AOT Bilayer Adsorption on Gold Surfaces: A Molecular Dynamics

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AOT Bilayers Adsorption on Gold Surfaces: A Molecular Dynamics Study Armen H. Poghosyan, Maksim P. Adamyan, Aram A. Shahinyan, and Joachim Koetz J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b11471 • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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The Journal of Physical Chemistry

AOT Bilayers Adsorption on Gold Surfaces: A Molecular Dynamics Study Armen H. Poghosyana*, Maksim P. Adamyanb, Aram A. Shahinyana, Joachim Koetzc aInternational

Scientific-Educational Center of National Academy of Sciences, M. Baghramyan Ave. 24d, 0019 Yerevan, Armenia

bNational cInstitut

Polytechnic University of Armenia, Teryan str. 105, 0009 Yerevan, Armenia

für Chemie, Universität Potsdam, Karl-Liebknecht-Straße 24-25, 14476 Potsdam, Germany

Abstract A molecular dynamics study was done to reveal adsorption properties of sodium dioctyl sulfosuccinate (AOT) bilayers on gold Au(111) surfaces. Examining the rotational mobility of AOT molecules, we track that the correlation time of AOT molecules on adsorbed layer is much higher. The data estimating the diffusive motion of AOT molecule shows a substantially lower rate of diffusion (~10-10cm2/s) in the adsorbed layers in comparison to other ones. The results show that an adsorbed layer is more rigid, while the outer layers undergo considerable lateral and vertical fluctuations.

Introduction The ability of surface active agents to adsorb on metal surface is crucial in many areas and is of considerable importance1, and a number of papers devoted to surfactant/metal surface systems have been published by many authors using various techniques2,3. Many studies on adsorption have been done by discussing gold surface4-6, as gold is important in the context of nanomedicine due to its tunable optical properties7 and great functionality by surface modification1,8. The unique architecture of surfactants leads to self-assembly when the latter is in polar or non-polar solvent, and are widely used nowadays, especially for technological issues. The level of surfactant adsorption on gold surface strongly depends on nature and structure of surfactants9. A variety of surfactants has been used as a template, benchmarking sodium dodecyl sulfate (SDS)10, cetyltrimethylammonium bromide (CTAB)11-13, alkyl thiols14,15, sodium bis(2-ethylhexyl) sulfosuccinate (aerosol-OT, AOT)16-20 and etc.21 to understand the mechanisms of growth of the nanoplates22. The adsorption properties of doubled-tailed surfactants, namely AOT, phosphatidylcholin (PC) and benzylhexadecyldimethylammonium chloride (BDAC) on gold surface have been intensively reported by many authors5,20,22-28 using different methods, including molecular dynamics20. AOT is a doubled-tailed anionic surfactant and widely used in many areas. It is known that dichain branched AOT at higher concentrations form a lamella29,30. The X-ray diffraction data of lyotropic liquid crystalline state of AOT-water (from 10% to 70% of AOT) have been reported by Fontell29, and according to experimental data, the 1 ACS Paragon Plus Environment

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area per molecule is ~65 Å2 (~50wt%) and the fundamental repeat distance is estimated to be ~ 39-40 Å. Similar results were obtained by Garza et al.30 by using the atomic force microscope technique, observing a lamellar interlayer spacing of 40.4±0.3 Å for the AOT (51.57 wt %)/water system. Atomistic molecular dynamics simulations allow to study a more detailed structure and the kinetic of the monolayer formation20,31. Recently, the series of MD simulations have been done20 to reveal an adsorption features of AOT20 and AOT/BDAC31 mixed systems adsorbed on Au(111) facet. Recently it was shown that AOT-stabilized gold nanoplatelets show special interesting features, e.g., in Raman scattering5,23. Therefore, this paper focuses on AOT bilayers adsorption properties on Au(111) surface, as well as the kinetic and orientational features of AOT multilayers by using atomistic molecular dynamics method. We organize the paper as follows: Section “Construction and Simulation details” describes all technical details concerning the construction of the mentioned systems, as well as all the information concerning the forcefield and simulation protocols. “Results and discussion” section includes all results, while “Conclusion” section summarizes the main issues of the paper.

