure, in agreement with the low position correlation. Similar chiral discrimination effects on the lattice structure have been found in the condensed phases of monoglycerol ethers that also give rise to oblique lattices in the enantiomeric monolayers and orthorhombic lattices, dependent on temperature an abrupt NN-NNN transition at medium surface pressure between 15 and 20 mN/m, in the racemic mixtures [70,71].

4. Crossover between homo- and heterochiral discrimination. Support by quantum chemical calculations Two types of interactions can occur at the molecular level in Langmuir monolayers. Homochiral interaction exists when D-D (or L-L) interaction is favored over D-L interaction, whereas in the case of heterochiral interaction the D-L interaction is favored compared with the D-D (or L-L) interaction. Two-dimensional systems provide opportunity to understand how molecular structure and interaction drive the chiral preference. There is a clear indication that the molecular structure, explicitly, chemical composition and spatial arrangement of

atoms or groups, and the packing of molecules in the two-dimensional lattice dictate the chiral preference. Several theoretical studies revealed, for the first time, interesting crossover from heterochiral to homochiral preference. These results are consistent with the experimentally observed discrimination effects. Chiral discrimination effects in the monolayer of 1-O-hexadecyl-racglycerol monolayers were theoretically investigated on the basis of a detailed coarse-grained molecular model [56]. On the basis of the effective pair potential (EPP) of a neighboring pair of molecules, the mutual orientation at the minimum of EPP was obtained. As result, significant orientation dependence of chiral discrimination was observed which indicated that discrimination effects are not only dependent on intermolecular separation but also on their mutual orientation. The theoretical results indicated the crossover from heterochiral to homochiral preference of 1-O-hexadecyl-rac-glycerol monolayers, corroborated by the experimentally observed discrimination effects. A further theoretical proof for a crossover from heterochiral preference to homochiral preference was obtained by studies of the chiral discrimination in 1-stearylamine-glycerol monolayers using the hybrid quantum mechanical/molecular mechanical method [72]. The intermolecular interaction profiles for enantiomeric and racemic pairs of the

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optimized structure of 1-stearylamine-glycerol were studied as a function of mutual tilt and azimuth for different values of intermolecular separation using the experimental data of the lattice structure [58]. The lowest energy structure of the enantiomer is identified by conformational analysis and optimized by the ONIOM ((abbreviation of the title “our n-layered integrated molecular orbital and molecular mechanics”) model. The study proves that, at shorter separation, the interaction profile of the racemic pair has deeper minimum than the enantiomeric pair, whereas at larger separation the minimum of the enantiomeric pair is deeper. This allows the conclusion to a crossover from heterochiral to homochiral preference in 1-stearylamineglycerol monolayers with the increase in the intermolecular separation corresponding to a larger area per molecule in the monolayer. Consequently, with gradual compression, the interactions between racemic pair dominate the experimental features, whereas, under nonequilibrium conditions at the beginning of the formation of the condensed phase, the experimental characteristics of homochirality are observable. The growth kinetics of the racemic N-palmitoyl-DL-threonine domains provides a further example for a mesoscopic transition from homochiral discrimination and chiral separation within the domain to a state with heterochiral preference [73]. The racemic N-palmitoyl-DLthreonine domains grow to a shape which suggests a new mesoscopic state with heterochiral preference without any indication for homochiral discrimination and chiral separation within the domain. Infrared reflection–absorption spectroscopic (IRRAS) studies [74] provided the basis for observation of changes from homo- to heterochiral preference in monolayers. In the case of the monolayer system Nhexadecanoylalanine on an aqueous subphase containing 1 mM ZnCl2 a change from homo- to heterochiral preference was observed by IRRAS measurements within a temperature range of 5 deg. as the temperature rises [75]. The results for N-octadecanoyltyrosine monolayers suggest a preferential heterochiral interaction at large area/molecule, while upon compression below about 0.35 nm2/molecule homochiral interactions appear to be more favorable [76]. 5. Conclusions Knowledge of the structure and phase behavior of condensed Langmuir monolayers in Å-scale progressed rapidly with the development of surface-specific X-ray diffraction techniques. The present review highlights the importance of synchrotron based GIXD for characterizing amphiphilic monoglycerol monolayers at the air/water interface and demonstrates the establishment of phase diagrams of amphiphilic monoglycerol monolayers on the basis of two-dimensional lattice structures and tilt directions of the molecules. The GIXD results of the racemic 1-monostearoyl-rac-glycerol monolayer reveal that with the appearance of an oblique intermediate phase (Obl) between the nearest neighbor (NN)- and next-nearest neighbor (NNN)-tilted orthorhombic phases symmetry breaking occurs at low temperatures. The generic lateral pressure − temperature phase diagram of racemic monoacylglycerol monolayers constructed on the basis of reliable two-dimensional lattice structures shows the essential temperature effect on the surprising existence of the oblique phase. The systematic overview on the structure and phase behavior of amphiphilic monoglycerol monolayers emphasizes the significant influence of the substituted polar groups, chemical structure variations at the position of the glycerol backbone and chirality on the lattice structure in Å-scale. The conspicuous effect of the position of the glycerol backbone at which the polar group is substituted is demonstrated by the monolayers of 2-monopalmitoyl-rac-glycerol which behave as those of 1-monomyristoyl-rac-glycerol having a two CH2 groups shorter alkyl chain. The comparison of the contour plots of the homologous enantiomeric monolayers indicates the stronger dominance of the chiral nature on the lattice symmetry in the longer chain homologues having a larger pressure range with three separated diffraction peaks at all

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temperatures. Theoretical studies revealed, for the first time, interesting crossover from heterochiral to homochiral preference using the experimental data of the lattice structure.

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