Ions Redistribution and Meniscus Relaxation during Langmuir Wetting

Feb 14, 2011 - Nonstationary kinetics of the ion redistribution within the meniscus region during deposition of a charged Langmuir monolayer after beg...
1 downloads 0 Views 990KB Size
Download PDF
ARTICLE pubs.acs.org/JPCB

Ions Redistribution and Meniscus Relaxation during Langmuir Wetting Process V. I. Kovalchuk,† M. P. Bondarenko,† E. K. Zholkovskiy,† V. M. Starov,‡ and D. Vollhardt*,§ †

Institute of Biocolloid Chemistry, 03142 Kiev, Ukraine Department of Chemical Engineering, Loughborough University, Loughborough, Leicestershire LE11 3TU, United Kingdom § Max Planck Institute of Colloids and Interfaces, 14424 Potsdam/Golm, Germany ‡

ABSTRACT: Nonstationary kinetics of the ion redistribution within the meniscus region during deposition of a charged Langmuir monolayer after beginning or stopping of the substrate motion is analyzed on the basis of the results of numerical simulations. The time evolution of the ions concentration profiles forming at the contact line and propagating toward the bulk solution is considered. It is shown that the diffusion front propagates much slower within the region of overlapping diffuse layers than outside of this region. At the beginning of the deposition process a region characterized by quasi-stationary behavior of the ion concentration and electric potential distributions is formed in close vicinity to the contact line. A stationary deposition regime is established when the region of quasi-stationary distributions reaches the external boundary of the Nernst layer provided that the substrate motion is not very fast. For the substrate velocities higher than the critical one the concentration near the contact line can decrease to such small values which do not allow a stable deposition process. The developed mathematical model allows addressing to transient regimes of the monolayer deposition which are very important for understanding the mechanisms leading to meniscus instability.

’ INTRODUCTION Fabrication of ultrathin organic or composite organic/inorganic films with nanometer-scale patterned structures is one of the most important branches in modern nanoscience.1-4 In particular the formation of transfer-induced regular patterns during wetting/dewetting processes attracts more and more attention.5-9 This simple, low cost, and highly efficient method is an alternative to the so-called top-down approach. It allows rapid and inexpensive patterning of a large (macroscopic) area with lateral resolution in the nanometer range. A broad spectrum of patterns with various geometric, physical and chemical properties obtained by this method is widely discussed in the literature.5-16 Also an important advantage of this method is that it can be used for deposition and patterning of very different materials including fragile (e.g., organic) materials. The interest to patterned interfaces produced during wetting/dewetting processes is explained by their potential applicability in electronics, bioanalytics, micro- and nanomechanics.5-9,17-22 For the effective control of the properties of the deposited patterned films, the mechanism of their formations should be precisely understood and the key process parameters should be identified. A variety of regular patterns can be formed at the substrate surface due to self-assembly of the deposited molecules or particles combined with the physical dewetting phenomena. The complicated interaction of the receding meniscus with the r 2011 American Chemical Society

substrate surface, which manifests themselves under essentially nonequilibrium conditions, is the most significant issue for understanding the mechanisms of patterns formation. In particular, a very important question is how this interaction can induce various meniscus instabilities during the dewetting process, which are responsible for the periodicity of the deposited structures. Therefore, the main efforts have to be concentrated on understanding the interaction of the meniscus interface with the substrate surface, analysis of the complicated nonstationary processes of matter and charge transfers within the meniscus region and their relationship to the meniscus instability and pattern formation. One of the most extensively used approaches to form patterned films is based on the so-called stick-slip behavior of the receding meniscus during dewetting processes.5,8,9,11,16 In this way regular striped patterns can be obtained with alignment parallel to the meniscus contact line (perpendicular to the film transfer direction). The parallel stripes of molecules or particles are formed because of periodical pinning of the three-phase contact line due to the interaction of the receding meniscus with the substrate surface. Though for different systems the interactions can be rather different in their nature, the mechanism of Received: December 29, 2010 Revised: January 25, 2011 Published: February 14, 2011 1999

