Foaming in Micellar Solutions: Effects of Surfactant, Salt, and Oil

Article pubs.acs.org/IECR

Foaming in Micellar Solutions: Effects of Surfactant, Salt, and Oil Concentrations Manas Ranjan Behera, Shailesh Ravi Varade, and Pallab Ghosh* Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati−781039, India

Pintu Paul and Ajay Singh Negi Hindustan Unilever Limited Research Centre, 64, Main Road, Whitefield, Bangalore−560066, India S Supporting Information *

ABSTRACT: Foaming in products based on micellar solutions has considerable importance in various consumer applications, such as washing and cleaning. In this work, the effects of surfactant concentration, oil content, and salts containing mono-, di-, and trivalent counterions on foam formation and stability were studied. The foams were generated by employing the Blender Test. The presence of salts caused a significant reduction in foam volume. Effectiveness of the salts followed the sequence Al3+ > Ca2+ > Na+. However, the foam collapse rate was slower in the presence of salt. The rate of adsorption of surfactant molecules at the air−water interface was augmented by salt. Oil reduced the foam volume and its stability. The entering, spreading, and bridging coefficients were calculated. These coefficients qualitatively explained the stability of foam in the presence of oil. adsorption.8 This ion-specific effect has been attributed to the difference in the hydrated radius of the counterions (which leads to the difference in the area occupied by the ions in the Stern layer), the effect of the counterions on the structure of water, and the London dispersion force.9 The presence of salt significantly affects the initial volume of foam and its subsequent stability.10−14 The stability of the thin foam film is one of the most important parameters in foam stability. This film is flanked by surfactant monolayers on both sides, as shown in Figure 1. Gravity, surface tension, and viscous forces play important roles in the drainage of foams.15−17 Electrostatic double layer (EDL), hydration, and van der Waals forces determine the stability of thin foam films.18 In addition to these forces, the initial volume of foam depends on the diffusional transport of surfactant molecules from the bulk solution to the air−water interface. An increase in

1. INTRODUCTION Foams of aqueous solutions of amphiphilic compounds are ubiquitous in a wide variety of applications in environment and meteorology, foods, geology, agriculture, materials science, biology, medicine, petroleum production, mineral processing, and home and personal care products.1 Generation of foam during consumer usage can be either desired or undesired depending on the application. For example, in a front-loader fabric washing machine or machine dish-wash, foam is undesirable. Antifoam materials are often added to the product so that they may effectively reduce the foam during washing. On the other hand, adequate foam generation during shampooing hair is a desirable proposition inasmuch as foam generation as well as the quality of foam dictate a sensory feel (e.g., superior and rich texture of foam provides a luxurious feel). In applications such as fabric hand-wash, generation of foam during the dissolution of detergent in water and soaking has a strong positive influence on the consumer. On the other hand, the extent of foam should be less (or no foam at all) during the rinse stage. The optimal level of foam in a mouthwash product should provide a good feeling in the mouth and yet be capable of delivering the antibacterial product on teeth and gum. Inorganic salts are either naturally present or added in many applications of foams. Salt influences the adsorption of surfactant molecules at the air−water interface and consequently alters the charge at the interface.2−7 Consequently, the formation and stability of foam are strongly affected by the presence of salt. The ions of different valence affect the adsorption of surfactant to different extents due to their varied effect on the screening of electrostatic charge. The binding of counterions can drastically reduce the potential at the air−water interface.7 Even salts having ions of the same valence (e.g., LiCl, NaCl, and CsCl) can lead to significant differences in surfactant © 2014 American Chemical Society

Figure 1. Foam film stabilized by anionic surfactant. The thickness of the film (h) is exaggerated for illustration. Received: Revised: Accepted: Published: 18497

September 10, 2014 November 5, 2014 November 10, 2014 November 10, 2014 dx.doi.org/10.1021/ie503591v | Ind. Eng. Chem. Res. 2014, 53, 18497−18507

Industrial & Engineering Chemistry Research

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(India). These chemicals were used as received from their manufacturers. The water used in this study was purified from a Millipore water purification system. Its conductivity was 1 × 10−5 S m−1, and its surface tension was 72.5 mN m−1. 2.2. Preparation of Prototypes and Their Solutions. The prototypes of micellar solutions were prepared as follows. The required amount of water was taken in a beaker. DOBS was added to it, and the mixture was mixed with an overhead stirrer [manufacturer: Remi Elektrotechnik (India), model: RQ122] at 300 rpm until a homogeneous solution was formed. Then SLES and methyl laurate were added to this solution and mixed at the same speed until the liquid became transparent. This process took ∼1 h. These prototypes were prepared 10 d before their use. The prototypes were diluted such that the total surfactant concentrations in the solutions were 0.5, 1, and 1.5 g dm−3. These solutions were prepared 1 d before their use. They were carefully homogenized for 6 h by slow stirring at 100 rpm in a magnetic stirrer [manufacturer: IKA (Germany), model: color squid]. These solutions were visually clear. However, some turbidity appeared in the presence of CaCl2 and AlCl3 due to precipitation of surfactant. 2.3. Foam Test. The Blender Test was employed to prepare the foams and measure their stability. The procedure adopted in our experiments was similar to that described in the ASTM Standard D3519-88. 200 cm3 of the dispersion was placed in a blender [manufacturer: Morphy Richards (India), model: Divo Essentials], and the same was stirred for 30 s at 15000 rpm. The foam thus generated was immediately transferred to a glass cell of 1000 cm3 capacity. The glass cell had flat walls. The initial volume of foam was measured. The decrease in foam volume with time was recorded for 1 h. Each experiment was repeated five times, and the mean values (with error bars) are reported in section 3. The foams had spherical bubbles (i.e., kugelschaum) in the initial stage. After some time, the foams became polyhedral in shape. Size of the foam bubbles was determined by following the procedure reported in the literature.28 Photographs of the foam were taken by a digital camera [manufacturer: Nikon (India), model: D5100]. The radius of the bubbles was measured from these photos by using the Digimizer software [manufacturer: MedCalc Software (Belgium)]. About 200 bubbles were analyzed for each system, and the size distributions were developed. 2.4. Measurement of Particle Size. The systems studied in this work were composed of self-assembled surfactant aggregates in the form of micelles and oil-swollen micelles. The diameter of these particles was measured by DLS in a particle size analyzer [manufacturer: Beckman Coulter (Switzerland), model: Delsa Nano C]. 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, from which the size distribution was obtained by fitting a multiple exponential to it by a nonnegative least-squares method. The hydrodynamic diameter was calculated by the Stokes−Einstein equation.

