Computer simulation studies of amphiphilic interfaces

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Computer simulation studies of amphiphilic interfaces Sanjoy Bandyopadhyay, Mounir Tarek and Michael L Klein Computer simulation has emerged as a powerful probe for analysing the behavior of amphiphilic systems. The past year has seen several novel applications, which have given important insights into the nature of the water/amphiphile interface as well as the behavior of amphiphiles at different liquidlvapor, Iiquidlliquid and liquid/solid interfaces. This review focuses on surfactants and lipids where simulations have revealed for the first time an atomistic level description of not only the hydration of polar head groups but also the comportment of the hydrophobic tails.

Address Center for Molecular Modeling and Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, USA Current Opinion in Colloid & Interface Science 1998,3:242-246 Electronic identifier: 1359-0294'003-00242

© Current Chemistry Ltd ISSN 1359-0294 Abbreviations AFM atomic force microscope DMPC dimethyl phosphatidylcholine DOPC dioleoyl phosphatidylcholine DPPC dipropyl phosphatidylcholine MD molecular dynamics SDS sodium dodecyl sulfate

Introduction The study of amphiphilic interfaces has a long history. Detailed understanding of such interfaces at a microscopic level has important implications for a wide range of scientific and technological research areas such as detergency, lubrication, molecular self-assembly, ion-transfer and drug delivery. With the advent of powerful computers and sophisticated methodologies, computer simulation has become an important alternative technique to experimental and theoretical approaches in investigating interfacial phenomena [1]. In this regard, computer simulation studies of water and aqueous-phase binary mixtures continue to be an area of interest, due to the universality of water as a polar solvent and it's vital role in biological systems. The focus of this review is the nature of the surfactant/water interface, and bilayer systems. Although simulation studies are often motivated by contemporary experiments, we focus almost exclusively on molecular dynamics (~ID) simulations. The readers interested in further details of the simulations and experiments are urged to investigate the cited references.

standing of nucleation phenomena in liquid-phase binary mixtures. The approach, which is based on atomistic models, contrasts with the classical capillary approximation where the droplets are usually described as spherical objects with a definite composition and their thermodynamic properties are used to derive the nucleation rate. Tarek and Klein [4°°] have reported an ~ID study of the shape and surface structure properties of water/ethanol droplets as a function of the droplet size. Tarek and Klein studied three different droplet sizes of -S, 11 and 16 A in diameter, with 12% ethanol mole fraction. From MD trajectories that spanned over a nanosecond, it was found that in each of the clusters ethanol aggregated at the droplet surface. In addition, the ratio of ethanol molecules at the surface to the number in the bulk increased as the size of the cluster decreased. Their analysis showed that significant shape fluctuations occur irrespective of the cluster size. Moreover, the clusters deviate severely from sphericity as size decreases, with characteristic shape fluctuations ranging from 30-120 ps, as found for spherical micelles in solutions. These calculations reveal aspects (atomic details) of the structure and dynamics of small clusters of binary mixtures and also suggest possible reasons for the discrepancy between nucleation rates predicted by existing theories [5] and the experiment [6]. First, the theoretical models do not allow for complete segregation of surfactant at the interface. Second, the thermodynamic description does not take into account the contribution of shape fluctuations to the surface free energy. It is likely, as in the case of micellar aggregates in solutions, that these fluctuations have to be taken into account in estimating the free energy required to form a mixed critical nucleus from the vapor phase. Laaksonen et al. [7] have reported ~ID simulations at room temperature for water-methanol binary mixtures over a wide range of compositions. They have studied in detail the effects of methanol on water structure and water on methanol structure; large structural changes were observed. In methanol-rich solution the water-water correlations were found to be very pronounced, while methanol retained most of its pure liquid structure. In water-rich solution, a high degree of ordering was observed with characteristic tetrahedral arrangements of water molecules around the hydroxyl group of the methanol. Recently, small angle neutron scattering has been used to study the properties of nanodroplet aerosols [SO].

Surfactant/water interfaces Surface properties of water and water/alcohol mixtures The surface properties of water [2,31, small water droplets, and water-alcohol mixtures are relevant to the under-

The molecular level study of the phenomena at the surfactant/water interface is an active field in the area of research with significant contributions from both experiment [9,10] and theory [11].

