Molecular Conformation and Bilayer Pores in a Nonionic Surfactant

Article pubs.acs.org/Langmuir

Molecular Conformation and Bilayer Pores in a Nonionic Surfactant Lamellar Phase Studied with 1H−13C Solid-State NMR and Molecular Dynamics Simulations Tiago M. Ferreira,† Daniel Topgaard,† and O. H. Samuli Ollila*,†,‡ †

Division of Physical Chemistry, Centre for Chemistry and Chemical Engineering, Lund University, Lund, Sweden Helsinki Biophysics and Biomembrane Group, Department of Biomedical Engineering and Computational Science, Aalto University, Espoo, Finland

‡

S Supporting Information *

ABSTRACT: The structure of the lamellar phase of aqueous pentaethylene glycol mono-n-dodecyl ether (C12E5) surfactant at various temperatures and molar fractions is studied by using united atom molecular dynamics simulations and nuclear magnetic resonance measurements. Namely, the simulation model is used to interpret the magnitude and temperature dependence of experimental C−H order parameter profiles in terms of the molecular conformation and orientation. Our simulations suggest that the low order parameters that are generally measured in poly(ethylene oxide) surfactant bilayers are due to the presence of bilayer pores throughout the entire lamellar phase region.

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INTRODUCTION Nonionic surfactants of the oligo(ethylene oxide) monoalkyl ether type, with the chemical formula CH3(CH2)m−1(OCH2CH2)nOH, are widely used in scientific and industrial applications such as drug delivery, emulsification, and protein crystallography.1−3 A common abbreviation for these molecules is CmEn, where n denotes the number of hydrophilic ethylene oxide groups and m denotes the number of carbons in the hydrocarbon chain. Because of the simple molecular architecture of these surfactants and the wide range of CmEn grades commercially available in high degrees of purity, such surfactants have been extensively used in basic scientific studies to understand the fundamental properties of amphiphilic systems.4,5 Furthermore, controlling their phase behavior is of great interest for various applications.5 Phase diagrams for a large number of CmEn surfactant grades have been determined and reported previously.5,6 Complex phase behavior is normally observed, as depicted in Scheme 1 for the specific case of the C12E5/water system. The general features of the CmEn phase diagrams are well known; however, the detailed structure of the lamellar phase has been under discussion. On the basis of EPR and FRAP experimental measurements, it has been suggested that the lamellar phases of C12E5 and C12E6 contain bilayers with defects of some kind throughout the entire lamellar phase region.7,8 However, NMR and scattering experiments have been interpreted such that the lamellar phases of C12E5 and C16E6 contain bilayers with porous defects at low temperatures whereas above certain transition temperatures they would be defect-free.9−12 With the work here presented, we attempt to clarify such ambiguity. © 2013 American Chemical Society

The lamellar phases of the CmEn type of surfactants have also been studied with various simulation methods.13−19 By using simple statistical thermodynamical calculations, Klose and Levine proposed that the hydrophobic−hydrophilic interface is less well defined in the lamellar phase of C12E4 than in lamellar phases made of phospholipid molecules.13 Nonideal bilayers have later been seen in atomistic molecular dynamics (MD)14 and coarse-grained simulations.18,19 In particular, coarse-grained dissipative particle dynamics18 and molecular dynamics simulation studies with the MARTINI model19 indicate that the nonionic systems are prone to form transmembrane pores, which would fit with the results by Klose and Levine.13 However, clear lamellar sheets without defects are obtained in coarse-grained molecular dynamics simulation studies with other models.15−17 Porous defects are also discussed in more general terms in the block copolymer field, where those are usually considered to be metastable structures.20,21 However, some simulation studies suggest that porous phases may be thermodynamically stable under certain conditions.21,22 On the basis of magic-angle spinning (MAS) NMR experiments and united atom MD simulations, we suggest in this work that the lamellar phases of C12E5 and similar surfactants contain a high density of transmembrane pores in thermodynamic equilibrium throughout the entire lamellar phase region. Received: July 13, 2013 Published: December 27, 2013 461

dx.doi.org/10.1021/la404684r | Langmuir 2014, 30, 461−469

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The order parameters SCH were calculated by assuming a tetrahedral geometry for CH2 groups and using the equation SCH = ⟨3 cos2 θ − 1⟩/2, where θ is the angle between the C−H bond vector and the membrane normal. The membrane normal was along the z axis in all of the systems except S7, in which the lamellar phase direction was tilted with respect to the simulation box. For this system, the order parameters were calculated for all possible directions of the coordinate system, using the coordinate system orientation that maximized the order parameters rather than the original coordinates. The fraction of trans conformers was calculated as the probability of the dihedral angle to have values smaller than −120° or larger than 120°. The number of water molecules in the vicinity of the headroup segments was defined as the average number of water molecules located at a distance shorter than 0.35 nm from the segment to any water atom. The repeat distance in the systems with the symmetry axis in the z direction (i.e., all systems except S7) was calculated as the average box side length in the z direction. For the system S7, the water density distribution was calculated along the membrane normal direction that was determined by finding the orientation with the largest order parameters. Then the repeat distance was determined by calculating the distance between two water density peaks from this distribution. In practice, we calculated the distance between the water density maximum and minimum and then multiplied this by a factor of 2. Simulated Systems. We simulated seven different systems at three different temperatures and three different water/surfactant molar fractions, all within the lamellar phase region according to the phase diagram by Mitchell et al.6 as shown in Scheme 1 where the simulated systems are marked as 1−7. Details about the composition and time lengths of these systems are listed in Table 1. The files used to run the simulations are publicly available.34

Scheme 1. Chemical Structure of C12E5 and Phase Diagram of C12E5/Water Taken from Reference 6a

a

The phases are labelled as follows: (W) dilute surfactant solution, (L 1 ) micellar solution, (H 1 ) normal hexagonal phase, (V 1 ) bicontinuous cubic phase, (Lα) lamellar phase, (L2) reverse micellar solution, (L3) sponge phase, and (S) solid surfactant. The systems studied in this work are marked with red squares.

