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Molecular Dynamics Simulations of Specific Anion Adsorption on Sulfobetaine (SB3-14) Micelles Diego Paula Santos, and Ricardo L. Longo J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b12475 • Publication Date (Web): 24 Feb 2016 Downloaded from http://pubs.acs.org on February 26, 2016

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The Journal of Physical Chemistry

Molecular Dynamics Simulations of Specific Anion Adsorption on Sulfobetaine (SB3-14) Micelles Diego P. Santos, Ricardo L. Longo*

Departamento de Química Fundamental, Universidade Federal de Pernambuco, 50.740-560 Recife - PE, Brazil. E-mail: [email protected]; Fax: +55 81 2126 8442; Tel: +55 81 2126 8459

Abstract

Structures of zwitterionic micelles of 61 sulfobetaine SB3-14, CH3(CH2)13[N+(CH3)2](CH2)3SO3– , molecules in water and in 0.15 mol L–1 and 0.015 mol L–1 NaX (X = F–, Cl–, Br–, I– and ClO4–) aqueous solutions were investigated by atomistic molecular dynamics simulations. The micelle presents a near-spherical shape and an average angle of 110° between the zwitterionic headgroup and the alkyl tail that provides an L-type shape for the surfactants and exposes the positive charged ammonium groups to the solution. This allows anions to adsorb at the surface of the micelle and the amount of adsorbed anions follows the Hofmeister series: F– < Cl– < Br– < I– < ClO4–, which directly correlates to the measured values of the zeta-potential. The size and shape of the micelle are not affect by the salt, except the solvent accessible surface area that decreases for strongly adsorbing anions such as ClO4– due to the approximation of the headgroups between

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adjacent surfactants. Indeed, the ion distribution show the formation of ion-pair-type species between ammonium and perchlorate, which indicates that the adsorption is directly related to the easiness of the anion to partially lose its hydration shell.

KEYWORDS: Hofmeister series, atomistic simulations, zwitterionic surfactant.

Introduction Zwitterionic-based materials have many applications because they can be designed with unique and tuned properties, they are relatively easy to obtain and synthesize, and are quite versatile.1-6 Indeed, the presence of cationic and anionic groups with charge densities that can be tuned and separated by controlled distances and spacers provides a very large number of designing possibilities of new zwitterionic materials. Interestingly that, in spite of being intrinsically neutral, zwitterionic micelles and surfaces can adsorb ions, preferably anions.7-15 This adsorption generates a negative electrical charge at the surface of the micelles and causes the incorporation of cations in the micellar pseudophase.16-21 The increase of ion concentration has been explored in the catalysis of several chemical reactions and other processes (e.g., acid dissociation) mediated by zwitterionic micelles.16-28 The adsorption of anions is specific in the sense that weakly hydrated anions such as large monovalent species (PF6–, ClO4–) are adsorbed preferably to strongly hydrated anions (F–, OH–).14,19 This ion specific adsorption followed by the incorporation of cations into the micellar pseudophase due to electrostatic effects is called chameleon-like nature of zwitterionic micelles.16-21 The dependence of the chameleon nature of sulfobetaine micelles upon the nature of the aqueous electrolytes and the chemical structure of the zwitterionic surfactant have been


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investigated

employing

equilibrium

and

kinetics

effects,

several

physical-chemical

measurements and techniques. Indeed, the adsorption properties of sulfobetaine micelles depend on the structure of the zwitterionic surfactants, namely, CnH2n+1N+R2(CH2)mSO3–, where for R = CH3, they are denoted simply as SBm-n. The effects of the linear alkyl tail length (n = 10 to 16), the size of the methylene tether group (m = 2 to 4), and the N-alkyl group length (R = CkH2k+1 with k = 1 to 4) have been investigated.8,11,14-16,22 In addition, the effects of substituents in the tether group, the nature of the positively charged group in the zwitterionic headgroup, and the dipole orientation of the headgroup upon the properties of sulfobetaine micelles were also explored.20-21,30 Regarding the dependence on the electrolyte, for instance, the anion adsorption at SB3-14 decreases as ClO4− > I− > Br− > Cl− > F−,14,19 which follows a Hofmeister-type series31-33 and may be related to the softness of the anions and their hydration energies. These observations are intriguing because a proposed idealized structural model7-13 of (near-)spherical micelle formed by (quasi-)linear monomeric surfactants,