Computer simulation studies of amphiphilic interfaces Bandyopadhyay, Tarek and Klein

Shelley et 01. [12·] have performed [\10 simulations using both non polarizable and polarizable models to study the structure and electrostatics of the surfactant/water interface. In particular, they have studied the sodium octanoate-water system in the lyotropic liquid crystalline rnesophase E, where the surfactants form very long columnar micelles which pack in a two-dimensional hexagonal pattern. Their simulations were carried out at a constant room temperature and constant pressure and the system consisted of a single columnar micelle containing 54 octanoate ions, 54 Nat counterions, and 503 water molecules. These simulations predicted that the interfacial region has a narrow water/hydrocarbon interface, about 4A thick, and the Nat ions and carboxylate head groups of the octanoate ions make up a characteristic electrostatic double layer around these micelles. It was shown that the contributions of the individual components to the overall potential drop are large (about 7 V). Because of the collective nature of dielectric screening, however, significant cancellation of the individual contributions occur to give a net potential drop of less than 0.3 V across the interface. The structural properties of another important and commonly studied surfactant, sodium dodecylsulfate (SOS), in mesophase E has also been studied recently (S Bandyupadyay, l\lL Klein, GJ Martyna, 1\1 Tarek, unpublished data). This simulation was carded out on a large system, which consisted of two columnar micelles, each containing 128 dodecylsulfate anions, 256 Nat counterions, and 4350 water molecules. A 260 ps trajectory was generated at constant temperature (333 K) and constant pressure (P = 0 atrn) using a new advanced molecular dynamics package, PINY-MO (GJ Martyna er 01., unpublished data). Figure 1 displays a configuration of the system taken from this simulation. The simulation study revealed that there is a small but distinct deviation from two-dimensional hexagonal symmetry of micelles in mesophase E, in agreement with experiment [13]. It was only possible to demonstrate such a distortion because of the presence of two distinct cylindrical micelles in the simulation system.

Surfactants at Iiquidlliquid and liquid/vapor interfaces Proper understanding of the behavior of arnphiphilic molecules at liquid (in particular at water) interfaces is crucial in order to obtain an insight into processes such as lubrication, detergent action and molecular self-assembly [14]. Recently, there have been several experimental studies of monolayers at the water/vapor interface which provide data to test the computer simulations. For example, vibrational sum frequency generation [15] and neutron reflection studies [16,17] are carried out on monolayers of SOS and also on other types of surfactants. There are also a number of 1\10 studies reported over last few years on different amphiphiles at water/vapor interfaces. Recently, Berkowitz and co-workers [18·] have reported 1\10 simulations to study the effect on the

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Figure 1

Curr~n t

O Plf1 .o11 In Colloid & lnlerlacf'I S Clfloce

Snapshot of the final configuration of two cylindrical SOS micelles taken from an MO simulation of SOS micelle in mesophase E. The surfactant head groups are highlighted and the counterions are shown as black spheres. Water molecules have been omitted for visual clarity and the simulation cell boundaries have been outlined.

properties of SOS of various substances at interface. They carried out two simulations, one at the water/vapor (mostly water) interface and the other at the water/CCl 4 interface. From the simulation they observed a significant difference in the configurational properties of the amphiphile at the two interfaces. The preferred orientation of the surfactant molecules at low surface coverage was different in the two systems. The water/vapour interface provides an attractive surface for the hydrocarbon tail of the amphiphile and therefore it prefers a bent conformation. This allows the head group region to be inserted into the aqueous phase and achieve maximum solvation while the hydrocarbon tail lies down flat at the interface maximizing its van der Waals interaction with the surface water. In contrast to this, the van der Waals interaction between the SOS and the solvent is rather uniform, which was indicated by the fact that on average the chains prefer to remain straight at the water/CCl4 interface with an inclination of about 40· from the surface normal. Urbina-Villalba et 01. [19] have studied the interfacial behavior of a model heptane-water system in the presence of nonylphenol triethoxylated surfactants. They reported the dependence of the superficial energy and entropy of the system as a function of surfactant concentration using an RUV-2 model [20] of ternary oil-water-surfactant systems. They also found that the surface free energy changes linearly with temperature but shows a minimum with respect to the concentration of the surfactant.