surfactant concentration and addition of salt both can affect the diffusion of surfactant and, hence, the volume of foam. The elasticity of the air−water interface, in some cases, can also be an important factor in foam stability.19,20 Foams generated from oil-in-water emulsions, microemulsions, and micellar solutions are more complex than foams which do not contain oil. The presence of oil dramatically affects its stability. Spreading of oil at the air−water interface is an important factor in the stability of the foam.21 One of the widely accepted theories of the role of oil on foam stability states that the oil droplets enter and spread at the air−water interface of the foam film and rupture it. Entering and spreading coeff icients are used to determine whether the oil enters the air−water surface of the film and spreads to form a weak spot in the film, thereby causing foam rupture.22 These coefficients are determined from the surface tensions of the oil and the aqueous phases, and the interfacial tension between these two phases. It has been reported23,24 that oil can stabilize as well as destabilize foam. The size of the oil droplets can also be an important parameter in foam stability.25 The properties of oil affect the stability of foam as well.26,27 The role of oil on foam stability has remained a subject of active research. Very few works have systematically studied the foaming characteristics of micellar solutions in the presence of salts that contain mono-, di-, and trivalent counterions (e.g., Na+, Ca2+, and Al3+). The main objective of this work was to study this aspect. In addition, the effects of surfactant and oil concentrations in the micellar solutions on foam formation and stability were also studied. The Blender Test was employed to study foams. The characteristics of the micellar solutions were analyzed by dynamic light scattering (DLS) and zeta potential. The dynamic and equilibrium surface tensions and the interfacial tension between the aqueous and oil phases were measured. The spreading, entering, and bridging coefficients were calculated, and they were correlated with foam stability.

2. MATERIALS AND METHODS 2.1. Materials. Three prototypes containing micellar solutions of surfactants were used in this work. Two of these prototypes contained oil. Sodium dodecyl benzenesulfonate (DOBS) and sodium lauryl ether sulfate (SLES) were used as the surfactants. Methyl laurate was used as the oil. The compositions of these three prototypes are given in Table 1. Table 1. Compositions of the Prototypesa prototype no.

DOBS (%)

SLES (%)

water (%)

methyl laurate (%)

P1 P2 P3

7.35 7.35 7.35

13.65 13.65 13.65

79.0 78.0 77.0

0.0 1.0 2.0

a

All the compositions are in weight percentages (on 100% basis of the materials).

DOBS [CH3(CH2)11C6H4SO3Na, 100% assay] was purchased from Sigma-Aldrich (Bangalore, India). SLES [C12H25(OCH2CH2)2.5OSO3Na, 70% assay] was purchased from Galaxy Surfactants (Mumbai, India). The other compounds present in SLES were 0.3% sodium chloride, 3% sodium sulfate, and 4% unsulfated materials, and the rest was water. Methyl laurate [CH3(CH2)10CO2CH3, 98% assay] was purchased from KLK Oleo (Selangor, Malaysia). Sodium chloride (99% assay), calcium chloride (98% assay), and aluminum chloride (98% assay) were purchased from Merck

d=

kT 3πμD

(1)

The size distributions of the particles in the three prototypes are given in Figure S1 (Supporting Information). 2.5. Measurement of Zeta Potential. The zeta potential at the air−water interface was measured by a zeta 18498

