Stabilization of Silicone Oil-in-Water Emulsions by Ionic Surfactant and

Article pubs.acs.org/IECR

Stabilization of Silicone Oil-in-Water Emulsions by Ionic Surfactant and Electrolytes: The Role of Adsorption and Electric Charge at the Interface Krishnamurthy Sainath† and Pallab Ghosh* Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India ABSTRACT: The stability of silicone oil-in-water emulsions in the presence of a cationic surfactant (viz. cetyltrimethylammonium bromide) and salts containing mono-, di-, and trivalent ions (viz. NaCl, CaCl2, and AlCl3) is reported in this work. Addition of salt enhanced the adsorption of surfactant at the oil−water interface and had a significant effect on the zeta potential. The evolution of droplet size with time was measured by dynamic light scattering. The experimental data were fitted by a model based on drop coalescence. The stability of the emulsions increased in the presence of salt. The effectiveness of the salts investigated was in the sequence AlCl3 > CaCl2 > NaCl. Coalescence of a single silicone oil drop at a flat oil−water interface was studied. Addition of salt increased the drop coalescence time.

1. INTRODUCTION Silicone oils are used in a wide variety of materials such as lubricants, plasticizers, fabric softeners, drilling fluids, defoamers, printing ink formulations, and high-performance coatings. Basic silicones are inert polydimethylsiloxanes. They are distinctly different from organic hydrocarbons. They have a backbone of silicon−oxygen linkages similar to that found in quartz, glass, and sand, whereas hydrocarbons are based on a backbone of carbon-to-carbon bonds. The molecular backbone in silicone oils is much stronger than the typical C−C chain and is more resistant to large fluctuations in temperature, oxidation, shear stresses, and chemicals. In addition, silicone oils have moderate surface tensions and are noncorrosive and waterrepellent.1 Apart from the applications mentioned above, silicone oils have also found extensive use in cosmetics and pharmaceuticals.2 Some of their important applications are in antiperspirants, deodorants, hand creams and lotions, hair sprays, lipstick, shampoos and conditioners, and shaving creams. The excellent spreading and film-forming properties of silicone oils provide gloss and a dry, nonsticky feel. Many of the industrial applications of silicone oils are in the form of either oil-in-water or water-in-oil emulsions.3 Because these emulsions are used to a great extent, their stability is of paramount importance. The free energy of formation of emulsions is greater than zero as a result of the energy associated with the large interfacial area of the droplets within the emulsion, which is given by the product of the interfacial energy and the total surface area of the droplets. This energy term outweighs the entropy of formation associated with the formation of droplets from the bulk constituents.4 Therefore, emulsions are thermodynamically unstable colloids, and they have a tendency to break unless stabilized. Emulsions, however, are kinetically stable because of the presence of an adsorbed layer of surfactant molecules at the oil−water interface. This layer provides electrostatic or steric repulsion when the droplets approach each other within a very close proximity. This repulsion serves to stabilize the thin film of liquid between the droplets. Coalescence occurs when this thin film becomes © 2013 American Chemical Society

unstable and then ruptures. Therefore, the presence of adsorbed surfactant molecules can reduce the likelihood of coalescence. As a result of their thermodynamic instability, emulsions have a tendency to reduce their total free energy by increasing the droplet size and, hence, reducing their total interfacial area. This leads to the degradation of the emulsion (i.e., coarsening). An emulsion can degrade by mechanisms such as coalescence, Ostwald ripening, creaming (with or without aggregation), and aggregation (with or without creaming). Coalescence and Ostwald ripening involve a change in the droplet size distribution and can result in complete phase separation into oil and water. Coalescence requires the droplets to be in close proximity. Ostwald ripening, on the other hand, does not require the droplets to be close because the process occurs by transport of dissolved matter through the dispersion medium. Important properties of emulsions such as viscosity, yield strength, flowability, pumpability, and texture are influenced by the droplet size distribution. When ionic surfactants are used to stabilize silicone oil emulsions, the electrical properties of the silicone oil−water interface play an important role in emulsion stability. The presence of electrolytes can alter the electrical properties because the thickness of the diffuse part of the electrostatic double layer is reduced with increasing electrolyte concentration. Sometimes, binding of counterions can substantially reduce the potential at the oil−water interface. Salts containing di- and trivalent ions can enhance the adsorption of surfactant at the interface more effectively than salts containing monovalent ions.5−7 This alters the electric charge at the interface.8 Various types of inorganic salts are usually present in applications in salty media (e.g., oil recovery).9 Drilling fluids almost always contain saline environments where salts of Received: Revised: Accepted: Published: 15808