Solid/liquid interfaces Understanding the adsorption mechanism of surfactant molecules at the solid/liquid interface is a key is-

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Thermodynamic and theoretical aspects

sue towards modeling technological processes that use arnphiphilic molecules, such as detergency, lubrication and water purification. The self-assembly of amphiphilic molecules in bulk solutions have been well-studied; however, less is known about how the aggregation process of surfactant molecules is influenced by the presence of a solid boundary surface. Recent studies of self-assembled structures formed by different types of surfactants (anionic and cationic) at solid/liquid interfaces using the atomic force microscope (AF~l) have revealed that many of the novel aggregate geometries observed in bulk solution also occur near-an interface [21,22,23·,24"]. AF~1 images show interfacial aggregates with a high degree of curvature and periodic structure, in contrast to the assumption by the classic textbooks that all monolayers and bilayers were flat. The surface has a strong influence on the organization of these amphiphiles. The interfacial structure of the cationic surfactant tcrradecyltrimethylamrnoniurn bromide has herni-cylinders on hydrophobic surfaces (such as graphite), full cylinders on mica and spheres on silica [22]. To date, no computer simulation study has attempted to use a fully atomistic model to study such interfacial aggregation, mainly due to the large time-scale involved in such processes. Recently, Bandyopadhyay et 01. (unpublished data) have carried out a lengthy 1\10 simulation to characterize the aggregate structure of arnphiphiles at a hydrophobic surface/water interface. In particular, they have studied molecular organization of cetyltrimethylarnmonium bromide at the interface of a flat hydrophobic surface and water. Their simulation system consisted of two such surfaces separated by a distance of 70 A. One of the surfaces was covered with 94 surfactants initially arranged in a monolayer, while the other surface was covered with two herni-cylindrical micelles each containing 64 monomers (at a surface coverage of 45 A2 per molecule). The surfactant layers were then separated by a water layer with appropriate dimensions. The simulation was carried out at constant volume and temperature. The initial configuration and the configuration after 500 ps of the system is shown in Figure 2. It was observed that the monolayer at the interface rearranged into roughly herni-cylinder micelles, while the initial herni-cylinders remain almost intact with slight deformation in shape. The simulation suggests that cationic surfactants like ceryltrimethylammonium bromide form stable herni-cylindrical aggregates at hydrophobic surface/water interfaces, which is in agreement with AF~1 measurements [21]. Recently, Wijmans and Linse [25] have reported Monte Carlo simulations using a lattice model to study the adsorption of arnphiphilic oligomeric surfactant molecules at hydrophobic interfaces. They observed that when the surfactant solution is brought into contact with a hydrophobic surface, the surfactant tails, which are hydrophobic in nature, get adsorbed by this surface

Figure 2

(a)

..,.

.11I b

Profile snapshots of the (a) starting configuration, and (b) the configuration after 500 ps, obtained from an MD simulation of cetyltrimethylammonium bromide (C 16TAB) confined between two hydrophobic surfaces. The surfactant head groups are the black spheres. Water molecules and counterions have been omitted for viewing clarity. The hydrophobic surfaces are at the top and bottom of the simulation cell, which is indicated by a box.

with hydrophilic head groups sticking out into the bulk solution.

Bilayers in confined geometries A detailed understanding of the properties of thin surfactant films within confined geometries is technologically important in different processes such as lubrication, adhesion, friction and modification of surfaces [26,27]. It is well-known from experiments [28,29] that the rheology of thin lubricants can be significantly different from that of the bulk. Kong et 01. [30··) recently carried out non-equilibrium 1\10 simulations on bilayers of dioctadecyldimethylarnrnonium chloride adsorbed between two solid surfaces. This cationic surfactant is commonly used as fabric softner,

Computer simulation studies of amphiphilic interfaces Bandyopadhyay, Tarek and Klein

They studied the behavior of one ordered monolayer of the surfactant adsorbed on a surface as it moves across a second layer of adsorbed surfactant. These simulations were performed at room temperature with a fixed wall separation at different shearing velocities (between 1 ms- I and 100 ms- I ). The interlayer force in the direction of the shear is measured and used to estimate a friction coefficient which compared well with experiment. The friction coefficient was found to decrease with increasing normal force, increase with decreasing amphiphile density, and increase with increasing shear velocity.

