ARTICLE pubs.acs.org/IECR
Coalescence of Bubbles in Aqueous Alcohol Solutions Ayanavilli Srinivas and Pallab Ghosh* Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India ABSTRACT: Coalescence of bubbles in alcohol solutions has widespread industrial applications. This work presents studies on coalescence in aqueous solutions of pentanol, methyl isobutyl carbinol, hexanol, heptanol, and octanol. The effects of the alcohol concentration, hydrocarbon chain length, adsorption at the airwater interface, electrolyte, and intermolecular and surface forces on bubble coalescence were investigated. The origin of stability of the thin aqueous films was studied. The zeta potential at the airwater interface was measured in the presence of these alcohols and a 1:1 electrolyte. The charge at the interface decreased in the presence of alcohol. It was found that the electrostatic double layer force does not have any significant effect in the stability of these thin films. The major stabilization comes from the hydration force. In aqueous heptanol and octanol solutions, the hydrophobic force plays a vital role so that the stabilizing effect of these alcohols is partly incapacitated. The coalescence time data were compared with the predictions of seven film-drainage models. The coalescence time distributions were fitted by the stochastic model. The model parameters were analyzed with the physical properties of the systems.
1. INTRODUCTION The coalescence of bubbles plays a vital role in the stability of gasliquid dispersions. A common example is foam, which contains bubbles of various size and shape that are separated by thin liquid films.1 The coalescence of bubbles is important in controlling the interfacial area in gasliquid contacting equipment,2 and in the generation of bubbles in microfluidic devices.3 The gasliquid dispersions are thermodynamically unstable, and show a tendency to destabilize. This instability is a result of the energy associated with the large interfacial area of the bubbles. This energy term outweighs the entropy of formation of the bubbles from the bulk constituents. Various surface-active agents (e.g., alcohols and surfactants) are used to produce and stabilize these dispersions. The adsorption of the surface active molecules at the airwater interface reduces the energy of the system, which provides stability to the dispersion. The surface-active compounds stabilize the thin aqueous films by various forces, such as electrostatic double-layer, steric, and hydration forces (depending on the nature of the compound).4,5 Some authors have suggested that the adsorbed surface-active compounds slow the hydrodynamic drainage of the films by the Plateau MarangoniGibbs effect.6 The presence of these compounds at the interface generates surface shear and dilatational viscosities. It has been reported in the literature7 that these surface viscosities have significant influence on the drainage of the thin liquid film. Therefore, the monolayers of surface active compounds act as barriers and prevent coalescence of bubbles, thereby providing kinetic stability to the gasliquid dispersion. The coalescence of bubbles and drops in the solutions of aliphatic alcohols has importance in varied applications such as mineral processing,8 enhanced oil recovery,9 and fermentation processes.10 In addition, alcohols are often used as cosurfactants in the preparation of microemulsions.11 Therefore, study of the coalescence in alcohol solutions has considerable industrial importance. Several works have reported coalescence of bubbles in aqueous solutions of aliphatic alcohols.10,1214 In these works, coalescence of two bubbles formed on the adjacent nozzles r 2011 American Chemical Society
(i.e., binary coalescence) was studied. The percentage of bubbles coalesced during the contact period was reported. The coalescences times reported in some of these studies13,14 were very small (viz, 98% purity. The properties of these alcohols are presented in Table 1.
3. RESULTS AND DISCUSSION 3.1. Adsorption of Alcohols at the AirWater Interface. The adsorption of alcohols at the airwater interface was studied by measuring the variation of surface tension with alcohol concentration. 796
dx.doi.org/10.1021/ie202148e |Ind. Eng. Chem. Res. 2012, 51, 795–806
Industrial & Engineering Chemistry Research
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The surface tension profile of heptanol is shown in Figure 1. The data reported in the literature29,30 are also shown in this figure, for comparison with our data. The surface tension profiles were fitted by the LangmuirSzyszkowski equation:19,31 γ ¼ γ0 RTΓ∞ lnð1 þ KL ca Þ
If the charge at the airwater interface is significant, the surface tension should be dependent on the surface potential and the surface charge density:32 Z γ ¼ γ0
ð1Þ
1 Γ ∞ NA
σ~ dψ
ð4Þ