dx.doi.org/10.1021/ie503591v | Ind. Eng. Chem. Res. 2014, 53, 18497−18507


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three prototypes were anionic in nature. The critical micelle concentration (CMC) of DOBS is ∼0.55 g dm−3,31 and the same for SLES is ∼0.3 g dm−3.32 The variation of equilibrium surface tension with total surfactant concentration is shown in Figure S2 (Supporting Information). It appears from these surface tension profiles that the aqueous solutions used for the foaming studies contained mixed micelles. However, micelles were not detected by DLS in some of these solutions, presumably because the DLS signals were too weak for the analysis of size distribution.33 In some samples, micelles were detected after increasing the pinhole size to 100 μm. The default value of the pinhole in the DLS equipment was 50 μm. When the intensity was below 3000 cps (due to the presence of a small number of particles of very small size), the pinhole size had to be increased. The effect of surfactant concentration on the foams generated from the samples of Prototype 1 is shown in Figure 2. The volume of foam slowly decreased with time. It has been reported27 that foams created by sparging gas through a porous frit into an aqueous surfactant solution showed an initial steep reduction in volume, which was followed by a very slow decay. However, the foams formed by the Blender Test did not show such a rapid collapse in the initial stage. It is evident from Figure 2 that the initial foam volume significantly increased with increasing surfactant concentration. This is expected because the increase in surfactant concentration increased the concentration of surfactant molecules at the air−water interface, which stabilized the foam film. In the presence of anionic surfactants, the air−water interface is negatively charged. The EDL developed at the film surfaces (see Figure 1) stabilized the foam film. A high concentration of the surfactant in the bulk solution may increase the rate of transport of the surfactant molecules toward the interface, as predicted by the Ward−Tordai equation.34

potentiometer [manufacturer: Beckman Coulter (Switzerland), model: Delsa Nano C] by following the procedure described in the literature.29 The electrophoretic mobility of micronanobubbles (which were generated in the aqueous solutions by ultrasonication) was measured, and the zeta potential was calculated from their mobility. All experiments were repeated five times, and the average values of these readings are reported. The water was free from any colloidal impurity, which was ascertained by DLS. The DLS studies of pure water clearly showed the absence of any material which would scatter the HeNe laser. 2.6. Measurement of Equilibrium Surface and Interfacial Tension. The surface tension of the samples was measured by a digital tensiometer [manufacturer: Krüss (Germany), model: K9] using a Wilhelmy plate made of an alloy of platinum and iridium. The sample vessels and the plate were methodically cleaned before each measurement. The plate was burned to red hot conditions in the blue flame of a Bunsen burner. It was dipped inside the aqueous phase up to the required depth. The air−water interface was allowed 1 h to reach equilibrium for surfactant adsorption. Thereafter, the surface tension was measured by slowly pulling the plate out (∼0.5 mm s−1) through the interface. The interfacial tension was measured by the same tensiometer. However, a du Noüy ring made of platinum and iridium was used. The procedure described in the ASTM Standard D1331-11 was followed. The ring was dipped inside the aqueous phase up to the required depth. The air−water interface was allowed 1 h to reach equilibrium for surfactant adsorption. Then the oil was poured over the aqueous phase very slowly and carefully along the wall of the sample vessel. The interfacial tension was measured by slowly pulling the ring out (∼0.5 mm s−1) through the oil−water interface. The measurements were repeated five times for each sample. The values of surface and interfacial tension measured by this procedure were highly accurate and reproducible. 2.7. Measurement of Dynamic Surface Tension (DST). A maximum bubble pressure tensiometer [manufacturer: Krüss (Germany), model: BP 100] was used to measure the DST. Gas bubbles were produced in the aqueous solution by means of a capillary, which was connected to a pressure sensor. The surface tension of the solution was calculated from the maximum bubble pressure. The principle of measurement of DST by the maximum bubble pressure method is available in the literature.30 The time from the beginning of the formation of the bubble to the time of detection of the maximum pressure is termed age of the surface. The bubble pressure tensiometer measured the DST as a function of surface age. 2.8. Measurement of Viscosity. The viscosity of the solutions was measured by a rheometer [manufacturer: Anton Paar (Austria); model: Physica 301] using the parallel plate configuration. Table S1 (Supporting Information) presents the viscosity of the solutions prepared from Prototype 1. The results were similar for the other two prototypes. 2.9. Other Experimental Details. All the experiments were conducted in an air-conditioned laboratory where the temperature was maintained at 298 ± 1 K. The prototypes and their solutions were stored in the same room under identical conditions.

⎛ Dt ⎞1/2 ⎛ D ⎞1/2 Γ(t ) = 2cs0⎜ s ⎟ − ⎜ s ⎟ ⎝π ⎠ ⎝π⎠

∫0

t

cs(0, τ ) (t − τ )1/2

dτ (2)

Some works have indeed observed a faster adsorption with increasing surfactant concentration.35,36 To verify this, DST of the aqueous solutions was measured at different surfactant concentrations, as shown in Figure 3a. The rate of surfactant adsorption (as indicated by the slope of these profiles) did not significantly increase with increasing surfactant concentration. Nonetheless, the DST decreased with increasing surfactant concentration. The amount of foam collapsed (or retained) during a certain observation period is a measure of its stability. It is observed from Figure 2 that foams suddenly collapsed after some time in some of the samples. In Prototype 1 samples, this phenomenon was observed in the absence of salt. In general, the rate of foam collapse increased with increasing surfactant concentration. This is likely due to the decrease in the Gibbs elasticity of the foam film with increasing surfactant concentration. The Gibbs elasticity (EG) is defined as EG = 2A

dγ dA

(3)

Using the Langmuir and Gibbs adsorption equations, the Gibbs elasticity is given by20

3. RESULTS AND DISCUSSION 3.1. Effects of Surfactant and Salt Concentrations on Foam. The surfactants (i.e., DOBS and SLES) used in the

EG = 18499

2 2 4RT Γ∞ KLcs

h(1 + KLcs)2 + 2Γ∞KL

(4)