May 10, 2013 July 11, 2013 October 18, 2013 October 18, 2013 dx.doi.org/10.1021/ie401490c | Ind. Eng. Chem. Res. 2013, 52, 15808−15816

Industrial & Engineering Chemistry Research

Article

Figure 1. Experimental setup for measurement of the coalescence time of silicone oil drops.

different valences are encountered.10 Many cosmetic and laundry formulations contain mixtures of salts that are used to alter their interfacial properties. The electric charge at the oil−water interface is considered to be an important factor in the coalescence of oil droplets11,12 and, hence, in the stability of emulsions.13 A few works have studied the electrical properties of the silicone oil−water interface.14−16 However, these works did not include systematic studies of the role of surfactant adsorption in the development of interfacial charge; the effect of salts containing mono-, di-, and trivalent ions on this charge; and the role of the electrical properties of the interface in the stability of silicone oil emulsions. Considering the preceding facts, the main objective of the present work was to experimentally study the stability of silicone oil-in-water emulsions stabilized by a cationic surfactant, namely, cetyltrimethylammonium bromide (CTAB). The zeta potential at the interface was measured in the presence of CTAB and the salts NaCl, CaCl2, and AlCl3. The role of adsorption in the development of interfacial charge was investigated. Adsorption of the surfactant at the silicone oil−water interface was measured by interfacial tensiometry. The effects of salts on the stability of the emulsions were studied. The evolution of droplet size with time was monitored by dynamic light scattering (DLS). The kinetics of droplet growth was analyzed using a coalescence model. The effects of CTAB and salts on the coalescence of silicone oil drops were studied. The experimental values of coalescence time were compared with the predictions of seven film-drainage models reported in the literature.17

2.2. Measurement of the Zeta Potential. The zeta potential at the silicone oil−water interface was measured with a zeta potentiometer (model Delsa Nano C, Beckman Coulter, Nyon, Switzerland) from the electrophoretic mobility of the fine oil droplets dispersed in water. A small amount of the oilin-water emulsion (1 cm3) was transferred to the flow cell of the zeta potentiometer. The electrophoretic mobility of the oil droplets was measured, and the zeta potential was determined. All experiments were repeated five times, and the average values of these readings are reported. The aqueous phase was free of any colloidal impurity, as ascertained by DLS. DLS studies of the purified water clearly showed the absence of any material that would scatter the HeNe laser. 2.3. Measurement of the Adsorption of CTAB at the Silicone Oil−Water Interface. CTAB solutions were prepared by dissolving the surfactant in water and subsequently diluting the stock solution. CTAB does not have an appreciable solubility in silicone oil. The adsorption of CTAB at the interface was determined by interfacial tension measurements. Interfacial tension was measured with a digital tensiometer (model K9, Krüss, Hamburg, Germany) using a du Noüy ring made of platinum and iridium. The procedure described in ASTM Standard D1331-11 was followed. The sample vessels and the du Noüy ring were methodically cleaned before each measurement in accordance with the ASTM standard. The ring was burned to red-hot conditions in the blue flame of a Bunsen burner. The ring was dipped into the aqueous phase to the required depth. Silicone oil was then poured very carefully over the aqueous phase. The system was allowed to equilibrate for 1 h, and the interfacial tension was measured. The sample vessel was moved at a very low speed (∼500 μm s−1) during the measurements. The measurements were repeated three times at each set of surfactant and salt concentrations. The values of interfacial tension measured by this procedure were highly accurate and reproducible. A rigorous cleaning was performed to remove silicone oil from the glassware. First, the glassware was cleaned using a fresh chromic acid solution. This was followed by a thorough wash with tap water. Thereafter, the glassware was washed with acetone. Finally, the glassware was washed with Millipore water and dried in an air oven. 2.4. Preparation of the Emulsions. The silicone oil-inwater emulsions were prepared by putting 0.2 cm3 of silicone