dx.doi.org/10.1021/jp112355x | J. Phys. Chem. B 2011, 115, 1999–2005

The Journal of Physical Chemistry B

ARTICLE

are formed within the three-phase contact zone. These profiles begin to change immediately after any change of the substrate velocity, and this process continues during a rather long relaxation time. As major consequences, the processes of ion concentration and electric potential profile formations change the monolayer adhesion to the substrate, the contact angle and the composition of the deposited LB film. A rather long meniscus relaxation after stopping the substrate motion, as usually observed in experiments,33-37 is a manifestation of such slow relaxation of ion concentrations in the solution close to the contact line.32 In the present study, numerical simulations of transient processes were performed in a model system with the objective to understand the peculiarities of nonstationary kinetics of ions redistribution within the meniscus region during the Langmuir wetting process.

Figure 1. Hydrodynamic flow within the subphase during the monolayer deposition.

patterns formation appears rather similar. The periodicity of patterns is determined by the intrinsic relaxation processes characteristic for a particular system. To control the periodicity and other properties of such structures, the intrinsic relaxation processes have to be thoroughly studied. The well-known example of striped structures formed by a receding meniscus is the self-organized patterns produced during the Langmuir-Blodgett (LB) transfer of phospholipid (DPPC) monolayers.5,23,24 The stripes within the LB films of DPPC are formed when the surface pressure is kept close to the phase transition from the liquid-expanded to liquid-condensed phase. Other insoluble monolayers as well as their mixtures can also be used for LB-patterning on the substrate surfaces of a different kind.3,8,10,25,26 A particularly interesting example is the formation of striped patterns by charged fatty acid monolayers.27,28 In the case of charged monolayers, the film transfer process and the deposited film properties can be controlled by regulating the amounts of different counterions and co-ions within the subphase because of their effect on monolayer composition, surface charge, surface potential, and electrical double layer thickness.29,30 As well, due to the ion-exchange mechanisms, different metal ions (or other counterions) with specific properties (electronic, magnetic, etc.) can be incorporated into the patterned films in controlled amounts,31 which can be important for many applications. As shown in previous studies,28-32 the deposition of charged Langmuir monolayers is strongly dependent on ion redistribution kinetics within the thee-phase contact zone. The ion redistribution is the most important relaxation process for charged monolayers as the electrostatic interaction between the monolayer and the substrate surface is strongly affected by the ionic distributions within the solution in close vicinity to the thee-phase contact line. The ion redistribution during the LB deposition process is a consequence of the initial misbalance of convective ion fluxes within the solution.28-32 This effect is very similar to the concentration polarization effect well-known in electrochemistry, membrane science and electrokinetics of disperse systems. Under the conditions of the steady-state deposition, stationary ion concentration and electric potential profiles

’ FORMULATION OF THE MODEL Under typical conditions of the Y-type LB film formation we have solution surface and substrate surface covered by the same amphiphilic monolayer (Figure 1). The monolayers of ionic amphiphiles (e.g., fatty acids or fatty amines) contain dissociated molecules which produce a net surface charge at the both sides of a liquid film formed within the acute wedge between the monolayer and the substrate. During the LB film deposition the moving monolayer and substrate surface produce a circular hydrodynamic flow within the solution near the contact line. The counterions and co-ions distributed within the diffuse parts of the electrical double layers (DL) at both charged interfaces become involved in convective motion. However the convective ions transfer itself is not sufficient to support steady-state balances of the partial ions fluxes within the three-phase contact zone.29-31 As a result, due to initial misbalance of the ion fluxes, the ion concentration and electric potential profiles form within the solution. These profiles produce electro-diffusion ion fluxes acting additionally to the convective ion fluxes. After a certain relaxation time the electro-diffusion ion fluxes are adjusted in such a way that they provide steady-state balances of all partial ions fluxes and electroneutrality of the system as a whole in the course of monolayer transfer to the substrate. Under nonstationary conditions the conservation of the ith ion within the solution between the floating monolayer and the substrate surface is described by the equation DCi ¼ - r 3 ji Dt

