Controlled Silanization of Silica Nanoparticles to Stabilize Foams

ARTICLE pubs.acs.org/Langmuir

Controlled Silanization of Silica Nanoparticles to Stabilize Foams, Climbing Films, and Liquid Marbles Paul D. I. Fletcher* and Ben L. Holt Surfactant & Colloid Group, Department of Chemistry, University of Hull, Hull HU6 7RX United Kingdom

bS Supporting Information ABSTRACT: We describe a method for the synthesis of multigram amounts of silica nanoparticles which are controllably hydrophobized to different extents using a room temperature vapor phase silanization process. The extent of hydrophobization of the particles can be adjusted by changing the amount of dichlorodimethylsilane reagent used in the reaction. The method produces particles with good uniformity of surface coating; the silane coating varies from monolayer coverage at low extents of hydrophobization to approximately trilayer at high extents of hydrophobization. Acid
base titration using conductivity detection was used to characterize the extent of hydrophobization which is expressed as the percent of surface silanol groups remaining after silanization. Particles with %SiOH ranging from 100% (most hydrophilic) to 20% (most hydrophobic) were hand shaken with water/methanol mixtures and produced either a particle dispersion, foam, climbing films, or liquid marbles. The type of colloidal structure produced is discussed in terms of the liquid
air-particle contact angle and the energy of adsorption of the particles to the liquid
air surface.

’ INTRODUCTION Particles of the correct wettability adsorb strongly to liquid interfaces and hence can stabilize a variety of colloidal structures; these include not only emulsions (both simple and multiple) in oil + water systems but also foams, “dry water” or “liquid marbles”, and climbing films in water + air systems. For a spherical adsorbed particle which makes a contact angle θ with an aqueous liquid
fluid interface (measured through the aqueous phase), the energy of adsorption Eads from either fluid phase is given by1 Eads ¼
πr 2 γf1 ( cos θg2

ð1Þ

where r is the radius of the particle, γ is the tension of the water
fluid interface, and the + or
sign corresponds to either adsorption from the aqueous phase (
sign) or the nonaqueous phase (+ sign). For a particular system, particle adsorption is most favorable when the contact angle θ = 90°. For typical values of γ (in the range 10
70 mN m
1 for water
oil or water
air interfaces) with r larger than about 10 nm and θ fairly close to 90°, the adsorption energy is orders of magnitude larger than the particle thermal energy kT (where k is the Boltzmann constant and T is the absolute temperature) with the result that particle adsorption is irreversible under these conditions. The strongly adsorbed particle layer is responsible for the excellent kinetic stability of many of the different possible colloidal structures. The main features of particle-stabilized emulsion formation in particle/oil/water mixtures have been well established.1
4 For mixtures containing equal volumes of oil and water, hydrophilic particles with θ < 90° form oil-in-water (o/w) emulsions whereas r 2011 American Chemical Society

hydrophobic particles with θ > 90° form water-in-oil (w/o) emulsions. Emulsion stability with respect to droplet coalescence is greatest when θ is close to 90° and the strength of particle adsorption is maximum. Emulsion drop size is commonly minimum when θ ≈ 90° for reasons discussed in ref 5. The contact angle of the particle with the oil
water interface θ, and hence the concomitant particle-stabilized emulsion behavior, can be manipulated by (i) chemical modification of the particle surface to alter its wettability, (ii) changing the nature of the oil component and/or (iii) altering the particle wettability by changing factors such as pH, electrolyte concentration in the aqueous phase, or (iv) surfactant addition. For mixtures of particles with water (or different polar liquids) and air, the colloidal structure formed is expected to depend on the contact angle formed by the particle with the water
air interface. Possible colloidal structures include particle dispersions in water, foams consisting of air bubbles dispersed in water, water drops dispersed in air (so-called “dry water” or “liquid marbles”),6
10 and climbing particle films.11
14 In aqueous climbing film systems (they are also seen in oil + water + particle systems), shaking an aqueous dispersion of particles of the correct wettability produces thin liquid films stabilized by particles which spontaneously climb up the walls of the vessel when the shaking is ceased.14 Using particles hydrophobized to different extents by surface chemical modification, it has been established that very hydrophobic particles produce water-in-air Received: July 25, 2011 Revised: August 31, 2011 Published: September 02, 2011 12869

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Figure 1. Diagram of the apparatus for silanization of the silica nanoparticles.