2. MATERIALS AND METHODS 2.1. Materials. The cationic surfactant CTAB (C19H42BrN) was purchased from Sigma-Aldrich (Bangalore, India). The surfactant had >99% purity. Sodium chloride (>99% purity), calcium chloride (>98% purity), and aluminum chloride (>98% purity) were purchased from Merck (Mumbai, India). These chemicals were used as received from their manufacturers. Silicone oil was purchased from Loba Chemie (Mumbai, India). Its density was 965 kg m−3, and its viscosity was in the range of 370−390 mPa s. The water used in this study was purified with a Millipore water purification system. Its conductivity was 1 × 10−5 S m−1, and its surface tension was 72.5 mN m−1. 15809

dx.doi.org/10.1021/ie401490c | Ind. Eng. Chem. Res. 2013, 52, 15808−15816


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Figure 2. Electrostatic double layer at the silicone oil−water interface.

oil in 49.8 cm3 of water. The dispersion of oil in water was prepared in a liquid−liquid homogenizer (model T10 Basic Ultra-Turrax, IKA, Staufen, Germany). The liquid mixture was agitated for 10 min. About 4 cm3 of the emulsion was collected, and the size of the droplets in the emulsion was measured by DLS. 2.5. Measurement of the Droplet Size Distribution. The diameter of the droplets was measured by DLS at different time intervals. The samples were irradiated with a HeNe laser (wavelength = 632.8 nm), and the intensity fluctuations of the scattered light were analyzed to obtain the correlation function. The size distribution was obtained by fitting a multiple exponential to the correlation function by the non-negative least-squares method. All computations were performed using the software associated with the equipment. The DLS method gives the hydrodynamic diameter, which is calculated by the Stokes−Einstein equation as d=

kT 3πμD

2.7. Other Experimental Details. The pH of the aqueous phase was measured using a pH meter (model pH 2700, 93X218819 electrode, Eutech, Singapore). These values were measured once for every system. All experiments were conducted in an air-conditioned laboratory where the temperature was maintained at 298 ± 1 K.

3. RESULTS AND DISCUSSION 3.1. Adsorption of Surfactant and Development of Electric Charge at the Silicone Oil−Water Interface. When positively charged CTAB molecules adsorb at the silicone oil−water interface, an electrostatic double layer is formed, as illustrated schematically in Figure 2. The diffuse part of the double layer extends into the solution. The zeta potential represents the electric potential at the surface of shear, which is located a few molecular diameters from the Stern layer, although its location is not precisely known. Nonetheless, the zeta potential is the only experimentally measurable quantity for the electric potential developed at the interface. The silicone oil droplets in water, in the absence of the surfactant and salts, were negatively charged. The zeta potential of the oil droplets was about −35 mV. Similar results have been reported in the literature.14 The negative charge was due to the preferential adsorption of hydroxyl ions at the interface. Addition of a small amount of CTAB rendered the zeta potential positive, and increase in the concentration of CTAB had a tremendous effect on the zeta potential, as depicted in Figure 3. The zeta potential increased monotonically with CTAB concentration up to ∼0.9 mol m−3 concentration, which is the critical micelle concentration (CMC) of CTAB. Further increases in the surfactant concentration did not significantly increase the zeta potential. Addition of salt had a profound effect on the zeta potential. The effect of NaCl is shown in Figure 3. The zeta potential increased with the addition of a small amount of NaCl. The increase was steady up to ∼0.01 mol m−3 concentration of CTAB. Beyond this surfactant concentration, the increase in zeta potential with surfactant concentration was less

