Sodium Dodecyl Sulfate (SDS) - American Chemical Society

Jan 9, 2009 - YereVan, Armenia; Institute of Applied Problems of Physics of National ... 25, 0014 YereVan, Armenia; and Institut für Chemie, UniVersi...
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J. Phys. Chem. B 2009, 113, 1303–1310

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Molecular Dynamics Study of Poly(diallyldimethylammonium chloride) (PDADMAC)/ Sodium Dodecyl Sulfate (SDS)/Decanol/Water Systems Armen H. Poghosyan,*,† Levon H. Arsenyan,‡ Hrant H. Gharabekyan,† Joachim Koetz,§ and Aram A. Shahinyan‡ International Scientific-Education Center of National Academy of Sciences, M. Baghramyan aVe. 24d, 0019 YereVan, Armenia; Institute of Applied Problems of Physics of National Academy of Sciences, Hr. Nersessian str. 25, 0014 YereVan, Armenia; and Institut fu¨r Chemie, UniVersita¨t Potsdam, Karl-Liebknecht-Strasse 24-25, Haus 25, 14476 Potsdam (Golm), Germany ReceiVed: July 16, 2008; ReVised Manuscript ReceiVed: December 11, 2008

We have performed a 50 ns of molecular dynamics study of poly(diallyldimethylammonium chloride) (PDADMAC)/sodium dodecyl sulfate (SDS)/decanol/water systems. The influence of the cationic polyelectrolyte on the anionic SDS-based lamellar liquid crystalline system was investigated. The main structural parameters have been calculated and compared with experimental data. We obtain two types of PDADMAC conformation, a more folded structure A and a structure B where the PDADMAC molecule is adsorbed at the anionic head groups of the surfactant molecules. The polyelectrolyte-induced coexistence of two lamellar phases at a concentration of 2-3% of PDADMAC is observed, which is in agreement with experimental findings. I. Introduction Lyotropic liquid crystalline (LC) phases are formed upon the dissolution of amphiphilic surfactant molecules in a solvent, e.g., water.1 Biological membranes are a well-known example ofself-assembledphospholipid-basedLCphases,andprotein-phospholipid interactions play an important role in such biomembranes.2,3 Molecular dynamics (MD) simulations were already successfully used to characterize phospholipid bilayer structures in more detail.4 Generally speaking, polymers can be incorporated into the LC phases and change the properties. This is of special interest in different fields of application, for example, in medicine, cosmetical formulations, food processing, etc.5,6 Depending on the type of polymer used, the polymer can be solubilized in the hydrophobic or hydrophilic part of the bilayer system or between the two of them.7-13 Therefore, the bending rigidity of the bilayer and the swelling behavior can be influenced by the added polymer.14 For the investigation of polymer bilayer interactions different experimental techniques, e.g., optical microscopy with crossed polarizers,10 freeze-fracture electron microscopy,9 cryoscanning electron microscopy (CryoSEM),13 rheology,15 and NMR spectroscopy,16 and particularly small-angle scattering methods like small-angle X-ray (SAXS)9-11 and small-angle neutron scattering (SANS) can be used.15,17,18 Recently, more attention was paid on the role of electrostatic interactions in complex systems consisting of surfactants and chargedpolyelectrolytes.18,19 Inthiscontext,polyelectrolyte-surfactant complexes consisting of polystyrenesulfonate (PSS)/poly(acrylic acid) (PAA) and various surfactants have to be mentioned here, examined by Antonietti and co-workers,20 as well as complexes with poly(diallyldimethylammonium chloride) (PDADMAC).21 * Corresponding author. E mail: [email protected]. † International Scientific-Education Center of National Academy of Sciences. ‡ Institute of Applied Problems of Physics of National Academy of Sciences. § Universita¨t Potsdam.

