Controlled Fabrication of Hexagonally Close-Packed Langmuir

Article pubs.acs.org/Langmuir

Controlled Fabrication of Hexagonally Close-Packed Langmuir− Blodgett Silica Particulate Monolayers from Binary Surfactant and Solvent Systems Yudi Guo, Dongyan Tang,* Yunchen Du, and Binbin Liu Department of Chemistry, School of Science, Harbin Institute of Technology, Harbin 150001, China S Supporting Information *

ABSTRACT: We describe a controllable method to fabricate hexagonally close-packed Langmuir−Blodgett (LB) monolayers with stearic acid (SA) as co-surfactant and methanol as co-solvent. The optimal SA concentrations and volume ratios of chloroform to methanol are 0.8 mg/mL and 3:1 for particles of 140 nm, 0.50 mg/ mL and 4:1 for particles of 300 nm, and 0.05 mg/mL and 5:1 for particles of 550 nm, respectively. Additionally, SEM detections of the monolayers transferred at different surface pressures indicate that the monolayers deposited from the binary systems are more compressible. The experimental results indicate that the interparticle repulsions and particle−water interactions can be enhanced without decreasing the particle hydrophobicity by adding SA and methanol; thus, particulate monolayers with large hexagonally close-packed domains composed of small silica particles can be successfully fabricated using LB technique. We propose that the enhanced interparticle repulsion is attributed to the Columbic repulsion resulting from the attachment of SA molecules to the CTAB modified particles around the three phase contact line.

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densely packed particulate films from a single process, and deposition of films over large areas. These advantages have inspired many researchers to explore LB technique as a tool to prepare nanoparticulate films with well ordered structures.25−28 However, LB technique is frequently used to fabricate particulate monolayers at liquid−fluid interface and to investigate the interfacial properties of particles.29−32 For instance, Liggieri et al. have investigated the wide-frequency dilational rheology of the mixed silica-CTAB interfacial layers, and results indicated two relaxation mechanisms presented in the particulate layers obtained by the spontaneous accumulation of particles at the interface.33 Santini et al. have adopted LB technique to investigate the interfacial properties and structure of the mixed particle-surfactant layers.34,35 Moreover, the interactions between the particles and the surfactant molecules and the relationship between the particle hydrophobicity and the surfactant amount were both revealed. Recently, the possibility of fabricating two-dimensional colloidal crystals from spherical silica by LB technique has been studied, and mono- or multiparticulate films with hexagonally closepacked structure have been prepared successfully.36−39 For instance, Szekeres et al. fabricated two- and three-dimensional ordered silica particulate films by LB technique.40 In the literature, ionic surfactants were used to render the hydrophilic silica particle a hydrophobic surface, and the largest hexagonally

INTRODUCTION Colloidal crystals have drawn considerable interest from both fundamental and industrial considerations.1−4 Especially, twoand three-dimensional ordered colloidal crystals composed of monodisperse spherical particles on the mesoscale of 50−500 nm deserve much attention because of their potential applications in the field of photonic crystal,5−7 catalyst supports,8,9 electro-optics, biochemical sensors, etc.10−12 Among theses materials, silica colloidal crystals with hexagonally close-packed structure are often synthesized in terms of its significance in the potential photonic application,13 supports for lipid bilayers,14 and media for chemical separations.15 Over the past few decades, many techniques have been developed for fabricating two- or three-dimensional ordered particulate films. Up to now, more and more conventional techniques, such as spin coating,16 colloidal self-assembly,17 deposition with lateral capillary force,18,19 and solvent evaporation,20,21 have been explored to fabricate hexagonally close-packed films. However, the disadvantages, such as low reproducibility, substrate limited, and high cost, restrict their applications to some degree. The transfer of organized monolayers of amphiphilic molecules from air−water interface to a substrate is commonly known as the Langmuir−Blodgett (LB) technique, which enables the possibility to fabricate highly ordered monomolecular films with densely packed structures and precisely controlled thickness.22−24 Compared with those conventional techniques, LB deposited films have low mechanical strength and thermal stability. However, LB technique has some advantages, such as high reproducibility, low cost in fabricating large number of © XXXX American Chemical Society