(1)

2.6. Coalescence of Silicone Oil Drops. The coalescence of silicone oil drops in aqueous solutions of surfactant and salt was studied in a 10-cm-diameter coalescence cell made of glass, as shown in Figure 1. Silicone oil drops were formed by a specially designed glass buret that was inserted through the top of the cell. The drops were slowly formed at the tip of a fine needle fixed at the tip of the buret and released 5 cm from the oil−water interface. As the interfacial tension decreased, the size of the drops decreased slightly. Tips with different diameters were used to adjust the diameters of the drops to minimize the variation in their size. The size of the drops was determined by image analysis using ImageJ software, as described in the literature.18 The time during which a drop rested on the flat oil−water interface (i.e., the coalescence time) was measured using a video camera (model DCR-HC32E, 20× optical zoom, Sony, Tokyo, Japan) fitted with a timer with a resolution of 0.1 s. Coalescence times of 100 drops were studied in each experiment. 15810



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when dissolved in water, generates hydrated aluminum ions, [Al(H2O)6]3+.19 The hexaaquaaluminum ion is acidic because the electrons from the water molecules are strongly pulled toward the aluminum. This makes the hydrogens more positive and, as a result, susceptible to attack from solvent water, which acts as a base. The hexaaqua ion is deprotonated, causing the solution to be acidic from the formation of the hydroxonium ion. It has been reported in the literature that the zeta potential of surfactant-covered fluid−fluid interfaces usually increases with decreasing pH.20 Figures 3−5 clearly show that the amount of salt required to increase the zeta potential depends on the ionic strength of the solution. The effectiveness of the salts in increasing the zeta potential was in the sequence AlCl3 > CaCl2 > NaCl. The development of positive charge at the oil−water interface is a direct consequence of the adsorption of CTAB molecules at the interface. The concentration of surfactant at the interface increased with increasing concentration of CTAB. This increase was reflected in the decreasing interfacial tension, which is depicted in Figures 6−8. Similar results have been

Figure 3. Variation of the zeta potential at the silicone oil−water interface with the concentration of CTAB at different concentrations of NaCl.

pronounced. However, when the NaCl concentration was increased to 50 mol m−3, the zeta potential decreased significantly. Similar results were observed with CaCl2 and AlCl3, as shown in Figures 4 and 5, respectively. The increase in

Figure 6. Variation of the tension at the silicone oil−water interface with the concentration of CTAB at different concentrations of NaCl. Figure 4. Variation of the zeta potential at the silicone oil−water interface with the concentration of CTAB at different concentrations of CaCl2.

Figure 7. Variation of the tension at the silicone oil−water interface with the concentration of CTAB at different concentrations of CaCl2. Figure 5. Variation of the zeta potential at the silicone oil−water interface with the concentration of CTAB at different concentrations of AlCl3.

reported for other surfactants.21 The interfacial tension decreased significantly upon the addition of salt. With increasing salt concentration, the adsorption of CTAB molecules at the oil−water interface was complete at a lower concentration of the surfactant. This led to a reduction in the CMC.22 Consequently, the interfacial tension sharply decreased with surfactant concentration in the presence of salt and

zeta potential was higher in the presence of AlCl3 than NaCl or CaCl2. The pH of aqueous solutions containing NaCl or CaCl2 varied in the range of 6 ± 0.2, whereas the pH of aqueous AlCl3 solutions varied in the range of 3.5 ± 0.2. Aluminum chloride, 15811