, would

provide a negative surface, which would not be an environment to adsorb anions. However, by solving the Poisson-Boltzmann equation using this structural model it was shown that the electric potential within the headgroup region is positive and thus could attract anions,7-9,12-15 which then may penetrate the surface and form an ion-pair with the quaternary ammonium.7-10,13,17,19,21 Because this structural model assumes that anions penetrate the surface of the micelle, it predicts that the surface coverage should be small and that the anion should lose its hydration and its mobility. Indeed, for one of the most adsorbing anion (ClO4−) the surface coverage is ca. 0.2-0.3, 15,16

whereas a clear relationship between anion adsorption and its hydration enthalpy has been

observed. However, XAFS, NMR and other measurements suggest that adsorbed anions at zwitterionic micelles are partially hydrated and quite mobile.16,34-35 To reconcile these


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observations, modifications to this idealized structural model, such as bending of headgroup with respect to the aliphatic chain as well as intra and intermolecular interactions within and between the headgroups, were proposed.10,20-22 For instance, the bending of the headgroups could expose the positive charge of zwitterion and explain the specific anion adsorption. On the other hand, a robust structural model should also explain the changes in the properties of zwitterionic micelles caused by addition salts in the aqueous solution. In fact, a detailed structural model and a microscopic understanding of these systems is important to explain their physical and chemical properties, which requires detailed information about the interactions of the ions with micelles and solvent, the equilibrium structures and fluctuations (dynamics) of the micelle-ion systems, in addition to many-body effects. The properties of these systems have been probed by several experimental techniques, such as, NMR,36-38 radiation scattering,39-41 zeta-potential,42-43 neutron diffraction,44-45

electrophoresis,12,46

vibrational47-48

and

time-resolved

fluorescence

spectroscopies.49-50 Even with this wide variety of experimental methods used to probe micelleion systems, it is still very difficult to obtain detailed structural data and information about surface phenomena at the molecular level. To this end, molecular simulations have become an important and efficient tool to describe the structure, fluctuation and dynamics of micelles.51-55 Molecular dynamics (MD) methods using atomistic or coarse-grained potentials are the most commonly used approaches,56-59 because they provide results (structures, distributions, diffusion, viscosity, etc.) at the molecular and macroscopic levels. This allows direct comparisons with experimental data and microscopic interpretations and comprehension. In particular, during the last two decades, the interactions of molecules and ions with micelles have been studied in detail by molecular simulations.60-64 For instance, micelles of sodium dodecyl sulphate (SDS) and their interactions with sodium ion65 and pyrene66 have been investigated by MD methods. Micelles


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with 72 monomers of SB3-12 was simulated by atomistic MD in the presence and absence of the spin probe methyl 5-doxylstearate.67 Micelle formation from 50 monomers of CH3-(CH2)11N+(CH3)2-CH2-CH(OH)-CH2-SO3– randomly distributed in water was investigated by coarsegrained MD simulations.63 So far, these are the only reported simulations of sulfobetaine micelles in addition to the investigations of monomeric sulfobetaines (CH3)3N+-(CH2)2-SO3– and RR’R”N+-(CH2)-SO3– (R, R’, R” = H and CH3) in ionic aqueous solutions.68-71 Thus, MD simulations of SB3-14 micelles in several salt aqueous solutions were performed. We aimed at evaluating and quantifying the effects of ion concentrations and ion nature on the structure and surface properties of the micelles. A structural model risen from these simulations can reconcile several experimental observations and the specific anion adsorption was characterized and comprehended at the molecular level. Simulation Details Building the systems The

zwitterionic

micelles

were

CH3(CH2)13N+(CH3)2(CH2)3SO3–

constructed

with

61

molecules

of

SB3-14,

(N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate),

according to the average experimental aggregation number of 61±2.19 Because the sulfobeine micelles are rather insensitive to the ionic strength, the same number of SB3-14 was employed in both high (0.15 mol L–1) and low (0.015 mol L–1) ionic strengths simulations. The initial structures of the micelles were built with the packmol program72 such that the methyl groups of the hydrophobic tail were inside a 5 Å radius and the remaining of the molecules within 30 Å radius. The ions of the salts NaF, NaCl, NaBr, NaI and NaClO4 were distributed randomly inside the simulation box according to the ionic strength. The dimensions of the simulation boxes (ca.


