Molecular Dynamics Study of Intermediate Phase of Long Chain Alkyl

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A Molecular Dynamics Study of Intermediate Phase of Long Chain Alkyl Sulfonate/Water Systems Armen Hamlet Poghosyan, Levon H. Arsenyan, and Aram Shahinyan Langmuir, Just Accepted Manuscript • DOI: 10.1021/la302378r • Publication Date (Web): 07 Dec 2012 Downloaded from http://pubs.acs.org on December 16, 2012

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A Molecular Dynamics Study of Intermediate Phase of Long Chain Alkyl Sulfonate/Water Systems Armen H. Poghosyan* 1, Levon H. Arsenyan2 and Aram A. Shahinyan1 1

The International Scientific – Educational Center of National Academy of Sciences of Armenia, M. Baghramyan ave. 24d, 0019 Yerevan, Armenia

2

The Institute of Applied Problems of Physics of the National Academy of Sciences of Armenia, Hr. Nersisyan 25, 0014 Yerevan, Armenia

ABSTRACT: Using atomic level simulation we aimed to investigate various intermediate phases of long chain alkyl sulfonate/water system. Overall, about 800ns parallel molecular dynamics simulation study was conducted for a surfactant/water system consisting of 128 sodium pentadecyl sulfonate and 2251 water molecules. The GROMACS software code with united atom force field was applied. Despite some differences, the analysis of main structural parameters is in agreement with X-ray experimental findings. The mechanism of self-assembly of SPDS molecules was also examined. At T=323K we obtained both tilted fully interdigitated and liquid crystalline-like disordered hydrocarbon chains, hence presence of either gel phase that coexists with a lamellar phase or metastable gel phase with fraction of gauche configuration can be assumed. Further increase of temperature revealed that the system underwent a transition to a lamellar phase, which was clearly identified by the presence of a fully disordered hydrocarbon chains. The transition from gel-to-fluid phase was implemented by simulated annealing treatment and the phase transition point at T=335K was identified. The surfactant force field in its presented set is surely enabled to fully demonstrate the mechanism of self-assembly and the behavior of phase transition making it possible to get important information around the phase transition point. ACS Paragon Plus Environment

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KEYWORDS: Surfactant self-assembling; long chain alkyl sulfonate; molecular dynamics study

1.INTRODUCTION

Nowadays amphiphilic materials are important in many fields from medicine to industry arousing interest in their properties. The surfactant molecules consist of a hydrophilic polar head group and hydrophobic apolar chains and depending on concentration, temperature, as well on alkyl chain length the different phases can be seen1,2. For many surfactant systems depending on temperature and the concentration the structures classified as crystalline, gel, coagel, liquid crystalline, etc. are observed and the phase transformation between the structures is accompanied by the configurational changes in molecular packing modes3. The characteristic feature of coagel phase is the penetration of tilted hydrocarbon chains into each other (close packaging with the rotation restricted along the chain axis) in the presence of a slim water layer (~1-1.5nm)4. The latter is different to the gel phase, where the water layer can be around 100nm. In coagel phase tightly bound water molecules are confined between the hydrophilic regions of lamellas whilst the gel phase is characterized by a much larger water slab identified as intermediate, i.e. tightly bound and bulk water. It is also stated that the number of structured water molecules becomes greater for longer alkyl chains and the hydrophobic chain length above 14 carbon atoms shows intermediate metastable gel phase4 . The previous studies reported that the mentioned coagel and gel phases can be seen at surfactant-rich content and temperatures above the Krafft point and it is shown that for R-SO3 surfactant systems with R=C15 the Krafft point is estimated to be ~48oC and the corresponding CMC value is ~0.725mM5-8 . The surfactant systems have been intensively studied for many years both experimentally4-11

and

theoretically12-14 . Alkyl sulfonates and alkyl sulfates are quite similar from structural point of view, the ACS Paragon Plus Environment

