02 colsua powell

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Colloids and Surfaces

A: Physicochemical and Engineering Aspects 206 (2002) 241251

Wetting of nanoparticles and nanoparticle arrays

C. Powell, N. Fenwick, F. Bresme, N. Quirke *

Department of Chemistry, Imperial College of Science, Technology and Medicine, South Kensington SW7 2AY, UK

Abstract

We report investigations of nanoparticulate wetting carried out using molecular dynamics simulations. Despite theirsmall size, model Lennard Jones nanoparticles in simple Lennard Jones solvents exhibit well defined contact

angles, which for high surface tension interfaces have been shown to obey Youngs equation with surprising accuracy.

In this paper we present new results for fully molecular models of nanoparticles at a water surface, where again

well-defined contact angles are evident. Pressure area curves obtained in Langmuir trough experiments on (nano)

particulates have been used in the past to determine contact angles. We have recently demonstrated by molecular

dynamics and theory that, contrary to expectations, the collapse pressure measured in this experiment should be

independent of contact angle and that the initial collapse mode is by surface buckling. We present new molecular

dynamics results for arrays of nanoparticles with a contact angle of 72 at a liquidvapour interface which confirm

our earlier work and offer more information on the details of the structure of collapsed nanoparticle arrays. 2002

Elsevier Science B.V. All rights reserved.

Keywords: Surface tension; Nanoparticles; Wetting

www.elsevier.com/locate/colsurfa

1. Introduction

Three phases in contact exhibit a range of

interesting behaviour. For example in the case of

a solid in contact with liquid and vapour it is

possible to distinguish two cases: one where the

liquid forms a uniform (macroscopic) film cover-

ing the solid surface, the complete wetting state

and a second where the liquid forms a droplet onthe surface, partially covering the surface, the

partial wetting state. For a solid particle in con-

tact with two bulk fluid phases, the completely

wet (or dry) phase corresponds to the particle

being completely immersed in one phase whilstthe partially wet state has the particle sitting inthe interface between the two fluid phases. Suchstates and the transitions between wetting states

are of both scientific and industrial interest, forexample in determining the order of wetting tran-sitions or in tailoring wetting properties for highspeed printing or emulsion stabilisation. Currentlythere is very strong interest in the description ofwetting of nanoscale materials and of nanostruc-tured/nanopatterned materials. Examples includethe behaviour of nanoparticles at fluid interfacesand its connection to self-assembly [1], the wettingof micron-sized channels [2] or nanopores for theformation of nanowires [3], and the wetting ofnanoscale patterned interfaces with application tonanolithography.

* Corresponding author.

E-mail address: [email protected] (N. Quirke).

0927-7757/02/$ - see front matter 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 7 - 7 7 5 7 ( 0 2 ) 0 0 0 7 9 - 1
mailto:[email protected]:[email protected]


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C. Powell et al. /Colloids and Surfaces A: Physicochem. Eng. Aspects 206 (2002) 241 251242

In this paper we report investigations of

nanoparticulate wetting carried out using molecu-

lar dynamics simulations. In previous work [46]

we have shown that despite their small size, model

LennardJones nanoparticles in simple Len-

nardJones solvents (at both liquidvapour, LV

and liquid liquid interfaces) exhibit well defined

contact angles, which for high surface tensioninterfaces (in particular the liquid liquid case)

have been shown to obey Youngs equation with

surprising accuracy.

Our initial results [46] were for a LV interface

with a surface tension of 3 mN m1 (using LJs

units for argon), which is low compared with the

water/air surface tension, 72 mN m1 but larger

than say a microemulsion oil interface 0.1

mN m1. By measuring the free energies of the

simulated fluid and solid interfaces we were able

to show that Youngs equation is accurate for thisinterface for nanoparticles of diameter E3 nm.

For smaller nanoparticles Youngs equation was

less accurate and the simulations showed a wet-

ting transition at 1 nm not predicted by

Youngs equation. Youngs equation disregards

one of the four interfaces in the system: the fourth

interface being the line that separates the three

phases. The free energy associated with this line,

the line tension (~), may influence the contact

angle. We estimated [46] ~ from our simulations

and found it to be of the order of 1012 N.

