A molecular-dynamics-investigation-of-the-stability-of-a-charged-electroactive-polymer-monolayer

A molecular dynamics investigation of the stability of a charged electroactivepolymer monolayer

D.A. Morton-Blake ⁎, D. LeithSchool of Chemistry, Trinity College, Dublin 2, Ireland

a b s t r a c ta r t i c l e i n f o

Article history:Received 6 June 2008Received in revised form 31 July 2008Accepted 14 August 2008Available online 4 September 2008

Keywords:Molecular dynamicsElectroactive polymerCharged monolayerInterfacial tension

In a molecular dynamics simulation a monolayer is investigated consisting of an amphiphilic polythiopheneon a sodium chloride solution. When the monolayer is allowed to become chemically reduced the π-conjugated thiophene rings assume overall negative charges which are compensated by excess Na+

concentrations in the aqueous layer. One result of endowing the polymer with net negative charge is a loss ofthe planarity of the monolayer, leading to a buckling and eventual rupture. A second is the attraction of the‘excess’ Na+ ions to the interface, to occupy sites near the thiophene rings. At low redox reduction levels ofthe polymer the ions in these sites are much more mobile than the Na+ ions in the NaCl solution layer andrespond to applied electric fields by jumping between sites, showing energy barriers of 0.33 eV. Gouy–Chapman theory is applied to discuss the eventual instability of the polymer membrane.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Among the chemical structures resulting from the current interestin nanotechnology and self assembled monolayers are amphiphilicpolymers that form a monolayer on water. One such polymer is asubstitution derivative of polythiophene, which forms chains inwhichthe thiophene rings are linked in the 2 and5positions starting from thesulphur atom (Fig. 1). As a result the orientations of the rings alternatein an anti sequence (sulphur-up, sulphur-down) along the polymerchain. Each ring is substituted in its 3 positionwith two species of sidechains which alternate in successive thiophene rings. The side chainspecies are chosen to have different degrees of associationwith water,conferring amphiphilicity on the polymer; the alternate side chainsare octyl −C8H17 and an oligo ethylene-glycol −(CH2−O−CH2)4CH2OHwhich will be referred to as ‘glycol’. The figure shows that the anticonformation of the polythiophene backbone gives rise to a structurein which the two species of side chain point in opposite directions (upand down) from the main chain. The hydrophilic glycol chains arein the water layer while the hydrophobic octyl chains are directedinto the vacuumproducing a polymermonolayer on thewater surface.In this work the polymer will be referred to as the ‘amphiphilicpolymer’. Suchmonolayer systems have been prepared [1] and reportsof previous work from this laboratory [2,3] describe the use ofmolecular dynamics (MD) to investigate the response of the system tothe application of surface and hydrostatic pressures, simulating theisotherm curves and the eventual rupture of the monolayer.

Another interesting property of the polythiophene monolayerfollows from the redox electroactivity of the parent polymer thatendows it with electrical conductivity when the polymer is chemicallyoxidized or reduced [4]. When polythiophene and its derivativesundergo charge transfer in a suitable oxidizing or reducing mediumthey form a polyelectrolyte with electric charges on the π componentsof the thiophene rings of the main chain. The greater the extent ofthe oxidation or reduction of the polymer, the greater the charges onthe thiophenes. In order to encourage the synthesis of organic nano-materials and devices based on electrically charged monolayers weexamine the structural feasibility of these novel systems and in-vestigate their potential to function as charged membranes in nano-scale devices. If the electroactivity of the amphiphilic monolayer isto be exploited for this purpose it is important to ascertain how itsstructural stability responds to various factors.

Agents that accomplish the redox process might be chemicalcomponents of the aqueous layer such as iodine (oxidation) or tintriethyl hydride (reducing). But as MD does not monitor chemicalreactions, the redox agents will not be explicitly included in oursimulation. Instead, the polymer's excess charge will be compensatedby creating an appropriate imbalance of the Na+ and Cl− ions in theaqueous layer.