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difference is only in one oxygen, however, the molecules exhibit large differences in Krafft point value, micelle aggregation number, critical micelle concentration (CMC), etc. Sodium alkyl sulfonates have excellent detergent and foaming properties5 and their catanionic mixtures are much soluble in water than those of alkyl sulfate15 . During last decades, with the increase of hardware power, in silico experiments (namely the molecular dynamics simulation - MD) became an excellent tool for explanation of conformational properties and structure of complex systems making more understandable physics of such fundamental process as phase behavior. Recently, a number of MD studies in surfactant systems have been reported16-25 focusing predominantly on micellar structures of sulfate-based surfactant systems17-21 . A simple model of sodium dodecyl sulfate (SDS) micelles was suggested by Woods et al19 . A series of simulation of SDS micelles were performed by various groups18-21. Computer simulations of thin SDS films - “Newton black films” were performed by Faraudo et al22, where the properties of water and SDS molecules have been investigated. A series of long run of liquid crystalline SDS/water bilayers has been published16 discussing correspondingly the all atom and the united atom models of SDS. In fact, these studies were conducted on alkyl sulfates, whereas as of now there are no reports on long chain alkyl sulfonates. The purpose of this study was to analyse in detail intermediate phases of anionic sodium pentadecyl sulfonate (SPDS) /water systems using molecular dynamics simulation with an estimation of the transition point. Kinetics of phase transformation is essential for material design with specific features and functions. Thus in order to understand it , and to test the existing SPDS force field, we set out to perform a series of cooling/heating simulation annealing and long MD runs at fixed temperatures. The determination of such key parameters, interlayer spacing, area per molecule and tilting angle of surfactant with respect to lamella normal was also carried out with subsequent comparison with existing experimental X-ray diffraction data in order to verify the accuracy of MD simulations, paying particular attention to conformational features of hydrocarbon chains of surfactant.

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2. CONSTRUCTION AND SIMULATION DATA

Construction details As a surfactant molecule the sodium pentadecyl sulfonate (C15H31SO3Na) was used, the schematic presentation of which is shown in Figure 1. The SPDS molecule was extracted from our previous simulation23 and 128 replicates were positioned in the unit cell by random rotation, translation and shift. The 128 sodium ions, or the counterions, were modeled at ~0.3nm from sulfurs of the headgroups. The system was solvated by means of GROMACS24,25 genbox module with corresponding 2251 water molecules to give n wat ≅ 17 , where n wat is the number of water molecules per surfactant. After construction of the whole simulation cell, the energy was minimized using steepest descent method for 5000 steps to remove high-energy contacts that might have formed during the construction process.

Simulation details The force field parameters for SPDS molecule were made for the GROMOS53a5 force field using Dundee PRODRG Server26. The partial charges of the surfactant molecule were set according to Huibers27, where, in contrast to alkyl sulfate, the partial charge on alkyl sulfonate surfactants is distributed both negative into the SO3− and the first methylene (CH2) group of hydrocarbon chain ( α methylene). The methylene (CH2) and methyl (CH3) groups were modeled as unified atoms (GROMACS UA forcefield). The simple point-charge (SPC) molecules were used as a water model28 and all bonds were held fixed with LINCS constrain algorithm29. Temperature was set to 323K for Series I, II (i.e. TserI = TserII = 323K ) and 343K for Series III ( TserIII = 343K ) and maintained using V-rescale algorithm

with the time constant τ = 0.1 ps. Temperature of surfactants, the counterions and solvent were controlled independently. For Series I isotropic normal pressure was used and for Series II and Series III the corresponding semi-isotropic pressure was applied (1atm) with surface tension γ = 35dyn / cm . To ACS Paragon Plus Environment