Received: December 13, 2012 Revised: February 4, 2013

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dx.doi.org/10.1021/la3049218 | Langmuir XXXX, XXX, XXX−XXX

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Densely packed films composed of stearic acid (SA) and CTAB could be formed by LB technique because of the electrostatic attraction between the two kinds of molecules. Therefore, it might be a possible way to fabricate hexagonally close-packed LB particulate layers composed of very small particles by using the mixed surfactants of SA and CTAB. The interfacial structure and properties of the particle−surfactants composite system can be affected greatly by the interaction of the binary surfactants with the silica particle. Whatever, the interaction mechanism between the particles and the binary surfactants at the interface could be extended to other binary surfactant system, and it would have great value in the fabrication of functional LB nanoparticulate layers with small particle diameter. In this work, monodisperse spherical silica particles (140, 300, and 550 nm in diameter) were synthesized using Stöber method, and two-dimensional hexagonally close-packed particulate films were fabricated by LB deposition of particles modified by the adsorption of CTAB. Furthermore, the SA and methanol were added to the particle dispersions as the cosurfactant and co-solvent, respectively, with the aim to enhance the interparticle repulsions and particle−water interactions. We have attempted to display the effects of SA concentration and the composition of spreading solution on the orderting and packing of the monoparticulate LB layers in this report.

close-packed crystalline areas were obtained with methanol as co-solvent, which was also confirmed by Kim et al.41 Lee et al. have investigated the monolayer behaviors of silica particles at the water−air interface and suggested that a more ordered structure was easily obtained with particles modified by physical adsorption of surfactant instead of chemical graft of alkyl chains.38 They concluded that the enhanced particle−water interactions contributed to the ordered structures, which were attributed to the formation of partially hydrophobic particles because of the reversible adsorption of surfactant molecules to the particle surfaces. Besides ionic surfactants, silane coupling agents are always used to give the particle a hydrophobic surface.42,43 Whatever, it can be concluded that the hydrophobic/hydrophilic balance is one of the key factors in producing well ordered particulate films. These achievements promote the applications of LB technique in fabricating ordered particulate films. However, present studies indicate that most of the two- and threedimensional hexagonally close-packed particulate films can be successfully fabricated with particles in the size range 300−1500 nm using LB technique.37,38 And few works were considered for the preparation of well ordered particulate layers with particles of smaller diameter (even though it owns obvious advantages).13,39 From the theoretical point of view, the binding energy ΔE of a solid particle adsorbed to the fluid− fluid interface with the radius R, equal to ΔE = −πR2γ(1 ± cos θ)2 in terms of the surface tension γ and particle three-phase contact angle θ.44 Apparently, the value of ΔE strongly depends on the particle size and θ (i.e., particle surface wettability). Moreover, the particle surface wettability can also influence the interparticle interactions.30,45 Previous study indicates that a higher hydrophobicity is necessary for producing close−packed particulate monolayers because of the hydrophobic particles (θ > 90°) possessing the long-term stability at the interface.46 It is well-known that the high hydrophobicity would elicit strong van de Waals interparticle attractions and low ΔE, which favor the formation of particle self-aggregates.47 For particles of small diameter with high hydrophobicity, ΔE decreases sharply according to the equation aforementioned, which makes the fabrication of ordered and close-packed particulate monolayers more difficult. Besides, previous studies present that the weak interparticle attractions and long−range interparticle repulsions are the two dominant parameters in fabricating a well ordered and close packed monoparticulate layer.47 As a result, in order to fabricate well ordered particulate monolayers from small particles, the effective way is supposed to increase the interparticle repulsions. Experiments conducted by Horozov et al. revealed that, compared with the interparticle dipole−dipole repulsion, the interparticle Coulombic repulsion had significant effects on the ordering of monolayers.45 Szekeres et al. revealed that the negative charge on the bare silica particles could be neutralized by adsorbing cationic surfactants, which would decrease the surface potential.40 It is supposed that the cationic−anionic mixed surfactants may be used to render the particle partial hydrophobicity, as well as increase the free charges at the interface. Thus, it could be used to fabricate well ordered particulate monolayers from particles of small diameters. However, less study focuses on this system. A related investigation has been performed by van Duffel et al., and results indicated that hexagonally close-packed monolayer can be obtained by adding anionic surfactant sodium dodecylsulfate (SDS) to N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride modified silica particle dispersions.36

