Lipid Composite Films at

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Thin Film Formation of Silica Nanoparticle/Lipid Composite Films at the Fluid-Fluid Interface Michael Maas,* Chin C. Ooi, and Gerald G. Fuller Department of Chemical Engineering, Stanford University, Stanford, California 94305-5025, United States Received May 13, 2010. Revised Manuscript Received October 28, 2010 We report a new and simple method for the formation of thin films at the interface between aqueous silica Ludox dispersions and lipid solutions in decane. The lipids used are stearic acid, stearyl amine, and stearyl alcohol alongside silica Ludox nanoparticle dispersions of varying pH. At basic pH thin films consisting of a mixture of stearic acid and silica nanoparticles precipitate at the interface. At acidic and neutral pH we were able to produce thin films consisting of stearyl amine and silica particles. The film growth was studied in situ with interfacial shear rheology. In addition to that, surface pressure isotherm and dynamic light scattering experiments were performed. The films all exhibit strong dynamic rheological moduli, rendering them an interesting material for applications such as capsule formation, surface coating, or as functional membranes.

Introduction The self-assembly of nanoparticles leads to the formation of ordered structures on meso- or macroscopic scales and thus combines the advantages of large-scale structures with the special properties of the nanoscale. In this respect it is of foremost interest to investigate new ways for the preparation of self-assembled and at the same time durable nanostructured materials such as cohesive thin films. In a previous paper,1 we reported the growth of thin films grown at the interface between calcium hydrogen carbonate solution and stearic acid solution in dodecane. As a growth mechanism for those films, we proposed that the agglomeration of stearic acid and amorphous calcium carbonate particles at the interface leads to the formation a cohesive thin film. In order to test this hypothesis, for this paper, we took a more general approach and exchanged the calcium hydrogen carbonate phase with an aqueous dispersion of silica Ludox particles. In this way we were able to provide a system that is much simpler than the bio-inspired calcium carbonate approach, enabling us to understand the film-growth process on a more detailed level while providing results that are of interest to a broader group of scientists. A multitude of publications exist in the field of nanoparticle research. Because of this, only recent and exemplary publications are cited. The deposition of nanoparticles or nanoparticle films on substrates or at the air-water interface is well documented in the literature.2,3 Regarding the liquid-liquid interface, the adsorption of single nanoparticles at the interface is well understood.4-8 Concerning the adsorption and self-assembly of nanoparticles at liquid-liquid interfaces, many publications focus on the *Corresponding author: e-mail: [email protected]; Tel: þ1-650-725-3139; Fax: þ1-650-725-7294.

(1) Maas, M.; Rehage, H.; Nebel, H.; Epple, M. Langmuir 2009, 25, 2258–2263. (2) Sear, R. P.; Chung, S. W.; Markovich, G.; Gelbart, W. M.; Heath, J. R. Phys. Rev. E 1999, 59, 6255–6258. (3) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293–346. (4) Bresme, F.; Oettel, M. J. Phys.: Condens. Matter 2007, 19, 413101. (5) Bresme, F.; Quirke, N. Phys. Chem. Chem. Phys. 1999, 1, 2149–2155. (6) Schlossman, M. L. Curr. Opin. Colloid Interface Sci. 2002, 7, 235–243. (7) Binder, W. H. Angew. Chem., Int. Ed. 2005, 44, 5172–5175. (8) Lin, Y.; B€oker, A.; Skaff, H.; Cookson, D.; Dinsmore, A. D.; Emrick, T.; Russell, T. P. Nano Lett. 2002, 2, 583–587.

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investigation of Pickering emulsion systems.9-15 Several publications exist about the effect of Pickering emulsifiers on surfactant systems.16,17 The formation of thin films of nanoparticles at interfaces is addressed by several groups, which study the assembly of regularly ordered two-dimensional particle arrays (colloidal crystals).18-20 Of particular interest is one publication21 that discusses the behavior of nanoparticle monolayers at the liquidliquid interface under lateral pressure. In this work, Aveyard et al. observed that an otherwise well-dispersed nanoparticle monolayer agglomerates as soon as the aqueous phase contains a certain amount of surfactant. The authors do not propose an explanation for this phenomenon. However, it is evident that a similar process occurs during the formation of cohesive nanoparticle films that are presented in this paper.