R where the term σ ~ dψ is the free energy of the electrostatic double layer. The effect of electric charge, in the presence of alcohol, was investigated by adding a 1:1 electrolyte (viz, NaCl) to the aqueous solution. The addition of NaCl had a small effect on the surface tension of the alcohol solutions. This is depicted in Table 3 for the pentanol system. It has been suggested in the literature33 that the electrostatic double-layer interaction plays an important role, even in the aqueous systems of nonionic surface active compounds. These conclusions are mainly based on the reduction in the thickness of aqueous nonionic-surfactantstabilized films by the addition of electrolyte. However, the data obtained in the present work (Table 3), and the data reported in the literature,18,34 indicate that the effect of electrolyte on the adsorption of such compounds is small, even at the highelectrolyte concentrations. Interestingly, the effect of electrolyte on the adsorption of ionic surfactants is rather strong.27,35 For example, the surface tension of the aqueous solution of an ionic surfactant (e.g., sodium dodecyl sulfate (SDS)) can be reduced by 10 mN/m or more, when the NaCl concentration is increased from zero to 100 mol/m3. However, the reduction in surface tension for a nonionic surfactant solution is ∼1 mN/m (or less) for a similar increase in electrolyte concentration. The thickness of the diffuse part of the electrostatic double layer (i.e., the Debye screening length), k1, is related to the concentration of the ions present in the aqueous medium by the equation36 0 10:5 NA e2 ∑ zi 2 c∞ s, i i A k1 ¼ @ ð5Þ εε0 kT
The parameters KL and Γ∞, obtained by regression of our data, are presented in Table 2. From the value of Γ∞, the minimum surface area per adsorbed molecule (Am) was obtained from the equation31 Am ¼
Z Γ dμ~
ð2Þ
The value of Am is dependent upon the properties of the alcohol such as its structure and surface activity. The values of A m reported in Table 2 agree well with those reported in the literature.29 The surface tension profiles indicate that the structure of the alcohol molecules plays a very important role in adsorption. For a given concentration, the surface tension reduced as the number of carbon atoms in the hydrocarbon chain of the alcohol increased. In addition, branching in the hydrocarbon chain of MIBC had a significant effect on its adsorption, because the molecule occupied a larger area at the airwater interface. The surface excess concentration of alcohol can be calculated from the Langmuir adsorption equation, which is given by31 KL c a Γ ¼ Γ∞ ð3Þ 1 þ KL ca The variation of surface excess concentration (Γ) with alcohol concentration (ca) is shown in Figure 2. It is apparent from this profile that, at a given concentration of alcohol, the surface excess concentration increased as the number of carbon atoms in the molecule increased. A crossover of the profiles of pentanol and MIBC was observed at the high alcohol concentrations.
Figure 2. Variation of surface excess concentration with alcohol concentration.
Figure 1. Variation of surface tension of heptanol with its concentration.
Table 2. Parameters of the LangmuirSzyszkowski Equation alcohol
applicable concentration range (mol/m3)
KL (m3/mol)
Γ∞ (mol/m2) 106
Am (nm2)
Am (nm2) (literature)29 0.25
n-pentanol
0200
0.058
6.50
0.255
MIBC
0120
0.180
4.58
0.363
0.33
n-hexanol n-heptanol
030 08
0.170 0.471
7.46 8.52
0.223 0.195
0.22 0.20
n-octanol
02
1.550
8.76
0.190
0.18
797
dx.doi.org/10.1021/ie202148e |Ind. Eng. Chem. Res. 2012, 51, 795–806
Industrial & Engineering Chemistry Research
ARTICLE
Table 3. Predictions from the Film-Drainage Models for the Aqueous Pentanol Systema cs (mol/m3)
γ (mN/m)
1.0
100
1.0
300
1.0
ca (mol/m3)
tc,1 (s)
tc,2 (s)
tc,3 (s)
tc,4 (s)
tc,5 (s)
tc,6 (s)
tc,7 (s)
tExpt (s) c
71.6
86.7
57.1
2587.4
915.3
2.5
497.8
277.2
4.7
71.3
87.1
57.4
2603.8
921.0
2.5
500.7
278.9
2.9
71.0
87.6
57.7
2620.3
926.9
2.5
503.6
280.5
2.4
500
69.6
89.7
59.1
2699.8
955.0
2.6
517.6
288.3
1.9
68.4
91.6
60.3
2771.1
980.2
2.6
530.2
295.3
7.9
5.0
100
68.2
91.9
60.5
2783.3
984.5
2.6
532.3
296.5
2.9
5.0 5.0
300 500
67.9 67.2
92.4 93.5
60.9 61.6
2801.8 2845.7
991.1 1006.6
2.6 2.6
535.6 543.3
298.3 302.6
2.7 1.2
65.7
96.1
63.3
2943.7
1041.3
2.7
560.5
312.2
17.8
9.0
100
65.5
96.4
63.5
2957.2
1046.0
2.7
562.9
313.5
4.8
9.0
300
65.1
97.2
64.0
2984.5
1055.7
2.7
567.6
316.2
4.1
9.0
500
64.5
98.2
64.7
3026.2
1070.5
2.7
574.9
320.2
3.8
1.0
5.0
9.0
a
Physical properties of the systems: ΔF = 995.9 kg/m3, g = 9.8 m/s2, a = 1.1 mm, B = 1 1028 J m, and μ = 1 103 Pa s.
Table 4. Parameters of the Stochastic Model for the Aqueous Pentanol System ca (mol/m3)
cs (mol/m3)
1.0
SΓ
1330.5
6.6
0.32
100
1336.1
7.4
0.14
1.0
300
1341.7
8.1
0.11
1.0
500
1368.7
8.9
0.12
1392.7
5.5
0.33
5.0
100
1396.8
7.8
0.23
5.0
300
1403.0
8.1
0.25
5.0
500
1417.6
11.4
0.20
9.0
The Debye length at an NaCl concentration of [NaCl] = 100 mol/m3 is ∼1 nm, and at [NaCl] = 500 mol/m3, this reduces to