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Figure 3. Dynamic surface tension: (a) effect of surfactant concentration (in absence of salt), and (b) effect of NaCl at 0.5 g dm−3 total surfactant concentration.

from a higher surfactant concentration are expected to exhibit greater stability. Such stability was, however, not observed in the systems studied in this work. The initial foam volume decreased by the addition of salt. The effect of NaCl is depicted in Figure 2 at three surfactant concentrations. The rate of surfactant adsorption at the interface increased in the presence of salt, which is evident from the slope of the DST curves shown in Figure 3b. Similar results were observed at the other two surfactant concentrations (see Figure S3, Supporting Information). The increase in slope in the surface tension profiles with increasing salt concentration reflects screening of electrostatic repulsion between the surfactant molecules, which favors their adsorption.35 Therefore, the decrease in foam volume indicates that the foams underwent rapid collapse immediately after formation. The repulsive force due to EDL between two charged spherical colloid particles at a separation δ is given by41 ⎛ zeψ ⎞ FEDL ≈ 64πR sRTcκ −1 tanh2⎜ 0 ⎟ exp( −κδ) ⎝ 4kT ⎠ (5)

Figure 2. Variation of foam volume with time at different NaCl concentrations for Prototype 1. Surfactant concentration: (a) 0.5 g dm−3, (b) 1 g dm−3, and (c) 1.5 g dm−3.

A film having high elasticity has more stability.37 The film elasticity decreases with increasing surfactant concentration (cs),38 which leads to the rapid collapse of the foam. A similar observation has been reported for alcohol-based frothers.20 Nikolov et al.39,40 have reported stratification in thinning foam films formed from the micellar solutions of nonionic and anionic surfactants. The stratification of thin films has been explained by a layer-by-layer thinning of ordered structures of micelles formed inside the film. As per these works, the stability of the foam film increases with increasing concentration of the micelles because more layers are present in the aqueous film at the higher surfactant concentrations. Therefore, foams made

where the Debye−Hückel parameter, κ, is defined as ⎡ N e2 κ=⎢ A ⎢⎣ εε0kT

∑ i

⎤1/2

zi2ci ⎥ ⎥⎦

(6)

With increase in salt concentration, the Debye length, κ−1, decreases, which leads to the reduction in electrostatic repulsion between the headgroups. As a result, more surfactant molecules adsorb at the air−water interface and the surface tension decreases. The surface tension of an aqueous surfactant system is related to the surface excess concentration of 18500



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concentration of the salt, micellar structure inside the foam film is no longer possible and the stability of the film decreases. The decrease in initial foam volume with increasing salt concentration occurs due to the rapid coalescence of the foam bubbles by the rupture of the thin aqueous films as a result of reduction in EDL repulsion. However, at low surfactant concentration (i.e., 0.5 g dm−3), the foam volume slightly increased by the addition of a small amount of salt (i.e., 10 mol m−3 NaCl concentration). This shows that the enhancement in surfactant adsorption in the presence of salt favored formation of a large amount of foam. The viscosity of the aqueous solutions did not significantly vary with variation in the salt concentration. Therefore, it is apparent that the EDL played the most significant role in the stability of these foams. The rate of collapse of foams was slow in the presence of salt. At a given salt concentration, the foam collapse rate increased with increasing surfactant concentration. However, at the high salt concentration (i.e., 50 mol m−3), the foam collapse rate was small, and it did not significantly vary with the surfactant concentration. The variation of foam volume with time for the CaCl2 system is shown in Figure 4. CaCl2 was much more effective in reducing the foam volume as compared to NaCl. However, the former was more effective in reducing the surface tension of the solutions (Figure S2, Supporting Information). Therefore, it is apparent that a lower surface tension of the aqueous solution may not ensure a more stable foam. The zeta potential at the air−water interface was considerably reduced in the presence of CaCl2 (Table S4, Supporting Information). Therefore, it may be concluded that the stabilizing effect caused by the enhancement in the adsorption of surfactant molecules at the air−water interface was largely offset by the reduction in the electric potential at the air−water interface, which led to the destabilization of the foam film. Similar results were observed in the presence of AlCl3 (Figure 5). A small amount of the salt was sufficient for the reduction in foam volume. At 0.5 g dm−3 surfactant concentration and 0.5 mol m−3 AlCl3, the foam volume was less than 300 cm3, which is lower than any of the NaCl or CaCl2 systems. However, the effect of AlCl3 was less pronounced at the higher surfactant concentrations (viz. 1 and 1.5 g dm−3) at such a low salt concentration. The foam film may remain stable due to two factors. The film may be thermodynamically stable due to electrostatic repulsion. The thickness of the film in this case will be greater than the critical film thickness. In the presence of salt, the film thickness may be below the critical thickness, and yet the foam may be stable by forming the Newton black film.38,45 Therefore, rather than collapsing in the presence of salt as predicted by the Deryagin− Landau−Verwey−Overbeek theory, a transition to the stable form may occur. Counterion binding may play an important role in the formation of the stable foam film. The size of the bubbles in the freshly formed foams is also influenced by the concentrations of surfactant and salt. The bubble size increased with increasing surfactant concentration (Table S6, Supporting Information). A reverse trend was observed with increasing salt concentration (Figure 6). The size of foam bubbles reflects the effect of mixing during the preparation of the foam in the blender as well as the initial stability of the foam. The principles of mixing suggest that the size of the bubbles would decrease with decreasing surface tension at a given speed of agitation.46 This prediction has been observed with surfactant solutions as well.28,47 The reduction in surface tension with increasing surfactant concentration was rather small (see Figure S2, Supporting Information). There-

surfactant (Γ), the surface potential (ψ0), and the surface charge density (σ) as42 γ = γ0 −