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in the adsorption of the positively charged CTAB molecules at the oil−water interface in the presence of salt. Equation 3shows that the zeta potential at the interface increases as the charge density increases and decreases as the concentration of salt increases. Therefore, the increase or decrease in the value of ζ depends on the relative magnitudes of σ and c∞. At low salt concentrations, the increase in σ offsets the increase in c∞. However, at high salt concentrations, the large increase in c∞ results in a significant decrease in ζ. 3.2. Stability of Silicone Oil-in-Water Emulsions. Coalescence of the silicone oil droplets is the main mechanism by which silicone oil-in-water emulsions coarsen and destabilize. Ostwald ripening is unlikely to occur because of the low solubility of the oil in water.13 Creaming becomes important when the droplets become large. Coalescence of oil droplets in an emulsion depends on the stability of the thin aqueous film.25,26 The stability of this film is governed by hydrodynamic drainage and, subsequently, by intermolecular and interfacial forces (e.g., electrostatic double layer, hydration, and van der Waals forces). According to Deryagin−Landau− Verwey−Overbeek (DLVO) theory of the stability of electrocratic colloids, the variation of the net interaction energy of two silicone oil droplets in water (Φnet) with the distance between them (δ) is given by27

Figure 8. Variation of the tension at the silicone oil−water interface with the concentration of CTAB at different concentrations of AlCl3.

became almost invariant with the surfactant concentration thereafter. The adsorption of ionic surfactant molecules at the silicone oil−water interface was facilitated by the addition of salt. The electrostatic double layer repulsion between the positively charged surfactant head groups was reduced by the salt ions, which favored more adsorption of the surfactant molecules at the interface. This reduced the interfacial tension.11 The interfacial tension depends on the zeta potential (ζ) and the interfacial charge density (σ) as23 γ = γ0 −

∫ Γ dϕ − ∫ σ dζ

Φnet =

z 2e 2

⎛ zeζ ⎞ AHd ⎟ exp( − κδ) − tanh2⎜ ⎝ 4kT ⎠ 24δ (4)

The value of the Hamaker constant (AH) of silicone oil droplets in water is 3.3 × 10−22 J.13 Equation 4 suggests that the net interaction energy would strongly depend on the Debye− Hückel parameter, κ, and the zeta potential, ζ. The Debye− Hückel parameter depends on the valence of ions and their concentration as

(2)

The zeta potential is related to the charge density at the interface and the concentration of electrolyte in the solution. For a symmetric z:z electrolyte, the Grahame equation (derived from the Gouy−Chapman theory of electrostatic double layer)24 describes the relationship between the interfacial charge density and the potential at the interface ⎛ zeζ ⎞ ⎟ σ = (8RTεε0c ∞)1/2 sinh⎜ ⎝ 2kT ⎠

16πεε0d(kT )2

⎛ N e2 κ = ⎜⎜ A ⎝ εε0kT

(3)

⎞1/2

∑ j

⎟ zj 2c ∞ j ⎟ ⎠

(5)

The energy barrier to coalescence mainly depends on these two parameters, as the Hamaker constant is rather insensitive to the increase in salt concentration. With increasing salt concentration, κ increases, and this lowers the energy barrier. However, ζ increases with increasing salt concentration and subsequently decreases at high salt concentrations, as depicted in Figures 3−5. Equation 4 predicts a large increase in the energy barrier even when a small increase in ζ occurs (e.g., by a few millivolts). The stability of silicone oil-in-water emulsions was studied at a CTAB concentration of 0.01 mol m−3 in the presence of NaCl, CaCl2, and AlCl3. When coalescence is the main factor behind the coarsening of the emulsion, the increase in droplet diameter (d) with time (t) follows the equation28,29

The variation of the interfacial charge density with the concentration of CTAB in the absence and in the presence of NaCl is shown in Figure 9. It can be observed from this figure that the charge density increased with increasing concentration of NaCl, which is attributed to the enhancement

1 1 = 2 − kt̂ 2 d d0

(6)

The rate constant for coalescence, k,̂ is given by 2πω/3, where ω is the frequency of film rupture per unit surface area. The size of the droplets increases with time due to coalescence. When the coarsening of emulsions occurs by Ostwald ripening, the droplet diameter increases on a different scale, namely, d3 increases linearly with t. Equation 6 was developed based on