The effect of the line tension on the wetting

properties of the nanoparticle, such as the contact

angle, may be considered through a corrected

Youngs equation, that reduces to the Youngs

equation when ~=0 or 1/R0 i.e. the curvature

of the three phase line is negligible. Our data

[46] show that the corrected Youngs equation is

more accurate, but still breaks down for the

smallest nanoparticles (1.5 nm).

The situation may be different however for

interface with a high surface tension. To investi-

gate this question we simulated [4 6] nanoparti-

cles at a liquid liquid interface for which the two

liquid phases are immiscible, analogous to the

water/oil interface. In this case the surface tension

of the liquid liquid interface was 14 mN m1

(compared to 3 mN m1 for the LV case de-

scribed above). We analysed the accuracy of

Youngs equation in this case by varying one of

the surface tensions, kpf, keeping the other two

constant. Youngs equation predicts the contact

angle is a linear function of the surface tension.

We found Youngs equation to be very accurate

in predicting the wetting behaviour of the

nanoparticle at this interface (k=14 mN m1)

even for diameters approaching 1 nm. This con-trasts with the results obtained for the LV inter-

face. It might be that the line tension in these

systems is negligible compared with the LV case.

However our calculations [4 6] show that this is

not so, indeed it is larger by one order of magni-

tude at 1011 N. This can be understood by

looking at the corrected Youngs equation, which

predicts that the contact angle is a strong function

of the line tension only when the surface tension

of the fluidfluid interface is low. When the sur-

face tension is high the contact angle is insensitiveto line tension. From our studies [46] of model

LJs nanoparticles at LJs interfaces we conclude

that it is possible to describe the wetting of

nanoscale interfaces (down to 1 nm) using

Youngs equation when (a) the fluidfluid interfa-

cial tensions are large and (b) the three phase line

has a small curvature.

These general results have been obtained using

idealised structureless model nanoparticulates. In

nature however nanoparticles have internal and

surface structures, are not necessarily rigid and

the solvents may themselves be complex. For ex-

ample metal nanoparticles have received consider-

able attention during recent years due to the wide

range of potential technological applications.

These range from the use of dextran coated iron

clusters [8] in target specific magnetic resonance

imaging [9], to the use of lattices of organothiol

stabilised gold clusters as chemical vapour sensors

[10] and lithographic masks [11]. Particular inter-

est has been shown in colloidal gold as a catalyst,

especially since supported gold nanoparticles have

been used to catalyse the oxidation of carbon

monoxide [12]. In this paper we extend our work

by describing a realistic molecular model of a

thiol passivated gold nanoparticle at a water in-

terface and reporting preliminary molecular dy-

namics simulations of its wetting properties. In

the final section we move from individual particles


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C. Powell et al. /Colloids and Surfaces A: Physicochem. Eng. Aspects 206 (2002) 241 251 243

to nanoparticle arrays and consider the pressure

area curves obtained in Langmuir trough (LT)

experiments on such arrays. Since its original

conception by Pockels [13] in the late 19th century

and subsequent refinement by Langmuir [14] in

the 1920s the LT technique has become an impor-

tant experimental tool in fields, ranging from

biology to electronics. Surface pressure (P)area(A) isotherms obtained from LT experiments can

provide important data on the geometry and in-

termolecular forces [15 18] of a wide range of

materials. This includes the characterisation of

nanoparticles used in processes relevant to oil

recovery, flotation, anti-foaming, gas sensors,

biosensors pyro, piezo and ferroelectric dielectrics

and has often involved the determination of con-

tact angles and line tensions [1921]. Such charac-

terisations depend on the correct interpretation of

the pressurearea isotherm. We consider our re-cent molecular dynamics simulations and theoreti-

cal work [22] that, contrary to expectations,

shows the collapse pressure measured in this ex-

periment should be independent of contact angle

and that the initial collapse mode is by surface

buckling. We present new molecular dynamics

results for arrays of nanoparticles with a contact

angle of 72 at a LV interface which confirm our

earlier work and discuss the details of the struc-

ture of the collapsed nanoparticle arrays.

2. Results

2.1. LennardJones nanoparticles at infinite

dilution at fluid interfaces

We consider the use of molecular simulation

[23] to probe the wetting of nanoparticles at infi-

nite dilution in a fluid interface. The situation of

interest is displayed in Fig. 1. We consider partic-

ulates at fluid interfaces with diameters in the

range 1 nm upwards.