2. Structures

Fig. 2 shows the interface ab plane of the starting structure ofthe amphiphilic polymer monolayer. The polymer chains arealigned along the a axis and the yellow S identify the thiophenerings. The c axis is perpendicular to the interface. Alternate polymerchains along the b axis have been shifted half a structural unit (one

Journal of Molecular Liquids 144 (2009) 75–88

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thiophene ring) along c to minimize repulsion [2]. The glycol sidechains are in the aqueous NaCl layer (not shown in the figures). Thepolythiophene backbone lies largely in the interface and the octylchains are directed approximately along c into the vacuum. Thepolymers are stacked along b with alternate chains displaced by onethiophene ring along the direction of the main chain as in poly(3-alkylthiophenes) lattices — a structural feature that minimizespacking repulsions.

The polymer's structural repeat unit C4SH(C8H17)–C4SH(CH2O-CH2)4CH2OH consists of 70 atoms, and six such units constitute a 420-atom chain in the MD box. (The box functions as a unit cell, so withperiodic boundary conditions there are of course no chain ends.) Inaddition the MD box contains twelve polymer chains, and thereforeholds a total of 420×12=5040 polymer atoms. The a (46.56 Å) and b(49.20 Å) axes imply that the ab surface area of the MD box is2290.8 Å2. The aqueous layer was formed by initially superimposing awater ‘lattice’ over the hydrophilic glycol side chains and extending it38 Å ‘below’ them. The vacuum layer was extended 33.5 Å ‘above’ the

monolayer's octyl side chains. By replacing the appropriate number ofwater molecules uniformly throughout the water layer by ions thedesired NaCl concentration in the aqueous layer could be produced. Inmost of the monolayer systems generated the mole fraction of NaClwas 0.064, corresponding to a 3.56 M solution, which is half of themolarity corresponding to a NaCl solution saturated at 25 °C (6.11 M)[5]. These concentrated solutions were selected in order to permit theuse of large numbers of ions, which optimizes the statistical analysis ofthe molecular dynamics, while keeping the number of watermolecules at a practical level. The general results from the presentwork aremainly independent of the concentration of salt solution, butfeatures which arise from the present heavy NaCl concentrations willbe commented onwhen they occur. The aqueous layer thus contained2087 water molecules and initially 134 each of Na+ and Cl− ions. Theexcess charge in the electrolyte layer to compensate the overall chargeon the oxidized or reduced polymer was created by unbalancing theequality of the Na+ and Cl− concentrations. So an electrolyte solutioncompensating a negatively charged chain was created by converting

Fig. 1. Part of the amphiphilic polymer poly(3-octyl 3′ 4-ethyleneglycol thiophene). The yellow atoms are sulphur and the red are oxygen. (For interpretation of the references tocolour in this figure legend, the reader is referred to the web version of this article.)

76 D.A. Morton-Blake, D. Leith / Journal of Molecular Liquids 144 (2009) 75–88

the requisite number of Cl− into Na+ ions. The (Na+, Cl−) fractionalimbalance [n(Na+)−n(Cl−)] / [n(Na+)+n(Cl−)] was varied from 0 to 26%.From calculations on polaron and bipolaron formation in dopedpolythiophene [6] maximum negative charges on thiophene rings ofup to −0.33e were inferred, i.e. an electron charge spread over threerings. (Electrochemical and spectroscopic investigations conclude thata unit positive charge is spread over 4–6 rings in chemically reducedpolypyrrole [7]).

Note the use of two terms in this work: (i) ‘Charge imbalance’refers to the excess of Na+ over Cl− ions in the aqueous layer, exactlycompensating the negative charge of the polymer backbone. All thesystems simulated (polymer plus electrolyte solution) have zerooverall charge. (ii) The chemical terminology ‘reduced polymer’meansthe result of the process implied in (i) in which electrons have beentransferred to the polymer.