(w/a) “dry water” whereas more hydrophilic particles produce air-in-water (a/w) foams.9 Using a single particle type, phase inversion from a/w foam to w/a dry water by surfactant addition has been demonstrated. It was shown that this phase inversion from a/w to w/a occurs when the contact angle θ (measured for solid surfaces similar to the particles) is approximately 90°.10 One main aim of this work is to investigate how the type of colloidal structure changes as a function of the extent of hydrophobization of the particles in water/methanol mixtures of varying methanol content. The results for the loci of formation of the different colloidal structures are correlated with calculated values of the particle
aqueous solution
air contact angle and the energy of particle adsorption to the liquid
air surface. The particles used consist of fumed silica nanoparticles which are hydrophobized to different extents using dichlorodimethylsilane (DCDMS). Silanization of silica surfaces has been extensively investigated, primarily in the context of chromatography stationary phases.15
20 However, there remains a lack of information about methods for the synthesis and characterization of bulk (multigram) quantities of silica nanoparticles which are controllably hydrophobized to different extents. A secondary aim of this work is the development of methods for the preparation and characterization of such controllably hydrophobized silica nanoparticles.

’ EXPERIMENTAL SECTION Materials. Water was purified by passing through an Elgastat Prima reverse osmosis unit followed by a Millipore Milli-Q reagent water system. Its surface tension was 71.9 mN m
1 at 25 °C, in good agreement with literature. Hydrophilic, “bare” silica nanoparticles (HDKN20 grade from Wacker Chemie, Germany), possessing surface silanol groups (SiOH) and with a surface area of approximately 200 m2 g
1, are produced by hydrolysis of silicon tetrachloride in an oxygen
hydrogen flame at high temperature. In the flame process, molecules of SiO2 collide and coalesce to give smooth and approximately spherical primary particles of 10
30 nm in diameter. These primary particles collide and may fuse at lower temperatures to form stable aggregates of 100
500 nm in diameter. A sample of partially silanized silica (code HDKH20 with 50% of the surface SiOH groups silanized with DCDMS) was a gift from Wacker. Methanol (HPLC grade, Fisher Scientific), NaOH (AR grade, Fisher Scientific), R-(+)-limonene (97%, Aldrich), n-dodecane (>99%, Avocado), and dichlorodimethylsilane (DCDMS, >99.5%, Aldrich) were used as received. Methods. Silica powders were silanized to different extents using the apparatus shown schematically in Figure 1 which was mounted