In order to obtain general results we have

defined idealised nanoparticles using both Len-

nardJones spline (LJ/S) and Lennard Jones po-

tentials as shown in Fig. 2. In the first case the

particulate interacts with the LennardJones

atoms forming the fluid phases through a LJ/S

potential whose finite range and depth is indepen-

dent of the particulate diameter (Fig. 2 and Eq.

(1)). Thus even for an infinite diameter, where the

particulate represents a planar structureless wall,

the interaction with the fluid is the same. This

produces a clear separation between geometric

and energetic effects. The spline potential has the

form

Uij=

4mij |f

rsij

12 |f

rsij

6n0BrsijBrs

mij[a(rsijrc)2+b(rsijrc)

3] rsBrsijBrc

0 rcBr

(1)

where r is the separation between particles of

species i and j, and |f is the diameter of the fluidparticles. The other variables for the LJ/S poten-

tial are defined as follows:

rs=(26/7)1/6, rc=(67/48)

r, a=(24192/3211)/r s2,

b=(387072/61009)/r s3,

sff=0 and sfp=(|p|f)/2.

In the second case the fluid fluid, and fluid

nanoparticle interactions were modelled using

Lennard Jones potentials, with a cutoff at 2.5|f.

Note that now the fluid nanoparticle interaction

is stronger than the fluidfluid interaction.Using the model described above we performed

molecular dynamics simulations of particulates at

LV interfaces. For the present work a key quan-

tity is the contact angle the particulate makes with

Fig. 1. The system of interest, a particulate (labelled 1) diame-

ter | at a fluid interface, distance d from the fluid interface,

with contact angle q.


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Fig. 2. (a) The LJ/S potential, (b) the Lennard Jones potential for fluidfluid, fluid colloidal nanoparticle, colloidal nanoparticle

colloidal nanoparticle interactions.

the liquid phase. At any instant of time the sur-

face is discharged on the atomic scale; in addition

the particulate moves in and out of the (instanta-

neous) fluid interface during the simulation [24].

Our concern in the present work is however, not

with the fluctuation properties of the particulate/

fluid interface system (interesting as they are), but

with the thermodynamic properties which arise

from averaging the system. Thus we define a

contact angle using the average height d of the

particulate as a function of time, with respect to

the (average) LV equi-molar dividing surface. The

time average of the quantity d is then related to

the contact angle q (Fig. 1) using cos q=2d/|p. The contact angle defined in this way is consis-

tent with that estimated from Youngs equation

as discussed in Section 1. That a (time averaged)

contact angle exists on the nanoscale can be seen

from density profiles taken across the interface

with respect to the particulate centre of mass (Fig.

3). The profiles were calculated by considering a

cylinder of radius r and length x centered on the

particulate:

z(x,r)=n(r, x)/6here n(r, x) is the average number of solventmolecules in a shell of thickness dx=0.2|f, with

dr=0.15|f, with volume 6=2prdxdr. The figure

shows a particulate with |p=10|f (mfp/mff=1.5) in

a LJ/S LV interface at T*=0.75 for which k*=

0.17 (stars refer to the usual Lennard Jones

units). The contact angle is 90. The variation

of contact angle with particle size and interaction

strength has been described in detail for this

system elsewhere [47].

In Section 3 we will also discuss LJ/S particu-lates with |p=8|f, mfp/mff=1.25, at T*=0.5

(z*l=0.83, z*6=0.002, k*=0.58) for which the

contact angle is 10193. For the Lennard Jones

system (Fig. 2(b)) with |p=7|f, mfp/mff=2, molec-

ular simulations at T*=0.75 of a particulate in

the LV interface (z*l =0.76, r*6=0.012, P*=

0.008, k*=0.49) produced a contact angle of 75.


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2.2. Molecular models of passi6ated gold

nanoparticles at infinite dilution at a water/air

interface

In this section we report preliminary results for

a molecular model of a gold nanoparticle at a

water interface. Thiol passivated gold nanoparti-

cles are described by a truncated octahedral (OT)motif and can be specified by indices (n,m) wheren is the number of atoms along any edge which

joins two (1 1 1) faces and m the number along

any edge joining (1 1 1) and (1 0 0) faces [25]. Our

cluster consists of a face centred cubic lattice of

140 atoms, adopting the OT morphology with

n=4 and m=2.

In our calculations, we use the semi-empirical

many body SuttonChen [26] potential parame-

terised for gold [27].