Materials and Methods Chemicals. All chemicals were purchased from Sigma-Aldrich Co. at analytical grade and used without further purification. Stearic acid (octadecanoic acid), stearyl amine (octadecyl amine), and stearyl alcohol (octadecanol) were dissolved in chloroform (concentration=0.1 mM) or decane (concentration=0.1-5 mM). Silica Ludox HS-30 dispersions were diluted with Millipore water to 5% and 10%. Those dispersions exhibit a pH of 10 due to sodium hydroxide, which is used to stabilize the negatively charged silica particles. Silica Ludox TMA (which is a deionized (9) Melle, S.; Lask, M.; Fuller, G. G. Langmuir 2005, 21, 2158–2162. (10) Vignati, E.; Piazza, R.; Lockhart, T. P. Langmuir 2003, 19, 6650–6656. (11) Bon, S. A. F.; Chen, T. Langmuir 2007, 23, 9527–9530. (12) Binks, B. P.; Whitby, C. P. Langmuir 2004, 20, 1130–1137. (13) Arditty, S.; Schmitt, V.; Giermanska-Kahn, J.; Leal-Calderon, F. J. Colloid Interface Sci. 2004, 275, 659–664. (14) Levine, S.; Bowen, B. D.; Partridge, S. J. Colloids Surf. 1989, 38, 325–343. (15) Binks, B. P.; Clint, J. H. Langmuir 2002, 18, 1270–1273. (16) Ravera, F.; Santini, E.; Loglio, G.; Ferrari, M.; Liggieri, L. J. Phys. Chem. B 2006, 110, 19543–19551. (17) Wang, W.; Zhou, Z.; Nandakumar, K.; Xu, Z.; Masliyah, J. H. J. Colloid Interface Sci. 2004, 274, 625–630. (18) Mougin, K.; Haidara, H.; Castelein, G. Colloids Surf., A 2001, 193, 231–237. (19) Vonna, L.; Schmitt, T.; Haidara, H. Colloids Surf., A 2008, 331, 220–226. (20) Duan, H.; Wang, D.; Kurth, D. G.; M\€ohwald, H. Angew. Chem., Int. Ed. 2004, 43, 5639–5642. (21) Aveyard, R.; Clint, J. H.; Nees, D.; Quirke, N. Langmuir 2000, 16, 8820–8828.

Published on Web 11/10/2010

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Figure 1. 2D double Couette shearing system. version of Ludox HS) was diluted to 10% and had a pH of 7. An acidic solution (pH=1) of Ludox HS-30 was prepared by adding 1 mL of concentrated hydrochloric acid to 100 mL of 10% Ludox HS-30 dispersion with rapid stirring. Dynamic Light Scattering. The dynamic light scattering and zeta-potential experiments were performed with a Malvern NanoZS instrument equipped with a 4 mW He-Ne laser (633 nm) with a fixed detector angle of 173°. Samples were prepared by pipetting 8 mL of freshly centrifuged aqueous dispersion and 5 mL of 1 mM lipid solution in decane into a test tube. After letting the interfacial film grow overnight, small aliquots of both the bulk organic and the bulk aqueous phase were taken out of the test tubes for measuring. Pure solutions and the respective solvents were measured as well. The autocorrelation function of the scattered intensity was analyzed using the inverse Laplace transform program CONTIN. Characterization of Ludox Particles. Prior to the dynamic light scattering experiments, the particle dispersions were centrifuged at 14 000 rpm for 15 min. The diameter of the silica nanoparticles is 9 nm at pH = 10, 16 nm at pH = 7, and 18 nm at pH = 1, as determined by dynamic light scattering (size weighted by volume). The larger radii are a result of more pronounced aggregation due to a lower zeta potential of the particles at lower pH. Since bigger particle aggregates are removed from the dispersions by centrifugation, the smaller aggregates add to the perceived particle diameter. The zeta potential of the particles was -30 mV for Ludox HS-30 at pH = 10, -24 mV for Ludox TMA at pH=7, and -0.5 mV for Ludox HS-30 at pH=1. (It has to be noted that the zeta-potential measurement at pH=1 is not very reliable.22 As the size measurements show, the dispersions at pH = 1 are still reasonably stable.23-26) All DLS measurements for the characterization of the particles were carried out in dispersions with a particle concentration of 5%. Π-A Isotherms. The Π-A isotherms were recorded using a KSV Langmuir trough (custom two-phase model, dimensions: 5.4 cm  33 cm) equipped with double barriers. A platinum Wilhelmy plate (width: 11 mm) was positioned perpendicular to the barriers. The trough was filled with either 200 mL of particle dispersion or water. The pH of the nonparticle subphases has been adjusted using NaOH or HCl. 200 μL of 0.1 mM stearic acid, (22) Kosmulski, M. Colloids Surf., A 2003, 222, 113–118. (23) Binks, B. P.; Rodrigues, J. A.; Frith, W. J. Langmuir 2007, 23, 3626–3636. (24) Kim, S.; Sung, J. H.; Hur, K.; Ahn, K. H.; Lee, S. J. J. Colloid Interface Sci. 2010, 344, 308–314. (25) Meeren, P. V.; Saveyn, H.; Kassa, S. B.; Doyen, W.; Leysen, R. Phys. Chem. Chem. Phys. 2004, 6, 1408–1412. (26) Boussu, K.; Belpaire, A.; Volodin, A.; Van Haesendonck, C.; Van der Meeren, P.; Vandecasteele, C.; Van der Bruggen, B. J. Membr. Sci. 2007, 289, 220–230.