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within a fume cupboard. A mass of 37 g of hydrophilic, “bare” silica was added into the main vessel compartment. Two subcompartments were each loaded with 10 mL of 50 wt % aqueous NaOH which acts to (i) absorb the HCl product of the silanization reaction and (ii) maintain the relative humidity of the reaction approximately constant at 5%.21 Prior to addition of DCDMS, the powder was stirred for approximately 30 min to equilibrate to the reduced humidity. The third subcompartment was loaded with the required volume of DCDMS which evaporates and reacts with the silica powder as vapor. During the reaction, the powder was vigorously stirred using the Teflon impeller rotating at 200 rpm. This was important to ensure uniform silanization of the silica. The metal shaft of the impeller was earthed to reduce the buildup of static electric charge in the system. The reaction, at room temperature of 20 ( 2 °C, was allowed to proceed until evaporation of the DCDMS was complete (typically 24 h). Powder immersion times in water/methanol mixtures were measured by placing approximately 5 mg of the silica powder on the liquid surface (5 mL volume) and sealing the vessel. The time required for all the added silica powder to be wetted by the liquid (i.e., no powder was visible to the eye on the liquid surface) was recorded. Nitrogen adsorption and desorption isotherms were measured at 77 K using a Micromeritics surface area and porosity analyzer. The isotherms were analyzed to obtain the specific surface area according to the Brunauer
Emmett
Teller (BET) model. Glass microscope slides were cleaned with alcoholic KOH and dried prior to silanization. Six 1/2 slides were mounted on a rack, placed in a screw top jar containing 0.5 mL of liquid DCDMS (not in direct contact with the glass slides), and left at 20 °C for 24 h. The silanized slides were cleaned by washing in chloroform in an ultrasonic bath, rinsed with copious amounts of pure water, and dried in air. Liquid drop contact angles were measured using a Kruss DSA10 instrument at 20 °C. To avoid evaporation of the drops of water/ methanol mixtures, the slides were mounted in the closed cell of the DSA10 instrument containing a trough filled with a water/methanol mixture of the same composition as the drop in order to saturate the vapor space and prevent evaporation of the test drop. The slides were pre-equilibrated with this saturated vapor prior to contact angle measurements (10 min). Both static advancing and static receding angles were each measured twice and the average values recorded. The behavior of the partially silanized particles in water/methanol mixtures was examined by vigorously hand shaking stoppered vessels (35 mL total volume) containing 5 mL of water/methanol mixture plus 5 mg of the silica powder. It was visually observed whether the mixtures formed (i) stable dispersions, (ii) foams, (iii) climbing films, or (iv) liquid “marbles”. Optical micrographs of the foams were determined using a Leica DME microscope equipped with a Leica DFC 290 digital camera using Leica Application suite V3.3.0 software.

’ RESULTS AND DISCUSSION Synthesis and Characterization of Partially Hydrophobized Silica Nanoparticles. Using the apparatus shown in

Figure 1, silica powders silanized to different extents were prepared by reaction with DCDMS vapor obtained by evaporation of liquid DCDMS contained within one of the Teflon-jacketed subcompartments of the apparatus. The extent of silanization was varied by adjustment of the initial amount of liquid DCDMS added at the start of the reaction. The relative hydrophobicities of the final silica powders were characterized by measuring the time required for powder placed on the liquid
air surface of water/methanol mixtures to fully immerse in the liquid. Hydrophobic powders require high methanol concentrations 12870

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Figure 2. Silanized silica powder immersion times in water/methanol mixtures as a function of volume of DCDMS used in the silanization (upper plot). This plot also shows the data for a silica powder sample with 50% surface SiOH groups supplied by Wacker. For each curve, the near-vertical portion corresponds to zero immersion observed over 24 h. The lower plot shows a comparison of immersion times for samples of the same batch but taken from different positions in the silanization apparatus.

to immerse in short times whereas hydrophilic powders immerse rapidly in pure water. Figure 2 shows plots of immersion times versus methanol concentration for silica powders silanized using different amounts of DCDMS. It can be seen that samples prepared with large amounts of DCDMS require a high methanol concentration for immersion. Although the precise mechanism whereby the powders wet and immerse in the liquid mixtures is unclear, the simple measurement of immersion time is expected to depend on the uniformity of silanization of the powder in addition to factors such as size polydispersity and surface roughness. Uniformly coated, smooth particles of uniform size are expected to show steep curves corresponding to a sharp, steplike transition from zero wetting and immersion at low methanol content to full, rapid wetting and immersion at a critical methanol concentration. In Figure 2, it can be seen that the immersion time plots for the samples prepared here are significantly steeper than the plot for a commercial sample with an average extent of surface silanization of 50%. The measured times correspond to the immersion of a (poorly defined) mixture of particles, fused aggregates, and rough clusters and are likely to depend on the immersing particle cluster size and shape. However, because the commercial samples and those used here are both derived from the same type of original silica powder and so are likely to have similar morphologies, we tentatively ascribe the steeper immersion plots observed here to an increased uniformity of silanization for the sample prepared here