∫ Γdφ − ∫ σdψ0

(7)

The reduction of surface tension due to increase in salt concentration is shown in Figure S2 (Supporting Information). The surface tension reduced to lower values in the presence of CaCl2 and AlCl3 (as compared to the NaCl systems) as a consequence of the greater reduction in electrostatic repulsion between the surfactant headgroups. Addition of salt had a significant effect on the charge at the air−water interface. The surface potential is directly related to the charge density at the surface, as given by the Grahame equation (for a z:z electrolyte).43 ζ ≈ ψ0 =

⎡ ⎤ ⎛ 2kT ⎞ σ −1 ⎜ ⎟ sinh ⎢ ⎥ 1/2 ⎝ ze ⎠ ⎢⎣ (8RTεε0c) ⎥⎦

(8)

The surface potential, ψ0, may be approximated by the zeta potential, ζ. Equation 8 assumes that the Stern layer is absent. With increasing surfactant concentration, the charge density at the air−water interface (σ) increased, which increased zeta potential. With the addition of a small amount of NaCl (i.e., 10 mol m−3 concentration), the zeta potential increased due to the enhanced adsorption of the surfactant molecules in the presence of salt. However, at the higher salt concentration (i.e., 50 mol m−3 concentration), the zeta potential decreased due to the increase in concentration of ions near the interface, as predicted by eq 8. The zeta potential data are given in Tables S2−S5 (Supporting Information). In the presence of CaCl2 and AlCl3, the zeta potential considerably decreased due to the binding of Ca2+ and Al3+ ions on the negatively charged surfactant headgroups.7 The stability of a thin foam film depends on EDL, van der Waals, and short-range repulsive forces. The total disjoining pressure in a flat foam film is given by18 ⎛ zeψ ⎞ Π = ΠEDL + Π vdW + ΠSr = 64RTc tanh2⎜ 0 ⎟ ⎝ 4kT ⎠ AH × exp( −κh) − + C1 exp( −C2h) 6πh3

(9)

When the foam film becomes thin by hydrodynamic drainage, the double layers on each of the surfaces approach each other and begin to overlap. The counterions become confined to a narrow space due to the approach of the surfaces. This entropically unfavorable situation originates the EDL repulsion in the film between the two surfaces. The presence of salt ions reduces the Debye length (eq 6), which leads to a decrease in the electrostatic repulsion. In addition, binding of positively charged counterions (e.g., Ca2+ and Al3+) on the surfactant headgroups may significantly reduce ψ0.7 This would lead to a large reduction in the EDL repulsion. The van der Waals attractive force is rather insensitive to the addition of salt.41 The hydration force can strongly stabilize foam films even at high salt concentrations.44 Therefore, the foam film’s rupture mainly depends on the two opposite phenomena caused by the salt (i.e., increase in surfactant adsorption at the air−water interface but reduction in the EDL repulsion in the film). Nikolov and Wasan40 have reported that stratification in foam films is reduced in the presence of salt as a result of the reduction of the width of the EDL around the micelles. At a sufficiently high 18501



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Figure 4. Variation of foam volume with time at different CaCl2 concentrations for Prototype 1. Surfactant concentration: (a) 0.5 g dm−3, (b) 1 g dm−3, and (c) 1.5 g dm−3.

Figure 5. Variation of foam volume with time at different AlCl3 concentrations for Prototype 1. Surfactant concentration: (a) 0.5 g dm−3, (b) 1 g dm−3, and (c) 1.5 g dm−3.

fore, the effect of surface tension on bubble size was small in this case. An increase in bubble size with increasing surfactant concentration is probably due to the rapid coalescence of bubbles inside the foam. This resulted in the rapid collapse of foam, which has been discussed earlier in this section. On the other hand, the bubble size decreased with increasing salt concentration. The effect of surface tension was evident here, because the reduction of surface tension in the presence of salt was more significant. Therefore, the bubbles in the freshly generated foams were smaller in the presence of salt. The rate of film drainage and bubble size are related. The foam drainage 35 time decreases with increasing bubble size (τf ∝ R−2 b ). Therefore, smaller bubbles are expected to slow down the foam drainage and increase the stability of the foam. The mean values of the bubble size distributions are summarized in Tables S6 and S7 (Supporting Information). The size of the bubbles was smaller in the presence of CaCl2 and AlCl3 due to the lower surface tensions encountered in these systems (see Tables S8 and S9, Supporting Information).

