Dielectric Properties Tangential to the Interface in Model Insoluble

Apr 1, 2008 - Faculty of Chemistry, University of Sofia, 1 James Bourchier Boulevard, 1164 Sofia, Bulgaria. Langmuir , 2008, 24 (9), pp 4615–4624...
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Langmuir 2008, 24, 4615-4624

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Dielectric Properties Tangential to the Interface in Model Insoluble Monolayers: Theoretical Assessment Philip G. Shushkov, Stanislav A. Tzvetanov, Anela N. Ivanova, and Alia V. Tadjer* Faculty of Chemistry, UniVersity of Sofia, 1 James Bourchier BouleVard, 1164 Sofia, Bulgaria ReceiVed NoVember 19, 2007. In Final Form: February 6, 2008 Studies of insoluble monolayers built of phospholipids and various long-chained fatty acids or their glycerin esters are the major source for what is currently known about the relationship between monolayer composition and physicochemical properties. The surface pressure, dipole moment, dielectric permittivity, polarizability, refractivity, and other electrical and optical features are governed by the surfactant structural specificity and solvent organization at the microscopic level. To provide insight into the atomistic details of the interfacial structure, model monolayers at the air/water interface of two distinctly different in composition and isotherm profile surfactants are investigated by means of molecular dynamics all-atom simulations. Analysis of the computational results allows the estimation of empirically unattainable quantities such as tangential (di)electric properties, their decomposition to surfactant and water contributions, and their relationship with the changes in interfacial molecular organization at different surface concentrations. The employed theoretical approach provides a comprehensive description of interfacial phenomena at the molecular level where the traditional phenomenological investigations are ineffective.

Introduction Surfactant self-organization has been an attractive topic for scientific discussions in the past nine decades and is still a focal point of research.1 Specifically, insoluble monolayers (Langmuir films) formed by adsorption of amphiphilic molecules at gas/ liquid interfaces have the longest presence in the history of surfactant investigations. These systems can be natural or obtained in laboratory conditions and are studied in depth to augment the knowledge of membranes and pollutants as well as to enhance the design of Langmuir-Blodgett films with features suitable for modern nanotechnologies.2 Membranes of animal and human cells are built of lipid bilayers, providing the basis for bioenergetics, signal transduction, interand intracellular transport, resistance toward viral intrusion, etc.3 Biomembranes can be regarded as two weakly coupled phospholipid monolayers. Studies of monolayers render access to biologically relevant information which is experimentally unavailable for bilayers. On the other hand, surfactants act as water pollutants, a hot topic in environmental research.4 The specific electric, magnetic, optical, conducting, catalytic, and other properties of monolayers as well as their self-assembly aptitude offer a wealth of technical applications, some of which have already found practical implementation in Langmuir* To whom correspondence should be addressed. E-mail: tadjer@ chem.uni-sofia.bg. (1) (a) Meister, A.; Blume, A. Curr. Opin. Colloid Interface Sci. 2007, 12, 138. (b) Engelskirchen, S. Curr. Opin. Colloid Interface Sci. 2007, 12, 68. (c) Dluhy, R.; Shanmukh, S.; Morita, S. I. Surf. Int. Anal. 2006, 38, 1481. (d) Alonso, C.; Zasadzinski, J. A. J. Phys. Chem. B 2006, 110, 22185. (e) Meyer, E. E.; Rosenberg, K. J.; Israelachvili, J. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15739. (f) Thordarson, P.; Atkin, R.; Kalle, W. H. J.; Warr, G. G.; Braet, F. Aust. J. Chem. 2006, 59, 359. (2) (a) Gill, S.; Lobenberg, R.; Ku, T.; Azarmi, S.; Roa, W.; Prenner, E. J. J. Biomed. Nanotechnol. 2007, 3, 107. (b) Basabe-Desmonts, L.; Reinhoudt, D. N.; Crego-Calama, M. Chem. Soc. ReV. 2007, 36, 993. (c) Rosenblatt, C. Mol. Cryst. Liq. Cryst. 2004, 412, 1727. (3) Barenholz, Y.; Cevc, G. In Physical Chemistry of Biological Interfaces; Baszkin, A., Norde, W., Eds.; Marcel Dekker: New York, 2000; p 171. (4) (a) Singh, A.; van Hamme, J. D.; Ward, O. P. Biotechnol. AdV. 2007, 25, 99. (b) Nanotechnology and the Environement; Karn, B., Colvin, V., Alivasatos, P., Masciangioli, T., Zhang, W.-X., Eds.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

Blodgett films.5-11 Knowledge of the structure of insoluble monolayers along the entire surface potential/area isotherm is of critical importance for monolayer utilization in nanodevices and as nanoparticle building blocks. For instance, realization of magnetic ordering in molecular magnets could be accomplished by achieving predefined monolayer structural parameters.12 Selection of appropriate hydrophilic head and/or tail fragments allows optical control on monolayer magnetic and electric properties.13,14 Self-organization of light-sensitive surfactant molecules permits their use as long-wavelength light-harvesting systems in photovoltaic cells.15 Postadsorption functionalization of the tails enables the construction of films with a variety of attractive properties.16 A prospective application of self-organizing lipids is the building of nanosize vesicles for targeted drug delivery.17-19 Experimental studies of insoluble monolayers date back to the early 20th century20 and encompass a great arsenal of methods and techniques21-27 allowing assessment of various properties (5) Ashwell, G. J.; Urasinska, B.; Tyrrell, W. D. Phys. Chem. Chem. Phys. 2006, 8, 3314. (6) Onah, E. ACS Symp. Ser. 2006, 918, 384. (7) (a) Xu, J. M.; Ji, X. J.; Gattas-Asfura, K. M.; Wang, C. S.; Leblanc, R. M. Colloids Surf., A 2006, 284, 35. (b) Yin, F.; Kafi, A. K. M.; Shin, H. K.; Kwon, Y. S. Colloids Surf., A 2006, 284, 125. (8) Wang, T. X.; Wei, H. X.; Zeng, Z. M.; Han, X. F.; Hong, Z. M.; Shi, G. Q. Appl. Phys. Lett. 2006, 88, 242505. (9) Sakakibara, K.; Ifuku, S.; Tsujii, Y.; Kamitakahara, H.; Takano, T.; Nakatsubo, F. Biomolecules 2006, 7, 1960. (10) Pich, A.; Bhattacharya, S.; Adler, H. J. P.; Wage, T.; Taubenberger, A.; Li, Z.; van Pee, K. H.; Bohmer, U.; Bley, T. Macromol. Biosci. 2006, 6, 301. (11) Kushida, M.; Imaizumi, Y.; Harada, K.; Sugita, K. Thin Solid Films 2006, 509, 149. (12) Tyutyulkov, N.; Ivanova, A.; Dietz, F. Chem. Phys. 2003, 287, 71. (13) Balashev, K.; Panaiotov, I.; Petkov, I. Colloid Polym. Sci. 1998, 276, 984. (14) Balashev, K.; Panchev, N.; Petkov, I.; Panaiotov, I. Colloid Polym. Sci. 2000, 278, 301. (15) Gasser, A.; Raddatz, S.; Radunz, A.; Schmid, G. Z. Naturforsch. 1999, 54c, 199. (16) AdVanced Chemistry of Monolayers at Interfaces; Imae, T., Ed.; Academic Press: Amsterdam, Boston, 2007; Vol. 14. (17) Luengo, J.; Weiss, B.; Schneider, M.; Ehlers, A.; Stracke, F.; Ko¨nig, K.; Kostka, K.-H.; Lehr, C.-M.; Schaefer, U. F. Skin Pharmacol. Physiol. 2006, 19, 190. (18) Olivier, J. C. NeuroRx 2005, 2, 108. (19) Vauthier, C.; Couvreur, P. Pharm. Technol. Eur. 2007, 1, No. 04. (20) Langmuir, J. J. Am. Chem. Soc. 1917, 39, 1848.