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Table 1. Details of the Simulated Systems

METHODS

Force Field and Simulation Details. All simulations were performed using GROMACS 4.5.23 We used a united atom force field reported previously by Shang et al.24 that is a modification of the GROMOS 45a325 force field with the partial charges for the ether oxygens taken from the united atom OPLS.26 Two parameters were tuned by Shang and co-workers in order to reproduce the hydration enthalpy and conformer populations of 1,2-dimethoxymethane: the Lennard-Jones C12 term between water oxygen and ether oxygen and the 1−4 interaction scaling parameter for the Coulomb interaction. In this work, the simulations are used mainly to interpret experimental NMR C−H order parameters, thus the choice of using the GROMOS 45a3 force field is supported by the previous demonstration that this force field reproduces well the experimental order parameters for acyl chains in lipid bilayers.27 The surfactants and water were separately coupled to a heat bath at the target temperature by using the v-rescale scheme28 with a coupling constant of 0.1 ps. The box was separately coupled in the horizontal and planar directions to 1 bar with semi-isotropic Berendsen coupling29 with a coupling constant of 1 ps. Preliminary simulations with isotropic pressure coupling and Berendsen temperature coupling led to the same conclusions as the current simulations. The simulations were also started with anisotropic coupling; however, in some systems the simulation box experienced major deformations even though the surfactant phase remained unchanged. In conclusion, it seems that the essential results are independent of the coupling scheme, thus we use the results with the semi-isotropic coupling in this work. The cutoff radius for the van der Waals interactions was set to 1.4 nm. Particle mesh Ewald (PME) summations30 were applied for the long-range electrostatic interactions with a grid spacing of 0.12 nm, and a cutoff radius of 1.0 nm was employed for the real space summation. The time step was 2 fs using LINCS31 to constrain all bond lengths of the surfactants, whereas the SETTLE32 algorithm was used for the SPC water.33 The neighbor list with 1 nm cutoff was updated every 10th step. The atom coordinates were saved every 10 ps.

system

NC12E5a

NWb

wt % C12E5c

T (K)d

tsim (ns)e

tused (ns)f

S1 S2 S3 S4 S5 S6 S7

512 512 512 512 512 512 512

7699 4951 3168 7699 4951 7699 4951

60 70 80 60 70 60 70

298 298 298 320 320 333 333

136 188 200 137 183 137 187

76−106 58−180 50−200 47−137 60−123 47−137 103−187

a

Number of surfactant molecules. bNumber of water molecules. Weight fraction of C12E5. dSimulation temperature. eTotal simulation time. fTime frames used in the analysis. c

Initially, we constructed a typical starting structure for a lamellar phase simulation containing 128 C12E5 molecules forming a flat (nonporous) bilayer hydrated with 792 water molecules. First, we simulated a C12E5 molecule in a vacuum at 300 K, picked up the most elongated configuration, and then constructed a bilayer with 128 copies of this molecule displaced with hexagonal packing throughout the bilayer plane. Then a short simulation was run with position restraints at C1, the system was solvated with water, and a subsequent simulation (less than 500 ps) was run. The water molecules located in the hydrophopic region were then removed. Two simulations, with isotropic and anisotropic pressure coupling, were started from this configuration, and a transmembrane pore was formed during the first nanoseconds of the simulations in both cases. From the total simulation times of 240 and 87 ns for these system, we found that the C−H order parameters were significantly greater than the corresponding experimental values (Scheme 2). At this stage, we suspected that the transmembrane pore formation was limited by the simulation box size, thus we decided to increase the size of the simulated lamellar patch to have a 4-fold larger area (from roughly 5 × 5 nm2 to roughly 10 × 10 nm2) by multiplying the initial structure. This structure was used as a starting structure for the system labeled S3. To create the initial structures for the rest of the systems represented in Table 1, we added water molecules to S3. These 462


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spectroscopy35 using exactly the same experimental setup and data processing as described in Ferreira et al.36

Scheme 2. Order Parameters from the Simulation with 128 C12E5 Molecules and 512 C12E5 Molecules Compared to the Experimental Results (80 wt % Surfactant, T = 298 K)