3. Computation model

The molecular dynamics (MD) of the amphiphilic monolayer–aqueous system was conducted with the aid of the DL_POLY code ofSmith and Forester [8]. We used the DREIDING [9] and SPC/E [10] forcefields for the polymer and water sub-systems as used in our earlierwork, supplemented by the potentials of Lee and Rasaiah [11] for theNa+ and Cl− ions. The Lennard–Jones parameters of the heteroatomicpair potentials were derived from those of homoatomic pairs by theusual ‘geometric mean’method. The dynamics were conducted for 105

femtosecond time steps in a constant-temperature–pressure thermo-stat on a system with two-dimensional boundary conditions.

The atomic charges on the polymer were taken from the results ofDFT calculations on thiophene oligomers [12]. It was assumed thatcharge transfer associated with redox changes in ring-conjugatedmolecules involve only the carbon and sulphur atoms of the aromaticrings. The excess charges on the polymer monolayer were conse-quently distributed evenly between the existing charges on the ring Cand S atoms.

The surface charge density of the monolayer depends on thepolymer's degree of chemical reduction. A solution that is completelyion-balanced is defined by an MD cell containing 2087 water

molecules and exactly 134 ions each of Na+ and of Cl− ions,corresponding to a 3.56 M solution. The overall charge on the polymermonolayer would then be zero. Since the monolayer contains 12chains each of 12 thiophene rings, a Na+/Cl− imbalance resulting in Qelectronic charges implies that each of the thiophene rings in thepolymer acquires a charge of Q/144 electronic units. In this work arange of overall ring charges are considered, the most concentratedof which is when Q=70 unit charges. The charge on each thiophenering is then −0.4861e implying that a unit negative charge is spreadover about 2 thiophene rings. This maximum compares with the 3-ring and 4 to 6-ring spread of charge deduced for polythiopheneand polypyrrole respectively in refs. [6,7]. Our highest ring chargetherefore has a numerical value about twice those in the investiga-tions cited. From the 2-dimensional axes of the MD cell (a=46.56and b=49.20 Å), the surface charge density on the monolayer is−144×0.4861/(46.56×49.2)=−0.0306e per Å2 or −0.4896 C m−2.Because of familiarity with electronic units as quantities to describecharge distribution in molecules it will be convenient to monitorcharge imbalance using the net negative charge in electron units oneach thiophene ring. For the most-charged main chain just quoted,this would be −0.49e (the similarity between this numerical value andthe −0.4896 C m−2 surface charge density being largely fortuitous).

4. Results of the molecular dynamics

4.1. Monolayer structure

As observed in Refs. [2,3] the monolayer structure of theamphiphilic polymer that has no overall electrical charge remainsperfectly stable even after several hundred thousand time steps ofMD. However when the polymer is chemically reduced, Fig. 3 showsthat the overall negative charges conferred on the thiophene ringsresults in a destabilizing of the monolayer as evidenced by a ripplingand eventual destruction of the monolayer.

The effect is accompanied by the entry of Na+ ions (purple in thefigure) into the plane of the polymer backbone, where they take uppositions near the thiophene rings. These sites are reminiscent ofthose associated with the intercalation of alkali metal ions in graphite

Fig. 2.Monolayer packing assumed for the amphiphilic polymer chains lying on an aqueous surface. (For reasons of clarity the aqueous layer is not shown.) The monolayer is viewedfrom ‘above’, i.e. from the side of the hydrophobic octyl chains. On the right of the diagram the red oxygen atoms of those hydrophobic glycol chains which are unobscured by themain chain can be seen directed out of the MD box. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

77D.A. Morton-Blake, D. Leith / Journal of Molecular Liquids 144 (2009) 75–88

[13] and of various ‘dopant’ ions that form sites near π-conjugatedcentres of solid-state polymers as polyacetylene, polyparaphenylene,polythiophene and polypyrrole [14], sometimes retaining an ion

mobility in the matrix (and incidentally rendering the polymersystem electrically conducting). We turn to the properties of theseions next.