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explore a phase transition, the annealing simulation (heating/cooling) was also introduced from 323K to 343K, where the heating/cooling rate was 1.5K per ns. The pressure coupling Berendsen algorithm30 was used with τ = 0.1 ps time constant. The PME31, with 3D fast Fourier transform cell grid was used for long-range electrostatic interactions, and the van der Walls interactions were truncated at 1.2 nm. The equations of motion are integrated using leapfrog Verlet integrator32 with a timestep of 2fs. The cut-off distance for the short-range neighbor list was set to 1nm, and the short-range neighbor list was updated every 4 ns. Three-dimensional periodic boundary conditions were applied. Coordinates and velocities were saved every 0.1 ns and visualization of the molecular configurations of the system was generated with VMD package33 . The 128SPDS/2251 water system was subsequently subjected to the small simulation (~ 100ps) in NVT ensemble. Further, the long 800 ns (0.8 µs ) overall parallel MD run was carried out for 100ns in Series I in NP T ensemble and continuously for 300ns in Series II and 400ns in Series III in NPN γT ensemble. The parallel MD simulation was performed on ArmGrid sites (http://www.grid.am) and partially running on BlueGene/P supermachine at Bulgarian Supercomputating Centre (http://scc.acad.bg).

3. RESULTS AND DISCUSSION

Structural parameters

The interlayer spacing and the area per molecule, which can be extracted from real experiments, are the most important characteristics of the system. In Figure 2 (top) the time evolution of the interlayer spacing is plotted. The interlayer spacing is a layer repeat unit, i.e. a water layer plus a lamella thickness, where the latter is quantified from the distance of an average position of sulfur atoms in each layer. On the other hand, once the simulation with randomly located SPDS molecules in aqueous solution (Series I) started, the interlayer spacing assumed to be a value of z-axis. However, after ~7ns of the run self-


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organization of surfactant (system transition into a flat layers) occurs and before the end of 100ns simulation run, a stable SPDS bilayer could be tracked. Meanwhile it was stated34 that the self-assembling process takes longer for surfactants with long hydrophobic chains due to slower self-diffusion. Off note higher temperature than the room one increases the mobility of surfactant molecules and therefore speeds up aggregation. The self-organization process of SPDS surfactant can be divided into four stages: 1. fast aggregation (0.5-0.9ns) of SPDS molecules by forming “micelle-like” clusters (Figure 3a), i.e. oligomers have disordered structure surrounded by water domains; 2. clusterized aggregations tend to flatten like clusters linking by surfactant’s “bridges” (~1.5ns run point) that is to say, the small disordered clusters have been re-oriented and merged into flat like clusters, however, we still track the so called surfactant “gap” (or “bridges”); 3. swelling of interlamellar water followed by disappearance of bumpy surface; 4. formation of stabilized flat layers (since ~7ns of run), or the final bilayer ripening, which takes 57ns of simulation time. Thus, the formation of bilayer occurs via a mechanism described below. The simulation of Series I started from randomly located molecule was performed in NPT ensemble. Meanwhile, when the system dimensions are very small compared to macroscopic systems, the contribution of the surface tension to the pressure should not be neglected. Above the CMC the surface tension at the interface between water and SPDS molecules does not change significantly and in case of SPDS/water system it is estimated to be around 35dyn / cm 35. Therefore, for Series II the simulation in NPN γT ensemble with γ = 35dyn / cm was continued. The simulation at the constant surface tension

showed the variation of interlayer spacing value at the beginning of simulation, which is due to the reorientation of disordered chains. At 120ns the gradually increase of interlayer spacing (~7.25nm) is observed driven by the rearrangement of chains, after which the SPDS molecule hydrocarbon chains tend to become straight and as a result the decrease of interlayer spacing with upright located hydrocarbon chains occurred. The rearrangement of SPDS molecules leads to value of interlayer spacing ~ 6.3 ± 0.1nm ACS Paragon Plus Environment