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stearyl amine, or stearyl alcohol solution in chloroform was spread on the interface. After evaporation of the chloroform (5 min) the interfacial film was compressed at 3 mm/min while the surface pressure was recorded with the Wilhelmy plate. Measurements at extreme pH (1 or 14) proved to be unsuccessful due the delicate nature of the monomolecular films. Interfacial Rheology. The surface shear rheological properties of the films were determined by a TA Instruments AR-G2 rheometer, which was equipped 2D double Couette shearing system. The measuring cell consisted of a Teflon dish with an inner and outer edge and a ring (d=7 cm), which could be placed exactly at the interface between oil and water (see Figure 1). The dish was first filled with the aqueous phase up to a ledge at the half-height of the dish (ca. 10 mL). For measurements at the decane/water interface, the ring was first placed on the aqueous phase and the decane solution was added (8 mL) afterward. We measured the torque required to rotate the ring with a sinusoidal angular frequency ω and a deformation (or strain) γ. In such experiments, the two-dimensional elastic modulus G0 and the twodimensional vicous modulus G00 can be computed from the amplitudes and phase angles of the stress and deformation signals.27-30 Two kinds of tests were performed: time test (constant ω = 0.1 rad/s and constant γ = 0.1%) and strain test (constant ω = 0.1 rad/s). Frequency tests were not possible due to the high inertia of the measurement geometry with respect to the low moduli of the system. Even though the time tests were performed over long periods of time (10 h) there was no significant evaporation of decane, due to the relatively low vapor pressure of decane (0.13 kPa) and the presence of surfactants at the air/decane interface. Interfacial rheology is one of the very few techniques that provide the ability to investigate fluid interfaces in situ. Because of the typically low linear viscoelastic threshold (maximum strain) values and low moduli of interfacial films, the resulting stress (torque) values are always at the brink of the sensitivity of the rheometer. Because of that, the reproducibility of the measurements is limited to ∼1 order of magnitude. A comparison between two graphs recorded at the exact same conditions can be found in the Supporting Information. Scanning Electron Microscopy. For scanning electron microscopy (SEM) a silicon substrate was quickly dipped edgewise (27) Kr€agel, J.; Derkatch, S.; Miller, R. Adv. Colloid Interface Sci. 2008, 144, 38–53. (28) Biswas, B.; Haydon, D. A. Proc. R. Soc. London, Ser. A 1963, 271, 296–316. (29) Freer, E. M.; Yim, K. S.; Fuller, G. G.; Radke, C. J. J. Phys. Chem. B 2004, 108, 3835–3844. (30) Monteux, C.; Fuller, G. G.; Bergeron, V. J. Phys. Chem. B 2004, 108, 16473–16482.

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Figure 2. SEM micrographs of a silica Ludox/stearic acid film at different magnifications.

Figure 3. (a) Time and (b) strain test of an interfacial film grown at the interface between 5% Ludox HS-30 dispersion and 1 mM stearic acid in decane.

through the film and carefully pulled out again in a way that the lower side of the film was attached to the surface of the substrate. Excess solvent was quickly removed with a paper tissue that was applied to the edge of the film. The samples were dried in air and imaged with a FEI ESEM Quanta 400 scanning electron microscope.