10.1021/la703616c CCC: $40.75 © 2008 American Chemical Society Published on Web 04/01/2008

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Figure 1. Illustration of a surface pressure/area per molecule (Π/A) isotherm with notations of phase states of an insoluble monolayer (adapted from ref 28).

such as the dipole moment, polarizability, refractivity, dielectric permittivity, and other optical and electromagnetic characteristics. Monolayer rheology was explored originally by recording surface pressure/area (Π/A) and surface potential/area (∆V/A) isotherms.22 At low surface concentrations the surfactant molecules are relatively randomly distributed on the water surface in a manner resembling the gas (G) phase. In the early stages of compression accidental short-range ordering occurs (LE ) liquid expanded state). Further compression of the surface area leads to formation of organized structures in 2D solid domains (LC ) liquid condensed state) which may merge into a continuous 2D solid phase (SC ) solid condensed state).28 Surface pressure above a critical value results in collapse of the monolayer (Figure 1). Analysis of Π/A and ∆V/A isotherms is still a widespread approach for monitoring the variation of surfactant organization during monolayer compression. A number of analytical and phenomenological models for interpretation of the isotherms and for description of monolayer parameters are known.21,28-35 They have been tested by specially designed experiments, and their general validity has been confirmed. All of them, however, do not provide comprehensive molecular level interpretation of the interactions in the monolayer. Some open issues are the role of the solvent polarity for the monolayer formation, the solvent organization at the interface, the size of the interfacial layer, and the effect of surfactant-solvent interactions on the monolayer structure. These shortcomings are due to the lack of information about the microscopic characteristics of the molecules in the film. Some deficiencies can be overcome by means of theoretical simulations of insoluble monolayers. First, attempts for numerical assessment of monolayer organization are made with coarse models treating the surfactant molecules as rigid rotators positioned at the nodes of a certain lattice.27 Neglecting translational motion and simplifying head-head and head-solvent interactions, these (21) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; WileyInterscience: New York, 1969. (22) Erbil, H. Y. Surface Chemistry of Solid and Liquid; Wiley: New York, 2006. (23) Adamson, W. Physical Chemistry of Surfaces, 5th ed.; Wiley-Interscience: New York, 1990; p 113. (24) Moehwald, H. Annu. ReV. Phys. Chem. 1990, 41, 441. (25) Hoenig, D.; Moebius, D. J. Phys. Chem. 1991, 95, 4590. (26) Lee, L.; Mann, E.; Langevin, D.; Farnoux, B. Langmuir 1991, 7, 3076. (27) Kaganer, V. M.; Moehwald, H.; Dutta, P. ReV. Mod. Phys. 1999, 71, 779. (28) Moehwald, H. In Handbook of Biological Physics, Lipowsky, R., Sackmann, E., Eds.; Elsevier: Amsterdam, 1995; Vol. 1, Chapter 4. (29) Davies, J. T.; Rideal, E. K. Interfacial Phenomena; Academic Press: New York, 1961. (30) Demchak, R. J.; Fort, T. J. Colloid Interface Sci. 1974, 46, 191. (31) Vogel, V.; Moebius, D. Thin Solid Films 1988, 159, 73. (32) Vogel, V.; Moebius, D. J. Colloid Interface Sci. 1988, 126, 408. (33) Taylor, D. M.; Bayes, G. F. Phys. ReV. E 1994, 49, 1439. (34) Taylor, D. M.; Bayes, G. F. Mater. Sci. Eng., C 1999, 8, 65. (35) Taylor, D. M. AdV. Colloid Interface Sci. 2000, 87, 183.