Ui(r)=ma

r

nC

2

pinwhere pi=%

j arijm

(2)

the first term represents the repulsion between

atomic cores and the second term, pi, is the local

density of atoms. The parameters m, a and C are

determined by equilibrium lattice parameters and

lattice energies. The exponent pairs, n and m are

fitted to elastic constants. For gold, m=9.383265

kJ, a=4.080A, n=10.0, m=8.0 and C=34.408

[28]. In order to maintain the OT structure in the

passivated cluster the value of m was increased bya factor of 5 (see below).

2.2.1. The surface coating of the gold particle

The potentials used to model the interactions

between the gold, sulphur and alkyl chains can be

divided into bonded and non-bonded interactions.

The thiol chains were modelled using united-atom

potentials [29] the potential functions and

parameters used for describing the bonded inter-

actions [3032] are shown in Table 1.

For the molecular dynamics simulations of thepassivated gold cluster we used the code DLPOLY,

[33]. For computational convenience in using

DLPOLY, the bonded interaction between the gold

and the sulphur [34] was approximated as a non-

bonded Morse potential [35].

U(r)=E0

(nm)

mr0

r

nnr0

r

mn(3)

with E0=38.594 kJ, n=8, m=4, r0=2.9 A, .

Standard non-bonded interactions were mod-

elled with a Lennard Jones 12-6 potential [36].The appropriate parameters are shown in Table 2.

Parameters for unlike interactions were ob-

tained using LorentzBerthelot combining rules.

2.2.2. Cluster preparation

The thiol passivated gold cluster was prepared

using the method of Luedtke and Landman: [35].

Fig. 3. Density profil e o f a L J/S nanoparticle at the LV

interface. The figure shows a particulate with |p=10|f (mfp/mff=1.5) in a LJ/S LV interface at T*=0.75 for which k*=

0.17 (stars refer to the usual Lennard Jones units). The

contact angle is 90. Only the surface region of the liquid

phase immediately surrounding the nanoparticle is shown, the

greyscale variations show local density oscillations around the

nanoparticle extending down into the bulk liquid. As we move

from bottom to top across the LV interface (on the right and

left hand sides of the nanoparticle) the density falls until we

reach the vapour phase.


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Table 1

Potentials and parameters for describing carbon chain bonded interactions

Atom typesInteraction ParametersPotential functions

None, the bonds were constrained to a single, rigid lengthConstrained CH2CH3 r=1.54 A,bond

CH2CH2 r=1.54 A,

CH2SH r=1.82 A,

CH2CH2CH3Harmonic angle Kq=519.73 kJUbend(qi)=0.5k(uiu0)2

q0=114.4

CH2CH2SH Kq=519.73 kJq0=114.4

Dihedral angle Utorsion(I)=12 a1(1+cos )+

12 a2(1cos(2i))+

12 a3(1+cos(3i)) CH2CH2CH2SH a1=5.9046

CHxCH2CH2CH2 a2=1.134a3=13.1608

The bare, frozen, gold cluster was placed in a

solution of butanethiol molecules and the systemwas allowed to equilibrate by molecular dynamics

(1 500 000 timesteps, 1.5 ns) at low temperature

(200 K), allowing an excess of butanethiol to

absorb onto the gold surface. The temperature was

then raised to 500 K in steps of 50 K (all 25 000

timesteps, 0.025 ns) allowing desorption of excess

butanethiols and exploration of absorption sites, so

that the thiols were absorbed on the gold surface

in an HCP structure with 62 molecules absorbed in

total, in good agreement with other work [37]. The

butane of the butanethiol was then replaced bydodecane and equilibrated (all 500 000 timesteps,

1.25 ns). The gold atoms were kept frozen through-

out, resulting in a surface annealed cluster without

the structure of the gold nanoparticle being

changed. However on unfreezing the gold the

passivated cluster melted (the bare cluster was

microcrystalline at all temperatures considered

here). Due to the non-local nature of our AuSbond potential the sulphur atoms enter the gold

cluster. By increasing the Au Au interactions

(scaling mby a factor of 5) we forced the Au clusterto remain crystalline and the sulphurs bonded at

the surface. The modified passivated Au nanopar-

ticle was then equilibrated.