Lipid bilayers X-ray and neutron diffraction plus deuterium nuclear magnetic resonance (2H NMR) are powerful tools to obtain general structural information on biomembranes [31,32]. Experimental limitations along with the increase of computer CPU performance have encouraged implementation of novel computational methods in the study of biomembranes. Two recent reviews [33,34"] discussed the progress in this field in detail. We therefore restrict our discussion on publications that appeared last year.

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that even though the temperature, hydration level, and surface area per lipid for DOPC are lower than for DPPC, the two systems are almost identically disordered in the bilayer interior.' A strong interaction of the double bond in DOPC with water was indicated; this is responsible for membrane permeability and for stabilizing complex lipid-protein structures. Hyvonen er al. [38""] have reported a lengthy l\ID simulation study to examine the structure and dynamics of biologically important di-unsaturated I-palmitoyl-2-linoleoyl-sn-glycero-3 phosphatidylcholine bilayer. The structural properties of the phosphatidylcholine headgroup, the glycerol backbone, and the hydrating water were investigated. They have also studied in detail the structure of the double bond region and the effects of the diunsaturation on the bilayer interior. Lipids with double bonds in their alkyl chains are commonly found in all biomembranes. These simulations give the first atomistic descriptions of the interface between the lipids and water. The most important finding is that the double (unsaturated) bonds like to sample the interfacial region (water).

Conclusions Because of inadequate forcefields most of the previous MD simulations on lipids were restricted to those with saturated chains, namely dipropyl phosphatidylcholine (DPPC) and dimethyl phosphatidylcholine (D~IPC) [35,36]. Feller et al. [35] have performed a series of ~ID simulations on fully hydrated liquid crystalline DPPC bilayers as a function of molecular surface area. Their results support the value of 62.9 A2 per DPPC molecule as obtained from X-ray data. Hydrogen bonding between water and phosphatidylcholine was studied by Gierula et al. [36] using an MD simulation of a hydrated D~IPC bilayer membrane in the liquid crystalline phase. They observed that on average for each D~IPC molecule there were 5.3 H-bonds formed between water and D~IPC oxygens, four of these involved non-ester phosphate oxygens, 0.9 carbonyl oxygens and the remaining involved ester phosphate oxygens. They also showed the formation of many water bridges between D~IPC molecules to generate clusters. Recently, Husslein et al. (unpublished data) have carried out a constant pressure and temperature ~I 0 simulation of diphytanolphosphatidylcholine lipid bilayer in a hydrated liquid crystal phase. This is an important lipid which is widely used in the study of membrane channel activity. Their simulation results, (based on current generation force-field) for the electron density profiles agree reasonably well with X-ray diffraction data': Only recently have there been attempts to simulate unsaturated lipid biIayers, which are essential structural components of biomembranes. Feller et al. [37""] have proposed a potential energy function (force field) for unsaturated hydrocarbons and carried out 1\10 simulations of a water/octcne interface and a dioleoyl phosphatidylcholine (DOPC) bilayer at a low hydration of 5.35 water molecules per lipid. Comparison between DOPC and DPPC showed

The currently available simulation methodologies have the ability to offer visual images of amphiphilic interfaces. These images are a powerful complement to real experiments. In many of the examples cited above there is quantitative agreement with experiment, which gives added confidence to the simulators. The successes to date indicate that the use of atomistic computer simulations as a means of probing the behavior of complex fluids will inevitably continue.

Acknowledgements The authors acknowledge support from the National Institute of Health, the National Science Foundation and the Procter & Gamble company,

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19.

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1996. 34. ••

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Feller SE, Yin D, Pastor RW, MacKerell AD: Molecular dynamics simulation of unsaturated lipid bilayers at low hydration: Parameterization and comparison with diffraction studies. Biophys J 1997, 73:2269-2279. This paper reports MD simulation of unsaturated lipid bilayers, in particular, DOPC bilayer at low hydration._The important observation was the dramatic effects of unsaturation on bilayer structure. Hyvonen MT, Rantala n, Korpela MA: Structure and dynamic properties of diunsaturated l'palmitoyl-2-linoleoyl-sn-glycero3-phosphatidylcholine lipid bilayer from molecular dynamics simulation. Biophys J 1997, 73:2907-2923. The authors report MD studies of a diunsaturated lipid bilayer 1-palmitoyl-2linoleoyl-sn-glycero-3-phosphatidylcholine, which is known to be an essential part of biological membranes.They studied in detail the influence of the diunsaturation on the bilayer interior. 38. ••