Following the pioneering contributions by Ekwall,22 Friberg et al. have presented extensive work on the phase behavior of systems consisting of surfactant/water/long-chain alcohol.23,24 At a later time, polymer-modified systems have been intensively investigated by Koetz et al.9-13,25 The results show that, for example, the cationic polyelectrolyte PDADMAC can influence the swelling behavior of the bilayer in a characteristic way. That means, at a PDADMAC concentration >4% (by weight in the aqueous phase), a nonswelling bilayer with a constant interlayer spacing of 4 nm is observed. When the polymer concentration is fixed at 2-3% two corresponding interlayer spacings can be detected, i.e., a swelling and a nonswelling one.25 However, one can understand this unusual behavior much better in combination with microcalorimetric measurements. The results show a temperature- and polymer-dependent transition from a more swollen to a more compact multilamellar vesicle phase.17 Moreover, the coexistence of two lamellar phases is still a not well-understood phenomenon. Molecular dynamics studies are a new approach to get a more detailed physical insight in such self-assembled, multicomponent structures. During the past decade, the molecular dynamics simulations (MD) have become a great tool for the theoretical analysis of complex systems. The MD study of surfactant systems (micellar and lamellar) have been reported;4,26-29 however, the complex systems in the presence of polyelectrolytes remain poorly studied. It should be noted that the conformational properties of the PDADMAC molecule can be determined by combining MD and Monte Carlo (MC) methods,30 investigated a 40 repeating unit oligomer of PDADMAC with 40 chloride counterions in water solution, consisting of 500 water molecules. Taking this knowledge into account, the aim of this paper was to study molecular dynamics of the PDADMAC molecule in the liquid crystalline phase in more detail. Computer simulations seem to be useful to verify different conformations of the cationic polyelectrolyte in the LC phase. Therefore, the authors perform a MD study of the Na dodecyl sulfate/decanol system in aqueous solution (about 10 000 water molecules) with

10.1021/jp806289c CCC: $40.75  2009 American Chemical Society Published on Web 01/09/2009

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and without the PDADMAC molecule, and overall, about 60 ns of MD simulation was done. Finally, the structural parameters have been examined and compared to experimental findings of a coexistence of two different types of lamellar liquid crystalline phases, a swelling and a nonswelling one. II. Construction and Simulation Data The system was built in two steps: first, we took the already minimized 512 SDS bilayer with 512 chloride counterions27 from previous simulation and added 30 decanol molecules between leaflets. The second step was the insertion of two already prepared PDADMAC molecules containing 30 monomer units with 30 chloride counterions onto the surface of the bilayer. The decanol and PDADMAC molecules were constructed using Hyperchem (Hypercube Inc.) software. The PDADMAC molecule was constructed in vacuum by aligning 30 monomers. Therefore, the fcorresponding molar mass is 4.845 g/mol. A further few steps were done for equilibration. The created system was inserted into water bulk containing 10.000 water molecules of TIP3P water model.31 That means that the parameters used here are comparable to a polymer concentration of 2.6% and to the water:SDS decanol ratio of 1.18:1. It has to be mentioned here that these conditions are comparable with our SAXS and SANS data given in refs 17 and 25. The temperature was set to 298 K and the pressure to normal 1 atm. The pressure and temperature were controlled using Langevin dynamics32 with damping (or coupling) coefficient of 1 ps-1 and modified Langevin piston33 method, respectively. The PME34 with the precision of 10-6 was used for long-range electrostatic interactions and the van der Waals forces were cut off at 12 Å. The bonds were fixed at their equilibrium distances using SHAKE35 algorithm. The coordinates and velocities were saved every 0.1 ns, and the molecular graphics were built using VMD36 packages. After the energy minimization (1000 steps of Newton-Raphson algorithm), the final system was subjected to 50 ns MD simulation run with the 1 fs time step in NPT ensemble. In order to compare some features, we removed the PDADMAC molecule with chloride counterions and an additional 10 ns experiment was done on the SDS/decanol/water system. The simulations and energy minimizations were performed using NAMD software code with CHARMM27 all-atom modified force field.37 The decanol and PDADMAC molecules’ force fields were generated using Dundee PRODRG server38 with some modification. The entire simulations were performed in parallel on a Linux cluster. III. Results and Discussion The most important parameters describing the system and determining the structure of bilayer are the area per molecule and the interlayer spacing. These parameters can be compared with experimental data, which are available for the abovementioned systems.17,25 In Figure 1 the average area per molecule as a function of the simulation time is shown. The area per molecule is calculated simply by dividing the 〈x-y〉 plane projection of the mean area of leaflet 〈S〈x-y〉〉 by half the number of surfactants in the bilayer. As shown in the plot, the value of the area per molecule fluctuated from 0.274 to 0.288 nm2 and at the end of simulation it reached 0.281 nm2 (Figure 1). It should be noted that in the case of pure SDS/water system, the MD simulation result of area per molecule is about 0.40 nm2.39 At the end of the MD run, we removed the PDADMAC molecule and observed the decrease of area per molecule from 0.281 to 0.276 nm2 during the additional 10 ns of MD simulation (Figure 1). It is obvious that the presence of polyelectrolyte