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RESULTS Lamellar Phase Structures from Molecular Dynamics Simulations. Representative configurations of the different systems are shown in Scheme 3. The final structure of the Scheme 3. Representative Configurations from MD Simulations of the C12E5/Water Systema

systems were simulated up to 136−200 ns (details in Table 1). Indeed, the pores experienced major deformations in all systems during the simulations. The most significant deformations were seen in systems S1 and S2 where two pores connected to themselves through the periodic boundary conditions after ∼110 and ∼180 ns, respectively. After this event, the systems resemble a hexagonal phase; however, there is not real hexagonal packing, thus we consider these systems to be in the hexagonal-like phase. Also in systems S5 and S7 major deformations were observed during pore fusion at ∼123 and ∼60 ns. However, in these systems the deformations did not lead to the hexagonal-like phase; instead a new lamellar-like phase with pores is formed, having its normal direction at an angle of roughly 30° with the z axis of the simulation box. This happens because the original lamellar phase locally breaks and then fuses with its own periodic image in the z direction. The rest of the systems remained in the lamellar-like phase with transmembrane pores throughout the whole simulation time. We cannot distinguish if the deformations arise because the model that is used is too prone to form hexagonal phases or if the simulation box size is simply too small in comparison to the natural size of the pores. The origin of such an unrealistic feature could be further investigated by increasing the box size even more. However, such a procedure would require a heavy computational cost (the systems that were used already had 512 surfactant molecules), thus we limited ourselves to use only simulation frames that matched our experimental order parameters properly. In other words, we used the simulations as a source of model representations to find which ones could fit better with our experiments. Therefore, we considered only the time frames of systems S1 and S2 that were consistent with a lamellar phase. If these structures reproduce the experimental observables, then they can be considered to be reasonable representations of the local structure of the lamellar phase of C12E5. For the S5 and S7 systems, we use only the data before and after the tilt of the lamellar direction, respectively. For the rest of the systems, we analyze the last 90−150 ns of the simulation (details in Table 1). The analyzed properties did not essentially drift during the time used for the analysis. NMR Measurements of Order Parameters. The NMR measurements were performed on samples made of pentaethylene glycol dodecyl ether (C12E5) with a purity greater than 99.8%, purchased from Nikko Chemical Co. (Tokyo, Japan). Samples containing 62, 70, and 80 wt % surfactant were prepared by weighing the desired amounts of surfactant and milli-Q water into vials and mixing them in a vortex mixer. Each sample was then transferred to a 4 mm HR-MAS rotor (Bruker) with a micropipet and centrifuged to remove air bubbles. The experiments were performed on a Bruker Avance AVII-500 NMR spectrometer operating at a 1H frequency of 500.23 MHz and equipped with a standard bore CP-MAS HX probe. All experiments were done at the spinning frequency of 5 kHz. The order parameters were determined by performing R-PDLF MAS NMR

a

The simulation boxes are shown as top and side views as illustrated in the top/right panel. The snapshots are shown for a series of temperatures and compositions within the lamellar phase as indicated in Scheme 1. C12E5 headgroups and tails are represented as red and blue chains, respectively. Water is not shown for clarity. The horizontal size of all of the simulation boxes is ∼10 nm. The files required to visualize the structures with VMD37 are publicly available.34

simulation is shown for systems S4, S6, and S7 whereas for systems S1, S2, and S5 a representative structure before transformation to the hexagonal-like phase or tilting of the lamellar direction (Simulated Systems section) is shown. The final structures of S1 and S2 systems in the hexagonal-like phase are shown in the Supporting Information. Most importantly, the structures presented in Scheme 3 give order parameters close to the experimental values (next section), thus the bilayer inside the simulation box can be considered to be a reasonable structural model for a bilayer patch of a real system. 463


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Scheme 4. Order Parameter Profiles, |SCH| versus Carbon Number, from a Lamellar Phase of C12E5 for Several Compositions and Temperatures, Calculated from MD Simulations (Red) and Measured with R-PDLF NMR Experiments (Black)a

a

The top right corner shows the carbon labeling scheme used.

Table 2. Repeat Distances Calculated from MD Simulationsa

All of the structures contain transmembrane pores. However, further conclusions about pore size, density, and shape cannot be made because of the limited box size. Furthermore, we cannot exclude the possibility that our structures are not yet in equilibrium and that the model that is used would predict some other structures with longer simulation times, larger systems, or more suitable initial configurations. In particular, systems S1 and S2 evolve into a hexagonal-like phase; however, we cannot determine if this is because of the model parameters or the small box size compared to the natural size of the pores. Order Parameters. The simulated and measured order parameters for all studied systems are shown in Scheme 4. The experimental and measured order parameters are in relatively good agreement for all of the systems. The largest differences between simulation and experimental values are roughly ∼0.03, which is significantly smaller than ∼0.06 for the simulation with a smaller box size that did not allow the pore expansion (Simulated Systems). Repeat Distance. The lamellar phase of C12E5/water has been previously studied by X-ray scattering and the results were interpreted as containing signs of the formation of a porous lamellar phase with local ordering of the pores.10 Unfortunately, we cannot calculate the full scattering pattern because we have typically only two pores per simulation box, thus the lateral organization beyond this is artificially periodic in our simulations as a result of the periodic boundary conditions. However, the lamellar phase repeat distances calculated from MD simulations, shown in Table 2, can be compared to the experimental values. The experimental values for C12E5, 4.46 nm (73 wt %, 293 K)7 and 4.2 nm (73 wt %, 298 K),38 are 0.1− 0.8 nm lower than the simulation values in Table 2. Temperature Dependence. The temperature dependence of the order parameters for C12E5 and C16E6 surfactants in the lamellar phase had been previously measured and interpreted

333 K 320 K 298 K a

60 wt %

70 wt %

80 wt %

4.8 4.9 5.1

5.1 4.8 5.0

4.6

The unit is nanometers, and the error bars are ±0.2.