ð1Þ

where Ci is the local ith ion concentration, ji is the flux density vector, and t is time. The flux density can be represented as the sum of the convective and electro-diffusion fluxes ji ¼ vCi -

Di Ci rμeli RT

ð2Þ

where v is the local velocity vector, Di is the ith ion diffusion coefficient, μeli = ziFj þ RT ln Ci is the local electrochemical potential of the ith ion (according to ideal solution approximation), zi is the ion charge, F is the Faraday constant, R is the gas constant, T is the temperature, and j is the local electrostatic potential. Though concentration polarization is observed for any acute contact angle it is more significant in the case of small contact angle. At small contact angles the slope of the meniscus surface with respect to the substrate surface is small and the liquid film 2000

dx.doi.org/10.1021/jp112355x |J. Phys. Chem. B 2011, 115, 1999–2005

The Journal of Physical Chemistry B

ARTICLE

between these two surfaces can be considered locally as a quasiflat film. For any cross-section of such quasi-flat film the balance eq 1 takes the form32 ! Z h D Ci ðx, yÞ dy ¼ Dt 0 D Dx

Z

h

Z vðyÞCi ðx, yÞ dy - Di

0

0

h

! Ci ðx, yÞ Dμeli dy þ 2jiy jy ¼ 0 RT Dx

monolayer. By combining eqs 3 and 6 the unknown flux jHþy|y=0 can be eliminated and the conservation equation for hydrogen ions in each film cross-section can be obtained32 ! Z h D D 2X RH þ CHþ ðx, yÞ dy Dt Dt 0 D ¼ Dx

ð3Þ where x and y are Cartesian coordinates directed along the film and across it, respectively, h = h(x) is the local film thickness, slowly changing along the film, and jiy|y=0 is the flux density of the ions which are adsorbed (desorbed) at the monolayers (a symmetrical film is assumed). In a thin liquid film the equilibrium distribution of the ions across the film establishes much faster than along it, therefore the electrochemical potential gradient dμeli /dx in eq 3 can be considered as a function of the coordinate x only.29 The velocity distribution v(y) can be written in lubrication approximation as38  ! 1 y 1 2 vðyÞ ¼ U - þ 6 ð4Þ 2 h 2 where U is the velocity at the moving surfaces. Note, under stationary conditions the velocity distribution v(y) should give zero volumetric flow in each film cross-section provided water is not entrained by the deposited film. The flux density jiy|y=0 describes adsorption or desorption of the potential-determining counterions at partially ionized monolayers. We assume that local chemical equilibrium by formation of interfacial complexes is established sufficiently fast, so that equilibrium relationships for interfacial reactions can be used. We consider here the deposition of a fatty acid monolayer from the subphase containing a small amount of a strong inorganic 1:1 acid (e.g., HCl). In this case the local chemical equilibrium at the interfaces can be described by the equation39 X R- ¼

XR 1 þ KCSHþ

ð5Þ

where XR = XR- þ XRH is the total surface concentration of dissociated, R-, and nondissociated, RH, fatty acid, K is the equilibrium (association) constant, and CHþS = CHþ|y=0,h is the bulk concentration of hydrogen ions, Hþ, at the interfaces. For condensed fatty acid monolayers XR can be assumed as approximately constant, whereas concentration CHþS changes slowly along the interface because of the overlap of the diffuse parts of electrical double layers near the contact line.39 The local concentrations of the components at the interfaces changes due to both convective transfer of the components with the monolayer moving along the interface and dissociation/ recombination reactions with the dissolved ions. In particular, the balance of nondissociated fatty acid molecules at the interface is described by the equation32 2

DX RH DJ ¼ - RH - 2jHþ y jy ¼ 0 Dt Dx

ð6Þ

where JRH = -2UXRH is the flux of the nondissociated molecules moving with the monolayer along the interfaces and jHþy|y=0 is the flux density of hydrogen ions adsorbing (desorbing) at the

