10820
J. Phys. Chem. B 2005, 109, 10820-10829
Role of Electrostatic Interactions for the Domain Shapes of Langmuir Monolayers of Monoglycerol Amphiphiles K. Thirumoorthy and N. Nandi* Chemistry Department, Birla Institute of Technology and Science, Pilani, Rajasthan 333031, India
D. Vollhardt* Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam/Golm, Germany ReceiVed: March 3, 2005; In Final Form: April 11, 2005
The role of electrostatic interaction in the domain morphology of amide, ether, ester, and amine monoglycerol monolayers (abbreviated as ADD, ETD, ESD, and AMD, respectively) with systematic variation in the molecular structure of the headgroup region is investigated. Experimental studies using Brewster angle microscopy (BAM) and grazing incidence X-ray diffraction (GIXD) show that the characteristic features of the condensed monolayer phase, such as domain morphology, crystallinity, and lattice parameters, are very different for these monoglycerols. Therefore, the intermolecular interactions of the four amphiphilic monoglycerols are investigated in detail. First, the dipole moments of four monoglycerols of similar structure but with different functional groups are calculated by a semiempirical quantum mechanical technique. The dipole moments for monoglycerols follow the sequence AMD < ETD < ESD < ADD for the population of conformers of compounds investigated. The dipolar repulsion energies for the amphiphilic monoglycerols are also calculated for different possible mutual orientations between the dipoles. The calculated dipolar energies also follow the same trend for different possible headgroup orientations. These results can explain the domain shape of the monoglycerols observed experimentally. Second, ab initio calculations on the basis of the HF/6-31G** method are performed for representative monoglycerol headgroup segments. The results show that the intermolecular interaction energy related to dimer formation follows the order ETD < ESD < AMD < ADD segments, similar to that observed in experiment except in the case of the AMD segment. The relative importance of intra- and intermolecular hydrogen bonding in dimers is analyzed. The enhanced role of the intermolecular interaction relative to intramolecular interaction in the case of AMD contributes to the relatively high intermolecular interaction energy for the particular conformation of the dimer of AMD segment as observed from ab initio calculation. The present work shows that the variations in headgroup molecular structure alter drastically the domain shape, and the theoretical calculations conclusively reveal the important role of the electrostatic interactions for the mesoscopic domain architecture.
I. Introduction The molecular understanding of the domain shapes observed in the condensed phase Langmuir monolayers is yet to be complete.1-3 Such an understanding can provide important knowledge about the underlying molecular level interactions, which dictates the mesoscopic structure. Although it is established that the domain shape is principally controlled by the competition between the dipolar energy of the domains and the line tension energy (at the condensed phase/fluid phase boundary),4-7 the influence of the molecular structure on domain shape is not yet fully understood. The molecular structure of the amphiphile has a strong influence on the domain shape, and its manifestation could be diverse. A change in the molecular structure of the amphiphile results in a variation of the intermolecular interactions among the amphiphiles themselves as well as in the interaction with the water molecules in the aqueous subphase. The variation in the intermolecular interaction leads to the change in the molecular arrangement in the * To whom correspondence should be addressed. E-mail: nnandi@ bits-pilani.ac.in (N.N.);
[email protected] (D.V.). Fax: 91-1596244183 (N.N.); 49-331-567-9202 (D.V.).