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schemes prove to be an inadequate tool for description of fluid phases. The more realistic model of “beaded strings” provides more detailed structural information, e.g., the relationship between the molecular geometry and the experimentally observable tilting behavior.36 The road to the atomistic approach required the development of methods allowing adequate representation of inter- and intramolecular interactions in large systems within reasonable computational time limits. This instigated the proliferation of all-atom force fields tailored for specific or general applications.37,38 In parallel, two methods of statistical physics flourisheds molecular dynamics (MD) and Monte Carlo (MC). Further on, two-directional development of the theoretical approach was achieved: the force fields were improved,37,39-41 and the MD simulations became more reliable due to introduction of more ensembles, better thermostats, etc.42,43 Rapid hardware progress allowed simulations of substantially larger systems44,45 as well as the release of a variety of MD program packages working in a parallel environment. Some simulation problems have been overcome by the development of hybrid MC/MD approaches,46,47 which improved the phase space sampling and hence led to statistically more accurate results. Such molecular models permit optimization of the structure, organization, and molecular characteristics of amphiphilic molecules at the gas/water interface including all types of essential interactions within a monolayers van der Waals, electrostatic, hydrogen bonding, etc. Another advantage is the possibility of estimation of the normal and tangential components of various parameters of interest, i.e., the transition zone anisotropy is taken into account. Utilizing the accumulated atomistic data, new more accurate coarse grain models were developed allowing longer simulations of larger systems, thus reproducing more veritably the thermodynamic properties but lacking in intimate structural details.48 Therefore, the current study will adhere to the all-atom approach with explicit treatment of the solvent. The biological importance of lipid films makes them central to the investigation of insoluble layers. Typically, phospholipid bilayers are a primary subject of research. The study of bilayers offers easier molecular simulation protocols due to system symmetry, but comparison to experimental data is restricted. Monolayer properties can be monitored by a variety of instrumental techniques but are challenging for simulation. There are a number of communications dealing with theoretical simulation of lipid assemblies.48-54 The major part of them are (36) Swanson, D. R.; Hardy, R. J.; Eckhardt, C. J. J. Chem. Phys. 1996, 105, 673. (37) Weiner, S. J.; Kollman, P. A.; Case, D. A.; Singh, U. C.; Ghio, C.; Alagona, G.; Profeta, S., Jr.; Weiner, P. J. Am. Chem. Soc. 1984, 106, 765. Weiner, S. J.; Kollman, P. A.; Nguyen, D. T.; Case, D. A. J. Comput. Chem. 1986, 7, 230. Cornell, W. D.; Cieplak, P.; Bayly, C. I.; Gould, I. R.; Merz, K. M.; Ferguson, D. M.; Spellmeyer, D. C.; Fox, T.; Caldwell, J. W.; Kollman, P. A. J. Am. Chem. Soc. 1995, 117, 5179. (38) Jorgensen, W. L.; Maxwell, D. S.; Tirado-Rives, J. J. Am. Chem. Soc. 1996, 118, 11225. (39) Feller, S. E.; Yin, D.; Pastor, R. W.; MacKerell, A. D., Jr. Biophys. J. 1997, 73, 2269. (40) Feller, S. E.; MacKerell, A. D., Jr. J. Phys. Chem. B 2000, 104, 7510. (41) Jorgensen, W. L.; Chandrasekhar, J.; Madura, J. D.; Impey, R. W.; Klein, M. L. J. Chem. Phys. 1983, 79, 926. (42) Zhang, Y.; Feller, S. E.; Brooks, B. R.; Pastor, R. W. J. Chem. Phys. 1995, 103, 10252. (43) Chiu, S. W.; Clark, M.; Balaji, V.; Subramaniam, S.; Scott, H. L.; Jakobsson, E. Biophys. J. 1995, 69, 1230. (44) Lindahl, E.; Edholm, O. Biophys. J. 2000, 79, 426. (45) Becker, O. M.; MacKerell A. D.; Roux, B.; Watanabe, M. Computational Biophysics and Biochemistry; Marcel Dekker: New York, 2001. (46) Chiu, S. W.; Jakobsson, E.; Subramaniam, S.; Scott, H. L. Biophys. J. 1999, 77, 2462. (47) Chiu, S. W.; Jakobsson, E.; Scott, H. L. Biophys. J. 2001, 80, 1104. (48) Nielsen, S. O.; Carlos, L. F.; Srinivas, G.; Klein, M. L. J. Phys.: Condens. Matter 2004, 16, 481.

Dielectric Properties in Model Insoluble Monolayers

based on molecular dynamics and/or Monte Carlo statistical analysis with standard or topically designed molecular force fields.27 Either the interface is modeled as an abrupt boundary between two continuums with a substantial difference in dielectric constants or the water medium is treated explicitly. With no exception these simulations tend to reproduce a selected region of the Π/A isotherm or measurable properties exclusively in the direction normal to the interface.52 In an earlier study we reported MC simulations on periodic models of 1,2-dipalmitoylphosphatidylcholine (DPPC) and 1,2-didecanoylglycerole (dicaprin, DC) monolayers at different surface concentrations.55 The elementary cells (ECs) in this study were clusters of two, four, or nine surfactants at the air/water interface where air was replaced by vacuum. The results outlined the tendencies in the variation of structural and dielectric characteristics of the entire monolayer and separately of its components: normal and tangential contributions of the surfactant and water. These were rationalized in terms of structural reorganization of the film along the isotherm. This reorganization at a certain surface concentration was attributed to the reorientation of surfactant dipole moments. The employed computational protocol gave plausible results for the potential energy and the normal characteristic variations with area per molecule but failed to yield stable tangential behavior. To our knowledge, there are no communications discussing tangential lipid film conduct along the entire isotherm. While the normal component of dipole-dipole interactions between polar heads is more predictable, the tangential share of those is expected to vary significantly, thus governing the head-head and headsolvent organization at each degree of compression. We will show that the specific intermolecular interactions between the hydrophilic surfactant head and the closest surrounding water molecules may be critical for the overall behavior and tangential features of the monolayer since the water from the solvation shell contributes substantially to the microenvironment of the surfactant headgroups, in particular to the local dielectric medium, orientation of the polar parts, etc. The purpose of this study is to obtain a qualitatively realistic estimate of tangential structural characteristics and (di)electric properties of insoluble lipid monolayers at the air/water interface, vacuum modeling air, and to promote the surface water from a spectator to an active participant in the changes occurring upon compression. We emphasize the term qualitatiVe as our speculations are based on comparatively small models. Method DPPC and DC are used as model amphiphilic molecules (Figure 2, left). The choice of target molecules is dictated by two main arguments. First, we want to apply our scheme to two appreciably dissimilar cases. Dissimilarity in head features is reflected in the (49) (a) Ivanova, Tz.; Grozev, N.; Panaiotov, I.; Proust, J. Colloid Polym. Sci. 1999, 277, 709. (b) Brasseur, R. In Molecular Description of Biological Membrane Components by Computer-Aided Conformational Analysis; Brasseur, R., Ed.; CRC Press: Boca Raton, FL, 1990; Vol. 1, p 203. (50) (a) de Vries, A. H.; Mark, A. E.; Marrink, S. J. J. Am. Chem. Soc. 2004, 126, 4488. (b) Feller, S. E.; Venable, R. M.; Pastor, R. W. Langmuir 1997, 13, 6555. (c) Mauk, A. W.; Chaikof, E. L.; Ludovice, P. J. Langmuir 1998, 14, 5255. (d) Perera, L.; Essmann, U.; Berkowitz, M. L. Langmuir 1996, 12, 2625. (e) Snyder, R. G.; Tu, K.; Klein, M. L.; Mendelssohn, R.; Strauss, H. L.; Sun, W. J. Phys. Chem. B 2002, 106, 6273. (51) (a) Shinoda, W.; Fukada, T.; Okazaki, S.; Okada, I. Chem. Phys. Lett. 1995, 232, 308. (b) Stern, H. A.; Feller, S. E. J. Chem. Phys. 2003, 118, 3401. (c) Tieleman, D. P.; Berendsen, H. J. C. J. Chem. Phys. 1996, 105, 4871. (d) Nandi, N.; Vollhardt, D. J. Phys. Chem. B 2002, 106, 10144. (52) Kaznessis, Y.; Kim, S.; Larson, R. Biophys. J. 2002, 82, 1731. (53) Knecht, V.; Muller, M.; Bonn, M.; Marrink, S.-J.; Mark, A. J. Chem. Phys. 2005, 122, 024704. (54) Wohlert, J.; Edholm, O. Biophys. J. 2004, 87, 2433. (55) Tadjer, A.; Ivanova, A.; Velkov, Y.; Tzvetanov, S.; Gotsev, M.; Radoev, B. Int. J. Quantum Chem. 2007, 107, 1719.