Z

h

- DHþ 0

Z

h

vðyÞCHþ ðx, yÞ dy

0

CHþ ðx, yÞ DμelHþ dy - 2UX RH RT Dx

 ð7Þ

where, according to eq 5, the surface concentration of nondissociated fatty acid is XRH = XR(KCHþS)/(1 þ KCHþS). The co-ions A- do not bind at the monolayer, therefore the respective balance equation for co-ions takes the form ! Z h Z h D D CA - ðx, yÞ dy ¼ vðyÞCA - ðx, yÞ dy Dt 0 Dx 0 Z - DA0

h

CA- ðx, yÞ DμelAdy RT Dx

 ð8Þ

Thus, for describing the kinetics of the ion redistribution within the considered system, one has to solve the set of partial differential eqs 7 and 8 with the respective initial and boundary conditions. For a closed formulation of the problem the concentration and electric potential distributions within each crosssection of the film should be also given. As stated above, for a thin liquid film the equilibrium distributions of ions across the film establish much faster than along it. This allows one to use a quasiequilibrium approximation, i.e., to assume that the ion concentration and electric potential distributions within each crosssection of the film are approximately the same as within a flat film between charged interfaces being under equilibrium conditions.29,32 These quasi-equilibrium distributions should be obtained by solving the Poisson-Boltzmann equation for each film cross-section, assuming that the total numbers of counterions and co-ions (as well as their electrochemical potentials) in the cross-section slowly change along the film. It should be also taken into account that the surface charge density at the film interfaces is determined by the surface concentration of dissociated fatty acid molecules, given by eq 5, which also changes slowly along the film. So the formulated problem is rather complicated and can be solved only numerically. The complete problem formulation and its numerical solution procedure can be found in ref 32. Due to natural convection in the bulk the ion concentrations on the boundary between the meniscus region and the bulk solution should not be affected by significant changes. Therefore, according to the Nernst diffusion layer concept, we assume the ion concentrations to be fixed at a certain distance L to the contact line.29-31 It is obvious that the distance L should be of the order of the meniscus size, i.e., of about 1 mm. In the region between this boundary and the contact line the ion concentration profiles are developing which are the subject of subsequent considerations. The numerical results presented below are obtained for the following parameter set: equilibrium constant K = 6.54  104 dm3/ mol, total surface concentration of fatty acid XR = 8.3  10-6 mol/ m2, diffusion coefficient for hydrogen ions DHþ = 9.34  10-9 2001

dx.doi.org/10.1021/jp112355x |J. Phys. Chem. B 2011, 115, 1999–2005

The Journal of Physical Chemistry B

ARTICLE

Figure 2. Development of the concentration profile within the meniscus region for C0 = 0.03 mol/m3 and U/θ = 0.005 m/s, shown at different distances to the contact line corresponding to the film thicknesses h = 2  10-8 (1), 2  10-7 (2), 2  10-6 (3), 2  10-5 (4), 2  10-4 (5), and 2  10-3 m (6).

m2/s, and diffusion coefficient for the anions DA- = 2.04  10-9 m2/s.29-31 The calculated concentrations are functions of three parameters: effective time tef = θ2t, local film thickness h, and parameter U/θ.