monolayer and changes the domain shape. The change in the chemical structure may alter the line tension and dipolar interaction, and it leads to a change in the domain shape. Another well-known influence of molecular structure on the shape of the domain is the effect of the chirality of the molecule.3 The intermolecular orientational arrangement within the domain and the overall curvature of the domain shape can be significantly dependent on the chirality of the molecule. Experimental results show that the domain shape changes drastically with subtle variation in the molecular structure. Four types of racemic amphiphilic monoglycerols have been investigated in molecular detail using Brewster angle microscopy (BAM) and grazing incidence X-ray diffraction (GIXD) techniques to investigate how systematic alteration of the headgroup structure can affect the domain shape.8-12 The chemical structure of the four monoglycerol amphiphiles is shown in Figure 1a, the headgroups of which are monoglycerol amide, ether, ester, and amine (abbreviated as ADD, ETD, ESD, and AMD, respectively). The variation in the domain shape with the change in the molecular structure is rather dramatic, as seen in Figure 1b. It is useful to note the other feature of the domains, as demonstrated in Table 1. Out of the four compounds, the
10.1021/jp0510948 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/11/2005
Role of Electrostatic Interactions for Domain Shapes
J. Phys. Chem. B, Vol. 109, No. 21, 2005 10821
Figure 1. (a) Comparison of the chemical structures of the four monoglycerols. The variation in the chemical structures between the monoglycerol group and the alkyl chain is shown and compared in detail in the figure. (b) Representative BAM images of four diols.
TABLE 1: Comparison of the Molecular Structures and Domain Shapes of Monoglycerol Amphiphiles
crystalline nature increases from the amine (lowest) to the amide (highest). The amide domains are thin, are brittle and break up when interdomain collision occurs. Amide domains have the highest orientational correlation within the domain propagated up to a few millimeters. On the other hand, the monoglycerol amine domains are of fractal nature with a certain fluidity.
Correspondingly, the orientational correlation in monoglycerol amine domains is several orders of magnitude less than that in monoglycerol amide domains. The monoglycerol ester and ether domains have a crystallinity as well as an orientational correlation of a structure which falls between the monoglycerol amide and amine, as demonstrated in Table 1. The molecular azimuthal
10822 J. Phys. Chem. B, Vol. 109, No. 21, 2005 projection changes within the domains in monoglycerol ester monolayers, and such a change in director direction is also observed in monoglycerol ether. However, the domains of the monoglycerol ester are closed and round shaped, whereas the domains of the monoglycerol ether are branched and with elongated arms. The study of amide and amine glycerols is particularly interesting because amide and amine groups are an integral part of the general structure of a sphingolipid.13,14 The hydrogen bonding patterns of sphingolipids are well investigated due to their enigmatic properties.15 Racemic mixtures of amphiphiles are used in all cases, as the effect of chirality on the domain shape is expected to be less significant than the two main factors controlling the domain shape, namely, line tension and electrostatic repulsion. Little understanding is available about the line tension of monoglycerol monolayers at molecular detail. However, a comparison of the dipolar interaction can provide insight into the role of dipolar repulsion in the present case. Therefore, we calculated the average dipole moment using quantum mechanical methods and its components using experimental information about the molecular orientation and lattice structure at the interface from the GIXD data. The calculation of the out-of-plane and in-plane dipole moments and the area occupied by a molecule is expected to provide information about the dipolar repulsion in such systems. With this end in view, the dipole moments of different energy-minimized conformations of four monoglycerol compounds are calculated. Subsequently, the contributions of the in-plane and out-of-plane dipole moments are analyzed using the GIXD data. The lattice structural information is used to determine the relative dipolar repulsion within neighboring molecules of the four types of monoglycerol monolayers. Afterward, the dipolar interaction energies are calculated using the dipole moments obtained for different molecular conformations and for different possible intermolecular arrangements. The details of calculation are presented in the following section, and the results are presented in the subsequent section. II. Experimental Section 1-Octadecylamine-rac-glycerol (3-octadecylamino-1,2-propandiol, AMD) (purity >99%) was synthesized by reaction between octadecylamine and glycidol in methanol according to refs 12 and 16. 1-Hexadecylamide-rac-glycerol ((2,3-dihydroxypropyl)-hexadecanamide, ADD) (purity >98%) was prepared and purified according to ref 17. 1-Monohexadecanoylrac-glycerol (ESD) and 1-O-hexadecyl-rac-glycerol (ETD) (purity g99%) were obtained from Sigma. Ultrapure water produced by “Purelab Plus” was used as the subphase. A Brewster angle microscope (BAM-1, NFT, Go¨ttingen) was used to observe the monolayer morphology with a lateral resolution of ∼4 µm. Image-processing software was used to correct the distortion of the digitized images, as the BAM images are distorted due to the observation at the Brewster angle. The grazing incidence X-ray diffraction (GIXD) experiments were performed using the liquid-surface diffractometer on the undulator beamline BW1 at HASYLAB, DESY, Hamburg, Germany. A monochromatic synchrotron X-ray beam was adjusted to strike the helium/water interface at a grazing incidence angle. The diffracted intensity was recorded as a function of the horizontal scattering angle. All characteristics of the 2D lattice were calculated from the intensities as a function of the in-plane component and out-of-plane component of the scattering vector (for experimental details and the calculation, see ref 18).