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108 Å × 108 Å × 108 Å) were chosen to have at least a 20 Å layer of water molecules around the micelle, with the number of water molecules consistent with a density of 1 g cm–3. System parameters The GLYCAM 06 parameters73 were used for all atoms of the SB3-14 molecule, except the sufonate group that was described by the GAFF parameters.74 The atomic partial RESP charges were obtained with the HF/6-31G(d) method using the R.E.D. IV Tools.75 The SPC/E model76 was used for the water molecules, whereas the modified Aqvist parameters for ions in SPC/E water were employed for all ions,77 except for ClO4–.78 A complete set of parameters employed in these simulations can be found in the Supplementary Material file. Molecular Dynamics Simulation Protocol Steepest-descent and conjugate gradient combined methods within the sander module of the AMBER 1279 program were used to minimize the energy of the initial structures. A force constant of 20 kcal mol–1 was employed to keep the structure of the micelle fixed and thus relaxing the water molecules and ions during the initial energy minimization step, which was followed by an energy minimization of the entire system without any constraints. For the equilibration step, MD simulations were performed during 10 ns in a NPT (P = 1 atm, T = 298 K, N = 61 SB3-14 + 22811 H2O + 61 NaX for 0.15 mol L–1 and 61 SB3-14 + 22811 H2O + 6 NaX for 0.015 mol L–1) ensemble with the Berendsen thermostat and barostat (coupling constants of 2 ps) using a 2 fs time-step. At the end of the equilibration, the main properties (energies, density within 0.99-1.05 g cm–3, volume and box dimensions) have converged. A


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production MD run of 100 ns was performed for each system following the same protocol used in the equilibration step. The intermolecular interactions were calculated with the Lennard-Jones potential within an 11 Å cutoff radius and the Coulomb contribution to the potential energy was computed with the PME (Particle Mesh Ewald) method and cubic periodic conditions were employed. During the MD simulations, all bonds involving hydrogen atoms were kept fixed by the SHAKE algorithm. The trajectories were saved every 5 ps for analysis. All MD simulations were performed with the PMEMD module of AMBER 12 program installed in GPUs. All analyses were performed using the last 50 ns of the trajectories.

Results and discussion Before describing the details of the structure and adsorption on SB3-14, it is relevant to justify the computational model. Micelles of sulfobetaines, especially SB3-14, are highly stable in the sense that their structure and aggregation number are practically not disturbed by changes of pH nor the presence of cations and anions up to concentrations of ca. 0.05 mol L–1. Indeed, the aggregation number under several conditions do not vary more than 10% from the average observed value of 61.19,80 In addition, their globular structures were determined by different techniques with hydrodynamics radius practically invariant under quite distinct conditions.19,80 Thus, we have chosen an initial spherical structure with 61 SB3-14 extended units. We have chosen the fully extended conformation of the sulfobetaines, so that no biases were introduced and the simulation would provide the most probable structures for the SB3-14 units.


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Care must be exercised when comparing these simulations with experiments. For the high concentration of salt (0.15 mol L–1), the number of ions in the simulation box is coincidently the same number of SB3-14 surfactants and thus, they have the same concentration. Whereas in the low salt concentration (0.015 mol L–1), the surfactant concentration is 10 times larger than the ion concentration. In experimental measurements involving SB3-14, the surfactant concentration is, for instance, 0.05 mol L–1,19 while the salt concentration varies from 0 to 0.04 mol L–1. Therefore, the comparisons with experimental should be performed for the largest salt concentration because it approaches the concentrations used in the 0.15 mol L–1 simulation. Whereas simulations employing 0.015 mol L–1 of NaX should be compared to experimental data corresponding to 0.05 mol L–1 SB3-14 and 0.005 mol L–1 of salt. Thus, considering the assumptions on constructing the model of the SB3-14 micelle63,67 and changes in the aggregation number for high concentrations of NaX (X = I–, ClO4–, BF4–, PF6–),19 the results of these simulations and their predictions are limited to salt concentrations lower than 0.15 mol L–1. Because this is the first molecular simulation of sulfobetaine-type micelles, we present initially a detailed analysis of structural properties of the SB3-14 micelle in water and then in NaX (X = F–, Cl–, Br–, I–, and ClO4–) aqueous solutions. In addition, dynamical properties of the micelle and its surroundings were investigated. The reliability of the simulations was assessed by comparisons with experimental macroscopic quantities, which validated the microscopic details that may provide explanations for the observed properties of this micelle, especially its adsorption behavior. Structural Properties of Aqueous SB3-14 Micelle


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Figure 1 shows the initial and final (after 100 ns) structures of the SB3-14 micelle in pure water. Clearly, the (near-)spherical globular structure of the micelle is observed, in agreement with

the

packing

parameter

of

the

SB3-14

molecule,

namely,

⁄ 404 Å 19.21 Å 369 Å 0.05 1⁄3, where 27.4 26.9 Å and 1.5 1.265 Å are the volume and maximum length of the hydrophobic tail, respectively, for carbon atoms, and is the headgroup area obtained by the ratio between the total area of a sphere of radii 19.21 Å and the aggregation number of the micelle.81

Figure 1. Molecular dynamics simulation of the initial (t = 0 ns, left panel) and final (t = 100 ns, right panel) structures of the SB3-14 micelle in pure water. The water molecules are not shown to aid visualization. Notice the different choices of atom sizes used to emphasize the groups at the surface (spheres) and in the core (tubes).