3.2. Effect of Oil on Foam. The oil was solubilized by the self-assembled aggregates of the surfactants. A typical size distribution of these particles is shown in Figure 7. The size of the particles in the samples was larger than that in the original prototypes (Figure S1, Supporting Information), and three size zones were usually observed. The effect of oil (i.e., methyl laurate) on foam is depicted in Figure 8 for the Prototype 2 samples. The initial foam volume decreased in the presence of oil. These results corroborate the findings reported in the literature.25 In several samples (especially those prepared from Prototype 2), the foam suddenly collapsed to a large extent after a certain time. Similar phenomena were observed in the samples prepared from Prototype 3, which are shown in Figure 9. Like the oil-free systems discussed in section 3.1, the initial foam volume increased with increasing surfactant concentration at a given concentration of NaCl and oil. In addition, the initial foam volume decreased with increasing NaCl concentration at a given surfactant and oil concentration, with few exceptions. 18502



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Figure 7. Particle size distribution in Prototype 3 in the presence of 1 g dm−3 surfactant and 50 mol m−3 NaCl. Z-average diameter = 696.1 nm.

black film.38 The foam volume profiles in some of the systems were highly nonlinear. Therefore, the time of observation is important for measuring the foam stability. A short observation period may lead to a different conclusion in such systems. From the thermodynamic equilibrium of bulk foams, two parameters, termed spreading and entering coef f icients, are used to predict the stability of foams in the presence of oil. The entering coefficient (E) is defined as26,48 E = γw + γo / w − γo

(10)

The spreading coefficient (S) is defined as S = γw − γo / w − γo

(11)

If E > 0, the oil enters the air−water interface of the film. If S > 0, the oil spreads as a duplex film on each side of the original film, which forms a weak spot on the film, leading to its rupture. Garrett48,49 developed a model dealing with film destabilization by an oil bridge. He defined a bridging coefficient (B) given by B = γw2 + γo2/ w − γo2

(12)

Positive values of the bridging coefficient necessarily mean positive entry coefficient, although the reverse is not true. Positive values of B correspond to unstable bridges and rupture of the foam film. The stability of the foam decreases with the increasing values of E, S, and B. The values of E, S, and B are given in Table 2 for the NaCl system and clearly support the observation that the oil destabilized the foam under the given conditions. The rate of collapse of the foams decreased with increasing NaCl concentration, which is corroborated by the decrease in the value of these coefficients with increasing salt concentration. In CaCl2 and AlCl3 systems, oil reduced the foam volume more than the NaCl systems. These results are given in Figures S4−S7 (Supporting Information). The effectiveness of the counterions in reducing the foam stability followed the sequence Al3+ > Ca2+ > Na+. Very low foam volumes were observed in the presence of oil and AlCl3. However, these lowvolume foams were quite stable. The entering, spreading, and bridging coefficients were negative in the systems which contained high amounts of salt, as a result of the low values

Figure 6. Effect of salt on bubble size distributions for Prototype 1 samples: (a) no NaCl, (b) 10 mol m−3 NaCl, and (c) 50 mol m−3 NaCl concentrations. Surfactant concentration = 1.5 g dm−3.

A comparison of the samples prepared from Prototypes 1 and 2 shows that the stability of foam decreased in the presence of oil. However, a further increase in the concentration of oil in the prototype did not result in a lower stability of the foams. In most of the Prototype 3 samples, the rate of foam collapse was slower. For both Prototypes 2 and 3, at a given salt concentration, the foam collapse rate increased with increasing surfactant concentration. In Prototype 2 samples, the rate of foam collapse was high at 10 mol m−3 NaCl concentration. However, at 50 mol m−3 NaCl concentration, the foam collapse rate was small, and it varied little with the surfactant concentration. This is likely due to the formation of Newton 18503



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of surface and interfacial tension in these systems. Therefore, the values of E, S, and B (Tables S10 and S11, Supporting Information) qualitatively corroborate the stability of foams in the presence of calcium and aluminum chloride. The bubble size increased in the presence of oil in the absence of salt, and in the presence of NaCl (Tables S6 and S7, Supporting Information). This is due to the loss of stability of foam in the presence of oil. However, the bubble size was smaller in most of the Prototype 3 samples as compared to the Prototype 2 samples. The bubble size was significantly smaller in the CaCl2 and AlCl3 systems (Tables S8 and S9, Supporting Information), presumable due to their lower surface tension.

4. CONCLUSIONS The following conclusions were reached based on the experimental results and their analysis. The initial foam volume increased with increasing surfactant concentration. However, it decreased with increasing salt concentration. The EDL repulsion plays an important role in these phenomena. AlCl3 was more effective than CaCl2, and the latter was more effective than NaCl in reducing the foam volume. The same trend was observed in the reduction of zeta potential and surface tension. The rate of surfactant adsorption at the air−water interface did not significantly increase with increasing surfactant concentration. However, it increased with increasing salt concentration 18504



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absence of salt (Table S6); bubble size in foams in the presence of NaCl (Table S7); bubble size in foams in the presence of CaCl2 (Table S8); bubble size in foams in the presence of AlCl3 (Table S9); entering, spreading, and bridging coefficients in the presence of CaCl2 (Table S10); entering, spreading, and bridging coefficients in the presence of AlCl3 (Table S11). This material is available free of charge via the Internet at http:// pubs.acs.org.

Table 2. Entering, Spreading, and Bridging Coefficients without Salt and in the Presence of NaCl surfactant conc (g dm−3)

salt conc (mol m−3)

E (mN m−1)

S (mN m−1)

B (mN2 m−2)

0.5

0 10 50 0 10 50 0 10 50

11.6 8.9 5.6 10.5 7.8 4.9 9.4 6.1 4.6

1.2 1.7 1.0 1.5 1.6 0.3 0.8 0.1 0.2

429.0 340.0 202.3 394.7 296.8 158.7 332.1 193.5 146.0

1.0

1.5

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +91.361.2582253. Fax: +91.361.2690762. Notes

The authors declare no competing financial interest.