Langmuir, Vol. 24, No. 9, 2008 4617 unlike trendlines of experimental Π/A and ∆V/A isotherms of DPPC and DC (Figure 2, right). While three distinct branches are recognizable in the DPPC curves and speculations for phase transitions are readily rationalized (Figure 2, top right), the DC isotherms offer little support for such contemplations (Figure 2, bottom right). The second reason for selecting DPPC and DC in our models is the fact that these molecules form insoluble monolayers of great importance for fundamental studies and for practice-oriented science.56,57 Prior to MD simulations, the constructed models were subjected to MC relaxation (ca. 106 steps, NVT ensemble, 300 K) with a combination of force fields following a protocol described previously.55 The models consist of 9 partially hydrated surfactants (no less than 50 water molecules per lipid head) with a rectangular arrangement in periodic boundary conditions. The TIP3P41 model is used for representing water molecules. The lattice parameters provide areas per molecule corresponding to seven selected points of the isotherm for each target molecule. To account for the bulky heads, equal intersurfactant spacing is assumed for DPPC clusters in both directions of the interfacial plane (xy) (Figure 2, top center; for the side view see Supporting Information Figure S1, left; also see Supporting Information Table S1), whereas DC molecules are placed at nonidentical distances in the xy plane, scaling as 2:1 in compliance with the molecular dimensions (Figure 2, bottom center; for the side view see Figure S1, right; also see Table S1). The lowest energy structure from the relaxed part of each MC trajectory is chosen as the initial configuration for subsequent MD simulations in periodic boundary conditions. To model a system closest to real monolayers, standard periodic boundary conditions are applied in directions x and y, while the periodicity along z is interrupted by introducing vacuum slabs (Table 1 and Table S1) between the lipid-water clusters in neighboring ECs. The initial structures were subjected to geometry optimization with CHARMM22/27,41,58-60 and this force field was used for all further treatments as it is specially parametrized for lipids. The force field contains terms describing the electrostatic interactions in monopole approximation and the van der Waals interactions with 6-12 LennardJones potential with the parameters adjusted to reproduce correctly the thermodynamic behavior of lipid layers.40,50b Thus, the force field accounts explicitly for the orientational share of polarization and implicitly for the electronic one. To avoid an undesirable decrease of the water density, a harmonic potential along the normal direction is applied to the oxygen atoms of the water molecules at the pure gas/water boundary.52 Testing of different force constants yielded similar results; thus, an arbitrary value of 100 kcal mol-1Å-2 was adopted. The simulation was carried out in three stages: heating (0-300 K for 30 ps), equilibration (150 ps), and production simulation (5 ns). The time step was set to 2 fs in each stage, and the NVT ensemble was preserved throughout. The bonds in water were frozen using SETTLE,61 and all the remaining hydrogen-containing bonds were rigidized by means of SHAKE62 as implemented in NAMD 2.6. A (56) (a) Physical Chemistry of Biological Interfaces; Baszkin, A., Norde, W., Eds.; Marcel Dekker: New York, Basel, 2000. (b) Chen, P.-J.; Liu, Y.; Weiss, T. M.; Huang, H. W.; Sinn, H.; Alp, E. E.; Alatas, A.; Said, A.; Chen, S.-H. Biophys. Chem. 2003, 105, 721. (c) Sachs, J. N.; Petrache, H. I.; Woolf, T. B. Chem. Phys. Lipids 2003, 126, 211. (d) Hasegawa, T.; Ushiroda, Y.; Kawaguchi, M.; Kitazawa, Y.; Nishiyama, M.; Hiraoka, A.; Nishijo, J. Langmuir 1996, 12, 1566. (57) (a) Ivanova, M.; Svendsen, A.; Verger, R.; Panaiotov, I. Colloids Surf., B 2000, 19, 137. (b) Nannelli, F.; Puggelli, M.; Gabrielli, G. Colloids Surf., B 2002, 24, 1. (c) Gargouri, Y.; Pitroni, G.; Rivicre, C.; Sarda, L.; Verger, R. Biochemistry 1986, 25, 1733. (58) MacKerell, A. D., Jr.; Bashford, D.; Bellott, M.; Dunbrack, R. L., Jr. Evanseck, J. D.; Field, M. J.; Fischer, S.; Gao, J.; Guo, H.; Ha, S.; JosephMcCarthy, D.; Kuchnir, L.; Kuczera, K.; Lau, F. T. K.; Mattos, C.; Michnick, S.; Ngo, T.; Nguyen, D. T.; Prodhom, B.; Reiher, W. E., III; Roux, B.; Schlenkrich, M.; Smith, J. C.; Stote, R.; Straub, J.; Watanabe, M.; Wiorkiewicz-Kuczera, J.; Yin, D.; Karplus, M. J. Phys. Chem. B 1998, 102, 3586. (59) Schlenkrich, M.; Brickmann, J.; MacKerell, A. D., Jr.; Karplus, M. In Biological Membranes: A Molecular PerspectiVe from Computation and Experiment; Merz, K. M., Roux, B., Eds.; Birkhauser: Boston, 1996; p 31. (60) Feller, S. E.; MacKerell, A. D., Jr. J. Phys. Chem. B 2000, 104, 7510. (61) Miyamoto, Sh.; Kollman, P. J. Comput. Chem. 1992, 13, 952.

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Figure 2. Target molecules in this studysDPPC (top left) and DC (bottom left). Top view of DPPC (top center) and DC (bottom center) arrangement in the periodic box. Surface pressure/area (Π/A) and surface potential/area (∆V/A) isotherms of DPPC (top right) and DC (bottom right) insoluble monolayers recorded at 25 °C in ultrapure water [courtesy of Dr. Tz. Ivanova]. Table 1. Linear Dimensions (in the Normal Direction) of the Studied Monolayers and Their Constituentsa lipid height area per monolayer water width in the submergence in the molecule, slab depth, Å Å2 width, Å monolayer, Å monolayer, Å 40 50 60 70 80 100 120

35.5 32.5 32 28 27 27 27

DPPC 17.0 17.5 17.5 14.5 13.0 17.5 19.5

33 30 29 25.5 24 24.5 24.5

14.5 15.0 14.5 12.0 10.0 15.0 17.0

40 50 60 70 80 100 120

28 25 24.5 23 22.5 22 20

DC 13.5 14.5 14.5 12 13.5 15.5 14

23 22 21 20 19 18 18

8.5 11.5 11 9 10 11.5 12

a These dimensions are considered in eq 2 and throughout for dipole moment estimations (error range (0.5 Å).

force field consistent nonbonded switched cutoff at 10 Å was applied. The switching function was invoked at 8 Å. The same cutoff is used for the direct part of the electrostatic interactions. Long-range electrostatic interactions were treated by means of PME.63 Temperature control was maintained by the Langevin thermostat (damping coefficient 5 ps-1) implemented in the program package NAMD (62) Ryckaert, J.-P.; Ciccotti, G.; Berendsen, H. J. C. J. Comput. Phys. 1977, 23, 327. (63) Essmann, U.; Perera, L.; Berkowitz, M. L.; Darden, T.; Lee, H.; Pederson, L. J. Chem. Phys. 1995, 103, 8577.