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During the dynamics, we observed changes in the surface mainly associated with burying the alkyl groups and exposing the polar groups (ammonium and sulfonate). Despite this disordering, the micelle preserves its globular shape. The eccentricity (ε) provides a measure of the shape of an object, where a value approaching zero describes a nearly spherical object, whereas values close to one correspond to very thin oblate or prolate objects. The calculated principal moments of inertia of the micelle can yield the eccentricity as,82 ε 1 −

!"# ⁄〈

〉, where

!"#

is the smallest eigenvalue of the principal

moment of inertia tensor and 〈 〉 is the average of three principal moments. The average eccentricity is 0.097 indicating a nearly spherical shape of the SB3-14 micelle, with a slightly prolate

&

>

≅

deformation according to the average relationship 1.21:1.08:1.12.

Size of the SB3-14 Micelle The hydrodynamics radius, usually obtained by light scattering, is an important experimental measurement for describing micelles. This radius is also relevant for creating models of the micelle that can be used in the calculation of electric potential and explains its interactions with ions and values of zeta-potential. For near-spherical micelles, the hydrodynamics radius can provide their effective sizes and used to comprehend several properties such as their electrophoresis behavior. In computational simulation, it is possible to calculate the radius of a sphere that circumscribes the micelle, which is denoted as radius of gyration, )* , by the root mean square distance from each atom of the micelle to their centroid. The average micelle radius, )+ , can be obtained as, )+ ,5⁄3 )* ≅ 1.291)* , for a rigid sphere with constant density.83 To correlate the average micelle radius ()+ ) with the hydrodynamics radius, the position of the first peak in the radial distribution function between the water oxygen and the


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sulfur atom in the headgroup, -./012 , is added to )+ and the radius of the water molecule (1.4 Å) is subtracted.84 This provides the effective micelle radius that can be estimated, for instance, by SAXS measurements and is one of the primary structural features of micelles. In the case of the SB3-14 micelle, we obtained )+ = 22.53 ± 0.18 Å and -./012 = 2.67 Å, thus yielding an effective radius of 23.8 ± 0.18 Å (see Figure 2B), which is in very good agreement with the reported hydrodynamics radius within 24–26 Å.80,85 We have also calculated the average radius of the hydrophobic tail for the SB3-14 micelle (20.4 ± 0.18 Å), which is very close to the value (21.0 Å) calculated for the fully extended alkyl chain (see Figure 2A). The difference between the average micelle radius ()+ ) and this alkyl chain provides an estimated value of 2.1 Å for the SB3-14 micelle hydrophilic head average radius. This small value of the polar head radius suggests that headgroups are tilted with respect to the alkyl chains or that the headgroups are coiled or folded due to the self-interactions between the sulfonate and ammonium groups. This is an important structural feature of sulfobetaine micelles because it affects directly at their ability to generate electric fields responsible for adsorbing anions. It is noteworthy that the average size of the surfactants in the micelle (22.6 Å), calculated by averaging the distances between the centre-of-mass of the oxygen atoms in the headgroups and the carbon atom in the termini CH3 group of the alkyl tail, is very close to the effective radius of gyration (22.5 Å), which corroborates the small values of the eccentricity and the micellar shape being nearly spherical.


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(A)

Page 12 of 39

(B)

Figure 2. SB3-14 surfactant fully extended size (A) with intramolecular distances (COM stands for centre-of-mass) and (B) the representation of the effective radius of gyration.

Tilt angle between head and tail The tilt angle (θ) is determined by the vectors of the polar head and the apolar tail. The headgroup vector is defined by the nitrogen and sulfur atoms, whereas the alkyl tail vector by the nitrogen and the carbon atoms at the termini CH3 group, as depicted in Figure 3. The distribution of the tilt angle is almost normal with an average value of 107° ± 8° (Figure 3), indicating that in the micelle, the monomer is far from fully extended (θ = 180°) and the tilting of the headgroups improves their intermonomers electrostatic and van der Waals attractive interactions. It is noteworthy that this tilting of the headgroups with respect to the alkyl chain exposes both charged groups (sulfonate and ammonium) to the environment, which improves hydration and also allows adsorption of weakly hydrated ions.