’ RESULTS AND DISCUSSION As soon as the monolayer and the substrate begin to move with the velocity U they produce convective flow within the solution (Figure 1). As the hydrodynamic relaxation time is much shorter than the diffusion relaxation time, the hydrodynamic field produced by the moving interfaces (eq 4) can be considered as a steady-state field. The hydrogen ions, Hþ, and the anions, A-, distributed within the solution between the two charged interfaces are forced to move according to the velocity field distribution: the ions move toward the contact line near the interfaces and back to the bulk in the central part of the film. However, the ions near the charged interfaces are distributed nonuniformly. Therefore their direct convective flux (toward the contact line) is different from their back flux (toward the bulk solution). Moreover, the convective flux of hydrogen ions is not sufficient to compensate their amount bound by the dissociated fatty acid molecules near the contact line and transferred to the substrate with the deposited film. Thus, due to the misbalance of the ion fluxes, ion concentration profiles begin to form in immediate vicinity of the contact line after beginning of the monolayer transfer.29,32 This process can be demonstrated by variation of the quasiequilibrium concentration, C = (CA-CHþ)1/2, shown in Figure 2 for different distances to the contact line. In the initial time moment the concentration is constant everywhere in the meniscus region: C = C0. The concentration profile begins to develop at the contact line (curve 1) and propagates as a diffusion front toward the bulk solution (curves 2-5). As discussed above, at the external boundary of the meniscus region the concentration C remains fixed (curve 6). It is seen that the curves 3 to 5 in Figure 2 have approximately the same slop. These curves correspond to sufficiently thick regions, where the electrical double layers at both interfaces occupy only a rather small part of the film cross-section. In this case, the concentration profile propagates with a velocity which is determined by the average ionic diffusion coefficient in electroneutral

Figure 3. Propagation of the concentration profiles within the meniscus region for C0 = 0.03 mol/m3 and U/θ = 0.0005 (a) and 0.005 m/s (b), shown for different time moments: tef = 4  10-6 (1), 1.15  10-4 (2), 2.7  10-3 (3), 5.8  10-2 (4), 1.26 (5), 26.6 (6), and 1033 s (7) in (a) and tef = 4  10-6 (1), 1.06  10-4 (2), 2.08  10-3 (3), 6  10-2 (4), 1.8 (5), 53.6 (6), and 2636 s (7) in (b).

solution. The curves 1 and 2 have a smaller slop. They are related to very thin regions near the contact line, where the diffuse parts of the electrical double layers formed at the two interfaces overlap. In these regions the diffusion front propagates much slower as its velocity is limited by the diffusion of co-ions whose concentration is strongly reduced here because of electrostatic repulsion. As local electroneutrality should hold within the solution32 the counterions cannot move faster than the co-ions which results in a rather small effective diffusion coefficient. It should be noted that because of the decreasing concentration C the thickness of the electrical double layers increases and the region with overlapping diffuse layers expands gradually. As we assume that anions, A-, are not adsorbed at the interfaces their flux with the deposited film crossing the contact line is zero. However the total convective flux of anions is not zero in each cross-section of the solution region near the contact line. Anions are removed from the region near the contact line as their amount in the back convective flux is higher than in the direct flux (due to electrostatic repulsion at the interfaces). Hence the amount of anions remaining in the space between a chosen cross-section of the solution and the contact line should decrease gradually. The amount of cations decreases too as they are removed by the deposited film faster than they are transferred toward the contact line by the convective flux. 2002

dx.doi.org/10.1021/jp112355x |J. Phys. Chem. B 2011, 115, 1999–2005

The Journal of Physical Chemistry B

Figure 4. Concentration decrease near the contact line (h = 2  10-8 m) with time for C0 = 0.03 mol/m3 and different velocities: U/θ = 0 (1), 0.0005 (2), 0.0015 (3), 0.005 (4), and 0.01 m/s (5).

According to eq 1 the ion concentrations should change faster in the regions where the divergence of the ion fluxes is larger. The total convective fluxes of anions and cations through each crosssection are almost constant in the solution regions sufficiently far from the contact line, where the diffuse parts of the electrical double layers formed at two charged interfaces do not overlap. Therefore, initially, the ion concentrations almost do not change in these regions. This is seen from the concentration profiles shown in Figure 3. However, in the solution region close to the contact line, where the diffuse parts of the electrical double layers overlap, the total convective fluxes of the ions change fast with the distance to the contact line. In particular, the convective flux of anions decreases up to zero at the contact line. Therefore, a concentration profile begins to form here and propagates toward the bulk (Figure 3). Due to the decreasing concentration (and decreasing ions electrochemical potentials) electrodiffusion ion fluxes appear within the solution near the contact line, which restore the balance of the total ion fluxes. Initially the electrodiffusion ion fluxes compensate the misbalance of the total ion fluxes in immediate vicinity to the contact line. Because of such compensation of the ion fluxes quasistationary conditions establish in a small solution region close to the contact line.32 Under such conditions the ion concentrations and electric potential distributions in this small region become close to those which would be in a steady-state regime with fixed concentrations at the external boundary of this region. However, as the maximum of the divergence of the ion fluxes moves gradually toward the bulk solution, the concentrations at the external boundary of the steady-state region continue to change, and, hence, the ion concentrations and electric potential distributions in this region should change correspondingly. The quasi-steady-state region expands gradually until the external boundary of the meniscus region will be reached where the ion concentrations are fixed. After that true steady-state establishes within the whole meniscus region. Such behavior corresponds to the sequence of curves shown in Figure 3a. In such a case, the concentration in immediate vicinity to the contact line does not decrease to a critically small value. Figure 3b shows, however, a qualitatively different behavior which corresponds to higher deposition rates. In this case, before the steady-state establishes within the meniscus region, the concentration