Fig. 3. The effect of overall charge on the monolayer consisting of an amphiphilic polymer on the surface of aqueous NaCl. The net charge qring on each thiophene ring is shown as amultiple of e, and is balanced by excess Na+ charges in the solution (a) q=0.0, (b) q=–0.0694, (c) q=–0.0972, (d) q=–0.2083, (e) q=–0.4167, (f) q=–0.4861. The polymers are viewedalong the direction of their main chains, identified by the thiophene rings' yellow sulphur atoms. The green Cl− ions are visible in the aqueous layer while at high qring some (purple)Na+ have entered the region of the interface and become intercalated between the thiophene rings. (For interpretation of the references to colour in this figure legend, the reader isreferred to the web version of this article.)


4.2. Interface sodium ions

The system corresponding to the most highly reduced polymer,with a thiophene charge of −0.2778 in the aqueous layer was firstselected for investigation. It has 154 Na+ and 114 Cl− ions in theaqueous layer. After ten thousand time steps, of the 154 Na+ ions thatwere initially in the aqueous layer 12 had broken through to thepolymer main chain and occupied positions near a thiophene ring.

We should like to ascertain any difference in behaviour betweenthe Na+ ions that are in the aqueous solution and the twelve which

have become ‘intercalants’. The two ‘positional species’ of Na ion willbe distinguished by referring to the sodium ions that are intercalatedin the interface as ‘Na1+’ and to those in solution as ‘Na+’. Both weregiven the same atomistic parameters for simulation. We wish tounderstand the origin of the intercalate sites of the sodium ions at theconditions for higher polymer reduction. Is their incursion into theinterface region a consequence of Coulomb attraction by the extranegative charge of the thiophene rings or of monolayer deformationby the mutual repulsion of these rings, thereby rupturing themonolayer ‘membrane’ and allowing passage to the sodium ions?

Fig. 3 (continued).


Fig. 4 shows the result, on the ‘−0.2778 monolayer’, of removingthe overall charges on the thiophene rings, although the ring atomsretain their positive or negative charges appropriate to the unchargedpolymer. By 20×103 time steps the twelve Na1+ ions have started tofile out of their intercalation sites and by 40×103 these sites are fullyabandoned. This would appear to have occurred because of the

elimination of the electrostatic forces from the polymer interfacerather than the monolayer deformation.

MD runs of 2×104 time steps (20 ps) at temperatures between 0and 100 °C produced insignificant differences in the computeddiffusion coefficients of the two sodium ion species at the highertemperature range. Inspection of the molecular snapshots strongly

Fig. 3 (continued).


suggested that this was because under these conditions some of theNa+ and Na1+ species had become interchanged, and shorter run timesfailed to catch any difference between the species. It might beexpected that with a smaller degree of chemical reduction of thepolymer, resulting in a lower negative charge on the polythiophenebackbone the Na1+ ions might become mobile. The diffusion

coefficients plotted in Fig. 5 in which the charge on the thiophenerings was reduced to −0.1389 show a clear distinction between thepositional species, the coefficients of the ions in the interface beingnearly an order of magnitude greater than those in the bulk solution.An Arrhenius plot ln D vs T−1 indicated an energy barrier of 0.33 eV forthe migration of the Na1+ ions.

Fig. 3 (continued).


The overall charge on the polymer was then completely removed,leaving only the charges on the thiophene ring atoms corresponding toanunreducedpolymer.WhenNa1+ ionswere ‘manually’ introduced intothe interface (as they would not spontaneously enter the region), thebarrier was found to be 0.10 eV. The exercise indicates that any sodiumions that succeed in entering the stacked subspace of the monolayerwould possess significant thermal mobility for moderate degrees of

chemical reduction of the polymer. With a chemical reduction of thepolymer that is even slightly greater than that conferring a ring charge of−0.1389 the sodium ions become quite firmly bound in their sites andmigration is severely reduced. In comparison with ions in intercalatedalkali metals in graphite and conjugated polymer matrices measuredactivation energies for ion transport are highly diversewithmost valuesin the range from 0.1 to 2 eV [14,15].

Fig. 3 (continued).