Figure 3. Assignment of the axes in the elementary cell (left) and scheme of the dipole moment components with respect to the interface (right). 2.6.64 Data were collected every 100 steps, i.e., every 0.2 ps. The stability of the MD trajectory is verified by convergence of the total energy to a constant average value (Supporting Information Figure S2). The results presented in Figures 4, 6, and 7 were averaged over the entire trajectory, whereas Figure 5 contains the probability distribution of the corresponding data. The dipole moment was a major objective of this study and was calculated in the monopole approximation. The contributions of water molecules and surfactants to the total dipole moment were computed for each elementary cell, and all three vectors were decomposed into the respective normal (µz ≡ µ⊥) and tangential (µ|) components (µ| ) (µx2 + µy2)1/2, µtotal ) (µx2 + µy2 + µz2)1/2). The assignment of the x, y, and z axes and the dipole moment decomposition are illustrated in Figure 3. For assessment of the dielectric permittivity tensor elements (RR) the formula of Kirkwood-Froehlich was applied:51b (64) Phillips, J. C.; Braun, R.; Wang, W.; Gumbart, J.; Tajkhorshid, E.; Villa, E.; Chipot, C.; Skeel, R. D.; Kale, L.; Schulten, K J. Comput. Chem. 2005, 26, 1781.

Dielectric Properties in Model Insoluble Monolayers

( )

| 0 0  ) 0 | 0 0 0 ⊥

RR ) 1 + 4πHRR/V HRR )

1 (〈µ 2〉 - 〈µRR〉2) kBT RR

Langmuir, Vol. 24, No. 9, 2008 4619

(1) (2)

(more than 10-5 g/cm3) up to constant bulk density (1 g/cm3). An alternative approach is to monitor the electrostatic potential jump. Double integration of the average charge distribution in the normal direction to the interface yields the electric potential profile (Supporting Information Figure S5) across the monolayer (one-dimensional Poisson equation):68

(3)

where µRR is a dipole moment component (µ⊥ or µ|) and V is the volume considered in each particular case (Table 1). Equations 1-3 were used for quantification of the dielectric permittivity components in the normal (⊥) and tangential (|) directions resolved to lipid and water contributions. All methods used for the periodic MD simulations and for computation of dipole moments were employed as implemented in NAMD 2.6.64 Original scripts for construction of the elementary cell, batch calculations, vector decomposition, and statistical analysis were used.

Results and Discussion Our hypothesis is that tangential properties of monolayers play a leading role in the film organization. The structuring of interfacial water depends mainly on the head composition, geometry, and tilt at a certain area per molecule and tends to smooth the surfactant reorganization upon compression. Since the focus is on the effect caused by the different surfactant heads, we need to make sure that no “contamination” from dissimilar tail contributions is present. Therefore, we selected molecules with the same type of tailssalkyl chains which have a negligible dipole moment but participate in attractive van der Waals interactions. The inequality of the tail chain length is of no consequence for these estimations as it was shown earlier for surfactant prototypes with saturated hydrocarbon tails that homologues with a six-carbon-atom tail skeleton reproduce trustworthily the dipole moment of long-tail surfactants.34,65,66 Other data55,65 are in support of this statement as well: the dipole moment values of the heads do not differ from those of the whole molecules. Thus, the tendencies in behavior of the two targets are expected to vary significantly due as much to the heads as to the different patterns of water self-assembly around them. For validation of the simulation reliability, the probability distributions of dipole moment components were calculated along the MD trajectories and fitted (Supporting Information Figure S3) to their anticipated behavior as fluctuating measurable characteristics.67 As defined in statistical thermodynamics, the magnitude of a vector quantity (a scalar), has a MaxwellBoltzmann distribution while its components (vectors) obey a Gaussian distribution. The Gaussian distribution of the normal component of the dipole moment vector (µz) and the MaxwellBoltzmann distribution of the tangential and total dipole moment magnitudes were well reproduced. To define the monolayer size, we chose two complementary strategies for formulation of the bulk/surfacial water boundary. One approach to defining different regions in the system is based on density estimates (Supporting Information Figure S4). The latter clearly outline valuable monolayer structural information, which will be the topic of an upcoming paper. A possible definition of the term “interfacial water” frequently used in this paper is the water content of the monolayer ranging from negligible values (65) Ivanova, A.; Tadjer, A.; Tyutyulkov, N.; Radoev, B. J. Phys. Chem. A 2005, 109, 1692. (66) Ivanova, A.; Tadjer, A.; Radoev, B.; Panayotov, I. SAR QSAR EnViron. Res. 2002, 13, 237. (67) Landau, L. D.; Lifshitz, E. M. Statistical Physics, 3rd ed.; ButterworthHeinemann: Oxford, 1980; Part 1, Chapter XII.

φ(z) - φ(0) ) -

∫0zdz′ ∫0z′dz′′ Fc(z′′)

1 0

(4)