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EXPERIMENTAL SECTION

Materials. Tetraethyl orthosilicate (TEOS), ammonia hydroxide (25% aqueous solution of NH3·H2O), hexadecyltrimethylammonium bromide (CTAB), stearic acid (SA), concentrated sulphuric acid (98%), and hydrogen peroxide (30%) were purchased from Kemiou Chemical Co. (Tianjin China). 2-Propanol, chloroform, toluene, ethanol, methanol, and acetone were supplied by Xilong Chemical Industry Incorporated Co., Ltd. (Santou, China). Water used in all experiments was purified by means of a Milli-Q plus Unit (Millipore) and had a resistivity of 18.2 MΩ cm. 3-Aminopropyl triethoxysilane (APTES) was purchased from Aladdin Reagent Co. Ltd. (Shanghai, China) and used as amination modification agent for glass substrate Preparation of APTES Coated Substrates. Microscope glass slides measuring 39 mm × 12.5 mm × 1 mm were modified with APTES and used as substrates for layer deposition. First the slides were thoroughly cleaned by sonication for 30 min in acetone, ethanol, a mixture of ammonia (1 vol.) and hydrogen peroxide (1 vol.), and a mixture of deionized water (4 vol.), concentrated sulfuric acid (1 vol.), and hydrogen peroxide (1 vol.) solutions sequentially and finally rinsed with deionized water thoroughly. Further, the cleaned substrates were immersed in the mixture of concentrated sulfuric acid (6 vol.) and hydrogen peroxide (4 vol.), and then heated to boiling point for 1 h in order to create more activated hydroxyl groups at the surface of the substrates. Afterward, the substrates were rinsed with deionized water for three times, and finally dried at room temperature. The typical preparation of APTES coated substrates was performed as follows. The activated substrates were immersed into 1% APTES solution dissolved in the saturated toluene for 1 h. Subsequently, the substrates were rinsed with toluene, acetone, ethanol, and deionized water, respectively. Preparation of Monodisperse Silica Particles. Monodisperse silica particles were prepared by the modified Stöber method. As described in Table 1, the typical synthesis of particles of 140 nm (represented as Si-1) was as follows. A total of 2.0918 g of TEOS and 50 mL of ethanol were mixed together by rapid stirring at room temperature (solution A). Next, 3.9 mL of ammonia and 3.0843 g of H2O were mixed with 40 mL of ethanol thoroughly (solution B). Then, solution B was added to solution A with rapid stirring at room temperature and reacted for 24 h. The silica particle solution was centrifuged at 6000 rpm for 30 min, the ethanol supernatant solution was discarded, and the residue was redispersed in fresh ethanol. The B


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solvent evaporate. And the monolayer at the air/water interface was compressed at a speed of 5 mm min−1 and the particles were deposited at a speed of 1 mm·min−1 in the upstroke direction at the selected surface pressure. Afterward, the deposited particulate layers were heated in a furnace under air atmosphere for 30 min at 370 and 500 °C, respectively. Characterization. FT−IR measurements of bare silica and CTAB modified silica were accomplished on an AVATER-360B FT−IR spectrometer (Nicolet Co., USA) in the range from 400 to 4000 cm−1. Surface morphologies of the deposited particulate monolayers were examined with a scanning electron microscope (SEM, QUANTA200F, FEI Co. USA). The static contact angle measurements were carried out by the sessile drop method at room temperature using a JC2000C3 contact angle goniometer (Shanghai Zhongchen Powereach Co., CN).

Table 1. Components of Solutions for the Synthesis of Monodisperse Silica Particles of Different Diameters A

B

no.