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This improved interaction between the headgroups of the surfactants can lead to higher stability and is thus responsible for the small value of the critical micellar concentration, for the small value of the aggregation number and for a decrease of the radius of gyration of the micelle. Indeed, it was experimentally suggested the formation of L-shaped intermolecular ion-pairs (nearly parallel to the surface in membranes) is the most favorable conformation and molecular packing of zwitterionic surfactants because it reduces hydrophibic repulsions and increases attractive intermolecular van der Waals and ion-pair interactions.86

Figure 3. Angle distribution between polar head and hydrophobic tail. The Gaussian fitted values are 〈θ〉 = 107° (average) and 34 = 8° (width). Solvent accessible surface area The Connoly method87 was used to obtain the solvent accessible surface area (SASA) of the SB3-14 micelle that can be separated into hydrophobic and hydrophilic contributions, where the latter SASA may be directly correlated to adsorption of ions. Indeed, a large hydrophilic SASA value would lead to a larger surface area for ion interaction and adsorption. Figure 4 presents the temporal dependence of the total SASA for the SB3-14 micelle as well as the hydrophobic (alkyl


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tail) and hydrophilic (zwitterionic head) contributions. The calculated average values are 19450 ± 740 Å2, 12397 ± 431 Å2, and 7053 ±440 Å2 for the total, hydrophilic and hydrophobic SASA, respectively. Thus, nearly 64% of the solvent accessible surface area is due to the comparative small hydrophilic part of the surfactant, which shows the large affinity of the zwitterionic headgroups towards water. This large hydrophilic SASA at the SB3-14 micelle also demonstrates its usefulness as catalyst in several organic reactions that involve polar and/or charged species.

Figure 4. Solvent accessible surface area (SASA) for the hydrophobic alkyl chain tail (red), the hydrophilic zwitterionic head (blue) and the total SASA (black) of the SB3-14 micelle. Radial Distribution Function of Atoms on Micelle SB3-14 Structural features of micelles can also be accessed and quantified by the radial distribution function of a given atom to the centre-of-mass (COM) of the micelle. More specifically, the carbon atoms in the alkyl chain of the apolar tail were grouped into C1-C10 and C11-C14, where the latter carbons atoms are the nearest to the nitrogen atom of the polar headgroup. Similarly, the carbon atoms in the propyl group of the zwitterionic head were also grouped for averaging


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the radial distribution functions. These results are presented in Figure 5 that shows a fast decrease in the density of carbon atoms of the alkyl chain tail from the centre-of-mass (COM) of the micelle. The zwitterionic headgroup represented by the nitrogen and sulfur atoms presents maximum densities at 20 and 22 Å, respectively, which corroborates the results and analyses obtained for the effective radius of gyration. In addition, we observed that both groups of carbon atoms near (C11-C14) and within (propyl) the polar head are very close to the surface, which explain the significant (36%) hydrophobic SASA contribution and improve the intermonomer interactions.

Figure 5. The average distribution of different groups with respect to the centre-of-mass of the micelle. Carbon atoms C1-C10 in the hydrophobic tail core are grouped (green) as well as carbon atoms C11-C14 in the alkyl tail nearest the polar head (pink). The zwitterionic polar head is


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presented separately, namely, the nitrogen (black) and sulfur (red) atoms, whereas the carbon atoms in the propyl group (blue) are grouped. The distribution of water molecules is shown (brown) up to 500; however, it increases until ca. 850 and saturates after 28 Å.

Hydrophobic End-to-End and Hydrophilic Head-to-Head Distances The distribution of distances between carbon atoms C1 (termini methyl group) and C14 (nearest to the ammonium group), denoted as hydrophobic end-to-end distances, provides information about the flexibility of the alkyl chains within the core of the micelle. For fully extended all-anti conformation this distance is ca. 16.4 Å (see Figure 2A) using standard geometric parameters. The observed peak in this distribution at 14.6 ± 0.4 Å shown in Figure S1 suggests that the alkyl chains in the SB3-14 micelle are mostly in the all-anti conformation. However, gauche conformations are also present to account for the shifted peak towards smaller value than that estimated for an all-anti conformation (16.4 Å). Indeed, these gauche conformations were observed during the simulation and they cause a stronger attraction between the alkyl tails, increasing the packing of the micelle structure and thus leading to a smaller average effective size of the micelle compared to that predicted by a fully extended sulfobetaine surfactant. The distribution of the distances between the sulfur and nitrogen atoms within the same zwitterionic headgroup, denoted as head-to-head distances, is presented in Figure S1. This distribution provides information about the intramonomer interactions within the polar headgroups of the SB3-14 micelle. In the absence of self-interaction between the ammonium and sulfonate groups, the propyl group would be fully extended and the distance between the nitrogen atom in the ammonium and the centre-of-mass of the oxygen atoms in the sulfonate