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at a given concentration of surfactant. The rate of collapse of foam decreased by increasing the salt concentration. However, the foam collapse rate increased with increasing surfactant concentration. The size of the bubbles in the freshly prepared foam increased with increasing surfactant concentration. However, it decreased with increasing salt concentration. The effectiveness of salts in reducing the bubble size followed the same sequence as mentioned above. The presence of methyl laurate decreased the foam volume. The oil was solubilized by the surfactant micelles. The foam stability decreased when the oil concentration was 1%. The size of the foam bubbles increased in the presence of oil. However, an increase in oil concentration to 2% did not have any significant effect on foam stability and bubble size. In many samples, the bubble size decreased and the foam stability increased at the higher oil concentration.

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ACKNOWLEDGMENTS The authors thank M/S Unilever Industries Private Limited (India) for financial support of the work reported in this article through Project No. Agr 161 BL 2012 (dated: 12 June 2012).

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ASSOCIATED CONTENT

S Supporting Information *

Particle size distributions in the three prototypes (without dilution): (i) Prototype 1 (Z-average diameter = 7.9 nm), (ii) Prototype 2 (Z-average diameter = 8.7 nm), and (iii) Prototype 3 (Z-average diameter = 8.3 nm) (Figure S1); variation of surface tension with total surfactant concentration in the absence of salt and in the presence of (a) NaCl, (b) CaCl2, and (c) AlCl3 (Figure S2); dynamic surface tension in the presence of NaCl: (a) 1 g dm−3 and (b) 1.5 g dm−3 total surfactant concentrations (Figure S3); variation of foam volume with time at different CaCl2 concentrations for Prototype 2. Surfactant concentration: (a) 0.5 g dm−3, (b) 1 g dm−3, and (c) 1.5 g dm−3 (Figure S4); variation of foam volume with time at different CaCl2 concentrations for Prototype 3. Surfactant concentration: (a) 0.5 g dm−3, (b) 1 g dm−3, and (c) 1.5 g dm−3 (Figure S5); variation of foam volume with time at different AlCl3 concentrations for Prototype 2. Surfactant concentration: (a) 0.5 g dm−3, (b) 1 g dm−3, and (c) 1.5 g dm−3 (Figure S6); variation of foam volume with time at different AlCl3 concentrations for Prototype 3. Surfactant concentration: (a) 0.5 g dm−3, (b) 1 g dm−3, and (c) 1.5 g dm−3 (Figure S7); viscosity of the aqueous solutions of Prototype 1 in the presence of NaCl, CaCl2, and AlCl3 (Table S1); zeta potential at the air−water interface for Prototype 1 samples in the absence of salt (Table S2); zeta potential at the air−water interface for Prototype 1 samples in the presence of NaCl (Table S3); zeta potential at the air−water interface for Prototype 1 samples in the presence of CaCl2 (Table S4); zeta potential at the air−water interface for Prototype 1 samples in the presence of AlCl3 (Table S5); bubble size in foams in the

NOMENCLATURE A = film surface area, m2 AH = Hamaker constant, J B = bridging coefficient, N2 m−2 c = concentration of salt in the solution, mol m−3 cs = concentration of surfactant in the bulk solution, mol m−3 c0s = initial bulk concentration of surfactant, mol m−3 C1 = constant in eq 9, Pa C2 = constant in eq 9, m−1 d = particle diameter, m D = diffusion coefficient of particle, m2 s−1 Ds = diffusion coefficient of surfactant, m2 s−1 e = electronic charge, C E = entering coefficient, N m−1 EG = Gibbs film elasticity, N m−1 F = force, N h = thickness of foam film, m k = Boltzmann’s constant, J K−1 KL = equilibrium constant in the Langmuir adsorption equation, m3 mol−1 NA = Avogadro’s number, mol−1 R = gas constant, J mol−1 K−1 Rb = radius of bubble, m Rs = radius of surfactant headgroup, m t = time, s S = spreading coefficient, N m−1 T = temperature, K z = valence of ion

Greek Letters

γ = surface tension in the presence of surfactant, N m−1 γ0 = surface tension of pure water, N m−1 γo = surface tension of oil, N m−1 γw = surface tension of the aqueous phase, N m−1 γo/w = interfacial tension between oil and the aqueous phase, N m−1 Γ = surface excess concentration of surfactant, mol m−2 Γ∞ = concentration of surfactant at the air−water interface when the interface is fully covered, mol m−2 δ = distance between two spherical surfactant headgroups, m ε = dielectric constant of the aqueous phase ε0 = permittivity of free space, C2 J−1 m−1

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ζ = zeta potential, V κ = Debye−Hückel parameter, m−1 μ = viscosity of aqueous phase, Pa s Π = disjoining pressure, Pa σ = charge density at the air−water interface, C m−2 τ = dummy variable in eq 2, s τf = foam drainage time, s ϕ = chemical potential, J mol−1 ψ0 = potential at air−water interface, V