Results We will begin by discussing results concerning the formation of films grown at the interface between 5% Ludox HS-30 (pH=10) dispersion and 1 mM stearic acid in decane. Those results are then compared to findings with the other lipids or particle dispersions. When observed by scanning electron microscopy (Figure 2), the films appear to consist of several layers of cohesive particle agglomerates, which are engulfed in smooth areas that likely consist mainly of stearic acid or sodium stearate. The particles can be seen in Figure 2c. The sizes match those found with DLS (around 10 nm). The film growth can be monitored in situ using interfacial shear rheology. Figure 3a shows the dynamic moduli as a function of time. The film growth can be divided into two phases: the initial growth phase during the first hour of film growth and a subsequent, slower growth phase. In the first growth phase the moduli increase quickly, until the interfacial film is cohesive and mainly elastic (G0 > G00 ). In the second growth phase, the moduli increase more slowly. In this period, more material (lipid and particles) is accumulated onto the closed film at a constant rate until an equilibrium is reached between material that is dissolved in one of the volume phases and adsorbed material. The moduli at the end of film growth are rather high for an interfacial film, though the Langmuir 2010, 26(23), 17867–17873

films are rather brittle and tend to break irreversibly at very low deformations, as the strain test shows (Figure 3b). This is to some degree typical for interfacial films. In our case, the linearviscoelastic threshold (the strain at which the moduli are no longer parallel and the film structures tend to break irreversibly) is so low that it is very hard to clearly identify the threshold value. A close look at the strain sweep experiment in Figure 3b shows that G0 is never really parallel to G00 and is slightly degrading with strain. Up to 0.1% strain, though, G00 is constant. As a compromise between sensitivity of the technique and the least destructive strain, a constant strain of 0.1% was used in all presented experiments. In order to find out more about the contribution of stearic acid and Ludox HS-30 concentration on film growth and stability, the dynamic moduli were measured as a function of the concentrations of both aqueous and organic phases. Values for the moduli are taken from time tests after they reach equilibrium at about 10 h of film growth. At a fixed Ludox concentration of 5% (Figure 4a) the moduli stay constant above a stearic acid concentration of 0.3 mM. Measurements below this concentration are difficult due to a rapid decrease of the moduli. Therefore, it seems that as soon as the stearic acid concentration is high enough to form a closed film, additional stearic acid stays in the bulk phase or at least does not contribute to the stability of the films. At a fixed stearic acid concentration of 1 mM, the moduli stay constant above a Ludox concentration of 10% (Figure 4b). An additional measurement (Figure 4c) with 10% Ludox and 5 mM stearic acid revealed that there is no direct relation between the Ludox concentration and the stearic acid saturation at the interface. DOI: 10.1021/la103492a

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Figure 4. Dynamic shear moduli as a function of the stearic acid concentration in the decane phase (a) and the Ludox HS-30 concentration in the aqueous phase (b). (c) Time test with 10% Ludox HS-30 and 5 mM stearic acid.

Figure 5. Time tests of films grown at the interface between (a) 5% Ludox HS-30 and 1 mM stearyl amine, (b) 10% Ludox HS-30 and 5 mM stearyl amine, and (c) 5% Ludox HS-30 and 1 mM stearyl alcohol (right) in decane.

This means that while the moduli increase with increasing Ludox concentration (up to 10%), the moduli do not increase again upon increasing the stearic acid concentration. With stearyl amine instead of stearic acid, film formation can be observed as well, albeit with moduli that are more than an order of magnitude lower compared to stearic acid solution of the same concentration (compare Figure 3a to 5a and Figure 4c to 5b). Additionally, while films grown from stearic acid appear white to the unaided eye, films grown from stearyl amine are not visible. No film growth can be observed with stearyl alcohol (Figure 5c). Dynamic light scattering measurements were performed in order to look for particles/stearic acid agglomerates that either are solubilized in the decane phase or appear as slightly larger particles (particles with stearic acid attached to them) in the aqueous phase. No changes between the solutions before and after film formation could be measured. Therefore, it seems that all events leading to film formation occur at the interface only. In order to further elucidate the role of the lipids and their interactions with silica particles, Π-A isotherms were recorded with the three different lipids on particle dispersions (10% Ludox HS-30, pH=10, and 10% Ludox TMA, pH=7) and water with pH adjusted in accordance to the particle dispersions. Note that these measurements were performed at the air/water interface. Surface tension (or surface pressure) measurements at the decane/ water interface proved to be unsuccessful due to the solidlike behavior of the interfacial films. Ludox dispersions without lipids at the interface do not change the surface pressure at all, as is expected from a colloidal dispersion that is stabilized by surface charges.17 17870 DOI: 10.1021/la103492a