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should be ca. 6.0 Å (see Figure 2A). The observed peak in this distribution at 6.3 ± 0.6 Å indicates that the alkyl group in the zwitterionic headgroup is extended and thus, practically no self-interaction between the charged groups within the headgroup is present. Notice that the closest distances between the nitrogen in the ammonium of one headgroup with the centre-ofmass of the oxygen atoms in the sulfonate of another headgroup vary within 5.5 and 7.3 Å in the simulations. This shows that the inter-surfactant headgroups interactions are stronger than the self-interactions, which explains the high stability of the SB3-14 micelle.

Self-Diffusions in the SB3-14 Micelle The mobility of surfactants and carbon atoms within the micelle can be quantified by their diffusion coefficients using the Einstein relation for long simulation times.58-59 The average selfdiffusion coefficients of the surfactants within the micelle is 3.2 × 10–7 cm2 s–1, which is close to the experimental values 10–6–10–7 cm2 s–1 obtained for ionic and zwitterionic micelles using different techniques (NMR, voltammetry, electro kinetics chromatography).88-90 The carbon atoms within the SB3-14 have diffusion constants in the range [1–7] × 10–7 cm2 s–1 as depicted in Figure S2. Carbon atoms (C12-C14) nearest to the zwitterionic headgroup present the smallest mobility because the polar groups interact strongly amongst them, which keep these groups and their adjacent atoms more rigid. Whereas the termini carbon atoms (C1-C3) are much more mobile because their interactions are weaker (van der Waals) and slight conformational changes in the middle region of the alkyl chain can lead to large atomic displacements of the termini atoms. Structure of Ions around the SB3-14 Micelle and Their Adsorption


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As mentioned previously, zwitterionic micelles can adsorb specifically anions. In fact, the adsorbed amount of anions follows the Hofmeister series, for instance, perchlorate adsorbs more than chloride. This adsorption depends upon a balance between the interactions of the anion with the solvent and the surface of the micelle. Thus, it may be expected that the anions would affect the structure of the micelle, which can in turn influence the adsorption. We have analyzed the simulations of the SB3-14 micelle in ionic aqueous solutions considering the last 90 ns of the trajectories, because the first 10 ns were necessary for equilibration, after which the average properties remain constant. We begin by analyzing the radial distribution function (RDF) of anions at the surface of the micelle, namely, the RDF between the nitrogen of the ammonium group and the anion as depicted in Figure 6. Clearly, perchlorate anions penetrate deeply into the surface of the micelle with a sharp peak at ca. 2 Å in the RDF and two additional broader peaks at ca. 4.5 and 7 Å. The other anions present a broad peak at ca. 5 Å with intensity increasing in the following order F– < Cl– < Br– < I–. In fact, in the case of fluorine, there is no discernable structure in the RDF, indicating that these anions do not adsorb at the surface of the SB3-14 micelle and are actually dispersed in the solution. Indeed, the amount of anions around micelle can be quantified by integrating the corresponding radial distribution function up to its first minimum. Using the association number for fluoride as reference, the following relative values 1.0, 1.2, 1.3, 1.7 and 4.7 were obtained for F–, Cl–, Br–, I– and ClO4–, respectively, for a NaX salt concentration of 0.15 mol L–1. Thus, the concentration of perchlorate is ca. 5 times larger than that of fluoride at the surface of the SB314 micelle. This effect can be correlated directly to the measured zeta-potentials for the micelle


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in salt aqueous solutions, namely, –1.7, –17.1, –27.7, –52.1 and –69.8 mV,19 respectively for F–, Cl–, Br–, I– and ClO4–.

Figure 6. Radial distribution function (RDF) of anions with respect to the nitrogen atom in the ammonium group of the SB3-14 surfactant. Anions: fluoride (red), chloride (black), bromide (green), iodine (blue), and perchlorate (magenta). The NaX (X = F–, Cl–, Br–, I– and ClO4–) salt concentration is 0.15 mol L–1. Inset shows the convergence of the RDFs at large distances. These results demonstrate that as the amount of anions at the surface of SB3-14 micelle increases, the zeta-potential becomes more negative. In addition, they show that the anion adsorption, quantified by its association number, follows the Hofmeister series, namely, F– < Cl– < Br– < I– < ClO4–. This adsorption can be rationalized by the balance between the interactions of the anions with the zwitterionic headgroups and with the water molecules. Namely, weakly hydrated anions such as I– and ClO4– can (partially) lose their hydrating molecules and interact