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versus the commercial 50% SiOH sample. The silanization apparatus (Figure 1), incorporating vapor-phase silanization of a vigorously stirred powder, is designed to ensure uniformity of silanization throughout the entire silica powder sample. This was checked by measuring the powder immersion times for samples extracted from different positions (top, middle, and bottom with respect to height and close and far from the vessel wall) within the silanization vessel. As seen in the lower plot of Figure 2, the immersion time plots for powders extracted from different positions in the silanization vessel are virtually identical. This simple experiment provides qualitative evidence that all silica powder within the silanization vessel is uniformly exposed to and reacts with the DCDMS vapor. We have quantitatively determined the average extent of silanization of the surfaces of the silica particles by acid
base titration of the surface
SiOH groups using conductometric detection. The partially silanized silica particle surfaces contain a mixture of
SiOSi(CH3)2 and
SiOH groups. The
SiOH groups are weakly acidic; in the absence of added base, the silanol groups are virtually all in the form of
SiOH whereas deprotonation progressively converts them to
SiO
as the pH is increased. The extent of silanization is expressed as the percentage of surface silanol groups remaining after silanization with untreated silica corresponding to 100% SiOH and hydrophobizsed silicas having lower %SiOH values. Values of %SiOH were derived from measurements of the electrical conductivities of NaOH solutions with and without added particles. For these measurements, the solvent consisted of 66.7 vol % methanol in water to ensure good dispersion of all the silica samples with different hydrophobicities. The initial measured conductivity of the NaOH solution in the absence of added particles ko is given by22 k0 ¼ kNa þ kOH ¼ λNa CNa;0 þ λOH COH;0

ð2Þ

where kNa is the conductivity contribution from the sodium ions, λNa is the molar conductivity of the sodium ion, CNa,0 is the concentration of the sodium ions in the absence of added particles, and the OH subscript refers to the same quantities for the hydroxide ion. In general, the molar conductivity of an ionic species decreases slightly with increasing ionic strength (I) due to interionic interactions. For dilute solutions of strong electrolytes, the variation of λ with I is given approximately by Debye
H€uckel
Onsager (DHO) theory as23 pffiffi λ ≈ λ0
z I ð3Þ where λ0 is the molar conductivity at infinite dilution, I is the ionic strength of the aqueous phase, and z is a constant. As shown in the Supporting Information, separate measurements of conductivity as a function of concentration for solutions of NaOH in the mixed methanol
water solvent yielded the values of λ0NaOH (= λ0Na + λ0OH) and z. According to Shanks and Franses,24 in the context of DHO theory, charged micelles do not contribute significantly to the effective ionic strength of the aqueous medium. It has similarly been shown, by both theory and experiment, that charged micelles and their counterions do not contribute to the ionic strength in the calculation of the Debye length controlling the range of surface forces.25
27 Hence, the value of ionic strength in using eq 3 was taken to equal the concentration of NaOH; that is, the contribution to the overall ionic strength of the partially charged silica particles was assumed negligibly small. 12871

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Figure 3. Variation of surface concentration of deprotonated silanol groups (ΓSiO
) with hydroxide ion concentration. The key shows the volume of DCDMS in milliliter units used in the sample preparation.

In dispersions containing silica particles, the measured conductivity k, in principle, also contains an additional contribution from the charged
SiO
species on the silica particle surfaces. However, as seen for charged colloidal species such as micelles,28 this contribution is negligibly small because of the low mobility of the (relatively) large silica particles; that is, the molar conductivity of particle surface
SiO
groups λSiO is negligibly small. Hence, k ¼ λNa CNa þ λOH COH þ λSiO CSiO ≈ λNa CNa þ λOH COH

ð4Þ where CSiO is the concentration of particle surface
SiO
groups. Mass balance gives CNa ¼ COH þ CSiO