All three lipids show a different behavior under compression using a Langmuir trough. In the case of stearic acid (Figure 6a), the mean molecular area (Mma) is reduced slightly (about 4 A˚2) in the presence of HS-30 particles. The most likely reason for this behavior is the nucleation of sodium stearate crystals at the interface, which leads to the reduction of the amount of free stearic acid at the interface. The formation of sodium stearate is promoted by the sodium hydroxide that is used to stabilize the Ludox HS-30 (pH=10) dispersion. The difference between the graphs of Ludox TMA and pH=7 water is not significant. In the case of stearyl amine (Figure 6b), the Ludox TMA particles insert between the lipid molecules at the interface, shifting the isotherm to the right. This behavior was also observed with magnetite particles.31 The interaction is favored due to the negatively charged silica particles and the partially positively charged stearyl amine molecules. At higher pH, the amine head groups are uncharged and Coulomb interaction with the particles should be very weak. Therefore, no significant difference can be observed between the graphs of Ludox HS-30 (pH= 10) dispersion and water at the same pH. Stearyl amine on water (pH=7) exhibits a smaller Mma than stearyl amine on basic water (pH=10), despite the contribution of the charged head groups (at neutral pH) which should lead to a greater space requirement by the lipid molecules. This effect can be explained by the very low but significant water solubility of charged stearyl amine. (31) Degen, P.; Paulus, M.; Maas, M.; Kahner, R.; Schmacke, S.; Struth, B.; Tolan, M.; Rehage, H. Langmuir 2008, 24, 12958–12962.

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Figure 6. Π-A isotherms on pH adjusted water (pH = 7 and pH = 10), 10% Ludox TMA (pH = 7), and 10% Ludox HS-30 (pH = 10) disperions: (a) stearic acid; (b) stearyl amine; (c) stearyl alcohol.

Figure 7. Time tests of films grown at the interface between (a) concentrated NaOH (pH = 14) and 1 mM stearic acid, (b) 10% Ludox TMA and 1 mM stearyl amine, and (c) 10% acidic Ludox HS-30 (pH = 1) and 1 mM stearyl amine.

There is no significant change in the case of stearyl alcohol (Figure 6c). Stearyl alcohol is mostly uncharged throughout the pH range and thus is not prone to interact with silica particles. With additional rheological measurements, the role of sodium stearate formation was further investigated. The particle dispersion was exchanged by NaOH solutions of pH=10 (the same as the pH of the Ludox HS-30 dispersion) and pH=14 (Figure 7a). At pH=14, a strong film appeared within 1 h. In this case, at the interface, stearic acid is precipitated out of the organic phase and transformed into calcium stearate by the classic soap formation reaction. On the other hand, at pH = 10, no film growth could be observed without particles and stearic acid in the decane phase. This clearly shows that at this pH silica particles are necessary for film formation. In another measurement, no film growth could be observed with 1 mM stearic acid solution and (deionized) Ludox TMA dispersion (pH=7). So while film growth seems to be impossible without the presence of a basic medium, film formation cannot be explained purely by soap formation at moderate pH values. It is also important to note that the film growth in the presence of nanoparticles is much slower than when performed with concentrated NaOH as the aqueous phase. So it seems that the diffusion of the nanoparticles to the interface, as opposed to the diffusion of lipid, which usually saturates the interface in a matter of seconds, is the rate-limiting transport mechanism of the film growth at pH=10. At high pH, no film formation occurs with stearyl amine or stearyl alcohol in the absence of particles. In the case of stearyl amine, film formation can also be observed in the presence of Langmuir 2010, 26(23), 17867–17873

deionized Ludox TMA (Figure 7b). The moduli are much higher compared to films grown from the basic HS-30 dispersions (pH=10) (Figure 5a,b). Films grown from acidic HS-30 dispersions (pH=1) exhibit even stronger moduli (Figure 7c).