ARTICLE

Figure 5. Direct (1 and 2) and back (3 and 4) relaxations of the concentration near the contact line for C0 = 0.03 mol/m3 and U/θ = 0.0015 m/s at two distances to the contact line corresponding to the film thicknesses h = 2  10-8 (bold lines 1 and 3) and 2  10-5 m (thin lines 2 and 4).

in immediate vicinity to the contact line can reach a very small value, and the subsequent monolayer deposition can become impossible. With decreasing concentration near the contact line the electrostatic repulsion between the monolayer and the substrate surface increases what leads to a decrease of adhesion.28,29 At certain sufficiently small concentration, the adhesion work can decrease up to zero and the monolayer deposition process can be disrupted before steady-state is reached. As it is seen from Figure 4, the concentration decrease near the contact line is determined by substrate and monolayer velocity U (for a given contact angle θ). The higher the velocity U, the smaller is the concentration. For velocities smaller than the critical one the concentration decreases up to a steady-state value which is still sufficiently high (curves 2 and 3 in Figure 4). However, for the velocities larger than the critical one the concentration near the contact line can decrease to very small values which do not allow a stable deposition process (curves 4 and 5 in Figure 4). Such behavior was predicted earlier from the analysis of stationary deposition regimes.29 It is important that with decreasing concentration near the contact line the electrostatic repulsion between substrate and monolayer becomes stronger.29,30 This should lead to a decrease of the contact angle and, therefore, to a stronger concentration polarization within the subphase. Due to coupling between the electrokinetic processes and contact angle the meniscus becomes unstable when the local concentration at the contact line decreases to a critical value.28,29 The disruption of the monolayer deposition process at velocities higher than the critical one is very important as it can be used for patterning the substrate surface. In particular, in the presence of two potential-determining ions within the subphase it is possible to obtained striped patterns with alternating ionic composition of the stripes.27,28 To control the properties of such patterned films it is necessary to know the dependence of the maximum deposition rate on the subphase composition and the monolayer properties. We discussed above the formation of concentration profiles after beginning of the deposition process. When the deposition is stopped back relaxation begins in the solution within the meniscus 2003

dx.doi.org/10.1021/jp112355x |J. Phys. Chem. B 2011, 115, 1999–2005

The Journal of Physical Chemistry B region. During this back relaxation process the concentration profiles should disappear and the system should return to its equilibrium state, as shown in Figure.5. Such back relaxation of the meniscus was reported in a number of studies.33-37 It is important that the experimentally obtained relaxation times (usually of about 10 min or more) are in a good agreement with the times predicted by our numerical simulations.32