ð5Þ

Assuming that the concentration of free sodium ions is unchanged by particle addition (i.e., CNa,0 = CNa), combining eqs 2, 4, and 5 yields an expression for CSiO. CSiO ¼

ðk0
kÞ λOH

ð6Þ

The concentration of
SiO
groups per unit area of silica surface (ΓSiO) is then ΓSiO ¼

ðk0
kÞ Cp AλOH

ð7Þ

where Cp is the mass of silica particles per unit volume of solution and A is the silica surface area per unit mass of particles. To obtain ΓSiO using eq 7, the limiting molar conductivity of NaOH (λ0NaOH = λ0Na + λ0OH) in the mixed water/methanol solvent was measured separately together with the value for NaCl (see the Supporting Information). As described in the Supporting Information, λ0OH in the mixed solvent was derived from the two measured limiting molar conductivities using the assumption that the transport numbers of Na+ and Cl
ions29 were unaffected by the methanol content. The hydroxide ion limiting molar conductivity derived in this way was then corrected for the

Figure 4. Measured variation of %SiOH groups on the nanoparticle surfaces as a function of the amount of DCDMS used in the silanization. For each silanization reaction, the mass of silica nanoparticles used was 37 g and the reaction time was 24 h. The dashed line shows the variation expected if all the added DCDMS reacted to form a surface monolayer.

relevant ionic strength using the DHO correction (eq 3) to obtain λOH. The value of A was measured using nitrogen adsorption to be 206 ( 15 m2 g
1 (average of two measurements), in good agreement with the manufacturer’s quoted value of approximately 200 m2 g
1. Figure 3 shows the variation of ΓSiO with concentration of OH
ion for the “bare”, unmodified silica nanoparticles corresponding to 100% SiOH. The surface concentration of deprotonated silanol groups increases with increasing NaOH concentration to reach a plateau value corresponding to all the silanol groups being deprotonated. This occurs for pH values above approximately 12. From the plateau value, the total number of surface silanol groups per unit surface area of the silica is found to be 4.8 silanol groups per nm2, in good agreement with literature values for fully hydrated amorphous silica. The average value determined from measurements of 100 samples of silica samples with different specific surface areas and produced by a variety of methods was 4.9 silanols nm
2 (with indivdual values ranging from 4.2 to 5.7 OH groups nm
2).19 Despite the constancy of this value for different types of amorphous silica, many manufacturers quote much lower values of the silanol surface density. As noted by Zhuravlev,20 high temperature vacuum treatment progressively reduces the silanol surface density, and hence, it is likely that manufacturers’ quoted values refer to a (usually unspecified) high-temperature vacuum treated state rather than a fully hydrated state. Having established the hydroxide ion concentration required to produce >95% deprotonation of the available silanol groups, conductivity measurements were made with this hydroxide ion concentration with and without particles having different extents of silanization. The values of ΓSiO for samples prepared using different amounts of DCDMS are shown in Figure 3; the values were divided by ΓSiO for the unmodified silica to obtain values of %SiOH for each sample. The method described here was used to determine the %SiOH for a commercial sample of partially silanized silica supplied by Wacker; the measured value (49% SiOH) agrees well with the manufacturer’s quoted value of 50% SiOH. 12872

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Figure 5. Materials prepared by hand-shaking mixtures of 20% SiOH silica (5 mg), air, and 5 mL of water/methanol mixed solvent. (a) Appearance of the different materials 5 min after preparation. The type of material is given as well as the volume fraction of methanol in each sample, increasing from left to right. Scale bar equals 0.5 cm. (b) Top view of the unstoppered sample at ϕM = 0.0. Several liquid marbles are visible. Scale bar equals 0.5 cm. (c) Optical micrograph of the particle-stabilized foam in (a). Scale bar equals 100 μm.