Discussion Two main factors for film formation can be identified: the precipitation of charged lipids at the oil/water interface and the formation of lipid/silica particle agglomerates. At extreme pH, cohesive films are able to form without the addition of particles. Because of the extreme pH, the lipids transform into their salts. In the case of stearic acid, sodium stearate is formed at high pH in the presence of sodium hydroxide. In the case of stearyl amine, octadecylammonium chloride is formed at low pH in the presence of hydrochloric acid. In contrast, stearyl alcohol remains mainly uncharged at extreme pH and thus does not form cohesive solid films at the decane/water interface. The salts of both stearic acid and stearyl amine are not very soluble in either decane or water.32,33 This is why they precipitate out of the decane phase and remain at the interface once they get into contact with the basic (or acidic respectively) solutions and transform into their respective salts. The cohesive precipitate films are solid and therefore exhibit mainly elastic properties with a low linear viscoelastic threshold when investigated with interfacial shear rheology. At less extreme pH, apparently not enough lipids precipitate to form cohesive thin films. Although some of the (32) McBain, J.; Sierichs, W. J. Am. Oil Chem. Soc. 1948, 25, 221–225. (33) Jandacek, R. Lipids 1991, 26, 250–253.

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lipids are charged at moderate to neutral pH, there seems to be a certain quantity of lipids necessary for film formation that greatly exceeds a monolayer of lipids. This notion is also strengthened by the fact that quite a high amount of lipid is needed to form these thin films at the oil/water interface (8 mL of 1 mM lipid solution). In comparison, in order to form a solid-condensed monomolecular thin film at an interface with the same area as in our experimental setup, ∼20 μL of 0.1 mM lipid solution would be sufficient. The role of the particles during film formation can be appreciated by a discussion of the possible interaction forces between particles and lipids. The interactions between the particles can be described by the DLVO theory.4,34-38 Though the DLVO theory clearly has its limits, especially in systems with a high concentration of ions,39 it is a good starting point to qualitatively discuss how particle/lipid interactions could lead to film formation. In classic DLVO theory, the interaction potential between two particles is separated into Coulomb interactions and dispersive interactions. These are clearly the major interaction forces between particles. In order to refine this basic approach, the DLVO theory has been extended by the addition of several other minor interaction forces, including, but not limited to, hydration potentials, depletion potentials, and entropic contributions. Therefore, the interaction potential V according to the extended DLVO theory between two particles can be written as VDLVO ¼ VCoulomb þ VDispersion þ VHydration þ VDepletion þ VEntropy þ ::: In general, Ludox particles form stable dispersions in water due to the negative surface charges of the particles. Although Ludox dispersions are most stable in a basic medium, the particles maintain their negative surface charge in an acidic medium. In fact, in the case of the Ludox HS particles used in our experiments, the dispersions remained stable if the pH was changed quickly enough from basic to acidic. At neutral pH, Ludox dispersions tend to be unstable, unless salts are removed from the aqueous phase, as is the case in Ludox TMA. If the surface charges of the particles become shielded or neutralized, the dispersions become unstable. In the case of positively charged stearyl amine and the negatively charged Ludox particles, the surface charges on the particles become neutralized by adsorption of the lipids onto the particle surfaces.16,17,21,31 If the net-negative charge on the particles is sufficiently reduced, dispersive interactions (which are always attractive) will become dominant and the particles agglomerate. As the interactions between particles and lipids are restricted to the oil/water interface, a thin film forms. In the case of the negatively charged stearate molecules, the interaction with the particles is less intuitive. Nevertheless, the high concentration of lipids and soaps in proximity to the interface could lead to the shielding of the negative surface charges of the particles. In general, as the salt concentration in a particle (34) Behrens, S. H.; Christl, D. I.; Emmerzael, R.; Schurtenberger, P.; Borkovec, M. Langmuir 2000, 16, 2566–2575. (35) Dagastine, R. R.; Chau, T. T.; Chan, D. Y. C.; Stevens, G. W.; Grieser, F. Faraday Discuss. 2005, 129, 111–124. (36) Fernandez-Toledano, J. C.; Moncho-Jorda, A.; Martı´ nez-Lopez, F.;  Hidalgo-Alvarez, R. Langmuir 2004, 20, 6977–6980. (37) Hartley, P. G.; Grieser, F.; Mulvaney, P.; Stevens, G. W. Langmuir 1999, 15, 7282–7289. (38) Williams, D. F.; Berg, J. C. J. Colloid Interface Sci. 1992, 152, 218–229. (39) Bostr€om, M.; Williams, D. R. M.; Ninham, B. W. Phys. Rev. Lett. 2001, 87, 168103.