2.2.3. The water liquid/6apour interface

The water molecules in the water interface were

modelled using the extended simple point charge

potential [38]. The interface was constructed by

using a starting configuration comprising a liquidslab of 500 (and also 1000) water molecules in a

simulation cell with vacuum on either side and

equilibrating for 50 000 timesteps (or 0.125 ns) at

298 K. The surface tension was determined [23] by

integrating the difference between the normal and

tangential pressures across the interface giving a

value of 6495 mN m1, 10% smaller than the

experimental value [39] of 72.75 mN m1 at 298 K.

2.2.4. Introducing the particle to the interface

The passivated gold cluster was placed justabove the water interface and its trajectory fol-

lowed at 298 K for 325 000 timesteps (0.8125 ns).

Fig. 4 plots d, the position of the gold centre of

mass with respect to the average interface, the

particulate settles down to a position approxi-

mately 1.5 nm above the mean interface. The

contact angle is given by cos q=d/R, so that

given R, it can be estimated from the figure (see

below).

Table 2LennardJones parameters for non-bonded interactions

| (A, ) m (kJ)

CH3 3.930 0.9478

0.3908CH2 3.930

4.450 1.6629SH

2.737Au 3.2288


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Fig. 4. Height of the centre of mass of the gold core of the nanoparticle from the water interface (equimolar surface) as a function

of time at 298 K.

For a flexible nanoparticle the value of the

radius R fluctuates as the thiol molecules move in

response to thermal fluctuations. Fig. 5 is a den-

sity contour plot obtained in a similar manner to

Fig. 3, for the passivated gold nanoparticle at a

water interface. The nanoparticle clearly sits

above the (average) water interface with a well

defined radius which can be estimated from thefigure as 1.75 nm. Using cos q=d/R, the

data plotted in Fig. 4 predicts an average contact

angle of 150. Although there are significant

fluctuations of the contact angle on a timescale of

tens of picoseconds, this average value is in excel-

lent agreement with that estimated from the den-

sity plot taken from a subset of equilibrium

configurations, Fig. 5. The particulate as a whole

is partially dry in the water interface.

3. Nanoparticulate arrays

A popular method of characterising particulates

is to use a LT to produce surface pressure (P)

area (A) curves. Particulates are spread at the

fluid interface and confined to a given area by a

moveable bar (Fig. 6(a)).

The surface pressure on the bar is measured at

each area producing a curve similar to that shown

in the Fig. 7. A standard analysis [1921] of the

shape of the curve suggests that the size and

contact angle made by the particle with the fluid

surface can be obtained from the curve. In partic-

ular the analysis suggests that the collapse pres-

sure depends on the contact angle since it occurs

when particles are ejected from the monolayer to

form a bilayer. Hence we can measure nanoscale

Fig. 5. Density profile of the passivated gold nanoparticle at

equilibrium in the water interface at 298 K. The simulation

contained a slab of saturated liquid water comprising 1000

water molecules in a cell of dimension 6060100 A, 3. The

contour plot shows a half plane 25100 A, 2 (one scale point

to 0.25 A, ). These preliminary data were averaged over 566

equilibrium configurations.


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Fig. 6. The LT experiment and the analogous simulation cell.

tive particlefluid molecule interaction and a

short range repulsive particulateparticulate in-

teraction. For this model we take 16 nanoparticles

in 27 645 solvent particles with a contact angle

with the liquid of 101. In the second case there is

a LJs particulatefluid molecule interaction and

an attractive particulate particulate interaction

and we simulate 64 nanoparticles with a contactangle of 75 in a fluid interface composed of

43 888 solvent particles. By changing the y and z

dimensions of the simulation box (the system

density remains constant) it is possible to measure

surface pressures at different surface areas (from

difference in the surface tension of the two inter-

faces) thus obtaining a pressurearea isotherm.

The results of two simulations with particles

having different contact angles are plotted in Fig.

7. The surface pressure has been normalised with

respect to the surface tension of the LV interface.The simulation reproduces the typical shape of

the experimental isotherms. The transition is

marked by a knee in the curve beyond which the

surface pressure is essentially constant. Note that

the collapse pressure is equal to the interfacial

surface tension of the pure interface. Indeed

monolayers with different contact angles but in

the same solvent collapse at the same pressure. In

the standard interpretation based on particle pro-

motion out of the interface at the knee, they

should collapse at different pressures; the first sign

then that something is wrong with this view.