Figure 4 shows that the %SiOH decreases progressively with the amount of DCDMS used in the silanization reaction. The commonly accepted scheme for the low temperature silanization reaction is as follows.15,16 In the first step, the DCDMS adsorbed on the silica surface is hydrolyzed to the dimethyldisilanol species by water adsorbed on the silica surface: 2H2 O þ Cl2 SiðCH3 Þ2 f ðOHÞ2 SiðCH3 Þ2 þ 2HCl In the second step, the adsorbed alkylsilanol condenses with a surface silanol group (denoted SiSOH) to form a surface SiS
O
Si bond. SiS OH þ ðOHÞ2 SiðCH3 Þ2 f SiS OSiðOHÞðCH3 Þ2 þ H2 O

The surface-bound dimethylmonosilanol species SiSOSi(OH)(CH3)2 can then react further through a combination of two different reactions. It can either condense with an adjacent surface bound dimethylmonosilanol to form a cross-linked monolayer or condense with a further dimethyldisilanol species to form a multilayer coating. In Figure 4, the measured values of %SiOH as a function of the amount of DCDMS used in the synthesis are compared with what would be expected if one surface silanol group reacts with only one molecule of DCDMS to form a monolayer coating. This calculation uses the result obtained earlier that the silica particle surfaces contain 4.8 silanol groups nm
2. It can be seen that the silanization reaction

asymptotes to a monolayer film of DCDMS only at low extents of silanization (i.e., high %SiOH). As the extent of silanization increases, the coating apparently contains a progressively higher fraction of multilayer film. For the most hydrophobic silica with 20% SiOH, it is calculated that the total amount of DCDMS used in the synthesis corresponds to 3.1 DCDMS molecules per surface silanol group. If all the added DCDMS actually reacts with the silica, this result suggests that the silane coating corresponds approximately to a trilayer coating in this case. This result is similar to the observation, made using IR spectroscopy, that approximately 2.8 DCDMS molecules react per silica surface silanol group in a room temperature reaction followed by curing at 200 °C.16 Colloidal Structures Formed by Shaking Silanized Silica Particles with Water
Methanol Mixtures. Examples of the different types of colloidal structures formed are shown in Figure 5 for the case of the most hydrophobic particles (20% SiOH) shaken with different water/methanol mixtures. Stable dispersions are formed at low water/high methanol content mixtures when the particles have most affinity for the bulk solvent and least affinity for the liquid
air surface. Increasing water content, causing increased affinity of the particles for the surface, leads to the formation of foams, climbing films, and liquid marbles. Figure 6 maps the regions of formation of the different structures as functions of both the mixed solvent composition and the % SiOH of the particles. It can be seen 12873

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surfaces are estimated to be γdsa ¼ 0:200ð%SiOHÞ þ 22:0

ð11Þ

γpsa ¼ 0:331ð%SiOHÞ þ 0:92

ð12Þ
2

where the surface energies are in units of mJ m . The dispersion component of the methanol
air surface is 18.2 mJ m
2,37 compared with 21.5 mJ m
2 for the water
air surface.38 We assume here that γdla for water/methanol mixtures varies linearly between these two values with water vol % according to γdla ¼
0:0333ðvol %MeOH Þ þ 21:5 Figure 6. Measured loci of formation of dispersions, foams, climbing films, and liquid marbles as a function of both %SiOH and methanol concentration. The horizontal dashed line shows the minimum %SiOH value used here.

that the same progression of structures is observed for pure water as solvent in a series in which the %SiOH of the particles is increased. Qualitatively, one expects particle dispersions when the particle affinity for the surface is low. Structures requiring particle adsorption at the liquid
air surface require the opposite. The particle affinity for the surface depends on both the contact angle and liquid
air tension (eq 1); we calculate here how the particle adsorption energy varies with both %SiOH and water
methanol content and correlate the calculated adsorption energies with the “map” showing the loci of the types of structure formed. Despite a range of methods to measure contact angles for colloidal particles,30
32 there is currently no accurate method for particles of the small sizes as used here (diameter 10
30 nm). Hence, in order to estimate the adsorption energy of the particles, we use a theoretical approach based on resolution of the differenet components of the surface energies of the various surfaces present.33,34 According to Fowkes35 and Owens and Wendt,36 the surface tension of a liquid against air γ can be expressed as the sum of components due to dispersion forces γd and polar forces γp according to γ ¼ γd þ γp