Two large and opposite contributions from the surfactant and water result in a mild potential jump at the interface for DPPC films, involving only a small sector of the monolayer in the region of lipid heads. The water contribution governs the overall conduct of the potential (Figure S5, left). In DC films the situation is simpler: the lipid share determines both the sign and magnitude of the surface potential (Figure S5, right). Thus, the electric potential profiles provide information about the system stratification. The estimates from the two approaches were in very good agreement and allowed several normal to the interface structural parameters to be extracted (Table 1). For instance, using the density profiles (Figure S4), these parameters are defined as follows: (i) the monolayer slab width is the region ranging from a lipid density of more than 10-5 g/cm3 to constant water density (∼ 1 g/cm3); (ii) the water width in the monolayer is measured from a water density of more than 10-5 g/cm3 to constant water density (∼1 g/cm3); (iii) the lipid height in the monolayer is evaluated as the zone in which the lipid density exceeds 10-5 g/cm3; (iv) the submergence depth is the subspace spanning from a water density of more than 10-5 g/cm3 to a lipid density of less than 10-4 g/cm3 in the water slab. The data in Table 1 reveal a monotonous increase of the lipid height upon monolayer compression and thereof of the total monolayer slab width considered for assessment of the film electric properties. This is an expected relationship as with a decrease of the area per molecule the freedom of tail movement is reduced and they straighten, thus forming a taller lipid layer. Moreover, compressed polar heads produce a stronger electrostatic field, which protrudes farther in the aqueous phase and involves more water molecules in the interface region. However, the depth of the surfacial water and of surfactant submergence has a minimum in the middle of the isotherm at surface concentrations corresponding to LE bordering LE/LC phases of the particular lipid. It looks as if the lipids gradually emerge from water upon compression until they reach liquid structure alignment and then submerge again in the course of closer packing. Accordingly, the lipid-hydrating water gets thinner and then swells again. As this conduct is characteristic for both otherwise dissimilar targets, it has to be related to the peculiarities of phase organization rather than to surfactant specifics, the latter being reflected in the concrete area where the minimum occurs. This will be discussed in more detail below. The (di)electric properties of monolayers are described in terms of their dipole moment and dielectric permittivity. As mentioned earlier, this study addresses for the most part the tangential components of these characteristics. The dipole moment magnitude is quantified in atomic units per surfactant molecule (1 au ) 2.542 D). Figure 4 presents the profiles of the tangential and total dipole moments and the contributions of lipids and surface water for DPPC and DC monolayers. It is obvious that in DPPC layers the lipid share defines both the tangential and (68) Feller, S. E.; Pastor, R. W.; Rojnuckarin, A.; Bogusz, S.; Brooks, B. R. J. Phys. Chem. 1996, 100, 17011.

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Figure 4. Tangential (top) and total (bottom) dipole moment of DPPC (left) and DC (right) films. The total values (squares), as well as the contributions of water (triangles) and surfactant (stars), are shown.

total dipole moments while in DC films the water share is prevalent. The total dipole moment behaves likewise: in DPPC monolayers the lipid contribution dominates, the tangential and total moments being almost quantitatively identical at any area per molecule (emphasizing the negligible magnitude of the normal component). In DC films the water share is critical at low to medium surface concentrations; only for dense layers the lipid contribution in the curve trend becomes noticeable due to significant input from the normal component. DPPC Films: Dipole Moments. The tangential and total dipole moment contributions of DPPC display pronounced minima at 70 Å2/molecule, suggesting a radical change in lipid organization. Its origin will be discussed in more detail below. The overall conduct of the tangential and total moments of DPPC films can be interpreted in the following way: whenever a definite type of organization is established, the cumulative effect of the similarly oriented dipole moments of the separate molecules results in high values of the summed dipole moment contribution; at points of rearrangement or coexistence of structures the chaotic orientation of molecular dipoles leads to low values of the resultant moment. Thus, a maximum in the curve signifies a region of single-phase organization, while a minimum indicates a point of reassembly or phase coexistence. Accordingly, the dipole moment of the gas phase is low, and a twist in the curves at 100 Å2/ molecule marks the beginning of LE phase formation. The LE type of organization is most robust at 80 Å2/molecule with a dramatic change thereafter when the LC domains start to form. As the molecular orientation is too random to be attributed to a single phase, this should correspond to coexistence of the LE and LC states. At 60 Å2/molecule the LC state is already established but undergoes further rearrangement at 50 Å2/ molecule (possibly domain consolidation), thus converting to the LC/SC state. Of course, the model is too small and the areas per molecule studied are quite equidistant to claim that the values listed above are rigorous. Nevertheless, the drastic change in

molecular organization in the region 60-70 Å2/molecule is apparent and cannot be ignored as casual. Water contribution to the tangential moment grows monotonously (almost linearly, proportionally to the film water content) with the area per molecule in DPPC layers. A slight deviation in linearity occurs at 60, 70, and 80 Å2/molecule as a weak response to the changes in lipid behavior. The water contribution to the total dipole moment exhibits a similar trend, except that the mild maximum at 60 Å2/molecule is shifted to 50 Å2/molecule, apparently due to structural rearrangement in the normal direction (additional support for such an assumption is the enhanced lipid submergence at this area; see Table 1). Quantitatively, the values for water in the total moment contour are shifted up with respect to those in the tangential by approximately 1 au. Obviously, the normal water component is more or less constant and is roughly of this order of magnitude. The water share in the tangential and total moments has lower values than that of DPPC in the range 40-110 Å2/molecule, and only beyond this value water exceeds the lipid contribution, in agreement with the large amount of water interacting with deeply submerged surfactants (Table 1). A critical point in all curves of DPPC films is at 70 Å2/molecule, where the water share in the tangential moment equals that of DPPC; in the total moment profiles the value for water is even higher than that for DPPC at this surface concentration, which proves the important role of surfacial water in moderating the profound surfactant structural changes. DC Films: Dipole Moments. The DC share in the tangential moment decreases uniformly in the range 120-80 Å2/molecule, where a new pattern of organization arises, then stabilizes at 70 Å2/molecule in liquidlike ordering, and rearranges at 60 Å2/ molecule to a final stable alignment at 50 Å2/molecule, which is retained till film collapse. The water contribution to the tangential moment in DC monolayers also grows with the area per molecule and, remarkably, within almost the same range of values as in DPPC films. However, the deviations from linearity

Dielectric Properties in Model Insoluble Monolayers

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Figure 5. Tangential component (µ|) distributions with respect to the orientation φ of lipid molecules (top) and the magnitude (bottom, in atomic units per lipid molecule) of DPPC (left) and DC (right) monolayers.

are more pronounced. Kinks in the curve disclose a constant value of the water share at a low surface concentration of surfactant; there is a linear decrease from 100 to 70 Å2/molecule where major rearrangement of water molecules occurs as a response to the preceding lipid reorganization at 80 Å2/molecule and again a monotonous decline in the interval 60-40 Å2/ molecule. The summed tangential moment follows strictly the water profile in the range 120-80 Å2/molecule and then changes its slope of decrease to nearly constant behavior till 60 Å2/molecule. A maximum at 50 Å2/molecule designates a point of stability, and further compression leads to deterioration of the monolayer organization. Applying the same logical scheme as above, no definite intermediate phase states can be outlined in DC films. What seems to be the gas phase at 100 Å2/molecule reorganizes smoothly at 80 Å2/molecule to form the LE state, which is gradually compressed to the LC organization at 50 Å2/molecule. The total dipole moment of DC monolayers also reveals minor deviation from its tangential component as in DPPC films. The overall values differ quantitatively in the region of high compression mainly due to the difference in water behavior. Water contribution to the total dipole moment preserves virtually unchanged its tangential trendline except in the high-density region. Unlike the DPPC models, here the lipid curve retains the same profile but is lifted by an almost constant increment of ∼0.5 au from the normal component. The influence of the latter makes more pronounced the minimum in the DC curve at 80 Å2/molecule, indicating that molecular rearrangement at this surface concentration is sensed simultaneously in the normal and lateral directions. As a result, the total dipole moment of the DC film has a contour supporting the above observations: a poorly expressed maximum at 100 Å2/molecule marks the gasphase state, which undergoes reordering at 80 Å2/molecule to