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dispersion increases, the electronic double-layer surrounding the charged particles becomes more and more compressed, until dispersive forces again dominate and the dispersion becomes unstable. The same could happen to particles that reach the interface by diffusion. A complication of this approach is that the negatively charged interface (due to the presence of stearate) possibly presents a diffusion barrier to the particles. A more detailed understanding of the fine structure of the interfacial film in situ would be necessary to address these issues. In some multicomponent systems, depletion interactions40-42 can provide a driving force for particle agglomeration. For example, in a paper by Kline and Kaler,43 Ludox particle aggregation could be identified as a result of the depletion of n-alkyl sulfate micelles in the space between two particles. In this context, depletion results in a concentration gradient of micelles that leads to the osmotic drainage of the space between two particles and therefore to aggregation. Of course, in contrast to the films described in our paper, this micelle/particle system only exists in a three-dimensional volume phase. In order to speculate about similar forces acting during thin film formation, again, a more detailed understanding of the fine structure of the interface in situ is necessary. Some of our experiments with biomineral thin films at the oil/water interface1 gave rise to films that exhibited a morphology that was reminiscent of dried bicontinuous emulsions. This could not be observed from the SEM micrographs of the dried Ludox/lipid films presented in this paper (see Figure 2), but it is conceivable that a similar fine structure could exist in situ during film formation.

Conclusion In summary, we observed film growth with stearic acid at very high pH even without particles. At pH=10, films only form in the presence of particles. At lower pH, no films form with stearic acid. With stearyl amine, films do not form at pH=14 but start to form in the presence of particle dispersions of pH=10 or lower. The moduli of those films increase with decreasing pH. No film formation could be observed with stearyl alcohol. This leads to the conclusion that film formation is driven by a change of the charge of the lipid head groups that leads to the precipitation of the lipids out of the decane phase. The charge changes in the case of stearic acid from neutral to negative at high pH and in the case of stearyl amine from neutral to positive at low pH. The silica particles play a different role regarding film formation with each of the two film-forming lipids. In the case of stearyl amine, the positively charged molecules and the negatively charged silica particles interact with each other on the basis of Coulomb attraction, which promotes film formation. In the case of stearic acid, the negatively charged particles cannot interact directly with the negatively charged lipids. It is possible that the negative charges on the surface of the particles are shielded by the high concentration of lipids and their salts at the interface. More complex interactions like depletion forces could also play a role, but a more detailed understanding of the fine structure of the interface would be necessary to further elucidate the mechanism of film growth in this case. As stearyl alcohol remains mainly uncharged under varying pH conditions and Coulomb attractions with silica particles are (40) Goloub, T. P.; Koopal, L. K.; Bijsterbosch, B. H.; Sidorova, M. P. Langmuir 1996, 12, 3188–3194. (41) Paunov, V. N.; Binks, B. P.; Ashby, N. P. Langmuir 2002, 18, 6946–6955. (42) Bibette, J.; Roux, D.; Nallet, F. Phys. Rev. Lett. 1990, 65, 2470. (43) Kline, S. R.; Kaler, E. W. Langmuir 1996, 12, 2402–2407.

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unlikely due to the weakly negatively polarized headgroup of the lipid, no film formation occurs. As this approach of film growth is fairly easy to understand in most cases and works with inexpensive and nonhazardous materials, the next step in research would be to apply those principles to other systems, such as emulsions. Here, possible capsule formation and their use as drug carriers have to be investigated.

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Acknowledgment. Thanks are due to Prof. Dr. Heinz Rehage for providing the ability to perform the light scattering experiments and to the Deutsche Forschungsgemeinschaft for generous funding. Supporting Information Available: Comparison between two graphs recorded at the exact same conditions. This material is available free of charge via the Internet at http://pubs.acs.org.

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