J. Phys. Chem. B 2004, 108, 18793-18795
18793
Anomalous Temperature Dependence of Domain Shape in Langmuir Monolayers: Role of Dipolar Interaction Nilashis Nandi* Chemistry Department, Birla Institute of Technology and Science, Pilani, Rajasthan, 333031, India
Dieter Vollhardt* Max Planck Institute of Colloids and Interfaces, D-14424, Potsdam/Golm, Germany ReceiVed: August 24, 2004; In Final Form: September 23, 2004
Anomalous temperature dependence is observed in the shape of the condensed phase domains in Langmuir monolayer of amphiphilic monoglycerolether (1-hexadecyl-rac-glycerol) using Brewster angle microscopy. 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 Grazing incidence X-ray diffraction data reveal that the azimuthal orientation of the molecule enhances the regular arrangement of in-plane dipole moment at lower temperatures, which dictates the anomalous shape transition. The result has significance in controlling the shapes of mesoscopic aggregates by tuning temperature and molecular structure.
Introduction Condensed phase Langmuir monolayers display a variety of domain shapes.1-3 It is largely accepted that the shape transition is governed by the competition between the shape dependent dipolar repulsion, which favors elongated shapes, and the line tension between the condensed phase domain and the surrounding fluid phase, which favors compactness of domain. The shapes could be further anisotropic due to molecular chirality as well as the in-plane dipole moment which scales as anisotropic line tension. It is commonly observed that the domain shapes become more compact with decreasing temperature. The electrostatic repulsion between headgroups is expected to be reduced by the increase in the static dielectric constant of the subphase with decreasing temperature.4 The line tension is also expected to increase with decreasing temperature. Consequently, at the usual temperature dependence of the domain form, a compact shape would be favored with decreasing temperature whereas elongated shapes will be favored with increasing temperature. In the present letter, we report anomalous temperature dependence as observed in the condensed phase domains of 1-hexadecyl-rac-glycerol monolayers. Whereas compact domain shapes are formed at higher temperature, elongated arms or even long stripes are developed as the temperature is lowered. Such a behavior indicates increasing importance of the dipolar interaction at lower temperature. Therefore, the objective of the present paper is to analyze the role of the dipolar contribution to the observed temperature dependent domain shape change. Theoretical as well as experimental techniques have shown that the ratio of the difference in dipole density of the fluid and condensed phase of monolayer at a particular temperature and the fluid/condensed phase line tension dictate the transition in the domain shape.2,5-8 Thus, the observed anomaly is expected * To whom correspondence should be addressed. E-mail: (N.N.)
[email protected]; (D.V.)
[email protected].
to be explained on the basis of the dipole density difference at various temperatures. The dipole density difference at a particular temperature is dependent on the respective densities in the fluid and condensed phases. No significant average density difference is expected for the fluid phase at two different temperatures because the molecules are already randomly oriented at temperatures considered in the present case and the density is rather low compared to the condensed phase. The dipole density of the condensed phase could be responsible for the observed anomaly. In the present paper, we calculate the dipole moment of the monoglycerolether molecule with the variation in the temperature for different stable (energy minimized) molecular conformations. Subsequently, we analyze the in-plane and out-of-plane components of the dipole moment to understand the effect of the condensed phase dipole density. In the following section, we describe the theoretical calculation, and then, we discuss the results and correlate them with the anomaly experimentally observed. Theoretical Calculation Different conformations of headgroups are possible with different orientations and magnitudes. In the present work, we first compare the dipole moments of the same conformations at two different temperatures. The local effective dielectric constant for identical conformations of a molecule should be the same. Also, the dielectric constant changes by less than a factor of 1.1 only within the temperature range considered.4 Thus, we expect that the degree of scaling of electrostatic free energy by the local dielectric constant will be almost the same when identical conformations are considered in the temperature range investigated. The possibility of various tilts and magnitudes of headgroup dipoles exists. We, thus, calculated the dipole moment for different stable conformations of the molecule. The calculated dipole moments are then used to compute the inplane and out-of-plane components according to the experimentally observed molecular tilt and azimuthal orientation from
10.1021/jp0461697 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/13/2004
18794 J. Phys. Chem. B, Vol. 108, No. 49, 2004
Letters
TABLE 1: Grazing Incidence X-ray Diffraction Data for 1-Hexadecyl-rac-glycerol at 278.15 and 293.15 Ka temperature (K)
π (mN/m)
278.15
1.5 10 14 16 25 35 6 9.5 12 13.5 14 15 17 19 20 25 30
293.15
tilt direction
t (°)
A0 (Å2)
NN NN NN NNN NNN NNN NN NN NN NN NN NN NNN NNN NNN NNN NNN
41.2 37.3 36.0 33.2 30.3 26.1 39.3 35.8 33.6 32.7 35.3 34.3 31.7 29.2 29.6 27.7 26.6
19.6 19.6 19.6 19.7 19.7 19.7 20.1 20.1 20.1 20.2 20.0 19.8 20.1 20.0 20.1 20.0 20.1
a The azimuthal tilt direction is indicated as nearest neighbor (NN) or next nearest neighbor (NNN) direction. The tilt from the normal (t) is expressed in degrees. The cross section area of the alkyl chain (A0) is given in Å2.