C2H5OH/ mL

TEOS/g

NH3·H2O/ mL

H2O/g

C2H5OH/ mL

d/ nm

Si-1 Si-2 Si-3

50 50 50

2.0918 6.2576 8.3281

3.9 11.6 15.4

3.0843 9.0201 12.0001

40 40 40

140 300 550

centrifugation−redispersion procedure in ethanol was repeated at least three times. Finally, the silica particles were dried at 60 °C for 24 h. Silica particles with different diameters were obtained by changing the amounts of TEOS, NH3, and H2O. Preparation of Silica Particle Dispersions. Partially hydrophobic silica particles were achieved by surface modification with cationic surfactant of CTAB. In a typical reaction, 0.1200 g of bare silica particles was dispersed in 5 mL of 2-propanol under sonication for 24 h, and followed by adding 3 mg of CTAB. After sonication for another 3 h, the dispersion was left to equilibrate for 24 h. The dispersion was then centrifuged at 6000 rpm for 30 min. The centrifugation−redispersion procedure in ethanol was repeated three times to completely remove the unreacted reagents. Finally, the particles were dried at 60 °C for 24 h. The dispersions were prepared by dispersing CTAB modified silica particles in chloroform or a mixture of chloroform and methanol, and the silica particle concentrations were 20 mg·mL−1 in all experiments. SA was added to the mixture as cosurfactant. A series of silica particle dispersions were obtained by changing the SA concentrations and the volume ratios of methanol to chloroform. The recipes were listed in Table 2.

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RESULTS AND DISCUSSION Surface Characterization and Size Analysis of the Silica Particles. The bare and CTAB modified silica particles were investigated by means of FT−IR. The FT−IR spectrum of CTAB modified silica particles (b) is compared with that of the bare silica particles (a), and the spectra are shown in Figure 1.

Table 2. Compositions of the Silica Dispersions for LB Monolayer Deposition no.

diameter/ nm

CTAB−silica/ mg·mL−1

SA/ mg·mL−1

Si-1

140

20

0.60

Si-1

140

20

0.80

Si-1

140

20

1.00

Si-1

140

20

0.60

Si-1

140

20

0.80

Si-1

140

20

1.0

Si-2 Si-2

300 300

20 20

0.50 0.50

Si-3

550

20

0.05

Si-3

550

20

0.10

Si-3

550

20

0.15

medium chloroform/methanol = 4:1 chloroform/methanol = 4:1 chloroform/methanol = 4:1 chloroform/methanol = 3:1 chloroform/methanol = 3:1 chloroform/methanol = 3:1 chloroform chloroform/methanol = 4:1 chloroform/methanol = 5:1 chloroform/methanol = 5:1 chloroform/methanol = 5:1

Figure 1. FT−IR spectra of bare silica particles (a) and CTAB modified silica particles (b). Inset shows the spectra of (a) and (b) between 2800 and 2980 cm−1 and the photo images of the dispersions of CTAB modified silica (left) and bare silica (right) particles in the mixture of chloroform and methanol (5:1, v/v).

In both spectra, the characteristic peak at 1100 cm−1 can be attributed to asymmetric stretching vibration of Si−O−Si. The wide peak at 3400 cm−1 is assigned to the deformation vibration band of H2O adsorbed on the particles, and the peak at 1637 cm−1 is ascribed to bending vibration of the H−O−H. The optical images of the dispersions show that CTAB modified silica particles can be stably dispersed in the mixed solvent during 10 min, whereas the bare silica particles would settle down after 10 min. From the inset, Figure 1b shows strong peaks at 2853, 2927, and 2954 cm−1 which could be assigned to −CH2− and −CH3 groups. However, the weak peaks in Figure 1a could originate from the residual EtOH on the silica surface during the purification process. Additionally, in our experiments, the CTAB modified silica particles can be dispersed well in pure chloroform, while the bare particles cannot. From the above discussion, it can be confirmed that the CTAB has been introduced onto the silica surfaces.

Deposition of Particulate LB Monolayers. The silica particle dispersions were agitated in an ultrasonic bath at least 30 min prior to use. The film deposition and monolayer experiments were performed in a Langmuir trough (KSV mini-trough, KSV Instruments Ltd., Finland) with a working area of 364 mm × 75 mm on a vibration isolation table. The film pressures at the air/water interface were measured by the Wilhelmy plate attached to a microbalance. The water subphase temperature was always controlled at 20 °C by a thermostat (SDC-6, Ningbo Tianheng Instrument Factory, China). The particle dispersions were spread on a clean water subphase by using a microsyringe followed by a 30 min waiting period to let the spreading C