4.3. Electric field

For various reduction states of the polymer a set of short MD runs(5000 time steps) were made in the presence of constant electricfields of 0.1 v Å−1 applied along the three principal directions in the

monolayer — that of the polymer main chains (the a axis), the one inthe interface plane perpendicular to the chains (the b axis) and thetwo directions perpendicular to themonolayer (+c and −c). The resultsshow that if the net charge on the thiophene rings (reflecting thedegree of chemical reduction of the polymer) is more negative than

Fig. 3 (continued).


−0.1e the Na1+ ions in the interface have mobilities that are practicallythe same as those of the Na+ ions in the aqueous layer. At lowerreduction levels the behaviour of the ions a distinction in the

behaviours of the two positional species of sodium ions is observed.Fig. 6 demonstrates that with a net charge of −0.1 on the thiophenesthe mean square displacements of the Na+ ions in the aqueous bulk do

Fig. 4. The fates of the intercalant Na1+ ions (red atoms near the thiophenes) after changing the net charge on the thiophene rings from −0.2778e to zero. (a) initially, (b) after 20×103,and (c) after 40×103 time steps. In this time the sodium ions in the interface become depleted from this region and rejoin the other Na+ ions in the bulk aqueous layer. (Forinterpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)


not exceed 1 Å2 in the time interval considered. When the field isalong the +c axis, directed from the aqueous to the hydrophobic layer,the mobilities of the interface Na1+ ions are only slightly greater thanthose of the Na+ ions in the aqueous layer. In this direction the electricfield tries to propagate the Na1+ ions into the hydrophobic, ion-freeenvironment of the alkyl side chains or the vacuum. This is opposed bythe attractive forces from the net negative charges on the combinedpolythiophene backbone and aqueous NaCl layer, which tend to recallthe ion to this region. The Na1+ ions possess a greater facility torespond to an electric field if it is in the −c direction, which woulddisplace from the vacuum/hydrophobic chains into the polythiophenebackbone/aqueous layer. The Na1+ ions also respond readily to fields

applied in either of the principal directions in the plane of themonolayer. Figs. 1–4 explain the reason for this — along the a axis,which is parallel to the directions of the polymer chains, there areclear migration channels while along b the ions can pass through thegaps between the side chains on the thiophene rings.

In summary, the Na1+ ions in the interface demonstrate signifi-cantly greater mobility than the Na+ ions in the aqueous layer. Themobility is in directions parallel to the intercalation plane, where theions can jump between sites associated with thiophene rings or slipbetween the polymer side chains. The ions are also mobile in one ofthe two directions defined by the c axis that is perpendicular to theinterface.

Fig. 4 (continued).


5. Gouy–Chapman theory

Of the various descriptions of the configuration of charges near acharged surface the Gouy–Chapman model is probably the closest inits ability to describe the findings ofmolecular dynamics.We shall lookfor consistency between the predictions of the twomodels, exercisingcare in trying to identify the results of a granular atomistic treatmentobtained in MD with those associated with continuous matter.

As we are investigating a novel chargedmonolayer system it wouldbe interesting to use a charge distribution that is near the higher endof the range considered in the MD simulations. That for Q=60 ions(see Section 3 for the definition of Q) means a charge of −0.42e perthiophene and Fig. 3 shows that the resulting monolayer is stable.The rather large magnitude of the charge density on the monolayermembrane (−0.4197 C m−2) cautions us to eschew the use of thecommon linearized expressions in equations derived from Poisson

Fig. 4 (continued).


theory. The surface potential V0 on a plane with surface chargedensity σ is obtained from the Grahame equation [16]

σ ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi8c0ee0kT

psinh

eV0

2kTð1Þ

Here the bulk concentration of 3.56 M salt is c0=2.144×1027 NaClunits m−3. We use the value of dielectric constant (ε=10) proposed inRef. [17] for a heavy salt concentration in water. The decay of theelectric potential V(x) with distance x from a membrane is describedby

V xð Þ ¼ V0e−x=λD ð2Þ

where the Debye length λD (the distance characterizing the decay) isexpressed [18] as