form the LE state. Further compression results in no dramatic structural changes until closest packing at 50 Å2/molecule (LC). Dipole Orientation Issues. The fact that the summed values of tangential monolayer moments are higher than those of both constituents (water and lipid) along the entire isotherms of the two lipid monolayers indicates that the averaged orientations of the water and lipid molecules do not compensate at either area per molecule in the tangential direction. Rather than that, the angle between the orientations of the two contributions is apparently less than 90°. As the total dipole moment reproduces to a great extent the tangential component, an assumption could be made that water and lipid contributions do not compensate in the normal direction either (i.e., that they might have the same orientation). It should be pointed out that such an assumption is premature and possibly wrong. The hypothesis that extremes in the tangential and/or total dipole moments reflect areas of defined states (maxima) and structural reorganization (minima) of the films can be verified by means of statistical analysis of the tangential orientation of lipid molecules and of the lipid tangential moment magnitude. Two factors define the choice of these variables: (i) Figure 4 shows that the tangential moment determines to a great extent the total dipole momentsexclusively in DPPC monolayers and significantly in DC films; (ii) only the tangential component can change its orientation in the plane upon compression and thus provide information about the level of ordering, whereas the normal one maintains a fixed direction. The vector orientation distribution is based on the values of the angle φ between the tangential dipole moment of the lipid molecules and an arbitrary axis (the x axis) in the interfacial plane. Figure 5 reveals that for DPPC the low surface concentrations (120 and 100 Å2/molecule) are characterized by a uniform distribution of φ, i.e., by gas-phase behavior. A broad maximum

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emerges at 80 Å2/molecule, proving the appearance of a preferred orientation (although within a considerable range of values) as in a liquid state. Instead of getting narrower, the peak disappears in the next stage of compression (70 Å2/molecule), and again a random distribution is established, which is an indication of a drastic molecular reorganization. Further compression results in the reappearance of several preferred nonequally populated orientation peaks, which gradually get closer, narrower, and more populated until densest packing is reached. The most pronounced maxima at 80, 60, and 40 Å2/molecule are practically at the same φ value. The curve at 50 Å2/molecule resembles strongly that at 80 Å2/molecule, just displaced with respect to φ. If at 80 Å2/molecule the phase state is a typical LE state, this similarity in profiles is a symptom of a final rearrangement at 50 Å2/ molecule which is much milder than that at 70 Å2/molecule due to the reduced freedom of the molecules. The anticorrelated kink in the curves of both tangential and total moment profiles of lipid and water constituents at 50 Å2/molecule also prompts an additional rearrangement only in the lateral direction, providing an alternative alignment of the molecules of the two ingredients of the compressed layer. In other words, the switch from LE to LC at 70 Å2/molecule is labeled by a gas-type distribution, while the conversion from LC to SC at 50 Å2/molecule is marked by a liquid-type distribution. Three distinct and narrow maxima characterize the closest packing of the monolayer at 40 Å2/molecule. In contrast to the classical monocrystal structure, this distribution indicates the coexistence of one strongly preferred and two satellite asymmetrically displaced secondary orientations (+60° and -30° with respect to the central maximum). This may be attributed to insufficient simulation times, but experimental data51d provide evidence that DPPC monolayers preserve domain character till their collapse and that the molecules in the core of the domain have one orientation, whereas those along the periphery diverge in two different orientations, which leads to the formation of “fingers” with a certain symmetry. The DC tangential moment orientation features a completely different distribution upon compression. The chaotic gaseous distribution is preserved through the value of 80 Å2/molecule. A broad two-peaked profile emerges at 60 Å2/molecule; the preferred orientation of approximately 0° and 90° indicates the formation of regions with herringbone ordering.27 This alignment changes appreciably upon further compression, and the closest packing at 40 Å2/molecule features basically one preferred orientation (∼0°) with substantial dispersion, describing a glasstype rather than crystal ordering. The compression constrains the translational degrees of freedom but cannot hinder the rotational motion completely. This applies especially to the nonesterified hydroxyl group, which retains sufficient rotational freedom even at the closest packing. The symmetrically displaced satellite peaks ((30°) do not imply the formation of fingered structures but rather indicate the presence of two alternative favorable orientations of the free hydroxyl group. Tangential Moment Magnitudes. Analysis of the distribution of lipid tangential moment magnitudes shows that in the monolayers of both lipids the low level of organization is characterized by Boltzmann distributions starting from zero values of the dipole moment with maxima at dipole moments several times smaller than in denser phases, whereas the high level of compression features Gaussian profiles. With the increase of order, the fit to a Gaussian shape improves, the bandwidth of the curves lessens, and the maxima shift to higher values of the tangential moment. The better the order in the polar structure, the more uniform the alignment of dipoles and hence the higher