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The mean particle sizes were calculated based on the SEM images, and the values were listed in Table 1. The particle polydispersity is expressed as the standard deviation of the mean diameter, which is in the range of 4.4−5.8%. The size distributions of the silica particles are shown in Figure S1 of the Supporting Information. Wettability Estimating of Surfactant Modified Silica Particles. The extensive studies on the structural, dynamical, and thermodynamical properties of solid particles at the liquid interface demonstrate that the particle hydrophobicity is one of the crucial factors that determine the interfacial behaviors of particles.45,48,49 Usually, the behaviors are evaluated by the three-phase contact angle (θ).31,46 However, especially for very small particles, the three-phase contact angle cannot be obtained directly. As is well-known, θ is directly related to the particle surface wettability; thus, it is attractive to investigate the relationship between the two parameters. Lee et al. assessed the particle surface wettability by measuring the water contact angle on silica plates modified in a way similar to particles, and compared with the contact angles calculated from the collapse pressure.36 Maestro et al. have pointed out that the surface coverage and arrangement of surfactants adsorbed on the particle surface would be changed during the transfer process of a particle from the bulk to the fluid interface, which would influence the three-phase contact angle.50 Therefore, it is of value to evaluate the real wettability of particles at the fluid interface by the detection of the water contact angle on the hydrophobic part of the particle. In the case of spherical silica particle attached to the water− air interface, asymmetric particles would be formed with the apolar surface region covered with surfactant hydrophobic tails toward the air and the hydrophilic polar surface region toward the water, which results from the redistribution of surfactant on the particle surface.38 When the particulate monolayers are transferred to the glass substrates, hexagonally close-packed particulate films can be fabricated with the hydrophobic surface region toward air and hydrophilic surface region toward the glass side, which remains the original state of particles at the interface. As an alternative way, we proposed a simple method for assessing the particle surface wettability by measuring the water contact angle on hexagonally close-packed LB silica particulate films without sintering. Previous studies indicate that the wetting state of the hexagonally close-packed LB silica particulate film is dominated by the Wenzel state.37,39,51 It is well-known that the water droplet will completely wet the surface features if the surface is in the Wenzel state and the apparent contact angle, θreal, will be expressed in terms of the Young contact angle on the flat surface, θflat,52 i.e. cos θreal = r cos θflat

Scheme 1. Illustration of Water Droplet on the Wenzel State Surface and the Optical Images of Water Droplet on CTAB Modified Hexagonally Close-Packed LB Silica Particulate Monolayers with Particles of (a) 550 nm, (b) 300 nm, and (c) 140 nm

optical images of water droplet on the CTAB modified hexagonally close-packed LB silica particulate monolayers with particles of different diameters. The results indicate that CTAB modified silica particles of different diameters have the similar surface wettability, and the difference in the measured contact angles is due to the nonperfect geometry. When the particles are small enough and monodispersed, the gravitational effects can be neglected and the interface is planar right up to the particle surface. The three-phase contact angle θ can also be determined by measuring the “collapse” surface pressure in a particulate monolayer upon compression in the Langmuir trough.48 According to the theory deduced by Clint and Tayler,54 the collapse surface pressure (Πc) can be related to the hydrophobic characteristic of particles (i.e., contact angle θ) when the particulate monolayers are hexagonally closepacked. Πc = πγ(1 ± cos θ )2 /2 3

(2)

where γ is regarded as the surface tension of the subphase. The “ ± ” sign in the parentheses means that the particle moves toward the air phase (+), i.e., receding contact angle (θr), or water phase (−), i.e., advancing contact angle (θa). According to the contact angle measurements, the Πc should be almost identical for particles of different diameters. Π−A Isotherms of Particulate Monolayers at Water− Air Interface. During experimental process, we found that small particles cannot spread well at the water−air interface without co-surfactant and co-solvent and whitish domains can be observed by the naked eyes. Figure 2 presents the Π−A isotherms of the silica particulate monolayers with particles of different diameters prepared from different SA−methanol−silica spreading dispersions, where the surface pressure is plotted against the area/particle. Apparently, the liquid-expanded phase is observed in Figure 2a−c, whereas it is absent in Figure 2d. Determination of Π−A isotherms of monoparticulate layers at the water−air interface contributes fruitful information to particle interactions and wettability. The 2D arrays of particles attached at the interface are strongly dependent on the particle interactions and the particle surface wettability.36,38,40,42 The CTAB modified particles can reside at the water−air interface stably by adding SA to the spreading solutions, which indicates