Indeed by looking at computer graphics of the

model particles in the interface as the area is

reduced, a very different picture of what happens

at the knee emerges. For the LJ/S nanoparticles

(at T*=0.5) with contact angle of 101, the LV

interface containing the particulates, buckles [23],

creating new area at a free energy cost which is

lower than that required to promote a particle out

of the interface. Such buckling has been seen

experimentally for micron size particles [40].

For the LJs nanoparticles (at the higher tem-

perature of T*=0.75) rather than buckle, the LV

interface appears to roughen. Fig. 8(a) shows a

snapshot of a monolayer of 64 nanoparticles in

the vicinity of the knee. A layer of solvent parti-

cles surrounds the colloids, indicating that at the

collapse pressure the colloids remain strongly sol-

contact angles, (as well as line tensions) from this

simple experiment. Such characterisations depend

on the correct interpretation of the pressurearea

isotherm and it is this interpretation which we

have investigated using molecular simulation.

Fig. 6(b) illustrates the simulation method. We

have a film of liquid with two LV surfaces at one

of which we place model spherical nanoparticles(here with diameter 8 times the diameter of the

fluid molecules). We consider particulates of two

types as discussed in the previous section. In the

first case (Fig. 2(a)) there is a short range attrac-

Fig. 7. Pressure area curves from molecular dynamics of

nanoparticles at LV interfaces.


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Fig. 8. (a) Snapshot of the nanoparticle array at the knee of

the curve in Fig. 7 for a contact angle of 72 ; (b) nanoparti-

cle solvent radial distribution function at the collapse pres-

sure.

ticle MFP as a function of nanoparticle separa-

tion by considering a nanoparticle dimer at the

LV interface.

The results shown in Fig. 9 represent the contri-

bution to the MFP, DF due to the solvent only

(subtracting out the direct LJs interaction). The

curve shows features typical of those observed in

colloid depletion force plots, such as the maxi-mum at |f, and the oscillations at larger distances.

The force induced by the solvent is essentially

negligible for separations larger than 4.5|f. At

very short separations, DF remains positive and

repulsive, reflecting the preference of the nanopar-

ticle to be solvated. Clearly in these conditions the

collapse of the array under pressure involves sol-

vated nanoparticles with an effective size larger

than in vacuum. We expect a collapse surface area

which is significantly larger than the close packing

area, as indeed observed in Fig. 7.

4. Summary

From our studies of model LJs nanoparticles

at LJs interfaces we conclude that it is possible to

describe the wetting of nanoscale interfaces (down

to 1 nm) using Youngs equation when (a) the

fluidfluid interfacial tensions are large and (b)

the three phase line has a small curvature. These

general results have been obtained using idealised

structureless model nanoparticulates. In nature

however nanoparticles have internal and surface

structures, are not necessarily rigid and the sol-

vents may themselves be complex. Our prelimi-

nary results for passivated gold nanoparticles at

water interfaces indicate that such particles have a

well-defined mean contact angle with however

significant fluctuations on a timescale of tens of

picoseconds. Our simulations of nanoparticle ar-

rays in the LT configuration strongly suggest that

the collapse pressure measured in LT experiments

is independent of contact angle and that such

experiments cannot be used to determine contact

angles. We expect the array to collapse by surface

roughening when the surface pressure is equal to

the surface tension of the pure solvent interface.

In experiments where a different collapse pressure

is observed we suggest that contamination of the

vated. This conclusion is reinforced by Fig. 8(b),which shows the solvent nanoparticle radial dis-

tribution function for this monolayer. As sug-

gested by the snapshot there is a layer of solvent,

which is strongly adsorbed at the nanoparticle

surface. In addition we have analysed the mean

force potential (MFP) of the colloids due to the

solvent particles. We have calculated the nanopar-


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Fig. 9. Potential of mean force for solvated nanoparticles with a contact angle of 72.

fluid interface by surfactant or other components

of the system is likely to be responsible.

Acknowledgements

We thank EPSRC for support through grant

GR/M94427, GR/R39726.

References

[1] J.R. Heath, C.M. Knobler, D.V. Leff, J. Phys. Chem. B

101 (1997) 189.

[2] P.S. Swain, R. Lipowsky, Langmuir 14 (1998) 6772.[3] P.M. Ajayan, S. Iijima, Nature 361 (1993) 333.