Construction and simulation details Construction details: SERIES I - 84 AOT molecules were located such as to build a bilayer. The bilayer was installed on Au(111) facet and the system was solvated using GROMACS genbox module (with dimension of ~5.4x5.4x4.4nm3), where the number of water molecules were about 4700. A gold (111) facet was taken from GOIP-CHARMM package32. 84 Na+ counterions were loaded to maintain neutrality via GROMACS genion tool. The system was subsequently minimized for 5000steps and after a small NVT simulation run (~1ns) was done. A production run (200ns) was performed in NPT ensemble. SERIES II - Two AOT bilayers (multi-stacks) was extracted from simulation (Series I) and was installed in gold (111) facet. Further, the system was solvated via genbox module (with dimension of ~5.4x5.4x7.7nm3) and corresponding counterions (168 Na+) were added to neutralize the system. A geometric optimization was applied for 5000 steps (steepest algorithm) and then, a small NVT ensemble simulation (~1ns) was carried out. A production run was carried out in NPT ensemble for 200ns. Force fields and parameters: All simulations were performed via GROMACS software package33 and the CHARMM27 all-atom force field protocols for AOT surfactant were used as described by Abel et al.34. The GOIPCHARMM all-atom force field was used to describe the gold surface32. It is known that the mentioned forcefield conception considers two types of the gold atoms: a virtual (AUC/0.3 and AUI/0) and real atoms (AuS/-0.3 and AuB/-0.3) and the parametric set (Au to any atoms) was defined based on experiments and DFT/Moller-Plesset theories32. As the water models the SPC35 was used in all simulations. For production run, the Nose-Hoover thermostat36 with a coupling constant of 0.5ps was applied to fix the temperature (300K), and the well-known Parrinello-Rahman37 approach with a time constant of 1.0ps was used to couple the pressure. Note that the temperatures of all components were independently controlled. To fix all the bonds, LINCS algorithm38 was used. The PME39 with a cut-off at 1.1nm was used for electrostatic interactions and for van-der-Waals interactions, the truncation was 1.1nm. The timestep of simulation was set to 1 fs and the equation of motion was integrated using Verlet-integrator40. The coordinates/velocities were recorded at every 0.1 ns, while for subsequent analysis, the last 50 ns of production trajectories was used. The Linux clusters (IIAP NAS RA)41 and partially other resources were used. The visualizations have been represented by using VMD graphical package42.

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Results and discussion AOT bilayer on Au(111) surface: Before calculating the main structural parameters, the visual inspection of trajectories have been done. In Figure 1, the snapshots coming from last frames are shown for two cases: one bilayer at Au(111) and two bilayers at Au(111) surface.

a)

b)

Figure 1. The frames at 200ns timepoint from both simulations (a) a bilayer on Au(111) surface; (b) two bilayers on Au(111) surface. The colors are: gray - AOT, pink – gold surface. The AOT sulfurs: vdw spheres, the water, Na+ and hydrogen atoms were omitted for clarity. Captured via VMD package42.

As it is seen from provided snapshots in Figure 1, the AOT bilayer is adsorbed at a gold surface. Here, as in case of AOT micelles20, after a few nanoseconds of simulation, the AOT molecules moves to the surface and stay until the end of simulation, i.e., we track a fast adsorption within a few nanoseconds and the aggregate migrates to Au surface, where the distance of adsorbed AOT sulfurs and gold surface is estimated to be in the range of ~0.360.38nm. Note that less than 5 Å adsorption of AOT sulfurs has been reported43 on Ag facets. The parametric evidence of AOT lamella adsorption on gold surface was also done by calculating the following: (a) the parameter which describes the surface roughness; (b) the rotational correlation functions which describe how fast the surfactant rotates around its axis; (c) sulfur-to-sulfur distances depending on production time; (d) the various densities and the diffusion coefficients of molecules. To explore the residual roughness of the layers, the vertical deviations of sulfurs from layer center of mass were examined. The surface or layer roughness which estimates the surface quality and undulations is calculated as 

layer



layer 2 ( z ilayer  z com )

mean


(1)


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layer where z ilayer is the z coordinate of i sulfurs and z com -is C.O.M. of a layer. It is worthy to