Figure 9. Variation of the charge density at the silicone oil−water interface with the concentration of CTAB at different concentrations of NaCl. 15812



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several assumptions, as discussed by Deminière et al.29 This model is valid for droplet diameter variations below ∼10 μm. The Z-average diameter, obtained from DLS, was used to quantify the droplet size.30 It is the primary and most stable parameter produced by the DLS technique. This average is also known as the cumulant mean or harmonic-intensity-averaged particle diameter. The Z-average diameter will be comparable to the diameter measured by other techniques if the sample is monomodal (i.e., having only one peak), is spherical or nearly spherical in shape, is monodisperse (i.e., having a narrow distribution), and is prepared in a suitable dispersant. The Zaverage diameter can be sensitive to even small changes in the sample (e.g., the presence of a small proportion of aggregates). The Z-average diameter is a hydrodynamic parameter, and it is therefore applicable only to particles in dispersion or molecules in solution. A typical droplet size distribution is shown in Figure 10.

Table 1. Rate Constant for the Coalescence of Silicone Oil Droplets in the Emulsions salt − NaCl CaCl2 AlCl3

salt concentration (mol m−3)

k̂ (×106 μm−2 s−1)

coefficient of determination

− 10 50 5 20 5 10

5.42 4.13 2.70 4.76 2.51 3.64 2.66

0.97 0.95 0.93 0.98 0.97 0.95 0.95

for coalescence decreased with increasing concentration of NaCl. Thus, the addition of NaCl prevented the coalescence of silicone oil droplets, thereby slowing the coarsening of these emulsions. Similar results were observed with CaCl2 and AlCl3, which are presented in Figures 12 and 13, respectively.

Figure 12. Variation of 1/d2 with time at different concentrations of CaCl2 in 0.01 mol m−3 CTAB.

Figure 10. Distribution of the diameter of silicone oil droplets in water in 0.01 mol m−3 CTAB.

The variation of the size of silicone oil droplets in the emulsion with time in 0.01 mol m−3 CTAB is depicted in Figure 11. The initial droplet diameter, d0, decreased upon the addition of salt. This is expected because the interfacial tension decreased upon the addition of NaCl (as shown in Figure 6) and the droplet diameter decreased with decreasing interfacial tension.31 Equation 6 fit the data well. The values of k̂ obtained by linear regression are presented in Table 1. The coefficients of determination are also included in Table 1. The rate constant

Figure 13. Variation of 1/d2 with time at different concentrations of AlCl3 in 0.01 mol m−3 CTAB.

However, the quantity of CaCl2 used was smaller than that of NaCl, and the amount of AlCl3 was even smaller. The rate constants for coalescence in the CaCl2 and AlCl3 systems were comparable to those in the NaCl system. These results indicate that, apart from the zeta potential, the concentration of surfactant at the interface was also very important for the stability of the silicone oil-in-water emulsions. At low salt concentration, both the zeta potential and the adsorption of surfactant at the interface increased. However, at high salt concentration, the zeta potential decreased, but the adsorption of surfactant increased. The film-drainage models of

Figure 11. Variation of 1/d2 with time at different concentrations of NaCl in 0.01 mol m−3 CTAB. 15813



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Table 2. Comparison of the Mean Values of Coalescence Time Distributions with Predictions by Film-Drainage Modelsa salt

c (mol m−3)