λD ¼ 1e

ffiffiffiffiffiffiffiffiffiffiffiffiee0kT2c0

s

ð3Þ

which gives λD=0.56 Å. The electrostatic potential of the polymermain chain therefore decays very sharply with distance from theinterface plane with the electrolyte concentrations in the presentmodel. This is partly a feature of the necessarily high salt concentra-tion of 3.56 M, the requirement for which was explained in Section 2,but the rather small value that would also be obtained for a decimolarNaCl solution (λD=3.34 Å) implies that the potential at the boundaryof the bulk aqueous layer — that beyond the ends of the glycol sidechains for which x=16 Å in Eq. (3) — is practically very small.

5.1. Electrical capacity

The first derivative of the surface chargewith respect to the surfacepotential is the electrical capacitance C of the double layer formed bythe negatively charged polymer backbone and the positively chargedsodium ions which have entered the interface region. Eq. (1) lets usexpress the capacitance as

C ¼ dσdV0

¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2e2c0ee0

kT

rcosh

eV2kT

ð4Þ

Although V here is really the potential drop across the double layer,the small value we calculated for the Debye length λD in Eq. (3)suggests that V may be equated to the surface potential V0. Howeverwe should note the departure of the charge configuration in Fig. 4,generated by molecular dynamics, from the regular model of a perfectparallel-plate capacitance, that is inherent in the Gouy–Chapmandouble layer theory. As the capacitance is consequently likely to be

significantly smaller than the value predicted by Eq. (4) and which arelisted in Table 1, we tentatively propose that for use in estimating ball-park energies in the next section these capacitances be reduced by50%.

5.2. Stability of the monolayer

In Gouy–Chapman theory the electrostatic free energy density g(in J m−2) of the monolayer depends on the surface potential V0 asgiven [17] by

g ¼ −8c0kTλD cosheV0

2kT

" #−1

$ %

which using Eqs. (1) and (3) can also be written as

g ¼ −2kTσe

tanheV0

4kT

" #ð5Þ

As this expression reflects the attractive interaction between thepositively charged solution (modeled as a fluid continuum) and thenegatively charged rigid membrane, it becomes more negative withincreasing membrane (negative) charge, as seen in the data in thepenultimate column of Table 1.

Another way to describe the stability of the monolayer is throughthe effect of the surface charge or surface potential on the surfacetension. Application of the Gibbs–Duhem equation expresses a changein the interfacial tension γ in a monolayer with surface charge densityσ and inter-phase potential dV as

dγ ¼ − ∑ion types

iΓ idμ i − σdV

where Γi denotes the surface excess of the ith ion. The partial de-rivative of Aγ

AV ¼ −σ with respect to V provides the electrical capac-itance of the double layer,

A2γAV2 ¼ −

AσAV

≡ −C ð13Þ

integration of which leads to the Lippmann equation [19]

γ ¼ − ∫V0 CV 0dV 0 ð14Þ

γ ¼ γ0− 1=2CV2

The diminution in the interfacial tension γ−γ0 due to the chargeon the polymer can therefore be calculated by supplying values of the

Fig. 5. The diffusion coefficients (10−9 m2 s−1) of two positional species of sodium ion—

those in the bulk aqueous medium (Na+) and the intercalated ions (Na1+) in theinterface. The overall charge on each thiophene ring is −0.1393e.

Fig. 6. The response of the bulk aqueous (Na+, black dots) and interfacial (Na1+) ions toelectric fields applied in the directions a, b or c. The plotting points corresponding to thethree field directions for the Na+ ions in the bulk aqueous layer are not distinguished.