[4] F. Bresme, N. Quirke, Phys. Rev. Lett. 80 (1998) 3791.

[5] F. Bresme, N. Quirke, J. Chem. Phys. 110 (1999) 3536.

[6] F. Bresme, N. Quirke, Phys. Chem. Chem. Phys. 1 (1999)

2149.

[7] F. Bresme, N. Quirke, J. Chem. Phys. 112 (2000) 5985.

[8] T. Shen, R. Weissleder, M. Papisov, A. Boganov, T.J.

Brady, MRM 29 (1993) 599.

[9] R. Weissleder, et al., AJR 152 (1989) 167.

[10] S.D. Evans, S.R. Johnson, Y.L. Cheng, T. Shen, J.

Mater. Chem. 10 (2000) 183.

[11] F. Burmeister, C. Schafle, T. Matthes, M. Bohmisch, J.

Boneberg, Pleiderer, Langmuir 13 (1997) 2983.

[12] G.C. Bond, D.T. Thompson, Catal. Rev.-Sci. Eng. 41

(1999) 319.[13] A. Pockels, Nature 12 (1891).

[14] I. Langmuir, J. Am. Chem. Soc. 39 (1848) 1917.

[15] G. Gabrielli, G.G.T. Guarni, E. Ferroni, J. Colloid Inter-

face Sci. 54 (1976) 424.

[16] A. Doroszkowski, R. Lambourne, J. Polymer Sci. C. 34

(1971) 253.

[17] R. Aveyard, J.H. Clint, D. Nees, V. Paunov, Langmuir 16

(2000) 1969.

[18] H.M. McConnell, Annu. Rev. Phys. Chem. 42 (1991) 171.

[19] J.H. Clint, S.E. Taylor, Colloids Surfaces 65 (1992) 61.

[20] J.H. Clint, N. Quirke, Colloids Surfaces 78 (1993) 277.

[21] R. Aveyard, J.H. Clint, J. Chem. Soc. Faraday Transac-

tions 91 (1995) 175.[22] N.I.D. Fenwick, F. Bresme, N. Quirke, J. Chem. Phys.

114 (2001) 7274.

[23] N Quirke, Mol. Sim. 26 (2001) 1 and references therein.

[24] Readers may view a movie of such a simulation at our

website available from: http://www.ch.ic.ac.uk/quirke/

research.html.

[25] R.L Whetten, et al., Adv. Mater. 8 (1996) 428.

[26] A.P. Sutton, J. Chen, Philos. Mag. Lett. 61 (1990) 139.

[27] B.D. Todd, R.M. Lynden-Bell, Surf. Sci. 281 (1993) 191.
http://www.ch.ic.ac.uk/quirke/research.htmlhttp://www.ch.ic.ac.uk/quirke/research.htmlhttp://www.ch.ic.ac.uk/quirke/research.htmlhttp://www.ch.ic.ac.uk/quirke/research.html


11/11


[28] B.D. Todd, R.M. Lynden-Bell, Surf. Sci. 328 (1995)

170.

[29] J.P. Ryckaert, A. Bellemans, Faraday Discussions of the

Chem. Soc. 66 (1978) 95.

[30] J. Hautman, M.L. Klein, J. Chem. Phys. 91 (1989) 4994.

[31] S. Balasubramanian, M.L. Klein, J.I. Siepmann, J. Chem.

Phys. 103 (1995) 3184.

[32] W.L. Jorgensen, J. Phys. Chem. 90 (1986) 6379.

[33] DLPOLY developed by W. Smith and T.R. Forester.[34] H. Sellers, A. Ulman, Y. Shnidman, J.E. Eilers, J. Am.

Chem. Soc. 115 (1993) 9389.

[35] W.D. Luedtke, U. Landman, J. Phys. Chem. B 102 (1998)

6566.

[36] T.K. Xia, J. Ouyang, M.W. Ribarsky, U. Landman, Phys.

Rev. Lett. 69 (1992) 1967.

[37] W.D. Luedtke, U. Landman, J. Phys. Chem. 100 (1996)

13323.

[38] H.J.C. Berendsen, J.R. Grigera, T.P. Straatsma, J. Phys.

Chem. 91 (1987) 6269.

[39] CRC Handbook of Chemistry and Physics, 74th ed.[40] R. Aveyard, J.H. Clint, D. Nees, N. Quirke, Langmuir 16

(2000) 8820.

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