by changes in the average molecular orientation9−12 or in the internal structure of the molecules.36 The latter explanation is related to the more general behavior of the conformations of poly(ethylene oxide) polymers.36,39−42 The measured and simulated order parameters shown in Scheme 4 for the systems with 70 wt % surfactant are replotted in Scheme 5 as a function of temperature for each carbon segment. All of these simulated systems are porous lamellar phases, with order parameters in good agreement with experiments (previous sections). Both the experiments and the simulations show a decrease in the order parameters of the alkyl chain with increasing temperature and opposite behavior for the headgroup. In the middle of the surfactant chain (between segments −8 and 1), the experimental order parameters first increase throughout a temperature increase from 298 to 320 K and then decrease from 320 to 333 K. Such a nonmonotonic trend is weaker in the simulations. However, the changes in the order parameters as a function of temperature are relatively small and comparable to the error bars in the simulation results. Possible explanations for the temperature dependence of the headgroup order parameters are changes in the fractions of trans and gauche conformers as well as the hydration level.36,39−42 To elucidate the issue further, we have also analyzed these properties from the simulations. Scheme 6 shows the fraction of trans conformers for each C−C bond 464


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Scheme 5. Temperature Dependence of the Segmental Order Parameters in a C12E5 Lamellar Phase (70 wt % Surfactant) as Predicted by MD Simulations (Red) and Measured with R-PDLF NMR Experiments (Black)a

a

Each panel shows data from one CH2 segment as indicated by the carbon indexes on top (labeling in Scheme 4). The size of the error bars for order parameters calculated from the MD simulations was defined to be the change in the order parameter due to the rotation of the membrane normal by 10°.

Scheme 6. Simulated (Red) and Experimental (Black) Fraction of Trans Conformers as a Function of Temperature for Each C− C Bond as Indicated by the Carbon Indexes on Top of Each Panel (Where −1 Denotes Carbons in the EO Groups and the Labeling Is Given in Scheme 4)a

a

Experimental values come from Raman spectroscopy measurements by Tonegawa et al.43

Scheme 7. Simulated Number of Water Molecules in the Vicinity of Headgroup Segments as a Function of Temperature for the Headgroup Atoms Indicated on Top of Each Panel

water molecules in contact with alkyl tail groups is so small that they are not shown.

calculated from our MD simulations together with available experimental data from the literature.43 With increasing temperature, the simulated trans fractions increase for the headgroup O−C−C−O dihedrals and decrease for the tail. The behavior for the tail is qualitatively similar to the experimental results. However, the simulated trans fractions are systematically higher than the experimental ones, as pointed out previously for the GROMOS force field.44 The extent of headgroup hydration is analyzed by calculating the number of water molecules in the vicinity of each segment as a function of temperature as shown in Scheme 7. The values vary between two and six water molecules, which is in agreement with experimental estimations.45,46 The number of

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DISCUSSION Order Parameters. The order parameters for C12E5 and C12E4 in the lamellar phase are a factor of ∼2 smaller than those observed for lipids and ionic surfactants, as already pointed out in previous studies.41,47,48 To emphasize this difference, we have plotted the measured and simulated order parameters for the hydrophobic tails of phospholipids, potassium palmitate, and C12E5 in Scheme 8. The most obvious explanation for the lower order parameters in the nonionic surfactant bilayers would be the larger area per hydrocarbon chain.47 By measuring the repeat distance and assuming ideal planar bilayers, X-ray 465


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experiments. From a visual inspection of the simulation configurations, it is clear that this agreement between simulation and experiment is achieved for structures that consistently contain defects (Scheme 3). During the construction of the initial structures, we also simulated a smaller system that could only form a small pore but did not allow for pore expansion. Such a smaller system had far larger order parameters than the measured ones. Reasonable agreement between simulation and experiments was achieved only after increasing the system size, which leads to pore expansion and a consequent decrease in the order parameters as shown in Scheme 2. In conclusion, on the basis of our results and previous simulation results13,14 we suggest that the low order parameters for the CmEn type of nonionic surfactants originate from bilayer pores. The good agreement between calculated and measured surfactant order parameters indicates that the molecular-level surfactant layer structure proposed by simulations would be reasonable. Thus we assume that the slightly larger repeat distances in simulations arise from the water layer thickness between surfactant layers. For example, further pore expansion would decrease the repeat distance because the water would move from the interlamellar region into the pore. However, the pore expansion might be limited by the box size in our simulations. Temperature Dependence. Increasing temperature normally leads to a decrease in the order parameters for hydrocarbon chains.55 Opposite behavior is observed for the poly(ethylene glycol) headgroups in both experiments and simulations. Similar experimental observations36 have previously been interpreted by using the theory by Karlström to rationalize the effect of temperature on the solubility of poly(ethylene oxide) polymers in water.39−42 In this theory, the attraction between the poly(ethylene oxide) chain and water arises from the interaction between the dipole moments of water and the overall dipole moment of O−C−C−O groups in the gauche conformation.40,42 Furthermore, the theory proposes that with increasing temperature the hydration of the poly(ethylene oxide) chain decreases, which leads to an increase in the order parameter due to an increased fraction of trans conformations.36,41 The decreasing hydration and the increasing trans fractions and order parameters for the poly(ethylene oxide) chains are clearly seen in Schemes 7, 6, and 5, respectively. For the alkyl chain, the trans fractions and order parameters decrease with increasing temperature, which is in agreement with previous results for alkyl and acyl chains.43,55 A nonmonotonous temperature dependence of the experimental order parameters has been observed for the carbons in the middle of the C12E5 chain, with a maximum close to T = 320 K for the segments from −8 to 4; see Scheme 5 and Ferreira et al.36 The nonmonotonic temperature dependence was first observed with 2H NMR for the α carbon (carbon 1) and deuterated water.9,11,12 These results were interpreted as the existence of two distinct lamellar phases: the classical one without pores, where the order parameters display the expected decrease with temperature, and a perforated lamellar phase, where instead the order parameters increase with temperature because of a gradually decreasing fraction of the bilayer area containing pores. This interpretation was later questioned when more detailed experimental data showed that the majority of the alkyl chain segments display the normal temperature dependence throughout the temperature range. The non-