Thirumoorthy et al. TABLE 2: GIXD Data for the Four Racemic Monoglycerol Monolayersa monoglycerol
T (K)
rac-amine diol-18 293.15
rac-ether diol-16
293.15
rac-ester diol-16
293.15
rac-amide diol-18 298.15
π a (mN/m) (Å) 1 5 10 35 6 9.5 12 13.5 14 15 17 19 20 25 30 5.5 15 30 44 5 10 30
b (Å)
t (deg)
6.36 8.49 43 4.83 10.21 36 5.00 10.00 35 4.91 8.58 18 6.01 8.60 39.3 5.78 8.61 35.8 5.61 8.59 33.6 5.58 8.59 32.7 5.70 8.60 35.3 5.59 8.58 34.3 5.52 8.58 31.7 4.92 9.31 29.2 4.93 9.38 29.6 4.93 9.18 27.7 4.91 9.17 26.6 5.68 8.58 34.7 5.45 8.54 31.0 5.22 8.47 26.2 5.07 8.42 21.9 4.93 5.42 37 4.93 5.40 37 4.92 5.30 34
td NN NNN NNN NNN NN NN NN NN NN NN NN NNN NNN NNN NNN NN NN NN NN NN NNN NNN
Axy A0 (Å2) (Å2) 27.0 24.7 25.0 21.0 25.9 24.9 24.1 24.0 24.5 24.0 23.7 22.9 23.1 22.7 22.5 24.4 23.3 22.1 21.4 24.1 24.0 23.5
19.9 19.9 19.9 19.9 20.1 20.1 20.1 20.2 20.0 19.8 20.1 20.0 20.1 20.0 20.1 20.0 20.0 19.9 19.8 19.2 19.2 19.3
a Here, π is the surface pressure in mN/m, a and b are lattice constants, t is the polar tilt angle of the alkyl chains, Axy is the area per molecule, A0 is the cross-sectional area of the alkyl chains, and td is the azimuthal tilt direction.
III. Theoretical Calculation (A) Semiempirical Calculation of Dipole Moment Components. Whereas accurate information about the tilt of the amphiphilic alkyl chain can be obtained from GIXD data, such detailed experimental information about the headgroups is unavailable. Different conformations of headgroups are possible with different average molecular orientations and magnitudes of dipoles. We, thus, calculated the dipole moment for different energy-minimized conformations of each of the four amphiphilic monoglycerol molecules. The calculated dipole moments are then used to compute the in-plane and out-of-plane (perpendicular) components according to the experimentally observed molecular tilt and azimuthal orientation from the GIXD data (as shown in Table 2). As the GIXD data only provide the alkyl chain orientation, the present calculation assumes a rigid geometry of the molecule. The dipole moment was calculated using a standard molecular modeling technique with the help of the well-known quantum mechanical program package (CHEM 3D software),19 and the theory used is at the semiempirical PM3 level.20 The charges were obtained using the Mulliken population analysis (MPA) technique.21 Fifteen different energy-minimized conformations for each of the monoglycerol molecules are obtained, and the corresponding dipole moments are calculated. The orientations of the molecular dipole and the molecular tail are calculated for each molecule as the conformation and dipole moments are generated using the PM3 method in a medium with a dielectric constant of 2. The calculated dipole moments were used to estimate the in-plane µxy ()[µx2 + µy2]1/2) and out-of-plane (µz) components using the experimental azimuthal tilt and orientation at the interfacial plane as follows. First, the orientation of the molecule is calculated on the basis of the generated structure and made to coincide with the GIXD data for tilt and azimuthal projection of the molecule. The change in dipolar orientation due to molecular reorientation (corresponding to the process of coincidence of the arbitrarily generated molecular conformation to the orientations obtained from GIXD) is calculated. The dipole moment components are then recalculated keeping the molecular