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over the course of Series II run. On the contrary, the system heating (at fixed T=343K) is accompanied by the decrease of the interlayer spacing and at the end of simulation ~ 5.6 ± 0.2nm is obtained. Simultaneously, with the decrease of interlayer spacing, the system undergoes gel-to-liquid crystalline phase transition and as a result we see fully disordered hydrocarbon chains. In parallel with interlayer spacing the water layer and lamella thicknesses (sulfur-to-sulfur distance) were also calculated over the course of simulation (see Figure 2(bottom)). It should be emphasized that for Series I, i.e. for a tensionless system started from random distribution, the almost constant interlayer spacing is accompanied by a gradually increase and decrease of water slab and lamella thicknesses correspondingly. Meanwhile for Series II and III, the dynamic behavior of water slab and lamella thickness is identified by interlayer spacing behavior. For gel phase (Series II), at the end of simulation, the value of lamella thickness ~ 2.5 ± 0.2nm was obtained. Our earlier X-ray diffraction data11,36 (the experiments were performed at the composition region between 25 to 94 wt% SPDS) at low water concentration show that the lamella thickness is estimated to be d L ≅ 2.75nm , i.e. the MD results are in agreement with the provided experimental findings (see Supporting information, Figure S1). The area per molecule, calculated by multiplying x and y values of the system size and divided by the number of molecules in a layer, is shown in Figure 2 (middle). It is obvious, that this parameter makes no sense when the system is self-assembling into lamellar structure. Hence, the curves are shown starting from 7ns simulation point. The value of area per molecule is constant for Series I, however, for Series II, when the reorientation occurs, the area per molecule decreases and subsequently increase reaching about 0.32nm2. Upon heating (at fixed T=343K), the area per headgroup increases and becomes about 0.39 ± 0.1nm 2 at the end of simulation. In order to visualize the structural changes in more detail, we provide the snapshots of SPDS/water systems at the start point (random distribution) and at the end of simulation, accordingly for Series I, II and III. In Figure 4a,b,c and d, the cross-sectional view perpendicular to the bilayer plane is shown. At


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fixed T=323K the packing mode changes into interdigitated gel phase and an ordered state is observed with a tilt angle, as evidenced by the snapshot (Figure 4c), which is generally known as gel Lβ phase. The dynamic behavior of the curves provided in Figure 2 and the inspection of the snapshots from intermediate timepoints reveals that the following changes in packing mode of hydrocarbon tails: the initial step which impels the re-orientation of surfactant is assumed the increase of interlayer spacing. At the beginning of Series II simulation the surfactant molecules move towards the normal of bilayer, which is clearly seen on Figure 2 (at ~120ns time point), simultaneously the alkyl chains become straight, after which a drastic decrease of interlayer spacing, i.e. the penetration of alkyl chains located on opposite sides of the bilayer is observed. Thus, we see the fully interpenetration of the SPDS alkyl chains located on opposite sides of the bilayer (fully interdigitated gel state), however, together with ordered molecules, the disordered domains or so called “clusterized regions”

37

are also presented, hence we assume the

formation of the gel phase coexisted with a lamellar Lα phase. On the other hand, this kind of phases can be represented as a metastable gel phase with fraction of disordered domains, which are also observed in systems of phospholipid membranes38. Hence, the analysis of calculated parameters together with the snapshots implies either a coexistence of fully interdigitated and random phases or the presence of metastable gel phases. The visual inspection of trajectories and snapshots suggests the phase transition from gel to fluid like structures, we also observe a gel phase that coexists with a lamellar Lα phase. To explain the phenomena, an additional annealing simulation was carried out to find a transition temperature. We set out a series of annealing simulations (heating/cooling) with different starting configurations (equilibrated systems at 323 and 343K points) and different heating/cooling rates (data not shown) to statistically strengthen the data. In Figure 5, the interlayer spacing and the area per surfactant, which are sensitive to phase transition changes from annealing simulation are shown. However, it should be noted, that other parameters such as