(1)

r is the roughness factor, defined as the ratio of the real surface area to the corresponding projected surface area of a flat surface. According to hexagonally close-packed particulate monolayers, the value of r can be calculated by combining hexagonal packing with a hemispherical model, and the value is about 1.9 which is independent of particle diameters but only on the packing geometry.53 The water contact angles on hexagonally close−packed LB silica particulate monolayers are 126.5° (550 nm), 124.6° (300 nm), and 123.2° (140 nm), respectively. Values of θflat, calculated from eq 1, are 108.2° (550 nm), 107.4° (300 nm), and 106.7° (140 nm), respectively. Scheme 1 shows the illustration of water droplet on the Wenzel state surface and the D


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Figure 2. Π−A isotherms of the silica particulate monolayers with particles of different diameters prepared from different SA−methanol−silica spreading dispersions. (Si-1: (a) chloroform/methanol = 4:1, SA = 0.60, 0.80, and 1.0 mg/mL; (b) chloroform/methanol = 3:1, SA: 0.80 and 1.0 mg/mL. (c) Si-2: SA = 0.50 mg/mL, chloroform and chloroform/methanol = 3:1. (d) Si-3: chloroform/methanol = 5:1, SA = 0.05, 0.10, and 0.15 mg/mL).

that the interparticle repulsion is enhanced. Long-range electrostatic dipole−dipole repulsions are frequently considered as the dominant parameter determining the ordering of particulate monolayer at the interface.40,42,55,56 However, Horozov et al. proposed that the ordering of the monoparticulate layers is attributed to the interparticle Coulombic repulsions which result from the presence of surface charges at the interface for particles with large contact angle.45 When the silica particles are spread at the water−air interface, the SA molecules will be adsorbed onto the CTAB modified surface by ion exchange because of the higher hydration energies of H+ and Br−. It is likely that, due to the steric repulsion between the hydrophobic tails of the two surfactants, the adsorbed SA molecules will distribute uniformly around the three phase contact line as shown in Scheme 2. Thus, it can be assumed that the ion pair amphiphils (IPA) are formed and the extra net negative charges are expected to exist around the three phase contact line, and thus the interparticle Coulombic repulsions should not be neglected. Therefore, loosely ordered or even no particulate aggregations are formed at the interface after spreading because of the relatively weak interparticle attraction. Particles can rearrange and pack closely upon compressing the monoparticulate layers, thus liquid-expanded (LE) phase is observed. In contrast to small particles, large particles spread at the interface prefer to form orderly close-packed clusters because of the high binding energy. Figure S2 of the Supporting Information shows the orientation of particles of different diameters in different phase during the compression process. The surface pressure increases sharply because of the intimate contact of clusters at a very low trough area without

Scheme 2. Illustration of Behaviors of the Modified Particles Floating at the Water−Air Surface

rearrangement of particles during the compression process; in other words, LE phases are missing in the Π−A isotherms. Figure 2a,d demonstrates the effects of SA concentration on the particle Π−A isotherms. As shown in Figure 2a, the lowest A0 value is observed for Si-1 particulate monolayer with the SA concentration of 0.6 mg/mL, whereas Figure 2d shows that the A0 value increases with increasing the SA concentration. A0 is referred to the 2D gel point at which continuous layer is formed, which is determined by extrapolating the steepest constant−slope region of the isotherm to the zero surface pressure. At low SA concentration, the strong interparticle van de Waals forces would result in heterogeneous aggregates, thus high A0 value can be observed. However, much more SA will result in a stronger interparticle repulsion which prevents particles packing closely, and thus a higher A0 value is observed. E