Scheme 8. Order Parameter Profiles of Hydrophobic Chains, |SCH| versus Carbon Number, in Lamellar Phases of a CmEn Nonionic Surfactant (C12E5, 70 wt % Surfactant, T = 298 K), a Phospholipid (POPC in Excess Water, T = 298 K), and an Ionic Surfactant (Potassium Palmitate, 70 wt % Surfactant, T = 315 K)a

a

The data was taken from Ferreira et al.53 for POPC and from Davis and Jeffrey54 for potassium palmitate.

scattering measurements give areas per hydrocarbon chain of 0.34 nm2 for POPC,49 0.45 nm2 for C12E5 (70 wt %, 298 K),38 and 0.42 nm2 for potassium palmitate (62 wt %, 359 K).50−52 Because of the weak temperature dependence of the area per molecule for ionic surfactants,52 the potassium palmitate value is comparable in this discussion even though it has been measured at a higher temperature. On the basis of these results, the higher order parameters for POPC compared to those for C12E5 could be explained by the smaller area per chain. However, this is not the case for potassium palmitate. Klose and Levine approached this issue by using a simple statistical thermodynamic model for lamellar phases of pure and mixed C12E4 and POPC systems.13 The cross-sectional area per molecule is fixed in their calculations, and the corresponding order parameters are calculated. With this approach, the experimental area per molecule and order parameters for phosholipid bilayers are reproduced with relatively good accuracy. However, an area per molecule of 0.85 nm2 is required for the C12E4 bilayer to reproduce experimental order parameters. This value is almost a factor of 2 larger than the experimental area per molecule. The authors conclude that the low order parameters for C12E4 originate from hydrophobic− hydrophilic interfaces that are rather diffuse in comparison to the well-defined interfaces for lipid bilayers. This diffuse interface cannot be captured by their simulation model,13 thus leading to an unreasonably large simulated area per molecule to reproduce the experimental order parameters. In another simulation study with a more accurate model, the experimental area per molecule and order parameters were simultaneously reproduced.14 Indeed, in this simulation a nonzero density of ethylene oxide headgroups in the hydrophobic region was observed, indicating that the hydrophobic− hydrophilic interface is diffuse and/or uneven rather than ideally flat. This particular aspect of the results was not explicitly discussed by the authors,14 and no simulation snapshot of the simulated system was shown. In this work, we use MD simulations to generate structures that reproduce the order parameters in good agreement with 466


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simulations, which might be due to either the small simulation box size compared to the natural size of the pores or inaccuracies in the force field parameters. To determine the equilibrium structure predicted by the model that is used and validate this against scattering results by calculating the scattering pattern, one should simulate a system with a large number of pores or other defects and during a time interval long enough for the lateral diffusion of defects with the subsequent loss of long-range order. We expect that for this kind of simulation one would need at least a 4 times larger area, a 3 times larger height, and a simulation time interval of microseconds to remove the artificial periodicity by defect lateral diffusion. Regardless of the results of such a hypothetical simulation, the structures in Scheme 3 can be safely considered to be reasonable candidates for the lamellar phase structure of C12E5. Porous lamellar phase structures for nonionic surfactants have also been sketched in previous studies with the justification that they would explain the experimental obervables.8−11,57−60 The main advantage of our approach compared to previous studies is the atomistic-level resolution that allows the calculation of order parameters and thus a significantly more detailed comparison to the experiments. From a theoretical point of view, the porous phases are extensively discussed in the block copolymer field.20−22 Traditionally, the conclusion from mean-field theories has been that a porous phase should be metastable; however, recently several models have indicated that thermodynamically stable porous structures exist under certain conditions.18,19,21,22 For nonionic surfactants, the same lamellar phase properties are normally obtained when the temperature for the experimental investigations is reached by either heating or cooling, indicating that the structures are at thermodynamic equilibrium.9,59 Therefore, CnEm surfactants may also be interesting systems from this theoretical point of view because they seem to form thermodynamically stable porous lamellar phases.