γ (mN m−1)

tc,1 (s)

tc,2 (s)

tc,3 (s)

tc,4 (s)

tc,5 (s)

tc,6 (s)

tc,7 (s)

texpt (s) c

− NaCl

− 10 50 5 20 5 10

25.0 20.2 18.5 23.3 20.3 23.0 20.0

985.5 1272.7 1414.4 1072.3 1265.2 1089.2 1288.0

649.3 838.6 931.9 706.5 833.6 717.6 848.7

29550.8 40686.7 46421.8 32843.2 40386.5 33487.9 41298.6

10453.0 14392.1 16420.7 11617.6 14285.8 11845.6 14608.5

28.2 33.1 35.3 29.7 33.0 30.0 33.3

5674.8 7615.9 8598.3 6254.0 7564.2 6366.8 7721.2

3160.6 4241.8 4788.9 3483.2 4213.0 3546.1 4300.4

18.1 49.3 70.4 78.2 132.7 100.1 220.6

CaCl2 AlCl3 a

Physical properties of the system: a = 2.8 mm, B = 1 × 10−28 J m, g = 9.81 m s−2, Δρ = 35 kg m−3, and μ = 1 × 10−3 Pa s.

coalescence predict that the coalescence time increases with decreasing interfacial tension (see section 3.3). Therefore, the increase in surfactant adsorption at the interface led to increased stability of the silicone oil droplets against coalescence, which increased the stability of the emulsion. 3.3. Coalescence of Silicone Oil Drops. To investigate the roles of the surfactant and salts in the coalescence of silicone oil droplets in the emulsion, experiments on coalescence were carried out at a flat silicone oil−water interface using single drops of silicone oil, following a procedure similar to that described in the literature.12,32,33 In every experiment, a wide distribution of coalescence time was observed, similar to that reported in the literature.34 The mean values of these coalescence time distributions were compared with seven film-drainage models (i.e., eqs 7−13) given by Slattery17 tc,1 = 1.07

μa3.4(Δρg )0.6 γ 1.2B0.4

tc,2 = 0.705 tc,3 = 1.046 tc,4 = 0.37

tc,7 = 0.44

(7)

μa3.4(Δρg )0.6 γ 1.2B0.4

(8)

μa 4.5Δρg γ 1.5B0.5

(9)

μa 4.5Δρg

tc,5 = 5.202

tc,6 = 0.79

Figure 14. Distributions of the coalescence time of silicone oil drops in water in 0.01 mol m−3 CTAB at different concentrations of NaCl.

γ 1.5B0.5

(10)

μa1.75 γ 0.75B0.25

(11)

Figure 15. Distributions of the coalescence time of silicone oil drops in water in 0.01 mol m−3 CTAB at different concentrations of CaCl2.

μa 4.06(Δρg )0.84 γ 1.38B0.46

(12)

μa 4.06(Δρg )0.84 γ 1.38B0.46

(13)

The values of coalescence time are presented in Table 2. The coalescence time increased markedly with increasing concentration of salt, as shown in Figures 14−16. The coalescence times were higher in the AlCl3 system than in the other systems, which is probably due to the higher zeta potential at the silicone oil−water interface in the presence of AlCl3 (see Figure 5). These results are in contrast to the results reported in the literature for systems that did not contain a surfactant, where the coalescence time decreased with increasing salt concentration.21 The results from the coalescence experiments corroborated the increase in rate constant (k̂) with increasing salt

Figure 16. Distributions of the coalescence time of silicone oil drops in water in 0.01 mol m−3 CTAB at different concentrations of AlCl3.

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predicted by these models deviated considerably from the experimental data.

concentration, as depicted in Table 1. However, note that the size of the drops used in the coalescence studies was much larger than the size of the droplets in the emulsions and that the coalescence of the drops occurred at a flat oil−water interface. Therefore, it is likely that the time required for coalescence of silicone oil droplets in the emulsions would be different from that observed in the coalescence experiments. Nonetheless, the coalescence experiments clearly show the increase in the stability of the thin aqueous film against rupture in the presence of CTAB and salts. This stability of the films prevented coalescence of the droplets in the emulsions. The film-drainage models predict an increase in coalescence time with a reduction in the interfacial tension (see eqs 7−13). As the salt concentration was increased, the interfacial tension decreased (as presented in Figures 6−8 and Table 2), which increased the coalescence time. However, the values of coalescence time predicted by these film-drainage models differ widely from the experimental values. The coalescence time varies considerably from one model to another as well. Quantitative agreement between the experimental data and the values predicted by these models was not found. The stability of the thin liquid film (also known as an emulsion film) depends on the electrostatic double layer and other short-range repulsive forces,21,35 which were not considered in these filmdrainage models. In addition, these models do not correctly depict the dependence of the coalescence time on surfactant adsorption. These are the likely reasons for the deviations from the experimental results.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +91.361.2582253. Fax: +91.361.2690762. Present Address †