’ CONCLUSIONS In the present study nonstationary kinetics of the ion redistribution within the meniscus region during the deposition of a charged Langmuir monolayer is analyzed on the basis of the results of numerical simulations. A model system composed of an insoluble fatty acid monolayer deposited from the subphase containing an inorganic acid is considered. The developed mathematical model allows addressing to the transient regimes of the monolayer deposition which are very important for understanding the mechanisms leading to meniscus instability. It is shown that after beginning or stopping of the substrate motion, a complex relaxation process is initiated in the system. During this relaxation process both the electric potential and the ion concentration distributions in the meniscus region change gradually toward those corresponding to a steady-state regime for the given substrate and monolayer velocities. It is shown, that at the contact line, with the beginning of the substrate motion, a region with nearly zero co-ion flux forms and gradually expands toward the bulk solution. This region is characterized by quasi-stationary behavior of the ion concentration and electric potential distributions. These quasi-stationary distributions slowly change with time due to the expansion of the region of zero co-ion flux. For not very high deposition rates a stationary deposition regime is established when the region of quasi-stationary distributions reaches the external boundary of the Nernst layer. For high deposition rates the concentration near the contact line can decrease to very small values which do not allow a stable deposition process. After stopping the substrate motion, an opposite relaxation process is observed, during which the ion concentration and electric potential profiles gradually disappear. It has to be noted that the numerical results presented here, were obtained under the assumption of a flat meniscus profile, when the local thickness changes as a linear function of the distance: h(x) = θx. The real meniscus profile can be obtained from the consideration of the dynamic force balance with accounting for van der Waals and electrostatic forces. However the mechanism of concentration polarization does not depend on a particular meniscus profile. Therefore, for the sake of simplicity, we can assume constant slope of the meniscus profile, which is widely used in the works on wetting dynamics. The generalization of the obtained results to any particular meniscus profile h(x) is straightforward, provided that the slope dh/dx remains small. Moreover, the behavior of the system remains qualitatively the same also for the case that the slope is not small. ’ AUTHOR INFORMATION Corresponding Author

*E-mail:[email protected].

’ ACKNOWLEDGMENT Financial assistance by the Bundesministerium f€ur Bildung, Wissenschaft, Forschung und Technologie (BMBF) and the

ARTICLE

Ukrainian Ministry of Education and Science (common project UKR 07/007), and National Ukrainian Academy of Sciences (project 64/10-H) is gratefully acknowledged. V.I.K. thanks The Royal Academy of Engineering. The work was supported by COST D43 Action.