monotonic trends in the order parameters were then rationalized in terms of changes in molecular conformation rather than the effect of pore formation below a certain temperature.36 However, the changes in order parameters with temperature are relatively small (∼0.02, Scheme 5) compared to the differences between nonionic surfactants and other molecules in lamellar phases (∼0.1, Scheme 8) or to the changes due to the expansion of the pores resulting from the increase in the simulation box size (∼0.06, Scheme 2). Thus, we believe that the nonmonotonous temperature dependence of the order parameter in the middle chain region arises from some subtle details of the chain conformations near the hydrophobic− hydrophilic interface rather than differences in phase structure. This idea is supported by order parameter measurements of potassium palmitate, where a similar order parameter maximum was observed but the order parameters are rather large and pores are not expected to be found.54 Also, this maximum has been explained by changes in the molecular conformation,56 but the analysis is slightly different compared to that for nonionic surfactants.36 In principle, the atomistic molecular dynamics simulations could be used to interpret the molecular origin of the maximum. However, the model that is used is not accurate enough to reproduce the maximum properly, as seen in Scheme 5. In conclusion, we interpret the order parameter maximum as a function of temperature arising from changes in molecular structure close to the hydrophobic−hydrophilic interface instead of a transition between porous and nonporous lamellar phases. Structure of the Lamellar Phase. In the above sections, we have suggested that the low order parameters for the lamellar phases of CmEn surfactants indicate that the lamellar phase does not consist of ideal lamellar sheets. Similar conclusions have been reported previously.13,41,57 Interestingly, the low order parameters are measured throughout the wide temperature and concentration ranges in the lamellar phase of C12E5. In various experiments, including this work, the order parameters for C12E5 segments are always smaller than 0.1 between 57 and 75 wt % surfactant and between 300 and 335 K.36,43 Therefore, the results indicate that the lamellar phase of the C12E5 surfactant always contains defects. The same would also apply to other CmEn surfactants where low order parameters are measured, such as C12E4.47 This conclusion is in agreement with previous EPR7 and FRAP8 results, which also suggest that the lamellar phase contains defects at all temperatures for C12E5 and C12E6 surfactants. However, it contradicts previous interpretations of NMR results,9,11,12 which assume that the lamellar phase is porous only at low temperatures and that the pores disappear above a certain transition temperature given by the maximum in the order parameters for the α carbon and deuterated water. The possible structures of a defectuous lamellar phase have been widely discussed, and several possibilities that would be in agreement with experiments have been proposed.8−11,57−60 In this work, we suggest structures constructed by using the MD model by Shang et al.24 The structures shown in Scheme 3 are reasonable candidates for the local structure of the lamellar phase because they reproduce experimental order parameters with relatively good accuracy. However, it should be noted that these structures might not be the equilibrium structures predicted by the model that is used. In particular, systems S1 and S3 turn into the hexagonal-like phase at the end of the

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CONCLUSIONS We have performed NMR measurements and MD simulations of the C12E5/water lamellar phase as a function of temperature and composition. On the basis of our results, we suggest the following: (1) The much lower order parameters SCH in the lamellar phase of C12E5, compared to the ones found in phospholipid bilayers, are due to the existence of defects. (2) Such defects are present throughout the whole lamellar phase region of C12E5. (3) Structures with transmembrane pores, similar to the ones shown in Scheme 3, are realistic models for the local structures of these lamellar phases.

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ASSOCIATED CONTENT

* Supporting Information S

The final structures of S1 and S2 systems in the hexagonal-like phase. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: samuli.ollila@aalto.fi. Notes

The authors declare no competing financial interest. 467


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ethylene alkyl ether surfactants. J. Phys. Chem. B 2012, 116, 14353− 14362. (20) Matsen, M. W. The standard Gaussian model for block copolymer melts. J. Phys.: Condens. Matter 2002, 14, R21. (21) Beardsley, T.; Matsen, M. Monte Carlo phase diagram for diblock copolymer melts. Eur. Phys. J. E 2010, 32, 255−264. (22) Schultz, A. J.; Hall, C. K.; Genzer, J. Computer simulation of copolymer phase behavior. J. Chem. Phys. 2002, 117, 10329−10338. (23) Hess, B.; Kutzner, C.; van der Spoel, D.; Lindahl, E. GROMACS 4: algorithms for highly efficient, load-balanced, and scalable molecular simulation. J. Chem. Theory Comput. 2008, 4, 435−447. (24) Shang, B. Z.; Wang, Z.; Larson, R. G. Molecular dynamics simulation of interactions between a sodium dodecyl sulfate micelle and a poly(ethylene oxide) polymer. J. Phys. Chem. B 2008, 112, 2888−2900. (25) Schuler, L. D.; Daura, X.; van Gunsteren, W. F. An improved GROMOS96 force field for aliphatic hydrocarbons in the condensed phase. J. Comput. Chem. 2001, 22, 1205−1218. (26) Briggs, J. M.; Matsui, T.; Jorgensen, W. L. Monte Carlo simulations of liquid alkyl ethers with the OPLS potential functions. J. Comput. Chem. 1990, 11, 958−971. (27) Chandrasekhar, I.; Kastenholz, M.; Lins, R.; Oostenbrink, C.; Schuler, L.; Tieleman, D.; Gunsteren, W. A consistent potential energy parameter set for lipids: dipalmitoylphosphatidylcholine as a benchmark of the GROMOS96 45A3 force field. Eur. Biophys. J. 2003, 32, 67−77. (28) Bussi, G.; Donadio, D.; Parrinello, M. Canonical sampling through velocity rescaling. J. Chem. Phys. 2007, 126. (29) Berendsen, H. J. C.; Postma, J. P. M.; van Gunsteren, W. F.; DiNola, A.; Haak, J. R. Molecular dynamics with coupling to an external bath. J. Chem. Phys. 1984, 81, 3684−3690. (30) Essman, U. L.; Perera, M. L.; Berkowitz, M. L.; Larden, T.; Lee, H.; Pedersen, L. G. A smooth particle mesh ewald potential. J. Chem. Phys. 1995, 103, 8577−8592. (31) Hess, B.; Bekker, H.; Berendsen, H. J. C.; Fraaije, J. G. E. M. LINCS: a linear constraint solver for molecular dynamics simulations. J. Comput. Chem. 1997, 18, 1463−1472. (32) Miyamoto, S.; Kollman, P. A. SETTLE: an analytical version of the SHAKE and RATTLE algorithm for rigid water models. J. Comput. Chem. 1992, 13, 952−962. (33) Berendsen, H. J. C.; Postma, J. P. M.; van Gunsteren, W. F.; Hermans, J. In Intermolecular Forces; Pullman, B., Ed.; Reidel: Dordrecht, 1981; pp 331−342. (34) http://dx.doi.org/10.6084/m9.figshare.861071, 2013. (35) Dvinskikh, S. V.; Zimmermann, H.; Maliniak, A.; Sandstrom, D. Measurements of motionally averaged heteronuclear dipolar couplings in MAS NMR using R-type recoupling. J. Magn. Reson. 2004, 168, 194−201. (36) Ferreira, T. M.; Medronho, B.; Martin, R. W.; Topgaard, D. Segmental order parameters in a nonionic surfactant lamellar phase studied with 1H-13C solid-state NMR. Phys. Chem. Chem. Phys. 2008, 10, 6033−6038. (37) Humphrey, W.; Dalke, A.; Schulten, K. VMD − visual molecular dynamics. J. Mol. Graphics 1996, 14, 33−38. (38) Klose, G.; Eisenblaetter, S.; Galle, J.; Islamov, A.; Dietrich, U. Hydration and structural properties of a homologous series of nonionic alkyl oligo(ethylene oxide) surfactants. Langmuir 1995, 11, 2889−2892. (39) Andersson, M.; Karlström, G. Conformational structure of 1,2dimethoxyethane in water and other dipolar solvents, studied by quantum chemical, reaction field, and statistical mechanical techniques. J. Phys. Chem. 1985, 89, 4957−4962. (40) Karlström, G. A new model for upper and lower critical solution temperatures in poly(ethylene oxide) solutions. J. Phys. Chem. 1985, 89, 4962−4964. (41) Ahlnäs, T.; Karlström, G.; Lindman, B. Dynamics and order of nonionic surfactants in neat liquid and micellar solution from multifield carbon-13 NMR relaxation and carbon-13 NMR chemical shifts. J. Phys. Chem. 1987, 91, 4030−4036.