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tilting angle of C1-C15 with respect to lamella normal and sulfur-to-sulfur distances are also taken into consideration to validate these data. The so called transition point is estimated from the slope of presented temperature curves, which is around 335K (Figure 5). However, exploring the cooling plots both for area per molecule and interlayer spacing shows that the phase transition point is not so obvious. Also, the transition temperature point can be clearly seen from the tilting angle heating plot (Figure 6) – suggesting that the phase transition occurs at ~335K temperature. In other words the change in the slope of heating curves for all provided parameters corresponds to the 335K temperature point. As the phase transition is known to be a cooperative phenomenon studied by statistical physics, we have also testify the twice larger lamella system with same C wat / C SPDS concentration ratio (256 SPDS/~4500water) by carrying out a series of heating/cooling scans, and as expected, according to testing runs, we obtained similar results (see snapshots in Supporting information, Figure S2) which suggests, that a small system is sufficient for obtaining reliable results in regards to the phase transformation mechanism. The curves on Figure 5 represent hysteresis loops, which are characteristic of a system undergoing a first-order phase transition. Similar pattern is experimentally observed also for lipid bilayer38,39 . To better describe structural changes we analyzed hydrocarbon chains properties by determining following characteristics: tilting angle of C1-C15 with respect to lamella normal, C1-C8-C15 mean angle and order parameter. These characteristics were obtained from the ensemble average of trajectories over the last 50ns of the simulation run in each series to verify that the stability of each system is reached. The probability distribution of the titling angle θ between hydrocarbon chain vector and the lamella normal, defined by the vector between each C1 and terminal C15 in surfactant chain and the lamella normal points along the inside of the bilayer, is shown in Figure 7(a). The MD data shows that in Series II the orientation of surfactant alkyl chains of perpendicular to lamella plane is most likely to occur and the dominant angle is about 26o, i.e. there is a peak in the gel phase with narrow shoulders centered around


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26o, which fits well with the Gaussian distribution. The X-ray diffraction experimental estimation of tilting angle36 of SPDS molecule with respect to lamella plane is estimated to be 34o. Note, the forementioned the X-ray diffraction experiment was carried out on SPDS/water system with C wat / C SPDS concentration ratio which is similar to those given from MD runs. Overall, the MD simulation conforms well to experimental results. There is one peak observed in gel and fluid phases distribution: a narrow tilting angle distribution for gel θ max ≅ 26 o and a wide tilting angle distribution for fluid phase, where chains become completely random with θ max ≅ 51o . In order to get more information about the structure of alkyl chains, we also calculated the molecule chain length (C1 to terminal C15 vector) and the probability distribution of molecule chain length is monitored for three series of simulation runs (Figure 7b). If we consider the chain length of a SPDS molecule in all-trans configuration, consisting of n hydrocarbon atoms, then according to following formula l ≅ 1.25(n − 1) , the chain length of a SPDS molecule equals ≈ 17.5 A o .For gel phase we obtained a narrow distribution peak at ~ 15.6 A o at most, meanwhile, for fluid phase the dominant length decreases to ~ 14.06 A o , which suggests that the hydrocarbon chains are disordered and melted in fluid phase in contrast to gel phase where molecule chains are close to the extended configuration, as to be seen from snapshots below. We also estimated the full-width of the distribution at half maximum (FWHM) and peak maximum (PM)40 (data not shown) in three series of simulation runs, where a sharp increase in FWHM value between 323 and 343K is observed. To examine structural transformations in details, we determined the C1-C8-C15 mean angle (hereafter the deformation angle) and in Figure 8 the curve depending on simulation time is shown. To calculate the deformation angle, we defined the average angle between C1-C8 and C8-C15 vectors in the alkyl chains. The results show that in Series I, the mean deformation angle fluctuates in a narrow range of 135-140o, meanwhile it took about 200ns for the angle to stabilize for in gel phase. Increase in temperature (Series III) leads to a sharp decrease of C1-C8-C15 mean deformation angle and get equilibrated from 40ns of run.


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For the gel phase the mean angle is estimated to be around ~ 150 ± 2 o , which indicates that the hydrocarbon chains are mostly in upright configuration. Upon transition to fluid state, the value of deformation angle appears to be around 135o due to chain disordering. This remains consistent with our previous assumption and we conclude that the phase transition point lies along 323-343K temperature interval. Another important parameter characterizing the tail ordering is the order parameter, which is formulated as:

S mol =

3 1 < cos 2 α i > − 2 2

(1)

where α i is the angle between the z-axis of simulation box and molecular axis, defined as a vector from C i −1 to C i +1 atoms and the brackets denote the time and ensemble average. Assuming an axial symmetry of the segment motion S mol can be further related to the deuterium order parameter according to S mol = − S CD . The latter can be obtained experimentally using deuterium magnetic resonance spectroscopy and is usually used to calibrate molecular dynamics simulations. The deuterium order parameter is shown in Figure 9. The data shows that the value of calculated order parameter in Series II is larger than those in Series I and III. The data in Series II refers that the bilayer is in gel phase, where the molecular order is higher compared to the fluid-like structure. The values for carbons close to the sulfur atoms are low, also a decrease at the end of the chains (from the 11th carbon) is observed. Unfortunately, there is no comparison available due to the lack of experimental data on the gel and liquid crystalline phases. To describe the features of system surfaces we have estimated the surface roughness, which can be considered as the degree of the vertical displacement of SPDS molecule headgroup, defined as: →

→

→

→

ξ ( R) = < ( z ( r ) − z ( r + R)) 2 >

(2)


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→

→

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→

where z ( r ) and z ( r + R ) are the z-axis coordinates of two sulfur atoms in the same leaflet. The corresponding plots are shown in Figure 10 and produce a value of ξ ( R → ∞) ~ 2.74 A o for the surface roughness function in the gel phase. Off note, the theoretical and experimental calculations suggest a value of ~2.4A°

41

and ~2.7A°

42

correspondingly, assuming that the surface tension of monolayer is

~38mN/m. Further increase of the temperature up to 343K leads to the formation of bumpier surface with

ξ ( R → ∞) ~ 3.65 A o . To explore the interactions of the counterions and water with surfactant at the interfacial region, we calculated the three dimensional radial distribution functions between different atoms. Figure 11(top) shows the RDFs between surfactant sulfur atoms and sodium counterions in three series. It should be noted that there are no significant differences in all cases, besides a sharper peak in fluid phase in Series III. All curves show one sharp peak around 0.29nm followed by a second diffuse peak at ~0.57nm. In all cases more than 80% of sodium counterions are within the first shell covered at ~0.4nm from the surfactant headgroup, which suggests the counterion condensation. The RDFs between surfactant sulfur atoms and water oxygen are also shown (Figure 11 (bottom)). RDF curves show peaks at ~0.43nm and ~0.7nm implying that various shells of water are present. The experimental evidence of such a behavior has been previously reported by our group43 . The differential thermal analysis suggests that there are two types of bound water found in the SPDS-water system apart from bulk water (weakly and strongly bound water), which are identified by their evaporation temperatures (see Supporting information, Figure S3).

4. CONCLUSION The MD simulations of the long alkyl chain sulfonate/water system were carried out at different temperatures. We have investigated the self-organization behavior of surfactant by performing simulations using random distribution of surfactant in initial configuration. We have explored in detail the dynamics of bilayer formation, which is indeed quite interesting as related to the dynamic self-assembly ACS Paragon Plus Environment

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pathway leading to lamellar phase. The self-assembly process can be divided into several stages: fast aggregation of small “micelle-like” disordered structures, followed by a stage, when surfactants tend to separate from water domains by forming flat-like structures with a bumpy surface, and finally we a fully stabilized surfactant bilayer is observed. Flat-like structures, in fact, are formed by coalescence of “micelle-like” loops as plainly shown in snapshots. At fixed T=323K the interdigitated gel phase is formed, meanwhile, we assume that the gel phase coexists with fluid like phase, i.e. the presence of fluid like domains. For the gel phase we got the value of interlayer spacing ~ 6.3 ± 0.1nm and the corresponding value of lamella thickness was around 2.5 ± 0.2nm . The X-ray diffraction experiment, which was previously reported by our group11,36 revealed that at low water concentration the lamella thickness is estimated to be d L ≅ 2.75nm . Besides small differences these MD results are in agreement with the provided experimental findings. Moreover, the estimation of the tilting angle of C1-C15 with respect to lamella normal shows that in gel phase the alkyl chains are found mostly perpendicular to lamella plane with a corresponding angle about 26o. The above mentioned experimental finding on the tilting angle36 produces a value of ~34o, which is, in fact, consistent with the MD data. The visual inspection of trajectory movies reveals that in the gel phase the chains located on the opposite sides of the lamella are fully interdigitated, however, the so called “disordered domains” are also observed, which suggests either the coexistence or the metastable gel phase. The change in temperature (T=343K) leads to the formation of fluid like phase with fully disordered hydrocarbon chains, i.e. the system undergoes gel-to-fluid phase transition. Upon heating the interlayer spacing decreases in parallel with an increase of area per molecule, which leads to formation of fully disordered hydrocarbon chains as evidenced by the nature of characteristic curves of main structural parameters of hydrocarbon chains.