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the summed moment. A more exhaustive scrutiny of the curves provides in-depth information about the stages of lipid selfassembly. In DPPC layers the Boltzmann distributions at 120 and 100 Å2/molecule typical for a gas phase give way to two broad and ill-shaped Gaussians at 80 Å2/molecule with maxima at ca. 4 and 6 au, indicative of the establishment of flexible organization in the system (LE). Instead of becoming narrower and better defined, at the next step of compression again a Boltzmann distribution (much slimmer than the gas-phase distributions) marks the area of 70 Å2/molecule. However, in its shoulder emerges a small Gaussian with a peak at 4.5 au revealing the onset of ordered packing (LC type) in the disordered system. Further compression to 60 Å2/molecule strongly enhances the population of the Gaussian at 4.5 au, and a second one appears at 6.5 au which can be attributed to the presence of looser and tighter packing, the looser packing being predominant. At 50 Å2/molecule the two peaks get closer and sharper, and at 40 Å2/molecule they almost merge; i.e., a well-defined high and narrow peak centered at 6 au overlaps with a lower and narrower one at 6.5 au, the latter being responsible for the most compact selfassembly (SC type), which should be typical for the core of domains but is insufficiently populated due to the small size of the model. In DC films the analysis is much more straightforward and comprehensive. In the first stages of compression (120-80 Å2/ molecule) the magnitude distributions of the lipid tangential moment fit to Boltzmann curves with a continuous tendency of the profiles to get narrower and higher at each step, all of them having their maxima at a constant value of ca. 0.4 au. A broad well-shaped Gaussian peaked at 0.75 au labels the average area of 70 Å2/molecule characterizing a relatively ordered phase (LE type). The next step of compression restores the Boltzmann contour, and the peak recedes back to 0.5 au, retaining a comparatively compact shape as an indication of structural changes. This can be related to tilt variations in the process of film reorganization, leading to a phase with a distribution fitting to a well-defined Gaussian centered at 1.1 au for 50 Å2/molecule. The bandwidth of the profiles at 80, 60, and 50 Å2/molecule is the same, and this implies that the ordering at 50 Å2/molecule is somewhat loose, more typical for a compressed liquid than for crystalline alignment. Most surprising at first glance is the last step of film surface reduction featuring the broadest and most distorted Gaussian with maximum dispersion of values. Though centered at the same magnitude of 1.1 au, the values cover the entire range from 0 to 2 au, i.e., to the estimate of the total dipole moment of the lipid in vacuum. The state is apparently more dispersed than any of the earlier stages of compression and more liquidlike even than LE. The “mystery” of DC films at 40 Å2/molecule can be explained if we go back to the experimental results. In the ∆V/A isotherm this area per molecule is beyond the point of collapse (Figure 2, bottom right), and obviously the structural parameters correspond to a system that cannot be termed an “insoluble monolayer”. There are two possible events that can occur either separately or in combination: (i) part of the molecules are solubilized; (ii) multilayered structures are formed. To check which one is responsible for the behavior of the film at this surface concentration, we address the data in Table 1. Unlike DPPC, which retains a constant submergence depth at high surface concentrations, DC molecules emerge at 40 Å2/molecule, and the lipid layer height exceeds the most extended molecular length (∼19 Å), which indicates that certain molecules stick out of the layer. Thus, the second alternative seems more plausible in this case.

Dielectric Properties in Model Insoluble Monolayers

Figure 6. Variation of the lipid contribution to the tangential dipole moment (in atomic units per molecule) in monolayers of DPPC (top) and DC (bottom), averaged separately for each molecule (circles) and from the entire cluster (squares).

Cooperative Behavior and Dielectric Properties. Even though the simulation keeps track only of interactions between closest lipid neighbors, it is interesting to compare the values of the lipid contribution to the tangential moment estimated by averaging along the trajectory of (i) the individual values for the separate molecules and (ii) the values for the entire cluster scaled per lipid molecule (Figure 6). The comparison reveals a striking qualitative difference in the contours for DPPC and less expressed deviations for DC. In both cases the magnitudes based on individual values are higher than those quantified from the entire cluster; apparently the dipole-dipole interaction reduces the overall polarity of the layer. In idealized models the properties of the entire system are often represented additively, assigning equal contributions to each participant. It is obvious that the cooperative effects in the layers of the highly polar lipid (DPPC) are critical and the additivity of the properties is utterly ineffective, while for the less polar lipid (DC) it is more tolerable. Insoluble monolayers are characterized by pronounced anisotropy of properties. On the basis of eq 2, the tangential dielectric permittivity is assessed, a quantity completely out of reach for direct experimental measurements (Figure 7). The values shown in the figure are averaged over the monolayer slab width (Table 1) and are scaled per surfactant molecule. The estimates show that they vary appreciably depending on the degree of compression and type of surfactant. In DPPC monolayers the surfacial water contribution defines quantitatively the | value range, while the lipid share determines qualitatively the curve profile. The interpretation of extrema in tangential permittivity is just opposite to that of the tangential moment: maxima correspond to disorder, minima to organized structures. Inspection of the contour of the tangential permittivity of DPPC films reveals two clearly distinguishable branches, 4070 and 80-120 Å2/molecule, as if upon compression one phase improves its ordering to a certain point and with a jump converts

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Figure 7. Variation in the tangential component of the relative dielectric permittivity with surface concentration and the contributions of the participants in monolayers of DPPC (top) and DC (bottom), estimated according to eq 2. The total values (squares), as well as the contributions of water (triangles) and surfactant (stars), are shown; a rescaled DC profile is shown in the inset.

to another phase with a much more rapid increase of the organization degree. The behavior of the DPPC contribution is the same. At high DPPC surface concentration the hydration shell has a tangential permittivity several times lower than that of bulk water (82 for TIP3P water69); it grows with film decompression and reaches a saturated value in the LE and gas states, closer to the bulk water permittivity. In the regions of sustainable phase states (80 and 40-50 Å2/molecule) the two contributions partially quench each other (anticorrelated), whereas in the remaining part of the isotherm they are positively correlated. The picture is different in DC films: tangential contributions from lipid and water are relatively constant and anticorrelated along the entire contour. The domination of the water share is obvious both in quantitative and in qualitative respects; nonetheless, the total tangential permittivity is always lower than that of pure water. Even though the lipid polarity is not impressive, the low level of organization results in high values of the total tangential permittivity even at highest compression. Examination of the DC share curve shows best ordering at 80 Å2/molecule (LE) and growing disorder instead of improved crystallinity upon further compression (in support of the glass-type alignment suggested above). In both systems the simulation results describe correctly the tendency of a decrease of total tangential permittivity with a reduction of surfactant freedom upon compression.

Conclusions The study gives insight into the variations of tangential (di)electric characteristics in pure nonionic lipid films along the entire Π/A isotherm, based on two dissimilar surfactant targets. This is achieved by means of theoretical estimation and statistical (69) Kusalik, P. G.; Svishchev, I. M. Science 1994, 265, 1219.

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analysis of the tangential component of the dipole moment and dielectric permittivity. Molecular dynamics is used for generating trajectories of the monolayers. Extrema in the properties considered are rationalized in terms of structural rearrangement related to nucleation and phase stability. Tangential dielectric parameters are shown to dominate the respective total characteristics, and knowledge about their behavior is valuable. Moreover, they are inaccessible by direct experiments. Special attention is paid to defining the spatial range and estimating the conduct of interfacial water, which is demonstrated to participate in film reorganization. Ongoing simulations on larger systems will allow more detailed and accurate interpretation of insoluble monolayer features.

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Acknowledgment. Thanks are due to Prof. B. Radoev, Dr. Tz. Ivanova, and Dr. N. Grozev for the experimental isotherms and for the fruitful experiment-related discussions. Supporting Information Available: Size of the simulated systems (Table S1), initial elementary cells for DPPC and DC simulations (Figure S1), total energy evolution along the MD trajectory for the DPPC monolayer at 120 Å2/molecule (Figure S2), probability distributions of the calculated dipole moments (total, normal, and tangential) of a DPPC monolayer (Figure S3), density profiles of DPPC and DC monolayers in the direction normal to the interface (Figure S4), profile of the surface potential across DPPC and DC monolayers (Figure S5). This material is available free of charge via the Internet at http://pubs.acs.org. LA703616C