mention that given parameter is bit far from roughness classic treatment44 and in some manner it represents the vertical displacement of sulfurs from layer C.O.M. (center of mass) or average deviation of sulfurs from a plane fit of the surface. The layer roughness depending on production run is monitored in Figure 2. For Series I (an AOT bilayer on gold surface) we track that the roughness value is nearly ~ 0.3 Å for first layer, while the second layer indicates significant roughness. However, when the second bilayer is Figure 2. The surface roughness depending on production runs for all cases. embedded on Au(111) surface, the roughness value is decreases (Series -II). For the layer close to gold surface, the roughness value is low (~0.28 Å) and for layer–II, we see that in average, the roughness is nearly 1.15 Å. For both layers, we track that the roughness values are much lower as compared with Series–I. As for the other layers far from gold surface, we see large fluctuations and in average the following values are obtained: layer-III ~ 1.36 Å and layer-IV ~ 1.56 Å. Thus, we argue that in presence of gold surface the formation of well-ordered layers occur. Unfortunately, there is no direct Figure 3. The average rotational autocorrelation function for all layers (both experimental ways to cases - Series I and II) over long time. A second order of Legendre polynomial estimate and compare was applied. the roughness in mentioned context, although the amplitude/magnitude of layer fluctuations can be experimentally estimated18,4547 from reflectivity profiles. Studying the adsorption of a stack of AOT multilayers onto the sapphire surface, the authors conclude that an increase of the temperature in the range of 1530oC leads to the decrease of fluctuations18. The adsorbed layer roughness ~2 Å was 4 ACS Paragon Plus Environment

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reported47 at sapphire surface. The adsorption of sodium bis(2-ethylhexyl) phosphate (NaDEHP) (a phosphorus analogue of the surfactant AOT) at the alumina surface is reported and they claim that a NaDEHP is adsorbed on alumina as a bilayer in analogue with AOT, where the experiments show that a bilayer is a bit thinner and denser packed46. It should be also noted that similar parameter has been discussed in [12], where the MD study of adsorption properties of cetyltrimethyl ammonium bromide (CTAB) bilayer on gold facets have been reported. The authors studied the z-axis fluctuations of CTAB molecules in outer layer (layer-II), where the full width at half maximum (FWHM) of the distribution (CTAB on Au (111)) ~0.67 Å was obtained. Unfortunately, there is no information about the z-axis fluctuations in inner layer to estimate the differences between leaflets.

Figure 4. The bilayer thickness depending on production runs for all series of run.

Figure 5. The density profile for each case.

Examining the rotational features of AOT molecules, we have calculated the decay of the rotational 1 autocorrelation function where the following atom triplets were chosen: S - AOT sulfurs; C tail 2 terminal carbons for AOT chain and C tail -terminal carbon for another chain. Note that the

mentioned parameters were calculated via GROMACS g_rotacf module for all layers and subsequently averaged over all AOT molecules. The corresponding curves are monitored in 5 ACS Paragon Plus Environment


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Figure 3. We see that the rotational dynamics of AOT molecules of layer-II, III and IV is the fastest, while the correlation time of AOT on absorbed layer is much higher, i.e., AOT molecules which are far from Au surface rotates fast than others, and the rotational mobility increases with the further move along the z-axis. If we compare the results of layer – II for both cases (Series-I and II), we see that the presence of a second bilayer has a strong effect on the rotational mobility of AOT molecules, i.e. in case of Series –I, the AOT molecules in layer-II (red, dotted lines) rotate faster than layer-II AOTs in case of Series-II (red, continuous lines). To estimate bilayer structural parameters, we have determined the bilayer thickness by averaging the sulfur-to-sulfur distance over the production run. Note that the distance between two sulfurs have Figure 6. The distribution of angles of adsorbed AOTs for both cases. been calculated from different calculations: Averaged values coming from density curves (data not shown), as well as, dynamically depending on production run. The bilayer thickness for all cases is shown in Figure 4 as a function of production run. One can see that for bilayer-I, the value is almost the same (~1.61 nm) for Series-I and II, i.e. the second embedded bilayer has relatively little effect on structure of the adsorbed bilayer. However, the second bilayer in Series – II seems to change over the simulation time and corresponds to ~1.65 nm. Hence, from structural point of view, there is nearly no difference, i.e., the resulting thickness profiles are very similar and has the same values in both cases when we discuss the adsorbed bilayer. Meanwhile, for bilayer – II (close to bulk water), the thickness is a bit large. It should be noted that the density curves (see Figure 5) also show the similar behavior when we estimate the distance between two peaks of headgroup sulfurs. The density curve for Series-I shows two peaks: The first one is well-pronounced with trapped sulfurs close to gold surface, and the second one is more diffused. It is obvious that the first peak correlates with the distribution of adsorbed sulfurs at a distance of ~0.36-0.38 nm, while the second peak corresponds to a bumpy layer in lamella. The component density for Series-II also shows the layering of AOT sulfurs and therefore, four peaks appear indicating the trapped and fluctuated sulfurs atoms. To examine the orientation of hydrocarbon chains in the adsorbed layer (Layer-I), we define the following angles: i) α – hydrocarbon tail tilt angle by averaging the angle between AOT sulfur – terminal carbon and plane normal vector; ii) β (so called «stretching angle») 6 ACS Paragon Plus Environment