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Using simulated annealing we explored the temperature range and estimate the phase transition point. A Series of heating/cooling scans shows that the phase transition occurs at ~335K temperature. The analysis of main structural parameters is in agreement with X-ray diffraction experimental data. The estimation of surface roughness in both cases reveals a value of ξ ( R → ∞) ~ 2.74 A o in gel phase, which correlates with experimental data, while in Lα phase the surface becomes bumpier with the roughness function reaching up to ~3.65Ao. Thus, we conclude that the presented united-atom character forcefield for long chain SPDS surfactant correlates with experimental data. Our results are therefore intended to serve as a starting point for experimentalists to verify the MD data.

ASSOCIATED CONTENT Supporting information, figure S1, S2 and S3 were uploaded.

AUTHOR INFORMATION

Corresponding Author *Phone: (374) 91490028; Email: [email protected]

ACKNOWLEDGMENT This

work

was

supported

in

part

by

the

European

project HP-SEE (N261499).

REFERENCES ACS Paragon Plus Environment

Commission

under

EU

FP7

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(1) Zana, R. Dynamics of Surfactant Self-Assembles: Micelles, Microemulsions, Vesicles and Lyotropic Phases; CRC Press; FL, vol. 126, 2005. (2) Shah, D.O. (Ed.) Micelles, Microemulsions and Monolayers, Science and Technology; Marcel Dekker: New York, 1998. (3) Wu, F-G.; Yu, Z-W.; Ji, G. Formation and Transformation of the Subgel Phase in Dioctadecyldimethylammonium Bromide Aqueous Dispersions. Langmuir. 2004 27, 2349-2356. (4) Tsuchiya, M.; Tsujii, K.; Maki, K.; Tanaka, T. Statistical Mechanical Theory of the Coagel-Gel Phase Transition in Ionic Surfactant/Water Systems. J. Phys. Chem. 1994, 98, 6187-6194. (5) Weil, J. K.; Smith, F.D.; Striton, A.J.; Bistline, R.G. Long chain alkanesulfonates and 1-hydroxy-2alkanesulfonates: Structure and property relations. J. Amer. Oil Chem. Soc. 1963, 40, 538 -541. (6) Tartar, H. V.; Wright, K.A. Studies of Sulfonates. III. Solubilities, Micelle Formation and Hydrates of the Sodium Salts of the Higher Alkyl Sulfonates. J. Amer. Chem. Soc. 1939, 61, 539-544. (7) Wright, K.A.; Tartar, H. V. Studies of Sulfonates. IV. Densities and Viscosities of Sodium Dodecyl Sulfonate Solutions in Relation to Micelle Formation. J. Amer. Chem. Soc. 1939, 61, 544-549. (8) Saito, M.; Moroi, Y.; Matuura, R. Dissolution and Micellization of Sodium n-Alkylsulfonates in Water. J. Coll. Inter.Sci. 1981, 88, 578-583. (9) Shahinyan, A.A.; Khanamiryan, L.A.; Aivazyan, O.M.; Nalbandyan, Yu. E.; Grigoryan, J.D. Influence of water-soluble polymers on the properties on the aqueous micellar solutions of an anionic surfactant and on the process of micellar-emulsion polymerization of styrene. Colloid Journal. 1987, 49, 521-529. (in Russian)