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more strongly with the surface of the micelle. Whereas, anions that are strongly hydrated (e.g., F– ) have difficulties in releasing their hydrating molecules, which shield their interactions with micelle and thus hinders their adsorption.91 These assertions can be corroborated by comparing the average hydration number in pure water and in the presence of the SB3-14 micelle employing the same simulation protocols. For instance, the average hydration of ClO4– is 5.0 and 4.6 in water and in the presence of the SB3-14 micelle, respectively. Considering that 50% of the ClO4– anions are within 8 Å, we can estimate the average number of hydration of adsorbed ClO4– to be 4.0. For F–, these average hydration numbers practically do not change from water to the micelle. Regarding the distribution of the anions with respect to the center of mass of the SB3-14 micelle, it can be observed (see Figure S4) that ClO4– penetrates up into the headgroups, whereas the penetration of F– is basically restricted to the surface. It is noteworthy that the force-field used was not polarizable, which suggests that at least the qualitative description of the Hofmeister series for anion adsorption can be achieved without the polarizability effects of the interaction sites. Of course, individual surfactants described by nonpolarizable point-charges will have different polarizabilities depending upon their geometries, which may mimic partial polarizable responses. It is thus expected that the main effects of polarizable sites would be for the water-ions interactions. Indeed, this seems the case for simulations of ionic aqueous solution/air interfaces,92-94 however, there are no available simulations of adsorbing anions in zwitterionic micelles with polarizable force-fields. The RDFs can also be used to estimate the fractional charges around the SB3-14 micelle, calculated as the integral up to the minimum divided by the overall number of cations in


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solution. For sodium cation the fractional charges are practically independent of the counterion, namely, 0.06 and 0.12 for the first and second minima, respectively, and thus 0.82 remains in the bulk for a 0.15 mol L–1 of NaX (X = F–, Cl–, Br–, I– and ClO4–) salt. For these same solutions, the anions F–, Cl–, Br– and I– presented the following fractional charges: 0.012, 0.017, 0.025 and 0.046. Whereas, for anion ClO4–, Figure 6 shows three well-defined structured layers that yield fractional charges of 0.036, 0.070 and 0.076. This analysis thus suggests that ca. 18% of the perchlorate are within 8 Å from the ammonium group. Notice that negative charge of these adsorbed perchlorate anions cancels the positive charge of the sodium cations near the surface of the micelle. Quantitatively, this neutralization is expressed by the excess charge at the surface, calculated as the difference between the total fractional charges at the same layer of the anions and cations, which yields +0,048, +0,043, +0,057 and +0.014 for the NaF, NaCl, NaBr and NaI salts, respectively. We observe that in presence of NaClO4 and NaI, the excesses of charges are slightly neutral showing that the anions are most of the time adsorbed at the surface of the micelle. For low salt concentration (0.015 mol L–1) the same trends are observed, however, the fluctuations are larger because the much smaller number of ions. In the presence of NaClO4 and NaI, the excess charges are slightly more negative, considering the first peak of the Na+ RDF. Specific Anion Effects on the SB3-14 Micelle Surface The added salts to the aqueous solution have very small effects on the radius of gyration and eccentricity of the SB3-14 micelle (Tables S1 and S2). This seems reasonable because for zwitterionic micelles, the adsorption of anions should not affect significantly the size or shape of


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the micelle because the anions interfere only slightly with the inter-surfactant interactions, unlike the case of ionic micelles. However, the surface of the SB3-14 micelle is directly affected by its interaction with anions because the polar headgroups are exposed and in direct contact with the charged species. From the structural analysis of the SB3-14 zwitterionic micelle (see Figure 3), we observed that several positive charged ammonium groups are exposed to the solution and can interact with anions, which thus alter the properties of the surface of the micelle. Indeed, its interaction with perchlorate anions causes a significant decrease of the total, hydrophilic and hydrophobic SASA. This is consistent with the experimental observation that the surface area of monolayers of DPPC decreases in the presence of perchlorate compared to the other anions such as chloride and bromide.91 The dependence of the SASA upon the nature of the NaX (X = F–, Cl–, Br–, I– and ClO4–) salt is presented in Figure 7 for a concentration of 0.15 mol L–1.