Figure 1. Area per molecule as a function of simulation time, in the presence and absence of PDADMAC.

Figure 2. System thickness, i.e., the interlayer spacing, depending on simulation time, in the presence and absence of PDADMAC.

suppresses the SDS headgroup surface and correspondingly increases the area per molecule. The next parameter is the system thickness, i.e., the interlayer spacing, which is the bilayer thickness plus the water layer. The thickness of the system fluctuated from 8.1 to 8.6 nm and in the last 10 ns it equilibrated at 8.35 nm (Figure 2). After the 10 ns of MD run without PDADMAC molecules, one can observe the sharp decrease of the interlayer spacing up to 7.7 nm, and then the value increases, reaching up to 8.21 nm (Figure 2). By means of SAXS experiments, an interlayer spacing of 6 nm can be determined at a comparable water:SDS decanol ratio of 1.18:1.17 It has to be mentioned here that an interlayer spacing of 8 nm can be detected experimentally only at a significant higher water content of about 2.3:1.17 After incorporation of PDADMAC, we experimentally observed under the conditions given here (2.6% polymer concentration) the coexistence of two Bragg peaks corresponding to an interlayer spacing of about 6 nm (swollen structure) and 4 nm (nonswollen structure). In addition, our SANS experiments have shown that at a polymer concentration of 3% the transition from the nonswollen to the swollen state can be induced by increasing the temperature from 15 to 20 °C. We have also estimated the so-called sulfur-to-sulfur distances as a function of time. This parameter is associated with the bilayer thickness. In Figure 3 the bilayer thickness in the range of 3.6-4.3 nm is monitored. From the equilibrium value of 3.85 nm one can conclude that the water layer is about 4.5 nm. In Figure 3 the sulfur-to-sulfur distance in absence of the PDADMAC molecule is shown, and as a result, a slight increase of the value up to 4.05 nm is observed. However, this value is in perfect agreement with the Bragg peak of the nonswollen structure.17 As already mentioned above, it is conditioned by

PDADMAC/SDS/Decanol/Water Systems

Figure 3. Sulfur to sulfur distance depending on simulation time, in the presence and absence of PDADMAC.

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Figure 5. Decanol mean layer depending on simulation time, in the presence and absence of PDADMAC.

Figure 6. (a) Percent of upright located decanol molecules depending on simulation time. (b) Mean angle between the vector connecting the first and last atoms of upright decanol and bilayer normal versus a simulation time.

Figure 4. Radial distribution functions from (a) SDS’s sulfur to sodium atom and (b) SDS’s sulfur to water oxygen.

the absence of PDADMAC molecule, and due to hydrophilicity of polar head groups, we assume that sulfur atoms of the SDS molecule fill the so-called “PDADMAC caused void”. It should be noted that S-to-S parameter is calculated using the mean sulfur surface for each leaflet. However, the sulfur surface is strongly bumpy, and where the polyelectrolyte is located, the neighbor SDS molecule is penetrated into the bilayer core, and correspondingly in this region the bilayer thickness is reduced twice up to 2.2 nm. In order to see the counterion distribution, we have calculated the radial distribution function from sulfur to sodium counterions, and according to the RDF curve, only one peak appears at 0.4 nm and about 75% of sodium counterions are bound in the first shell, which is a hint for sodium counterion condensation. In Figure 4a (solid line) the RDF curve is shown. As for the SDS/decanol/water system, a sharp peak appears at 0.3 nm (Figure 4a, dashed line), which indicates that more than 80% of counterions are in the first shell. In case of the pure SDS/water system, two peaks at 0.4 and 0.6 nm are observed.27 To quantify the hydration, the RDF for sulfur and water oxygen was calculated and shown in Figure 4b. The profile exhibits a solvation peak at about 5 Å in the presence