grazing incidence X-ray diffraction (GIXD) data.9 As the GIXD data only provide the alkyl tail orientation, the present calculation assumes a rigid geometry of the molecule. We also calculated the effect of rotation of the amphiphile in a cone on the ratio of in-plane/out of plane dipole moment. The calculation and results are shown in section A and Table 2 (a) and (b) of the Supporting Information. 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)10 and the theory used is at semiempirical PM3 level.11 Charges were obtained using the Mulliken population analysis (MPA) technique.12 Fifteen different energy minimized conformations of the molecule were obtained and the dipole moments were calculated. The calculated dipole moments were used to estimate the perpendicular (µz) and in-plane µxy () [µx2 + µy2]1/2) components using the experimental GIXD data (shown in Table 1). Results and Discussion Figure 1 shows the domain shapes of 1-hexadecyl-racglycerol monolayers at three temperatures. The domains have a compact shape at higher temperature, develop elongated arms with curved directions at a lower temperature, and show only long stripes at the lowest temperature. The compactness of domain increases obviously with increasing temperature. This behavior indicates that the dipolar interaction dominates at the lowest temperature. The dipole moments of individual conformations are listed in Table 2 (a) and (b) of the Supporting Information for 278.15 and 293.15 K, respectively. The data clearly indicate that the ratios of in plane/ out of plane dipole moment for all conformers are higher at low temperature relative to the corresponding ratios at higher temperature. The GIXD data show that the tilt to the normal is gradually decreasing with increasing surface pressure. It is thus expected that the magnitude of the dipolar component to the normal (µz) should increase with increasing pressure and the in-plane component (µxy) should decrease. The lattice arrangement for a particular molecule has different temperature dependence. This dependence is again controlled by intermolecular interactions as well as interaction with the aqueous interface, as shown by theoretical studies.13 The data and molecular structures of all individual
Figure 1. Domain shapes of rac-1-O-hexadecyl glycerol monolayers at (a) 278.15 K, π ) 0.1 mN/m (b) 288.15 K, π ) 2.2 mN/m and (c) 308.15 K, π ) 18 mN/m. The chemical structure of an alkyl etherdiol molecule, where R ) (CH2)14(CH3), is shown in (d).
conformations are shown in Figure 3 of the Supporting Information. The µz/µxy ratio for one conformer is plotted in Figure 2. All of the plots, also that of Figure 2, show clearly the phase transition. Thus, the in-plane dipole moment is always dominant at lower temperatures in the pressure range investigated. The increase in the dominance of the in-plane dipole moment would lead to thinning of the domain in the corresponding tilt direction. This is in agreement with the experimental observation, as presented in Figure 1. The calculated effect of rotation of the amphiphile in a cone on the ratio of in-plane/out-of-plane dipole moment (as presented in Table 2 (a) and (b) of the Supporting Information) shows that the effect is not significant in general. McConnell et al. and Mayer and Vanderlick draw the attention to the fact that the in-plane dipole moment should tend to make condensed phase domains as long and thin, provided they are all oriented in a particular direction.5,14 Although experimental measurements of dipole density difference between fluid and condensed phase domains were carried out,15-17 quantitative comparisons between the theoretical prediction and
Letters
J. Phys. Chem. B, Vol. 108, No. 49, 2004 18795 monolayers is reported, and a theoretical explanation is presented. The work demonstrates for the first time that the enhancement of the importance of the regular arrangement of in-plane dipole moment at azimuthal tilt direction at lower temperature is the origin of the anomaly. The incorporation of the data of molecular orientation from X-ray diffraction measurements into the dipole moment analysis leads to the correct prediction of temperature dependence of shape from the theory. Acknowledgment. This work is partially supported by a grant from CSIR, India to N.N.