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Figure 7. Average total (black), hydrophobic alkyl chain tail (red) and hydrophilic zwitterionic head (blue) solvent accessible surface area (SASA) for the SB3-14 micelle in pure water and aqueous solutions (0.15 mol L–1) of NaX (X = F–, Cl–, Br–, I– and ClO4–) salts. We observe from Figure 7 that the SASA of SB3-14 micelle decreases significantly in NaClO4 solution compared to pure water. Whereas aqueous solutions of sodium halide practically do not affect the total SASA, however, the hydrophilic zwitterionic head SASA decreases as the ionic radius of the halide ion increases. The interaction of the exposed ammonium groups with perchlorate ions causes an approximation of the headgroups of the surfactants and thus a decrease of the surface area, especially the hydrophilic part, available to the solvent. Indeed, the average number of contacts within 7 Å between the nitrogen atoms of the headgroups is 12.6 and 24.5 for the SB3-14 micelle in water and in 0.15 mol L–1 NaClO4(aq), respectively. It has been proposed that the surface area per surfactant could be employed as a quantitative probe for the effects of ions upon surfaces.90 This proposal is corroborated by the results in Figure 7, where a decrease of the SASA can be correlated to the adsorbed amount of anions at the SB3-14 surface. In addition to the effects in the SASA and inter-headgroup interactions, the specific adsorption of anions affects the tilt angle (θ) between the polar head and the apolar tail, which changes from an average value 〈θ〉 of 107° in water to 108° and 116° in 0.15 mol L–1 NaF(aq) and NaClO4(aq), respectively; however, the widths of these distributions remains the same (34 = 8°). Regarding the reliability of the simulations, we have also investigated the structure of the SB314 micelle with LIPID14 force-field.95 One of the main differences is related to the smaller values of the Lennard-Jones parameters for alkyl sites (CH2 and CH3) employed in LIPID14 in order to improve the values of self-diffusion coefficients of hydrocarbons. As a result, these


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smaller parameters cause a slight increase of the hydrophilic SASA compared to the GLYCAM 06 (see Table S4) possibly by decreasing the repulsions between the methylene (-CH2-) groups in the core of the micelle. However, the main structural and dynamical features of the micelle are practically the same when either force-field is employed in the MD simulations (see Table S5 and Figure S4). Conclusions Atomistic molecular dynamics simulations showed that the neutral zwitterionic SB3-14 micelle in water and ionic NaX (X = F–, Cl–, Br–, I– and ClO4–) aqueous solutions has a large surface area available to the solvent that decreases upon adsorption of ClO4–. However, it still large enough to have catalytic properties due the increase of local concentration of ions. The adsorption of anions may be rationalized by the L-type shape of the surfactants in the micelle, namely, an average angle of 110° between the zwitterionic headgroup and the alkyl tail, which exposes the positive charged ammonium groups to the solution, especially to weakly hydrated anions (e.g., I– and ClO4–). The amount of adsorbed anions, quantified by the integral of their radial distribution functions, follows the Hofmeister series, namely, F– < Cl– < Br– < I– < ClO4–, which directly correlates to the measured values of the zeta-potential. The structure of the anions around the micelle shows the formation of ion-pair-type species between ammonium groups and perchlorates, which indicates that the adsorption is directly related to the easiness of the anion to (partially) lose its hydration shell. ASSOCIATED CONTENT Supporting Information. Eccentricity, effective total radius of gyration, and force field parameters (Tables S1-S3), accessible solvent surface area (SASA), total and hydrophobic


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effective radius of gyration comparison between LIPID14/GAFF and GLYCAM/GAFF force fields (Table S4-S5), distributions of distances, diffusion coefficients and snapshots of the structures of the SB3-14 micelle using the LIPID 14/GAFF and GLYCAM/GAFF parameters (Figures S1-S3). This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author * Departamento de Química Fundamental, Universidade Federal de Pernambuco, 50.740-560 Recife - PE, Brazil. E-mail: [email protected]; Fax: +55 81 2126 8442; Tel: +55 81 2126 8459. ACKNOWLEDGMENT The Brazilian Agencies CNPq, CAPES, FACEPE and FINEP are acknowledged for providing financial support under grants Pronex APQ-0859-1.06/08 and INCT-INAMI Proc. no. 573986/2008-8. D.P.S. thanks CNPq for a graduate scholarship. We thank CETENE-CNPq (Prof. Borko Stosic - UFRPE) and CENAPAD-PE for providing partial computational resources. We also acknowledge Prof. Faruk Nome (UFSC) for his incentive and for calling our attention for these interesting systems. REFERENCES (1)

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Contents Graphic and Synopsis

F– < Cl– < Br– < I–

Molecular Dynamics Simulations of Specific Anion ... - ACS Publications

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