ð8Þ

The interfacial tension between two condensed phases a and b is then expressed in terms of these two components for each phase. qffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffi p p γab ¼ γa þ γb
2 γda γdb
2 γa γb ð9Þ For a liquid
air system in contact with a solid surface, the contact angle θ between the liquid
air interface and the solid surface is given by γ
γsl cos θ ¼ sa ð10Þ γla where the subscripts on the surface energies indicate the relevant interfaces (solid
air, solid
liquid, and liquid
air). As described in ref 33, the dispersion and polar components of untreated and hydrophobized silica
air interfaces as a function of %SiOH have been estimated by adjustment of the values to obtain a “best-fit” between measured contact angles of various liquids on suitable reference surfaces and values calculated using eqs 2
4. Using this procedure, the dispersion and polar components of the partially hydrophobized silica

ð11Þ

The polar component of the water
methanol mixtures was calculated from literature values of the water/methanol-air surface tension39 which varies with vol% water according to: γla ¼ 9:06  10
7 ðvol %MeOH Þ4
2:42  10
4 ðvol %MeOH Þ3 þ 2:54  10
2 ðvol %MeOH Þ2
1:52ðvol %MeOH Þ þ 71:9 ð12Þ Using eq 8, eqs 11 and 12 combine to the give the following polynomial expression for the polar component of the liquid
air surface energy as a function of the vol % of water. γla ¼ 9:06  10
7 ðvol %MeOH Þ4
2:42 p

 10
4 ðvol %MeOH Þ3 þ 2:54  10
2 ðvol %MeOH Þ2
1:55ðvol %MeOH Þ þ 50:4 ð13Þ This theoretical analysis enables the estimation of the liquid
air
silica contact angle as functions of both the water/methanol liquid composition and the %SiOH of the silica particles. As noted above, this approach has been used successfully to calculate the %SiOH giving θ = 90 in particle, oil, and water systems with different oils.34 These values correspond to the %SiOH value at which particlestabilized emulsions are observed to undergo phase inversion. Although the approach is valid for pure liquids, it is questionable whether it can be applied to liquid mixtures like the water/methanol mixtures used here. The approach is expected to be valid only when the surface compositions (at both the liquid
air and solid
liquid surfaces) are approximately equal to the bulk overall composition; that is, adsorption is absent. Methanol is known to be weakly absorbing from water/methanol mixtures to the liquid
air surface; the adsorption at the silica
liquid interface is not known. In order to test the validity of the approach, we have compared calculated liquid
air
silica contact angles with values measured for untreated (i.e., 100% SiOH) and maximally silanized (i.e., 0% SiOH) silica glass microscope slides. Figure 7 shows a comparison of the measured and calculated contact angles, and it can be seen that the contact angle variation with both %SiOH and vol % methanol are reasonably well described. To compare with the loci of formation of dispersions, foams, climbing films, and liquid marbles as functions of both %SiOH and methanol content seen in Figure 6, Figure 8 shows how the particle contact angles and energy of adsorption vary. Although the boundaries separating the regions of different structure formation do not map exactly onto the “contour” lines of constant 12874

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Figure 7. Comparison of measured and calculated contact angles for drops of water/methanol mixtures on untreated (100% SiOH) and maximally silanized (0% SiOH) glass slides. The plotted contact angles refer to the average of the measured advancing and receding static angles; the error bars indicate the measured hysteresis between the advancing and receding values.