ACKNOWLEDGMENTS We acknowledge Roberta Pigliapochi for useful discussions. This work was financially supported by the Swedish Research Council (grant numbers 2005-2936, 2009-6794, and 20114334). OHSO acknowledges the Osk. Huttunen and Emil Aaltonen Foundations for financial support. T.M.F. acknowledges FCT for fellowship SFRH/BD/48622/2008. We thank the Center for Scientific and Technical Computing (LUNARC), Lund University for computing resources.

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REFERENCES

(1) Jiao, J. Polyoxyethylated nonionic surfactants and their applications in topical ocular drug delivery. Adv. Drug Delivery Rev. 2008, 60, 1663−1673. (2) Dong, R.; Hao, J. Complex fluids of poly(oxyethylene) monoalkyl ether nonionic surfactants. Chem. Rev. 2010, 110, 4978−5022. (3) Rigaud, J.-L.; Chami, M.; Lambert, O.; Levy, D.; Ranck, J.-L. Use of detergents in two-dimensional crystallization of membrane proteins. Biochim. Biophys. Acta 2000, 1508, 112−128. (4) Evans, D. F.; Wennerstöm, H. The Colloidal Domain: Where Physics, Chemistry, Biology, and Technology Meet; Wiley-VCH: New York, 1999. (5) Holmberg, K.; Jönsson, B.; Kronberg, B.; Lindman, B. Surfactants and Polymers in Aqueous Solution; John Wiley & Sons: Hoboken, NJ, 2003. (6) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. Phase behaviour of polyoxyethylene surfactants with water. Mesophase structures and partial miscibility (cloud points). J. Chem. Soc., Faraday Trans. 1 1983, 79, 975−1000. (7) Paz, L.; Di Meglio, J. M.; Dvolaitzky, M.; Ober, R.; Taupin, C. Highly curved defects in lyotropic (nonionic) lamellar phases. Origin and role in hydration process. J. Phys. Chem. 1984, 88, 3415−3418. (8) Constantin, D.; Oswald, P. Diffusion coefficients in a lamellar lyotropic phase: evidence for defects connecting the surfactant structure. Phys. Rev. Lett. 2000, 85, 4297−4300. (9) Fairhurst, C. E.; Holmes, M. C.; Leaver, M. S. Structure and morphology of the intermediate phase region in the nonionic surfactant C16EO6/water system. Langmuir 1997, 13, 4964−4975. (10) Imai, M.; Saeki, A.; Teramoto, T.; Kawaguchi, A.; Nakaya, K.; Kato, T.; Ito, K. Kinetic pathway of lamellar −> gyroid transition: pretransition and transient states. J. Chem. Phys. 2001, 115, 10525− 10531. (11) Baciu, M.; Olsson, U.; Leaver, M. S.; Holmes, M. C. 2H NMR evidence for the formation of random mesh phases in nonionic surfactant-water systems. J. Phys. Chem. B 2006, 110, 8184−8187. (12) Medronho, B.; Miguel, M. G.; Olsson, U. Viscoelasticity of a nonionic lamellar phase. Langmuir 2007, 23, 5270−5274. (13) Klose, G.; Levine, Y. K. Membranes of palmitoyloleoylphosphatidylcholine and C12E4a lattice model simulation. Langmuir 2000, 16, 671−676. (14) Schneider, M. J.; Feller, S. E. Molecular dynamics simulations of a phospholipid-detergent mixture. J. Phys. Chem. B 2001, 105, 1331− 1337. (15) Shinoda, W.; DeVane, R.; Klein, M. L. Multi-property fitting and parameterization of a coarse grained model for aqueous surfactants. Mol. Simul. 2007, 33, 27−36. (16) Shinoda, W.; DeVane, R.; Klein, M. L. Coarse-grained molecular modeling of non-ionic surfactant self-assembly. Soft Matter 2008, 4, 2454−2462. (17) Klein, M. L.; Shinoda, W. Large-scale molecular dynamics simulations of self-assembling systems. Science 2008, 321, 798−800. (18) Denham, N.; Holmes, M. C.; Zvelindovsky, A. V. The phases in a non-ionic surfactant (C12E6)-water ternary system: a coarse-grained computer simulation. J. Phys. Chem. B 2011, 115, 1385−1393. (19) Rossi, G.; Fuchs, P. F. J.; Barnoud, J.; Monticelli, L. A coarsegrained MARTINI model of polyethylene glycol and of polyoxy468