’ REFERENCES (1) Shimomura, M.; Sawadaishi, T. Curr. Opin. Colloid Interface Sci. 2001, 6, 11. (2) Henzie, J.; Barton, J. E.; Stender, C. L.; Odom, T. W. Acc. Chem. Res. 2006, 39, 249. (3) Girard-Egrot, A. P.; Godoy, S.; Blum, L. J. Adv. Colloid Interface Sci. 2005, 116, 205. (4) Woodson, M.; Liu, J. Phys. Chem. Chem. Phys. 2007, 9, 207. (5) Gleiche, M.; Chi, L. F.; Fuchs, H. Nature 2000, 403, 173. (6) Xia, Y.; Qin, D.; Yin, Y. Curr. Opin. Colloid Interface Sci. 2001, 6, 54. (7) Yabu, H.; Shimomura, M. Adv. Funct. Mater. 2005, 15, 575. (8) Badia, A.; Moraille, P.; Tang, N. Y. W.; Randlett, M. E. Int. J. Nanotechnol. 2008, 5, 1371. (9) Tao, A. R.; Huang, J.; Yang, A. P. Acc. Chem. Res. 2008, 41, 1662. (10) Howland, M. C.; Johal, M. S.; Parikh, A. N. Langmuir 2005, 21, 10468. (11) Ghosh, M.; Fan, F.; Stebe, K. J. Langmuir 2007, 23, 2180. (12) Hirtz, M.; Fuchs, H.; Chi, L. J. Phys. Chem. B 2008, 112, 824. (13) Kondo, Y.; Fukuoka, H.; Nakano, S.; Hayashi, K.; Tsukagoshi, T.; Matsumoto, M.; Yoshino, N. Langmuir 2007, 23, 5857. (14) Demidenok, K.; Bocharova, V.; Stamm, M.; J€ahne, E.; Adler, H.-J. P.; Kiriy, A. Langmuir 2007, 23, 9287. (15) Lomova, E. M.; Turygin, D. S.; Ezhov, A. A.; Arslanov, V. V.; Kalinina, M. A. J. Phys. Chem. B 2009, 113, 8581. (16) Pignataro, B. J. Mater. Chem. 2009, 19, 3338. (17) Lenhert, S.; Zhang, L.; Mueller, J.; Wiesmann, H. P.; Erker, G. E.; Fuchs, H.; Chi, L. F. Adv. Mater. 2004, 16, 619. (18) Zhang, M.; Lenhert, S.; Wang, M.; Chi, L. F.; Lu, N.; Fuchs, H.; Ming, N. Adv. Mater. 2004, 16, 409. (19) Purrucker, O.; F€ortig, A.; L€udtke, K.; Jordan, R.; Tanaka, M. J. Am. Chem. Soc. 2005, 127, 1258. (20) Wu, X.; Lenhert, S.; Chi, L. F.; Fuchs, H. Langmuir 2006, 22, 7807. (21) Chen, X.; Lenhert, S.; Hirtz, M.; Lu, N.; Fuchs, H.; Chi, L. F. Acc. Chem. Res. 2007, 40, 393. (22) Watanabe, S.; Kimura, H.; Sato, T.; Shibata, H.; Sakamoto, F.; Azumi, R.; Sakai, H.; Abe, M.; Matsumoto, M. Langmuir 2008, 24, 8735. (23) Riegler, H.; Spratte, K. Thin Solid Films 1992, 210/211, 9. (24) Spratte, K.; Chi, L. F.; Riegler, H. Europhys. Lett. 1994, 25, 211. (25) Chen, X.; Lu, N.; Zhang, H.; Hirtz, M.; Wu, L.; Fuchs, H.; Chi, L. F. J. Phys. Chem. B 2006, 110, 8039. (26) Raudino, A.; Pignataro, B. J. Phys. Chem. B 2007, 111, 9189. (27) Mahnke, J.; Vollhardt, D.; St€ockelhuber, K. W.; Meine, K.; Schulze, H. J. Langmuir 1999, 15, 8220. (28) Kovalchuk, V. I.; Bondarenko, M. P.; Zholkovskiy, E. K.; Vollhardt, D. J. Phys. Chem. B 2003, 107, 3486. (29) Bondarenko, M. P.; Zholkovskiy, E. K.; Kovalchuk, V. I.; Vollhardt, D. J. Phys. Chem. B 2006, 110, 1843. (30) Bondarenko, M. P.; Kovalchuk, V. I.; Zholkovskiy, E. K.; Vollhardt, D. J. Phys. Chem. B 2007, 111, 1684. (31) Kovalchuk, V. I.; Bondarenko, M. P.; Zholkovskiy, E. K.; Vollhardt, D. J. Phys. Chem. B 2008, 112, 11333. (32) Bondarenko, M. P.; Kovalchuk, V. I.; Zholkovskiy, E. K.; Vollhardt, D. Colloids Surf. A 2010, 354, 226. (33) Gaines, G. L., Jr. J. Colloid Interface Sci. 1977, 59, 438. (34) Peng, J. B.; He, S.; Dutta, P.; Ketterson, J. B. Thin Solid Films 1991, 202, 351. (35) Petrov, J. G.; Angelova, A. Langmuir 1992, 8, 3109. 2004

dx.doi.org/10.1021/jp112355x |J. Phys. Chem. B 2011, 115, 1999–2005

The Journal of Physical Chemistry B

ARTICLE

(36) Petrov, J. G.; Angelova, A.; M€obius, D. Langmuir 1992, 8, 206. (37) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I.; Ye, X. Colloids Surf. A 1995, 94, 279. (38) de Gennes, P. G. Colloid Polym. Sci. 1986, 264, 463. (39) Kovalchuk, V. I.; Zholkovskiy, E. K.; Bondarenko, M. P.; Vollhardt, D. J. Phys. Chem. B 2001, 105, 9254.

2005

dx.doi.org/10.1021/jp112355x |J. Phys. Chem. B 2011, 115, 1999–2005