Role of Electrostatic Interactions for Domain Shapes orientation as identical with the GIXD data. In addition to the tilt from the normal to the surface and azimuthal projection on the surface, the consideration of the rotation of the molecule around the chain axis (often refereed to as “rotation in a cone”) is important to consider. The rotation of the molecule in a cone of angle θ implies that the molecule should have the possibility of variation of the tilt angle in the range [µ ( (θ/2)] and variation of the azimuthal angle in the range [R ( (θ/2)]. The variation of the tilt angle and azimuthal angle from the average values (µ and R) is more with an increase in the degree of rotation of the molecule in the cone (with an angle of θ). The rotation is energetically favorable or allowed as much as the interaction of neighboring molecules and the solvent molecules in the subphase with the central molecule allows. Naturally, such a rotation is more feasible in the gaseous or liquid expanded state of the monolayer but gradually becomes more restricted as the monolayer approaches the condensed state at higher surface pressures. As the area per molecule decreases, larger rotation would lead to nonbonded repulsion with neighboring molecules. Molecules, on average, are in a state with the average values of tilt and azimuth (µ and R) as observed in GIXD. Despite the amphiphilic molecules having dynamical freedom (depending on the temperature of the system as well as interaction with other molecules and molecules in the subphase), on average, the majority of the population of amphiphilic molecules is expected to be observed with most probable tilt (µ) and projection (R). Consequently, GIXD study gives certain average values of µ and R without a broad variation in these parameters. Also, for the same reason, BAM images show clear tilt directions within the domains. In the present paper, we are investigating the domain shapes, which are relevant to the condensed phase where the rotation is not significant for the reasons mentioned above and also not observed by GIXD or BAM. Table 2 cites the values of cross-sectional area per molecule (A0, Å2) obtained from GIXD data. Notably, these A0 values are larger than the cross-sectional area of an alkyl chain calculated on the basis of the molecular structure (Am ≈ 18.75 Å2). For example, an A0 value of 20.1 Å2 at 293.15 K for ETD16 corresponds to the so-called rotator phase. It is obvious that the effect of such rotation is more pronounced at higher temperatures. This is also observed in previous studies where the value of A0 is observed as higher at higher temperatures.27 Recently, the approximate angle of rotation (θ) was determined for ETD-16 for two different temperatures and the effects of the rotation on calculated dipole moment components were found to be insignificant.27 Table 2 shows that the A0 values of all diols are less or comparable in magnitude to ETD-16. Hence, the effect of rotation of the molecule around its axis (rotation in a cone) does not lead to the cancellation of the in-plane dipole moment. Hence, the result of the calculation of the present paper is not dependent on such a rotation angle. The in-plane and out-of-plane components of the dipole moments of all energy-minimized conformations are plotted in the form of a bar diagram. Such a diagram represents the distribution of the range of the magnitude of the dipole moment component for the corresponding molecule. We then compare the bar diagram of four monoglycerols corresponding to the same (or within a close range) surface pressure values. The comparison of the components of the dipole moments is presented in Figures 2 and 3. (B) Calculation of Dipolar Energy. The magnitude of the dipolar interaction energy is computed as follows. The interaction energy between two dipoles, µ1 and µ2, separated by a
J. Phys. Chem. B, Vol. 109, No. 21, 2005 10823
Figure 2. Comparison of the in-plane dipole moments (µxy) of the four monoglycerols. The dipole moments (in Debyes) for 15 conformations of each monoglycerol are plotted in the form of a bar diagram. The bar diagram shows the range of the in-plane dipole moment for the population of conformations investigated for the specific monoglycerol. The bar diagrams for different monoglycerols are compared for either the same or closely similar range of surface pressure values (indicated on the top of each bar diagram). Details of the computation of the dipole moment values are discussed in the text.