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The effects of methanol on the particle Π−A isotherms are shown in Figure 2c. When the methanol was introduced as the co-solvent, a higher collapse pressure (Πc) value is observed, which can also be found for Si-1 when the volume ratio of methanol is increased to 1/3, as shown in Figure 2b. Dékány et al. have proposed that the apolar component in the binary mixture is preferentially adsorbed to the hydrophiobic part of particle, while the polar component would be adsorbed to the hydrophilic part.42 Maestro et al. have demonstrated that the solvent molecules can be trapped at the particle interface during the spreading process, which would change the wettability of particle at the fluid interface.57 Therefore, during the solvent evaporation process, the surfactant hydrophobic tails pack closely and turn toward the air phase, while the hydrophilic part of the particle is dragged into the water phase. Consequently, a higher hydrophobicity would be achieved, which results in a higher Πc value. From Figure 2, the Πc values of 49.5, 50.8, and 51.1 mN/m are observed for Si-1, Si-2, and Si-3 with appropriate compositions of SA−methanol, respectively, which are agreement with the assumption aforementioned. The contact angles calculated from the Π−A isotherms are 97.7° for Si-1, 97.1° for Si-2, and 96.9° for Si-3, which are less than that obtained from contact angle measurements. The contact angles of the three particles should be assigned to receding angles because the hydrophobic particles would move to the air phase after collapse. Since the equilibrium contact angle should be between the receding and advancing angle, the contact angles evaluated from Π−A isotherms seem to be reasonable. Hysteresis Behaviors of LB Particulate Monolayers. The compression and re−expansion ability of particulate monolayers is very important in fabricating well ordered hexagonally close-packed silica particulate films, and can be estimated by examining the hysteresis curves shown in Figure 3. For all the three monolayers, the expansion curves locate on the left side of the compression curves, which illustrate that a more expanded state is easily achieved in the re-expansion stage because of the enhanced interparticle repulsions. Moreover, a larger hysteresis is observed in the LE phase and the surface pressure can decrease easily to the initial value before compression. The larger hysteresis in the LE phase can be attributed to the irreversible formation of ordered particulate clusters during the compression process which would decrease the number of free particles at the interface, and thus a lower surface pressure is observed at the trough area where the clusters detach from each other. The identical isotherms in gaseous phase during the compression and expansion processes indicate the high re-spreading ability of these particulate monolayers, which is ascribed to the enhanced interparticle repulsions in combination with the strong particle−water interactions. Surface Morphologies of the Particulate LB Monolayers. Figure 4 shows the SEM images of Si-1 particulate monolayers deposited from different SA−chloroform−methanol dispersions at the surface pressure of 40 mN/m. When the volume ratio of chloroform to methanol is 4:1, monolayers with poor ordered and heterogeneous structures were formed by adding less (0.6 mg/mL) or more (1.0 mg/mL) SA, as shown in Figure 4a,c. However, as shown in Figure 4b, well ordered and close-packed particulate monolayer with some silica particles floating on the monolayer surface can be fabricated with the SA concentration of 0.8 mg/mL. Long-range interparticle repulsions are considered to contribute to the

Figure 3. Hysteresis curves of the particulate monolayers with particles of (a) 140, (b) 300, and (c) 550 nm.

formation of ordered particulate monolayers,45 while the strong interparticle attractions usually result in the formation of heterogeneous clusters because no rearrangements occur during the compression process.58−60 According to small particles with high hydrophobicity, the poor ordered monolayer is observed with a lower SA concentration, which is attributed to the strong interparticle attractions. However, the long-range interparticle repulsions increase with increasing the SA concentration, thus particulate monolayer with large hexagonally close-packed domains can be obtained. By further increasing the SA concentration, the interparticle repulsions would become stronger, and poor ordered monolayer would be formed. The floating particles in Figure 4b are attributed to the weak particle−water interactions and the poor dispersibility with low volume ratio of methanol. By increasing the volume ratio of methanol to 1/3, the particulate monolayers became more orderly and closely, and monolayer with the largest hexagonally close-packed domains was fabricated with the SA concentration of 0.8 mg/mL, as shown in Figure 4b′. Apparently, the more ordered and closed structures are a F


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Figure 4. SEM images of Si-1 particulate monolayers prepared from different SA−chloroform−methanol dispersions (chloroform/methanol = 4:1 (v/v) (a−c) and 3:1 (v/v) (a′−c′); the SA concentration: 0.6 (a, a′), 0.8 (b, b′), and 1.0 mg/mL (c, c′)).