Langmuir

Article

(42) Lindman, B.; Karlström, G. Nonionic polymers and surfactants: temperature anomalies revisited. C. R. Chim. 2009, 12, 121−128. (43) Tonegawa, A.; Ohno, K.; Matsuura, H.; Yamada, K.; Okuda, T. A combined Raman and deuterium NMR spectroscopic study on the molecular and phase structure of a nonionic surfactant C12E5-water system. J. Phys. Chem. B 2002, 106, 13211−13223. (44) Siu, S. W. I.; Pluhackova, K.; Böckmann, R. A. Optimization of the OPLS-AA force field for long hydrocarbons. J. Chem. Theory Comput. 2012, 8, 1459−1470. (45) Nilsson, P. G.; Lindman, B. Water self-diffusion in nonionic surfactant solutions. Hydration and obstruction effects. J. Phys. Chem. 1983, 87, 4756−4761. (46) Carlström, G.; Halle, B. The state of water in non-ionic surfactant solutions and lyotropic phases. Oxygen-17 magnetic relaxation study. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1049−1063. (47) Klose, G.; Mädler, B.; Schäfer, H.; Schneider, K.-P. Structural characterization of POPC and C12E4 in their mixed membranes at reduced hydration by solid state 2H NMR. J. Phys. Chem. B 1999, 103, 3022−3029. (48) Schönhoff, M.; Söderman, O.; Li, Z. X.; Thomas, R. K. Internal dynamics and order parameters in surfactant aggregates: a 2H NMR study of adsorption layers and bulk phases. Langmuir 2000, 16, 3971− 3976. (49) Kučerka, N.; Tristam-Nagle, S.; Nagle, J. F. Structure of fully hydrated fluid phase lipid bilayers with monounsaturated chains. J. Membr. Biol. 2005, 208, 193−202. (50) Boden, N.; Jones, S. A.; Sixl, F. Solubilization in lyotropic liquid crystals: the concept of partial molecular surface area. J. Phys. Chem. 1987, 91, 137−145. (51) Ekwall, P. Composition, properties and structures of liquid crystalline phases in systems of amphiphilic compounds. Adv. Liq. Cryst. 1975, 1, 1. (52) Gallot, B.; Skoulios, A. Interactions électriques dans les phases mésomorphes des systémes amphiphile-eau: rôle de la teneur en eau, ̂ paraffinique, de la nature du cation, et de la de la longueur de la chaine température. Kolloid Z. Z. Polym. 1966, 208, 37−43. (53) Ferreira, T. M.; Coreta-Gomes, F.; Ollila, O. H. S.; Moreno, M. J.; Vaz, W. L. C.; Topgaard, D. Cholesterol and POPC segmental order parameters in lipid membranes: solid state 1H-13C NMR and MD simulation studies. Phys. Chem. Chem. Phys. 2013, 15, 1976−1989. (54) Davis, J.; Jeffrey, K. The temperature dependence of chain disorder in potassium palmitate-water. A deuterium NMR study. Chem. Phys. Lipids 1977, 20, 87−104. (55) Petrache, H. I.; Dodd, S. W.; Brown, M. F. Area per lipid and acyl length distributions in fluid phosphatidylcholine determined by 2 H NMR spectroscopy. Biophys. J. 2000, 79, 3172−3192. (56) Abdolall, K.; Burnell, E.; Valic, M. The temperature dependence of water and counter ion order in soap-water mesophases. A deuterium and sodium NMR study. Chem. Phys. Lipids 1977, 20, 115−129. (57) Capitani, D.; Yethiraj, A.; Burnell, E. E. Memory effects across surfactant mesophases. Langmuir 2007, 23, 3036−3048. (58) Rancon, Y.; Charvolin, J. Fluctuations and phase transformations in a lyotropic liquid crystal. J. Phys. Chem. 1988, 92, 6339−6344. (59) Funari, S. S.; Holmes, M. C.; Tiddy, G. J. T. Microscopy, X-ray diffraction, and NMR studies of lyotropic liquid crystal phases in the C22EO6/water system: a new intermediate phase. J. Phys. Chem. 1992, 96, 11029−11038. (60) Leaver, M.; Fogden, A.; Holmes, M.; Fairhurst, C. Structural models of the R3̅ m intermediate mesh phase in nonionic surfactant water mixtures. Langmuir 2001, 17, 35−46.

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