of PDADMAC and about 2 Å for the system without polyelectrolyte. To understand the changes of system thickness and the sulfur-to-sulfur distance, we have also estimated the mean decanol layer thickness changes. As shown in Figure 5, the decanol layer is reduced from 1.8 to 1.2 nm. Moreover, the additional 10 ns of MD simulation without PDADMAC leads to the further decrease of the decanol layer up to 1.05 nm (Figure 5). One can see that horizontally lying decanol molecules move to the hydrocarbon core of the bilayer, and therefore the number of decanol molecules is reduced and, correspondingly, the thickness of the decanol layer is decreased. At the end of simulation, about 60% of decanol molecules come to the “upright” position (see Figure 6a) and the decanol molecules are located between the SDS methyl group layers. It has to be noted that most of the oxygen atoms of the decanol are directed close to the head of the SDS molecules, and meanwhile the terminal group of decanol is in the middle of hydrocarbon chains. The average angle between the vector connecting the first and terminal atoms of “upright” decanol and the bilayer normal ranges from 30 to 15 over the simulation time (see Figure 6b). The percent of the upright decanol molecules increases up to 67%, and the average angle between the vector connecting the first and terminal atoms of “upright” decanol and the bilayer normal also decreases in the case when we remove the PDADMAC molecule (data not shown). From curves in Figures

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Figure 7. Density profile for the system SDS, decanol, and water molecules in the presence of the PDADMAC.

3 and 5, we can estimate also the length of SDS molecules and get information about the ordering of the hydrocarbon chains. Hence, we obtain the tail unit length as 1.35 or 1.5 nm, respectively with and without polyelectrolyte. If we consider the length (L) of a SDS molecule in all-trans configuration, which consists of n hydrocarbon atoms, L can be defined as

L = (1.265n + 3 Å)

(1)

where 3 Å is the length of the head of surfactant molecule, n is the number of carbon atoms and equals 12, and the length of SDS molecule equals 1.9 nm. Hence, we have obtained well disordered and melted chains for both cases, as to be seen from snapshots below. In order to estimate the behavior of hydrocarbon chains, the average hydrocarbon chain orientation order parameter has been calculated, which is defined as mol Szz )

3 〈cos2 Ri〉 - 21 2

(2)

where Ri is the angle between the z-axis of the simulation box and the molecular axis, defined as a vector from Ci-1 to Ci+1 carbon atom and the brackets denote the ensemble and time average. Assuming the axial symmetry in the segment motion we have calculated the SCD parameter, which is related to the mol . Together with the SDS orientation parameters as -SCD ) Szz order parameter, the data from the pure SDS/water system have been also compared (compare Supporting Information, Figure S1). The carbon atom position ranges from 0.345 for C1 up to 0.29 for the terminal carbon atom C12. These values are almost twice higher than the order parameter values obtained from pure SDS/water systems. The order parameter profile shows a plateau region from C1 to C11 and slightly shifting toward the end of the chain. Due to the lack of experimental data on the abovementioned system, we have no comparison with experimental NMR data. We have also computed the densities of various components of the system, including SDS, sodium, decanol, polyelectrolyte, and different atoms of the system. The curves are plotted by averaging all the conformation from MD trajectories. As one can see from Figure 7, system density maxima correspond with the PDADMAC and SDS sulfur maxima (Figure 8a). Therefore, one can assume that the system density maximum appears at the interface due to the contribution from the SDS and PDADMAC molecules. In the middle part of Figure 7, where the decanol molecules are located, the value decreases and a lower density is occurred. However, the values of the terminal section are strongly influenced by the bulk water.

Figure 8. Densities of different components of the system in the presence of the polyelectrolyte.