(16) Vandewalle, N.; Caps, H.; Delon, G.; Saint-Jalmes, A.; Rio, E.; Saulnier, L.; Adler, M.; Biance, A. L.; Pitois, O.; Addad, S. C.; Hohler, R.; Weaire, D.; Hutzler, S.; Langevin, D. Foam Stability in Microgravity. J. Phys. (Paris) 2011, 327, 012024. (17) Joye, J.-L.; Hirasaki, G. J.; Miller, C. A. Asymmetric Drainage in Foam Films. Langmuir 1994, 10, 3174. (18) Joye, J.-L.; Miller, C. A.; Hirasaki, G. J. Dimple Formation and Behavior during Axisymmetrical Foam Film Drainage. Langmuir 1992, 8, 3083. (19) Xu, Q.; Nakajima, M.; Ichikawa, S.; Nakamura, N.; Roy, P.; Okadome, H.; Shiina, T. Effects of Surfactant and Electrolyte Concentrations on Bubble Formation and Stabilization. J. Colloid Interface Sci. 2009, 332, 208. (20) Wang, L.; Yoon, R.-H. Effects of Surface Forces and Film Elasticity on Foam Stability. Int. J. Miner. Process. 2008, 85, 101. (21) Ross, S.; McBain, J. W. Inhibition of Foaming in Solvents Containing Known Foamers. Ind. Eng. Chem. 1944, 36, 570. (22) Robinson, J. V.; Woods, W. W. A Method of Selecting Foam Inhibitors. J. Soc. Chem. Ind. 1948, 67, 361. (23) Wasan, D. T.; Nikolov, A. D.; Lobo, L. A.; Koczo, K.; Edwards, D. A. Foams, Thin Films and Surface Rheological Properties. Prog. Surf. Sci. 1992, 39, 119. (24) Lee, J.; Nikolov, A.; Wasan, D. Surfactant Micelles Containing Solubilized Oil Decrease Foam Film Thickness Stability. J. Colloid Interface Sci. 2014, 415, 18. (25) Koczo, K.; Lobo, L. A.; Wasan, D. T. Effect of Oil on Foam Stability: Aqueous Foams Stabilized by Emulsions. J. Colloid Interface Sci. 1992, 150, 492. (26) Lee, J.; Nikolov, A.; Wasan, D. Stability of Aqueous Foams in the Presence of Oil: On the Importance of Dispersed vs Solubilized Oil. Ind. Eng. Chem. Res. 2013, 52, 66. (27) Simjoo, M.; Rezaei, T.; Andrianov, A.; Zitha, P. L. J. Foam Stability in the Presence of Oil: Effect of Surfactant Concentration and Oil Type. Colloids Surf., A 2013, 438, 148. (28) Sharma, M. K.; Shah, D. O.; Brigham, W. E. The Influence of Temperature on Surface and Microscopic Properties of Surfactant Solutions in Relation to Fluid Displacement Efficiency in Porous Media. AIChE J. 1985, 31, 222. (29) Srinivas, A.; Ghosh, P. Coalescence of Bubbles in Aqueous Alcohol Solutions. Ind. Eng. Chem. Res. 2012, 51, 795. (30) Fainerman, V. B.; Miller, R.; Joos, P. The Measurement of Dynamic Surface Tension by the Maximum Bubble Pressure Method. Colloid Polym. Sci. 1994, 272, 731. (31) Fachini, A.; Joekes, I. Interaction of Sodium Dodecylbenzenesulfonate with Chrysotile Fibers. Adsorption or Catalysis? Colloids Surf., A 2002, 201, 151. (32) Aoudia, M.; Al-Maamari, T.; Al-Salmi, F. Intramolecular and Intermolecular Ion−Dipole Interactions in Sodium Lauryl Ether Sulfates (SLES) Self-aggregation and Mixed Micellization with Triton X-100. Colloids Surf., A 2009, 335, 55. (33) Kim, H.-U.; Lim, K.-H. Sizes and Structures of Micelles of Cationic Octadecyl Trimethyl Ammonium Chloride and Anionic Ammonium Dodecyl Sulfate Surfactants in Aqueous Solutions. Bull. Korean Chem. Soc. 2004, 25, 382. (34) Chang, C.-H.; Franses, E. I. Adsorption Dynamics of Surfactants at the Air/Water Interface: A Critical Review of the Mathematical Models, Data, and Mechanisms. Colloids Surf., A 1995, 100, 1. (35) Marinova, K. G.; Basheva, E. S.; Nenova, B.; Temelska, M.; Mirarefi, A. Y.; Campbell, B.; Ivanov, I. B. Physico-chemical Factors Controlling the Foamability and Foam Stability of Milk Proteins: Sodium Caseinate and Whey Protein Concentrates. Food Hydrocolloids 2009, 23, 1864. (36) Dukhin, S. S.; Kretzschmar, G.; Miller, R. Dynamics of Adsorption at Liquid Interfaces; Elsevier: Amsterdam, 1995. (37) Xu, W.; Nikolov, A.; Wasan, D. T.; Gonsalves, A.; Borwankar, R. P. Foam Film Rheology and Thickness Stability of Foam-based Food Products. Colloids Surf., A 2003, 214, 13.

Abbreviations

CMC = critical micelle concentration DLS = dynamic light scattering DOBS = sodium dodecyl benzenesulfonate EDL = electrostatic double layer force P1 = prototype 1 P2 = prototype 2 P3 = prototype 3 SLES = sodium lauryl ether sulfate Sr = short-range hydration force vdW = van der Waals forces

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Foaming in Micellar Solutions: Effects of Surfactant, Salt, and Oil

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