distance, r, oriented at angles θ1 and θ2 with respect to the line joining the center of the dipoles and a mutual angle, φ, between them is given by
Vdipol(r,θ1θ2φ) ) -
µ1µ2 4π0sr3
(2 cos θ1 cos θ2 sin θ1 sin θ2 cos φ) (1)
where 0 is the vacuum permittivity and s is the dielectric constant of the medium in which the dipoles are placed. It is important to note that the GIXD data provide only information about the orientation of the alkyl tails and not about the headgroup segment. Different headgroup arrangements are possible under the aqueous subphase. Two such possible arrangements for a pair of racemic molecules are shown in Figure 4. Despite the fact that the molecules can have a tilt angle from the normal (to the interface) the same as that in the two arrangements shown in Figure 4a and b, the mutual arrangement of groups in the headgroup region could be opposite. The groups attached to the chiral center in the headgroup region are the alkyl chain (projected in the air) and three other groups designated as A, B, and C, respectively. The net dipole moment of the headgroup region is schematically indicated by the solid arrow directed toward the aqueous subphase. In the case of a racemic pair, the pair of molecules being mirror images of each other, the mutual orientation of the headgroups of neighboring molecules may remain in arrangements between the two limiting possibilities shown in
10824 J. Phys. Chem. B, Vol. 109, No. 21, 2005
Figure 3. Comparison of the out-of-plane dipole moment (µz) of the four monoglycerols. The dipole moments (in Debyes) for 15 conformations of each monoglycerol are plotted in the form of a bar diagram. The bar diagram shows the range of the out-of-plane dipole moment for the population of conformations investigated for the specific monoglycerol. The bar diagrams for different monoglycerols are compared for either the same or closely similar range of surface pressure values. Details of the computation of the dipole moment values are discussed in the text.
Figure 4a and b. However, these two limiting group arrangements and the resulting dipolar orientation may result in both cooperativity and partial cancellation of the dipolar interaction due to the in-plane and out-of-plane dipole components depending on their mutual orientation. This is shown in parts c and d of Figure 4, respectively. In one case (as shown in Figure 4c), whereas the µy and µz dipole directions give rise to repulsive interaction of the magnitude Vdipol(1) ) µy/z2/4π0sr3, the µx components of neighboring dipoles are attracted by energy of the magnitude Vdipol(2) ) -2µx2/4π0sr3. Similarly, in the other case (as shown in Figure 4d), whereas the µy and µz dipole directions give rise to repulsive interaction of the magnitude Vdipol(1) ) µy/z2/4π0sr3, the µx components of neighboring dipoles are repelled by energy of the magnitude Vdipol(2) ) 2µx2/ 4π0sr3. The net dipolar interaction energy in the two cases should be different. It is impossible to say which arrangement out of those two shown in Figure 4c and d dominates the population of headgroup arrangements on the basis of the knowledge of available GIXD data. Hence, we considered both arrangements as the two extreme limits of dipolar arrangements. The dipolar energies are calculated according to these two arrangements and are indicated as Vdipol| and Vdipol*, corresponding to the dipolar arrangements shown in Figure 4c and d. The magnitudes of Vdipol| and Vdipol* for four monoglycerols were calculated on the basis of dipole moments computed in the previous section. The results are presented in Table 3. Out of the 15 conformations generated, the highest and lowest dipolar energies are presented in all cases. The understanding of dielectric constant at the aqueous interface is far from complete.22-26 For a microheterogeneous
Thirumoorthy et al.