From the distribution of the PDADMAC molecule and sulfur atoms, one can conclude that the PDADMAC molecule is located near the sulfur plane (Figure 8a), meanwhile the chloride counterions are in the bulk water, as to be seen from the distribution profiles of sodium and chloride counterions (Figure 8b). Together with two sharp peaks, the sodium distribution profile also shows that the terminal backbone corresponds to chloride peaks. Therefore, one can assume electrostatic “bridges” between sodium and chloride atoms, i.e., negatively charged chloride atoms are bound to positively charged sodium atoms. From the sulfur and sodium atoms peak position one can conclude that most of the sodium counterions are located in the first shell, underlined by the radial distribution function (Figure 4a). The density profile of the C12 terminal atoms of hydrocarbon chains shows that the peak-to-peak distance is about 1.3 nm (Figure 8c), which is nearly identical to decanol layer thickness. It should be noted that in the system without PDADMAC the peak-to-peak distance equals to 1.1 nm (Figure 9b). The removal of the PDADMAC molecule leads to a decrease of the decanol layer thickness as clearly demonstrated in Figure 5. The density curves of the system and different

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Figure 10. Characteristic relation of PDADMAC depending on simulation time.

Figure 9. Densities of different components of the system in the absence of the polyelectrolyte.

fragments from SDS/decanol/water system are almost similar to that of the SDS/PDADMAC/decanol/water system, whereas a small shift of peaks and the hydrocarbon volume is obvious. After removing the PDADMAC molecule with chloride counterions, we see that the sodium counterions, which were bound to chloride, are moving from water bulk toward the bilayer interface, and we do not see the terminal backbones (Figure 9b). From the water density profile it can be concluded that the concentration of water molecules inside the hydrocarbon chains is very small, and the interior of about 2 nm length is devoid of water molecules at all, which can be also concluded from the RDF curve in Figure 4b. The decanol molecule’s oxygen distribution profile provides three peaks. The higher density peak was received due to the contribution of horizontally located decanol molecules at the middle part of the hydrocarbon volume; meanwhile the lower peaks correspond to the upright located decanol molecules (oxygen toward to the interface) (Figure 8c). As mentioned above, the removal of the polyelectrolyte leads to the increase of upright situated decanol molecules and therefore the lower peaks gain higher values (Figure 9b). In addition, we have examined the polyelectrolyte properties in more detail. Therefore, we calculated the characteristic relation Cn, the radius of inertia, as well as the end to terminal distance. One of the most important parameters characterizing the behavior of the unperturbed long-chain polymer without excluded-volume interactions between monomers is the characteristic relation Cn, which is formulated as40

Cn ≡

〈r2〉0 nl2

(3)

where r is the start-to-end vector, n is the number of monomers, and the l is the length of monomers. It has to be mentioned here that in addition the chain moments 〈r4〉0 and 〈r6〉0 have to be taken into account. In that case the relation becomes 〈r4〉0/

n2l4, and in contrast to Cn, it will depend on n. However, with increasing n, these relations are reaching asymptotic values. In Figure 10 the characteristic relation curves have been monitored, estimated from both PDADMAC molecules (from both layers). As to be seen from the curves, we have two various kinds of polyelectrolyte molecules. The values of PDADMAC A decrease starting from the 5 ns point and visually we see that the PDADMAC A molecule becomes more folded. At the end of simulation (50 ns), the PDADMAC A terminal groups are folded into a “ball” (see the Supporting Information, Figure S2). In contrast to PDADMAC A, the characteristic values for PDADMAC B remain almost in the same range (the linear fitting is applied, dotted line in Figure 10). One can conclude that the PDADMAC B is adsorbed onto the bilayer surface due to electrostatic interactions between the positively charged quaternary N-functions of the PDADMAC and the negatively charged sulfate head groups of the SDS molecules. Similar conclusions can be drawn from the PDADMAC distribution profile (Figure 8a). For PDADMAC A we receive a lower peak value with a broader peak width, and for PDADMAC B the profile shows a higher peak with narrow width. The radius of inertia of the two PDADMAC molecules has been also calculated. The radius of inertia is given as

s2 ) (n + 1)-1

n

∑ si2

(4)