Figure 2. Plot of the ratio of in-plane and out-of-plane dipole moments (µxy/µz) versus pressure of rac-1-O-hexadecyl glycerol at 278.15 and 293.15 K for an arbitrarily chosen conformation (conformation 1). All data follow the same general trend that the magnitude of the in-plane dipole moment is higher at a lower temperature. For details see the text. The data for all individual conformers are presented in Table 2. Two views of the relevant same conformation are also presented.
Supporting Information Available: The dipole moments of individual conformations (Table 2 (a) and (b)). The data and molecular structure of all individual conformations (Figure 3). This material is available free of charge via the Internet at http:// pubs.acs.org.
the experimental result were not made.14 Experimental measurement of dipole moment is complicated by several reasons, for example, the complex nature of the interfacial dielectric function, the orientation and magnitude of dipole moment, as well as assumptions involved in the theoretical models used to interpret the experimental data.18,19 The role of the in-plane dipole moment in controlling the domain shape has not been demonstrated so far due to this complicacy. However, the present work clearly demonstrates that the temperature dependence of molecular tilt and azimuthal orientation can have decisive influence on the contribution of the in-plane dipole moments aligned to a particular direction to the electrostatic repulsive interaction. Higher in-plane dipole moments at lower temperatures favors elongated shapes along the azimuthal tilt direction. The ordering of amphiphiles will be more at lower temperature, which is also expected to favor the alignment of dipoles toward the azimuthal direction and thereby increase the repulsive interaction in that direction. The study indicates that possibility of controlling the shapes of mesoscopic aggregates by designing the molecular structure and tuning dipolar interaction.
References and Notes
Conclusions The anomalous temperature dependence of the domain shapes of amphiphilic monoglycerolether (1-hexadecyl-rac-glycerol)
(1) Mo¨hwald, H. Annu. ReV. Phys. Chem. 1990, 41, 441. (2) McConnell, H. M. Annu. ReV. Phys. Chem. 1991, 42, 171. (3) Nandi, N.; Vollhardt, D. Chem. ReV. 2003, 103, 4033. (4) CRC Handbook of Chemistry and Physics, 73rd ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1992. (5) McConnell, H. M.; Moy, V. T. J. Phys. Chem. 1988, 92, 4520. (6) Vanderlick, T. K.; Mo¨hwald, H. J. Phys. Chem. 1990, 94, 886. (7) Deutch, J. M.; Low, F. E. J. Phys. Chem. 1992, 96, 7097. (8) Miranda, J. A. J. Phys. Chem. B, 1999, 103, 1303. (9) Nandi, N., Vollhardt, D., Brezesinski, G. J. Phys. Chem. 2003, 103, 4033. (10) CHEM 3D Ultra, version 8.0; Cambridge Soft Corporation, USA. (11) Stewart, J. P. J. Comput. Chem. 1989, 10, 209 (12) Jensen, F. Introd. Comput. Chem. 1999, 81-99. (13) Cai, Z., Rice, S. A. Faraday Discuss. Chem. Soc. 1990, 89, 211. (14) Mayer, M. A.; Vanderlick, T. K. J. Chem. Phys. 1995, 103, 9788. (15) Miller, A.; Helm, C. A.; Mo¨hwald, H. J. Phys. 1987, 48, 693. (16) McConnell, H. M. Biophys. J. 1993, 64, 577. (17) Petrov, J. G.; Polymeropoulos, E. E.; Mo¨hwald, H. J. Phys. Chem. 1996, 100, 9860. (18) Demchak, R. J.; Fort, T., Jr. J. Colloid Interface Sci. 1974, 46, 191. (19) Dynarowicz-Latka, P.; Cavalli, A.; Silva Filho, D. A.; dos Santos, M. C.; Oliveira, O. N., Jr. Chem. Phys. Lett. 2000, 326, 39.