Figure 4. Schematic diagram of a pair of mirror images of an amphiphilic molecule with a monoglycerol headgroup. Two possible mutual orientations of the pair of molecules are shown in which the dipoles are in nearly parallel and antiparallel directions, respectively. The explicit dipolar arrangements are shown in parts c and d, respectively. The solid arrow indicates the dipolar direction, and the dotted arrows indicate its X, Y, and Z components. The related dipolar energies are calculated and discussed in the text.
medium, like the headgroup region considered in the present case, the assignment of the dielectric constant is not straightforward. It could be a function of different variables. One such variable could be the separation from the air/water dividing surface. Different empirical functions are used in the literature to represent the distance dependent dielectric function.25,26 For example, recently, a hyperbolic tangent function was used to represent the dielectric function in the headgroup region.27 However, such a choice of an empirical function could not be unique and caution must be exercised in representing the dielectric function of the headgroup region. The low value of a dielectric constant of 2 is used in the present calculation for all monoglycerols to compare the dipolar interactions of all compounds. (C) Ab Initio Calculation of Intermolecular Interaction between Representative Headgroup Segments of Monoglycerols. Ab initio calculation of the intermolecular interaction between the enantiomeric pair of molecules of the four monoglycerols (ADD, ETD, ESD, and AMD) is computationally expensive due to the large number of atoms present in the molecules. The discriminating factors in the intermolecular interactions between pairs of molecules of each monoglycerol giving rise to the differences in the domain shape is principally arising from the headgroup region, as shown in Figure 1. We calculated the intermolecular interaction by considering the complete molecular structure of the headgroup of each monoglycerol with a shorter alkyl tail (with n ) 1 in the methylene group region and a CH3-CH2- group as a whole in the tail region). Such reduced representations of all monoglycerols are mentioned
Role of Electrostatic Interactions for Domain Shapes TABLE 3: Dipolar Interaction Energies in kJ/mol for Four Monoglycerols Corresponding to the Two Dipolar Arrangements (Vdipol| and Vdipol*) at the Surface Pressures Investigateda dipolar energy along the a axis Vdipol| Vdipol*
compound pressure (temp in K) (mN/m) AMD (293.15)
1 5 10 35
ETD (293.15 K)
6 9.5 12 13.5 14 15 17 19 20 25 30
ESD (293.15 K)
5.5 15 30 44
ADD (298.15 K)
5 10 30
lowest highest lowest highest lowest highest lowest highest lowest highest lowest highest lowest highest lowest highest lowest highest lowest highest lowest highest lowest highest lowest highest lowest highest lowest highest lowest highest lowest highest lowest highest lowest highest lowest highest lowest highest lowest highest
-0.4165 0.4053 -0.2827 1.1086 -0.223 1.0476 0.0014 1.7666 -0.1245 0.6957 -0.1164 0.931 -0.1156 1.1478 -0.1111 0.3904 -0.1186 1.0004 -0.1198 1.1131 -0.1061 1.3078 -0.6336 2.447 -0.6484 2.4163 -0.5389 2.5195 -0.488 2.602 -0.0664 5.8512 -0.0543 7.2319 -0.0366 9.0398 -0.0144 10.5621 0.8264 6.5661 0.8264 6.554 1.0091 8.0192
0.0225 0.6022 0.0792 1.6988 0.0714 1.5497 0.0386 1.9208 0.0449 1.507 0.045 1.7508 0.0453 1.9476 0.0445 1.9949 0.0454 1.8342 0.0473 1.9535 0.0453 2.0723 0.0487 2.5787 0.0496 2.5472 0.0454 2.6355 0.0441 2.7145 0.0137 5.999 0.0163 7.3705 0.0209 9.1545 0.0241 10.6517 1.8209 14.8999 1.8204 14.912 1.87 15.2633
dipolar energy along the b axis Vdipol| Vdipol* -0.1751 0.0095 0.1704 0.2531 -0.0299 0.0084 0.1174 0.1798 -0.0279 0.0089 0.131 0.1937 0.0003 0.0072 0.3311 0.36 -0.0425 0.0153 0.2374 0.5143 -0.0352 0.0136 0.2816 0.5297 -0.0322 0.0126 0.0954 0.5425 -0.0305 0.0122 0.3327 0.5468 -0.0345 0.0132 0.2913 0.534 -0.0331 0.0131 0.3078 0.5402 -0.0283 0.0121 0.3483 0.5518 -0.0935 0.0072 0.3611 0.3806 -0.0941 0.0072 0.3508 0.3698 -0.0835 0.007 0.3902 0.4082 -0.0749 0.0068 0.3994 0.4167 -0.0193 0.004 1.6976 1.7404 -0.0141 0.0042 1.8796 1.9156 -0.0086 0.0049 2.116 2.1429 -0.0031 0.0053 0.8343 2.3255 0.6219 1.3703 4.9414 11.2131 0.6288 1.3852 4.9873 11.3474 0.8072 1.496 6.415 12.21
a The highest and lowest limiting values of the dipolar energy are presented and chosen from the 15 conformations generated. Details are discussed in the text, and corresponding dipolar arrangements are shown in Figure 4.