0

where si is the distance of atom i from the center of mass and the n the number of monomers. The radius of inertia characterizes the configuration of the polyelectrolyte. The curves of radius of inertia depending on simulation time are shown in Figure 11. Similar effects can be observed, which means a folding of PDADMAC A, corresponding to a top down from 8.2 to 6.7 nm2, whereas the value of PDADMAC B is still constant and quite the same. To see the changes by z-axis and to estimate the penetration degree, we have calculated the time-averaged distance of PDADMAC’s nitrogen atom from the center of mass (COM) of the system. The plot is shown in Figure 12. The maximum of nitrogen atoms of PDADMAC A molecule along 5 z-axis is about 2 nm (IA ) N30 z - Nz ), which means the PDADMAC A molecule partly penetrates into the bilayer, meanwhile the nitrogen maximum of PDADMAC B along z-axis is about IB ) 1 nm, which claims the adsorption of the PDADMAC molecule B at the anionic surface of the SDS bilayer. To clarify the surface properties, we have defined the

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Poghosyan et al. ξ(Rf∞) ∼ 2.7 Å for layer B and about ∼4 Å for layer A. The removal of the PDADMAC molecule leads to the value of about ∼3.2 Å. One can conclude that the high value of the roughness function is caused by the behavior of polyelectrolyte molecule at the surface. Particularly, this can be also seen in the snapshots given in Figure 14. Moreover, the interdigitation and tilting of surfactant methyl groups, and the orientation of decanol and water molecules, are to be seen. In addition, the water properties were investigated as well as the water density and water mean cosine (data not shown). IV. Conclusions

Figure 11. Radius of inertia of PDADMAC depending on simulation time.

Figure 12. Time-averaged distance of PDADMAC’s nitrogen atom from COM of the system.

Figure 13. Roughness function calculated from two layers, as well as the average value from the system in the absence of PDADMAC.

surface roughness function, i.e., the vertical displacement of SDS molecules, which is represented as

b) ) √〈(z(b) ξ(R r - z(b r +b R))2〉

(5)

where the z(r b) and z(r b+ b R) are the z-axis coordinates of two sulfur atoms in the same layer. In Figure 13, the plots of the surface roughness function from two layers are shown, in comparison with the average value of the system without PDADMAC. The PDADMAC A molecule is located in layer A and correspondingly PDADMAC B in layer B. The theoretical calculation of this function in pure SDS/water system is about 2.4 Å41 and the experiment claims a value of 2.7 Å. Our calculation shows that the roughness function gets the value of

We have performed the MD simulation of a complex system, consisting of a cationic polyelectrolyte and an anionic surfactantbased LC phase. The results from the MD simulation provide us information on the dynamical and structural features of the abovementioned complex system. In addition, conformational properties of the polyelectrolyte and the surfactant molecules become available. The attractive nature between the anionic SDS bilayer and the cationic PDADMAC molecule leads to the so-called “screening” of the bilayer and maintains the sulfur-to-sulfur distance at 4 nm. However, this is in perfect agreement with the nonswelling interlayer spacing determined by SANS measurements.17 During the simulation, the horizontally located decanol molecules are moved between the hydrocarbon chains, with the oxygen of the decanol directed to the sulfate head groups of SDS molecule. At the end of a 50 ns run, over 60% of decanol molecules reorient to lie perpendicular to the SDS bilayer surface. The process of reorientation of decanol molecules continues even when we remove the PDADMAC molecule and up to 67% of decanol molecules move to the hydrocarbon volume during the additional 10 ns MD run. “Electrostatic bridges” form between sodium and chloride counterions in bulk water; i.e., negatively charged chloride counterions of the PDADMAC are bound to positively charged sodium counterions of the SDS. By removing the polyelectrolyte with chloride counterions, sodium counterions move close to the sulfur head-group surface. The order parameter profile of SDS methyl chains shows a plateau region. One part of the SDS chains is folded and the other part is penetrated (telescoped SDS chains). However, some random package and tilting is also observed. In the region where the polyelectrolyte is present, the interdigitation of chains up to four carbons can be plainly seen. Polyelectrolyte parameters, like the characteristics relation, the radius of inertia, show two different types of PDADMAC molecules, i.e., type A and B. Whereas PDADMAC A is folded into a “ball”, PDADMAC B is adsorbed at the bilayer surface. The surface characteristics, roughness function, show, therefore, two different layers A and B. However, the existence of two different types of PDADMAC molecules A and B in a more folded as well as a more flat conformation and the formation of two different layers A and B are in full agreement with our experimental findings (SAXS and SANS results) of the coexistence of two lamellar phases. Based on the MD simulation, one can conclude that PDADMAC type B flat adsorbed on the bilayer with the lower roughness function (layer B) can be directly related to the experimental finding of a nonswelling, more compact LC phase, which tend to form multilamellar compact vesicles, as already shown by cryoscanning electron microscopy.13 However, the more folded structure type A, leading to the layer A of significant higher roughness, can be directly related to the coexistent swelling bilayer detected by