Figure 5. SEM images of Si-2 particulate monolayers prepared at different transfer pressures from different solvent systems (chloroform, 25 (a), 30 (b), and 35 mN/m (c); chloroform/methanol = 4:1, 35 (d), 40 (e), and 45 mN/m (f)).

consequence of higher particle−water interactions. However, the line defects and the empty points in Figure 4b′ are supposed to be formed due to the mismatch between domains upon compression. In addition, the non-uniformity of the particle size is considered as another reason leading to the formation of point defects. Figure 5 exhibits the SEM images of Si-2 particulate monolayers deposited at different transfer pressures from different solvent systems. Using chloroform as solvent, 25, 30, and 35 mN/m were selected as the transfer pressures according to the collapse pressure (Πc) of 38 mN/m. From Figure 5a−c, the LB monolayers pack densely and 3D aggregates can be found with increasing the transfer pressure, and large hexagonally close-packed domains can not be observed. Introducing methanol as the co-solvent, 35, 40, and 45 mN/ m were chosen as the transfer pressures because of the high Πc

of 51.1 mN/m. Compared with the single solvent system, the LB monolayer deposited at any transfer pressure is more ordered and densely, and the largest hexagonally close-packed domains are obtained with the transfer pressure of 40 mN/m, as shown in Figure 5e. As mentioned above, the particle hydrophobicity and the particle−water interactions are enhanced by adding polar solvent, which results in the formation of ordered and densely packed LB monolayers. Figure 5 shows that, for the two solvent systems, the LB monolayers are more densely packed with increasing the transfer pressures, which illustrates that the monolayers deposited from the binary systems are more compressible. In the mean time, it proves that the clusters rearrange upon compression, which results in the formation of the largest hexagonally close-packed particulate LB monolayers. G


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Figure 6. SEM images of Si-3 particulate monolayer prepared with the SA concentrations of 0.05 (a), 0.10 (b), and 0.15 mg/mL.

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Figure 6 shows the SEM images of Si-3 particulate monolayers prepared at the surface pressure of 40 mN/m with different SA concentrations and with the mixed solvent of chloroform and methanol in the volume ratio of 5:1. From Figure 6, the most extended hexagonally close-packed domains can be accomplished with the SA concentration of 0.05 mg/ mL. Furthermore, the particulate monolayer becomes more loosely with increasing the SA concentration, which is attributed to the enhanced interparticle repulsions. And, the area per particle calculated from the SEM images is 0.0191 μm2 for Si-1, 0.0858 μm2 for Si-2, and 0.287 μm2 for Si-3, which are close to that obtained from the Π−A isotherms at the transfer pressure (0.0221 μm2 for Si-1, 0.0967 μm2 for Si-2, and 0.318 μm2 for Si-3). From the above discussion, it can be concluded that LB particulate monolayers with large hexagonally closepacked structures can be achieved from the binary surfactant and solvent systems, and the volume ratio of methanol to chloroform decreases with the increasing of the particle diameter.

*E-mail: [email protected]. Tel: +86-451-86413710. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Program for New Century Excellent Talents in University (NCET-08-0165).

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REFERENCES

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CONCLUSIONS In summary, we have successfully synthesized monodisperse spherical silica particles in the diameters of 140, 300, and 500 nm with narrow polydispersity in the range of 4.4−5.8%. The LB silica particulate monolayers with large hexagonally closepacked domains were fabricated from the binary surfactant and solvent systems. Results indicate that high hydrophobicity and strong interparticle repulsions can be accomplished concurrently by the addition of SA to the CTAB modified silica particle dispersions, and this is explained by the Columbic repulsion because of the SA molecules adsorbing to the CTAB modified particles around the three phase contact line. Additionally, the methanol can not only improve the particle−water interactions but also improve the particle hydrophobicity, and its fraction in the mixed solvents decreases with the increasing of particle diameter. The surface morphologies of the LB particulate monolayers indicate that the monolayers are more compressible, and the hysteresis examinations indicate that these monolayers have the high respreading ability. It can be concluded that the controllable fabrication of hexagonally close-packed LB monolayers can be achieved from binary combinations of anionic−cationic surfactant and polar−apolar solvent systems.

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AUTHOR INFORMATION

Corresponding Author

ASSOCIATED CONTENT

S Supporting Information *

Particle size distributions of silica particles based on SEM images and the orientation of particles of different diameters in different phase during the compression process. This material is available free of charge via the Internet at http://pubs.acs.org. H


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Controlled Fabrication of Hexagonally Close-Packed Langmuir

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