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(10) Minasyants, M.K.; Zakaryan, V.A.; Shahinyan, A.A.; Chistyakov, I.G. X-ray analysis of Nadodecylsulfate–water system. Kristallografiya. 1979, 24, 319-323. (in Russian) (11) Minasyants, M. K.; Shahinyan, A.A.; Chistyakov, I. G. X-ray structure study of pentadecyl sulfonate sodium lyotropic liquid crystal. Izv. Akad. Nauk Arm. SSR, Ser. Fizika, 1977, 12, 67-71. (in Russian) (12) Tabiryan, N. V.; Shahinyan, A.A. On the Role of fluctuations in the micelle-formation process. Colloid Journal. 1989, 51, 731-739. (in Russian) (13) Layn, K.M.; Debenedetti, P. G.; Prud’homme. R. K. A theoretical study of Gemini surfactant phase behavior. J. Chem. Phys. 1998, 109, 5651-5658. (14) Berthod, A.; Brooks, S. H.; Dorsey, J. G. Theoretical study of critical micelle concentration determination by flow injection analysis. J. Colloid and Interface Sci. 1988, 122, 514–520. (15) Chen, L.; Xiao, J.-X.; Ma, J. Striking differences between alkyl sulfate and alkyl sulfonate when mixing with cationic surfactants. Colloid Polym. Sci. 2004, 282, 524-529. (16) Poghosyan, A.H.; Yeghiazaryan, G. A.; Gharabekyan, H.H.; Koetz, J.; Shahinyan, A.A. A molecular dynamics study of Na–dodecylsulfate/water liquid crystalline phase. Molecular Simulation.

2007, 33, 1155–1163. (17) Poghosyan, A. H.; Arsenyan, L.H.; Gharabekyan, H.H.; Koetz, J.; Shahinyan, A.A. Molecular Dynamics Study of Poly (diallyldimethyl ammonium chloride) (PDADMAC)/Sodium dodecylsulfate (SDS)/ Decanol/Water systems. J.Phys. Chem. B, 2009, 113, 1303–1310. (18) Poghosyan, A.H.; Arsenyan, L. H.; Gharabekyan, H. H.; Falkenhagen, S.; Koetz, J.; Shahinyan, A.A. Molecular dynamics simulations of inverse sodium dodecyl sulfate (SDS) micelles in a mixed


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Na+ O O

S O

Sulfonate head

Alkyl chain

Figure 1. The schematically presentation of sodium pentadecyl sulfonate molecule.


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Figure 2. The interlayer spacing, area per molecule and sulfur-to-sulfur (layer) distance depending on simulation time for three Series of run.


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a) ~0.9ns point

c) ~2ns point

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b) ~1.5ns point

d) ~7ns point

Figure 3. Four stages (a,b,c and d) of SPDS bilayer self-aggregation process (Series I). For clarity, the periodic images were applied. The counterions and water hydrogens have been omitted for clarity. The sulfur is drawn as a VDW sphere, other components are given as sticks (sulfur – yellow; carbon – cyan; oxygen – red).


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assembling Self −  →

a) Starting configuration

c) Series II (last 300ns point)

b) Series I (last 100ns point)

d) Series III (last 400ns point)

Figure 4 (a,b,c and d). The cross-sectional view perpendicular to the bilayer plane. The counterions and water hydrogens have been omitted for clarity. The sulfur is drawn as a VDW sphere (color is yellow) meanwhile other components are given as sticks (carbons are cyan and the oxygens are red).


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Figure 5. The area per molecule and interlayer spacing heating curves obtained from annealing simulation. Solid and dotted profiles correspond to the heating and cooling scans, respectively. Heating/cooling rate is 1.5K per ns.


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Figure 6. The tilting angle heating curves obtained from annealing simulation. Heating rate is 1.5K per ns.


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a)

b)

Figure 7. The tilting angle (a) and molecule chain length (b) probability distributions.


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Figure 8. The C1-C8-C15 mean angle dependence on simulation time for three Series of run.


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Figure 9. Order parameter for three Series of run.


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Figure 10. The surface roughness function calculated in Series II and III.


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Figure 11. Radial distribution functions from sulfur to sodium counterions (top) and water oxygen (bottom) for three series.


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Molecular Dynamics Study of Intermediate Phase of Long Chain Alkyl

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