Figure 8. Calculated variation of the liquid
silica
air contact angle (upper plot) and particle adsorption energy (lower plot) as functions of both %SiOH and methanol concentration.

contact angle, it can be seen that particle dispersions are formed when the particles are strongly wetted by the liquid, that is, when θ < 30° or so. Foams are formed when 30 > θ < 60°, foams + climbing films when 60 < θ < 90°, and liquid marbles + climbing films are formed when θ approaches 90°. Although Figure 8 reveals the general pattern of behavior of how the microstructures

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formed change with increasing particle contact angle, it must be emphasized that, owing to the approximations noted above, the absolute values of the contact angles at the different boundaries are rather approximate. In addition, inversion from a foam structure (an air dispersion in water) to a liquid marble structure (a water dispersion in air) requires a change in sign of the “preferred” curvature of the stabilizing particle film, and hence, this particular transition is likely to occur when the particle contact angle equals 90°. However, in addition to the particle contact angle leading to the required curvature of the adsorbed particle film, the strength of particle adsorption must be sufficient to confer kinetic stability on the structure formed. As seen in eq 1, the strength of particle adsorption also depends on the liquid
air tension which is a function of the methanol content of the mixed solvent. Using the equations above, we have calculated the energy required to desorb the particles divided by their thermal energy, that is,
Eads/kT (where k is Boltzmann’s constant and T is the absolute temperature) as functions of both %SiOH and methanol content for a particle of radius r equal to 10 nm. When this parameter is .1, particle adsorption is strong and kinetic stabilization of the structure is expected. The lower plot of Figure 8 shows the particle adsorption energy landscape plotted as “contour lines” corresponding to selected fixed values of
Eads/kT. It can be seen that particle adsorption is strong (>100kT) for most of the bottom left-hand half of the diagram. The adsorption strength increases progressively as the particles are made more hydrophobic and the methanol content is reduced. The iso-adsorption energy contour lines change shape under conditions such at the contact angle passes through 90°. For contact angles less than 90°, the lowest energy to remove the particles from the surface corresponds to desorbing them into the aqueous phases. Above 90°, the lower desorption energy corresponds to desorption into the air phase. From the comparison of the lower plot of Figure 8 and the measured boundaries shown in Figure 6, it can be seen that stable microstructures are not necessarily formed under all conditions when the particle adsorption is strong. This suggests that kinetic factors, particularly the kinetics of how the particles become adsorbed to the surface, are important. This is consistent with the observation that low energy input (hand shaking as used here) produces, for example, the liquid marbles whereas much higher energy input (using a food blender) can produce dry water.9,10

’ CONCLUSIONS We have developed a method whereby lab-scale quantities of silica nanoparticles can be controllably silanized to different extents using silane vapor. Simple measurements of the particle immersion time are used to check the in-batch uniformity of the silanization. The extent of silanization, expressed as the fraction of silica surface silanol groups which remain unreacted, can be measured using an acid
base conductivity titration method and corresponds to a monolayer coating at low silanization extents and approximately a trilayer at high coverage. When mixtures of particles of different hydrophobicity are shaken with water/ methanol mixtures, the formation of stable particle dispersions, foams, climbing films, and liquid marbles is observed. Using a semiquantitative estimation of the particle contact angles and energy of adsorption, it is demonstrated that increasing the hydrophobicity of the particles by alteration of either the %SiOH or the liquid composition leads to microstructure formation in the sequence: dispersions, foams, climbing films, and liquid marbles. 12875

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’ ASSOCIATED CONTENT

bS

Supporting Information. Additonal table and figures. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION

ARTICLE

(35) Fowkes, F. M. J. Phys. Chem. 1963, 67, 2538. (36) Owens, D. K.; Wendt, R. C. J. Appl. Polym. Sci. 1969, 13, 1741. (37) van Oss, C. J. Interfacial forces in aqueous media; Marcel Dekker: New York, 1994. (38) Jasper, J. J. J. Phys. Chem. Ref. Data 1972, 1, 841. (39) Vazquez, G.; Alvarez, E.; Navaza, J. M. J. Chem. Eng. Data 1995, 40, 611.

Corresponding Author

*E-mail: [email protected].

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dx.doi.org/10.1021/la2028725 |Langmuir 2011, 27, 12869–12876