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Figure 14. Snapshots of (a) the last frame from the 50 ns experiment in the presence of PDADMAC and (b) the last frame from the 10 ns experiment without polyelectrolyte: blue, PDADMAC; red, decanol; cyan, C atoms; yellow balls, sulfur of SDS molecules.

1310 J. Phys. Chem. B, Vol. 113, No. 5, 2009 SAXS measurements.25 To the best of our knowledge, this is the first validation by means of computer simulation concerning the coexistence of two lamellar phases in surfactant-based complex systems induced by an oppositely charged polymer. Taking into account that the coexistence of the two lamellar LC phases strongly depends on the polymer concentration as well as the temperature, further MD simulations will be focused on the influence of the temperature and the polymer concentration. In addition, it seems to be of interest to extend the research to anionic and noncharged polyelectrolytes. Acknowledgment. The authors express gratitude to Dr. Vladimir Sahakyan for providing us with the access to the cluster (ARMCLUSTER). Supporting Information Available: Figure S1: the hydrocarbon chain orientation of SDS molecules. Figure S2: 50 ns time shot of PDADMAC A molecule’s terminal group, represented into WDW form. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Dubois, M.; Zemb, T. Curr. Opin. Colloid Interface Sci. 2000, 5, 27–37. (2) Lee, A. G. Biochim. Biophys. Acta, Biomembr. 2003, 1612, 1–40. (3) Marsh, D.; Horwarth, L. I.; Swamy, M. J.; Mantripragada, S.; Kleinschmidt, J. H. Mol. Membr. Biol. 2002, 19, 247–255. (4) Heller, H.; Schaefer, M.; Schulten, K. J. Phys. Chem. 1993, 97, 8343–8360. (5) Kwak, J. C. T. Polymer-Surfactant Systems; Surfactant Science Series; Marcel Dekker: New York, 1998; Vol. 77. (6) Rieger, M. M.; Rhein, L. D. Surfactants in Cosmetics; Surfactant Science Series; Marcel Dekker: New York, 1997; Vol. 68. (7) Freyssingeas, E.; Antelmi, D.; Ke´kicheff, P.; Richetti, P.; Bellocq, A-M. Eur. Phys. J. B 1999, 9, 123–136. (8) Singh, M.; Ober, R.; Kleman, M. J. Phys. Chem. 1993, 97, 11108– 11114. (9) Bechthold, N.; Tiersch, B.; Ko¨tz, J.; Friberg, S. E. J. Colloid Interface Sci. 1999, 215, 106–113. (10) Ruppelt, D.; Ko¨tz, J.; Jaeger, W.; Friberg, S. E.; Mackay, R. E. Langmuir 1997, 13, 3316–3319. (11) Kosmella, S.; Ko¨tz, J.; Friberg, S. E.; Mackay, R. A. Ber. Bunsenges Phys. Chem. 1996, 100, 1059–1063. (12) Kosmella, S.; Ko¨tz, J.; Friberg, S. E.; Mackay, R. Colloids Surf. A: Physicochem. Eng. Aspects 1996, 112, 227–231. (13) Ko¨tz, J.; Tiersch, B.; Bogen, I. Colloid Polym. Sci. 2000, 278, 164– 168. (14) Javierre, I.; Bellocq, A. M.; Nallet, F. Langmuir 2001, 17, 5417– 5425.

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