as corresponding monoglycerol segments. The geometry of the noncovalently bound pair of enantiomers of the four monoglycerol segments is optimized at the Hartree-Fock level of theory using the 6-31G** basis set (HF/6-31G**). The optimized structures for pairs of all four monoglycerols are given in Figure 6. Due to the proximity of the hydrogen bonded groups in the given geometry, the substantial part of molecular interaction is due to hydrogen bond formation. The hydrogen bond properties are group properties, and the binding energy of the hydrogen bond structure is greater than that of the sum of individual bonds. Consequently, the pair of molecules is expected to give a lower magnitude of binding energy compared to that in the aggregated state. However, the ab initio calculation of the energy of even a small part of the hydrogen bond cycle is unfeasible due to the high computational cost involved. In the present work, we calculate the energy of interaction of a pair of molecules and analyze the optimized geometry to obtain the hydrogen bonded
J. Phys. Chem. B, Vol. 109, No. 21, 2005 10825 parameters of dimers. The energy of interaction of a pair of enantiomers in the reoptimized geometry is denoted by EHB and is given by
∆EHB ) EDimer - (Emonomer1 + Emonomer2)
(2)
All calculations are performed using Gaussian 03W software.28 The ∆EHB values and the individual dimer and monomers are presented in Table 4. To understand the role of intra- as well as intermolecular hydrogen bonding interactions, we computed the Coulombic attraction between the donor and acceptor pairs of all monoglycerol headgroup segments. Isolated headgroup segments already have intramolecular interaction between the donor and acceptor groups in their individual energy-minimized structures. When two such molecules are brought together, intermolecular interaction develops and now the same hydrogen bond donor and acceptor groups are in competition to form intermolecular interaction. As a result, the strength of the intermolecular hydrogen bond is reduced if already the donor and acceptor groups are involved in strong intramolecular hydrogen bonding. The Coulombic interaction energies between the H atom and corresponding nearest atom of the acceptor group are calculated. The calculation is strongly dependent on the dielectric constant of the medium, which is effectively the local dielectric constant of the different monoglycerols. The local dielectric constant is difficult to estimate. However, we computed the ratio of intermolecular versus intramolecular Coulombic interaction (Ec) assuming that the local dielectric constant is not drastically different in the two cases for the same monoglycerol and the effect of the dielectric constant is canceled out as follows:
qiqj
Ec )
∑ij 4π r
0 s ij
qkql
∑ kl 4π r
0 s kl
qiqj
)
∑ij r
ij
qk ql
∑ kl r
(3)
kl
where the sum ij runs over all intermolecular donor-acceptor pairs and kl runs over all intramolecular donor-acceptor pairs. The ratio gives the relative weightiness of the inter- or intramolecular hydrogen bonding interaction in each monoglycerol. The results are presented in the last column of Table 5. IV. Results and Discussion Figures 2 and 3 clearly show that the magnitude of both the in-plane and out-of-plane components of a dipole increase in the order AMD < ETD < ESD < ADD for the population of conformations within the surface pressure range studied. The observed increase in the dipolar nature of the monoglycerol headgroup in the order AMD < ETD < ESD < ADD is in perfect agreement with the fact that the ADD molecules with the largest dipole magnitude develop thin and straight-armed, highly crystalline domains with the largest orientational correlation length (about 5-10 mm), whereas the AMD monolayers form larger, fractal-like domains with the least orientational correlation length (