calculated surface potential and our estimated capacitance of theGouy–Chapman double layer in Table 1. Taking the surface tension ofan aqueous surface that is covered by a surfactant as 30 mN m−1 (or30mJm−2) [20] the values of the interfacial tension γ calculated by Eq.(14) are shown in the final column of Table 1. Because of theuncertainties in the values assigned to, for instance, the local dielectricconstant of the salt solution and capacitance of the double layer in ourapplication of the Gouy–Chapman approach to the MD model, weshould not hope to obtain more than a semi-quantitative comparisonof the two approaches. While the surface free energy density g in theTable cannot reliably be applied to describe the stability of themonolayer surface, the trends and the existence of the sign-change ofthe interfacial tension γ are significant. The decreasing γ values reflectthe diminishing stability of the surface with increasing polymercharge, the decrease being proportional to the increasing interfacialarea, which is clearly observed by the folding of the surface in Fig. 3.The loss of the positive signs of the γ values when the net charge oneach thiophene ring becomes more negative than about −0.1e meansthat this degree of chemical reduction of the amphiphilic polymerimplies the elimination of the interfacial tension and the collapse ofthe monolayer structure. In this work we have found that monolayerswith thiophene ring charges that were twice this value, and whichsurvived for104 time steps had not undergone further distortion after105 time steps. The discrepancy is probably due to the difficulty ofestimating realistic values of the double-layer capacitance and of thedielectric constant. Fig. 3 shows that polymer monolayer systemswithgreater charges do indeed lead to the collapse of the surface.

6. Conclusions and discussion

The structure and redox properties of the amphiphilic electroactivepolymer poly(3-alkyl, 3′-ethylene-glycol, bithiophene) permit it toassume a greater surface charge density than is found in other nano-membranes. Its ability to form charged monolayers on an aqueouselectrolyte solution suggests the existence of a property that might beusefully be exploited for new types of nano-scale electronic devices.The work presented has shown that an induced redox change in thepolymer results in the creation of sites in the interface that are closelyassociated with the negatively charged thiophene rings, where ionsin the aqueous layer can migrate. Provided the degree of reduction(or presumably also oxidation) is mild, the sites are associated withshallow energy wells; barriers are about 0.33 eV and the ions havesignificant mobilities in the interface region, as a result of eitherthermal or electric field effects.

The ‘ripples’ in the monolayer that occur as a result of its overallcharge, and which imply a greater interface area, are explained by thepredicted lowering of the interfacial tension. There is an apparentdiscrepancy in the value of the thiophene ring's limiting charge atwhich the instability of the monolayer sets in: continuum theorypredicts qringN0.1e while the MD finds that the monolayers survive105 time steps with thiophene charges up to 0.42 e. This may be due tothe inherent incompatibilities between granular and continuumapproaches, or uncertainties in the values of dielectric constant anddouble-layer capacity for use in the Gouy–Chapman model. Of course,the possibility must always be considered that an extension of timesfrom 0.1 ns of MD to the minutes or longer required for mostinvestigations on this topic might indeed lead to the collapse of themonolayer as predicted by the Gouy–Chapman approach. This is anissue that can only be resolved by experimental investigation.

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Table 1Parameters describing the monolayer–salt layer systems considered in the molecular dynamics

System n (Na+) n (Cl−) Elects. trnsfd. qring (e) σ (C m−2) V0 (volts) C (F m−2) g (J m−2) γ (J m−2)

1 134 134 0 0 0 0 0 0 0.003002 139 129 10 −0.0694 −0.0699 −0.0410 1.028 −0.0014 0.002143 141 127 14 −0.0972 −0.0979 −0.0535 1.225 −0.0024 0.001254 146 122 24 −0.1667 −0.1679 −0.0770 1.807 −0.0055 −0.00245 149 119 30 −0.2083 −0.2098 −0.0875 2.183 −0.0075 −0.00546 154 114 40 −0.2778 −0.2798 −0.1016 2.831 −0.0109 −0.01167 169 99 70 −0.4861 −0.4896 −0.1297 4.830 −0.0250 −0.0376

Elects. trnsfd.=No. of electrons transferred to polythiophene main chain; qring=net charge on thiophene ring; σ=surface charge density on monolayer; V0=monolayer surfacepotential; C=double-layer capacitance; g=surface free energy of monolayer system; γ=surface tension of monolayer system.


A molecular-dynamics-investigation-of-the-stability-of-a-charged-electroactive-polymer-monolayer

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