Department of Chemical Engineering, BMS College of Engineering, Bangalore 560019, India.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Council of Scientific and Industrial Research (CSIR, Government of India) for financial support of the work reported in this article through Scheme 02/(0015)/ 11/EMR-II.

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4. CONCLUSIONS The following conclusions were reached based on the experimental observations and the analysis of the results: (1) The zeta potential at the silicone oil−water interface increased with increasing concentration of CTAB up to the CMC of the surfactant. Addition of a small quantity of salt increased the zeta potential. However, the zeta potential decreased at high salt concentrations. The effectiveness of salts in increasing the zeta potential was in the sequence AlCl3 > CaCl2 > NaCl. The magnitude of the zeta potential was higher in the presence of AlCl3 than the other salts, because of the reduction of the pH of the aqueous medium in the presence of AlCl3. (2) The adsorption of CTAB at the interface was enhanced by the addition of salt. The interfacial tension decreased markedly upon the addition of salt, and the CMC was reduced as well. The charge density at the interface was increased by the enhancement in the adsorption of CTAB in the presence of salt. (3) The coarsening of silicone oil-in-water emulsions occurred by coalescence of the silicone oil droplets. The droplet size increased with time. Addition of salt reduced the size of the emulsion droplets and decreased the rate of coalescence as well. The reduction in coalescence rate was due to the high concentration of CTAB at the oil−water interface and associated reduction of the interfacial tension. The variation of droplet size with time was described well by a coalescence model. (4) Coalescence times of silicone oil droplets at a flat oil− water interface followed wide distributions. At a given surfactant concentration, the coalescence time increased with increasing salt concentration. Seven film-drainage models predicted this behavior, qualitatively, through the reduction in the interfacial tension by the addition of salt. The values

NOMENCLATURE a = radius of drop, m AH = Hamaker constant, J B = modified Hamaker constant, J m c∞ = concentration of ions in the bulk solution, mol m−3 d = droplet diameter, m D = droplet diffusion coefficient, m2 s−1 d0 = initial droplet diameter, m e = electronic charge, C F = cumulative probability distribution of drop coalescence time g = acceleration due to gravity, m s−2 k = Boltzmann constant, J K−1 k̂ = rate constant for coalescence, m−2 s−1 NA = Avogadro’s number, mol−1 R = gas constant, J mol−1 K−1 t = time, s T = temperature, K tc = coalescence time, s z = ion valence

Greek Letters

γ = interfacial tension in the presence of surfactant, N m−1 Γ = concentration of surfactant at the interface, mol m−2 γ0 = interfacial tension in the absence of surfactant, N m−1 δ = separation between two droplets, m Δρ = density difference between the two liquids, kg m−3 ε = dielectric constant of the aqueous phase ε0 = permittivity of free space, C2 J−1 m−1 ζ = zeta potential, V κ = Debye−Hückel parameter, m−1 μ = viscosity of the aqueous phase, Pa s σ = charge density at the interface, C m−2 ϕ = chemical potential, J mol−1 Φnet = net interaction energy of two droplets in the aqueous phase, J ω = frequency of rupture of the film per unit surface area, m−2 s−1

Abbreviations

CMC = critical micelle concentration CTAB = cetyltrimethylammonium bromide DLS = dynamic light scattering DLVO = Deryagin−Landau−Verwey−Overbeek 15815



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Stabilization of Silicone Oil-in-Water Emulsions by Ionic Surfactant and

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