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1 2 C tail  S AOT  C tail

1 2 made by the vectors S  C tail and S  C tail in AOT molecule. The normalized

angle distribution curves for both series of run have been shown in Figure 6. The molecular tilt or hydrocarbon tail tilt angle is around α~19o-20o for the AOT molecules in adsorbed layer. Note that the similar values have been also reported in literature12,14,15,48. Discussing the molecular tilting angle of alkyl thiol [SH(CH2)5CH3] molecules onto a gold substrate, the authors14 argue that the average molecular tilt is about 20 with respect to the surface normal, where the molecular tilt angle defined by the vector from the average headgroup position to the average tailgroup position. The computational study of the orientational and structural features of adsorbed alkanethiol chains on three different metal surfaces (Au, Ag, and Pt) have been examined by Alexiadis and coworkers48. The provided time evolution and normalized distribution curves of C16S tilt angle shows the following values: 30.9  1.8 (Au(111)), 20.2  3.0 (Pt(111)) and 16.8  5.0 (Ag(111))48. The nearly perpendicular orientation (~5o - layer-I and ~2o for layer-II) has been reported for CTAB molecules12 suggesting more tilting orientation of CTABs located in the layer close to gold surface. It is interesting to note that the mentioned studies were done on surfactant containing one chain, while the studies on doubled-tailed surfactants are lacking. To well understand the kinetics of AOT molecules, the diffusion coefficients ( D  and D )

were

also determined (based on last 50 ns trajectories) via g_msd GROMACS module. The D  and D are the diffusion coefficients Figure 7. The diffusion coefficients ( D  and D ) depending on layers for both cases. perpendicular and parallel to the bilayer plane normal. Note that the diffusion coefficients were determined from a linear fit to MSD (time interval between 5 to 45 ps). All data are summarized in Figure 7. To distinguish parallel and perpendicular direction diffusive motions, we represent the two different diffusion coefficient - D and D . As one can see from the plots, both components ( D  and D ) increase when we move from the first layer to the last. Note that the diffusion

coefficient of AOT molecules near bulk water is ~10 times greater than adsorbed AOTs.



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Conclusions A series of long MD runs have been carried out to reveal the adsorption properties, as well as the kinetic and orientational features of AOT bilayers onto gold surface. As expected20, an aggregate migrates to gold surface within a few nanoseconds (less than 10ns) and the distance of adsorbed AOT sulfurs and gold surface is estimated to be nearly 0.36-0.38 nm. The detailed study of z-axis fluctuations describing the layer roughness was done. We see that the roughness of adsorbed layer is much lower compared with other layers, which claims that is more rigid, while the outer layers undergo considerable lateral and vertical fluctuations. The data coming from rotational autocorrellation function shows that AOT molecules of layer-II, III and IV are the fastest, while the correlation time of AOT on absorbed layer is much higher, i.e., the rotational mobility increases with the further move along the z-axis. The estimated values for the bilayer thickness (sulfur-to-sulfur distance) show almost no differences, i.e. the second embedded bilayer has relatively little effect on the structure of adsorbed bilayer. The interesting information comes from the orientation of hydrocarbon chains in the adsorbed layer and we track that the AOT tail inclination angle is around α~19o-20o with respect to the surface normal. Note that the tilting angle is comparable to that of alkyl thiols14, while for CTAB molecules a nearly perpendicular orientation was reported12. We obtained the both components of diffusion coefficient for each layer and we claim that the diffusion of adsorbed AOT molecules is ~10 times slower than that in other layers. The existence of a more rigid adsorbed AOT-bilayer explains special features of AOTstabilized gold nanoplatelets very well. Due to the fast formation of the first AOT-bilayer on the Au(111) platelet surface the depletion flocculation is enhanced23, and onto the more rigid bilayer a reloading with oppositely charged components, i.e., polyethyleneimine (PEI)49 and BDAC5 becomes possible. This opens the possibility to form undulated superstructures in the AOT-bilayer49.

Acknowledgments The research has been co-funded (A.H.Poghosyan) by the European Commission under the H2020 Research Infrastructures contract no. 675121 (project VI-SEEM). Sincere thanks to Dr. Hrachya Astsatryan for providing us with the access to the computational resources.

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AOT Bilayer Adsorption on Gold Surfaces: A Molecular Dynamics

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