Influence of the foam film type on the foam drainage process

Colloids and Surfaces A: PhysicochemIcal and Engineering Aspects 94 (1995) 303-309

ELSEVIER

COLLOIDS AND SURFACES

A

Influence of the foam film type on the foam drainage process Khr. Khristov *, D. Exerowa Institute of Physical Chemistry, Bulgarian Academy ofSciences, Sofia 1113, Bulgaria

Received 3 August 1994; accepted 23 August 1994

Abstract The effect of the type of foam films: thin, common black (CBF) and Newtonian black (NBF), on the process of foam drainage is studied. The experimental method employed makes it possible to perform precise hydrodynamic investigations which indicate that the process of foam drainage is influenced by the type of foam films. The difference observed between the rate of drainage of foams with CBF or NBF allowed determination of the critical electrolyte concentration Cel •cr of the CBF/NBF transition. Keywords: Foam; Foam drainage; Foam films; Plateau borders

1. Introduction

Foam drainage, as it is well known, is a complex hydrodynamic process involving liquid flow from the foam films into the Plateau borders, distribution of the liquid along the foam column and its drainage under the influence of capillary and gravitational forces. In recent years the theoretical and experimental studies of the process of foam drainage (see, for example, Refs. [1-5]) have shown considerable improvement. However, the significant advance in the studies of foam drainage has not as yet given rise to a uniform theory able to describe this process entirely. No attempts to generalize the numerous experimental data accumulated about drainage of foams stabilized by various surfactants have even been reported. Furthermore the relationships between the rate of drainage and foam stability, the effect of surface layer mobility, etc. remain unclear. It is curious that no studies have

*

Corresponding author.

0927-7757/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0927-7757(94)03004-9

been reported about the influence that the type of foam films: thin liquid, common black (CBF) and Newtonian black (NBF), exerts on the kinetics of foam drainage. On the other hand evidence has been presented (see, for example, Refs. [1,4,6-8]) that the type of foam film plays an important role in the stability of disperse systems, especially of foams. Today it is hardly possible to succeed in the interpretation of the experimental data about foam stability without considering the type of foam films present. In that sense it could be expected that the process of foam drainage should also be influenced by the type of foam films in the foam. Moreover the three types of foam films exhibit different hydrodynamic behavior, for example: thin films drain continuously until an equilibrium state is reached, while CBF and NBF reach the equilibrium state by a sudden transition to a thinner stable film through formation of black spots in the draining thicker film [1,9-12]. This paper aims at studying the influence of the type of foam film (thin, CBF and NBF) on the

304

Khr. Khristov, D. Exerowa/Colloids Surfaces A Physicochem. Eng. Aspects 94 ( 1995) 303-309

kinetics of the process of foam drainage occurring in foams stabilized with sodium dodecyl sulfate (NaDoS).

2. Experimental

2.1. Materials

Aqueous sodium dodecyl sulfate solutions were used to obtain the foams investigated. The choice of this surfactant was prompted by both the vast amount of data reported in the literature about its use as a model in various studies of thin liquid films and foams, and by its ability to form all three types of foam films. Sodium chloride (NaCl) was added as an electrolyte and its concentration (Ced was varied so that formation of the different types of foam films was ensured. Both chemicals used (NaDoS and NaCl) were supplied by Merck. The NaCl was heated up to 550°C prior to use in order to eliminate any surfactant contaminations.

in Fig. 1. It consists of a measuring cell 10 - a glass cylinder the bottom of which is a porous plate 1. The construction of this cell allows insertion of electrodes 2 of different types (horizontal or vertical) at the chosen levels of the foam column. Thermosensor 3 controls the temperature. 4 is the inlet/outlet to the thermostat and 5 is a thermostatting glass jacket. The foam was obtained by a generator described in Ref. [8]. Then the measuring cell was filled up with the foam to a certain level, thus ensuring an equal foam volume in all experiments. Stopcock 6 connects the measuring cell to a vacuum system which creates the needed pressure difference LJPo. The rate of the drainage process can be regulated by applying different LJPo. 7 is a glass lid and 8 connects the electrodes with the computerized measuring and registering block 9. The rate of foam drainage was monitored by the electroconductivity method [14,15]. The liquid content (W) in the foam was calculated by substituting the values of the electroconductivity of the

2.2. Apparatus and methods

Foam drainage is a relatively slow process and depends not only on the hydrodynamic characteristics of the foam but also on several other factors, for instance: change in foam dispersity caused by coalescence of foam films and diffusion transfer of gas, i.e. foam stability, temperature, etc. However, significant acceleration of the process can be achieved by the method of applying an increased and regulated pressure in the foam liquid phase (see, for example, Refs. [1,4,8,11 ]). This makes it possible to study independently the processes of coalescence and drainage occurring in a foam which has been reported in several papers (see, for example, Refs. [1,4,8,11,13 ]). As mentioned in the introduction this method has been employed mainly for foam-stability investigations. In order to study the process of foam drainage a new apparatus was constructed which could register automatically the parameters investigated. A schematic diagram of the apparatus is given

Fig. 1 Schematic diagram of the apparatus used in the study of the process of foam drainage.

Khr. Khristov, D Exeroll'ajColloids Surfaces A: Physicochem. Eng. Aspects 94 ( 1995) 303-309

foam

Kf

305

in the following formula:

W (vol.%) = B(Kr/K s ) x 100

(1)

where K s is the specific electroconductivity of the bulk solution, and B is a proportionality coefficient accounting for the distribution of the liquid between the Plateau borders and the foam films. The theoretical considerations (see, for example, Refs. [15-18]) reveal that the values of coefficient B depend on the foam structure and may vary within the range 1.5-3. The structure of a real foam changes with time and so does, correspondingly, the value of coefficient B. Lemlich [19] has proposed a formula which relates the proportionality coefficient B and the ratio K D • The latter accounts for the foam structure (within the extremes spherical and polyhedral): B=3-2.5Kif3+0.5Kf>

(2)

where K D = Kr/K s • The apparatus constructed especially for these studies allows us to calculate automatically both the coefficient B at any time t using formula (2) and the water content W by formula (1). Thus the accuracy in the evaluation of W increases significantly. Both parameters can be determined over a large time range, from a second to any other chosen time. All this proves the apparatus to be a very convenient one for performing precise investigations of the foam drainage process.

Destruction after""" 24~

2

a -1

-2 Destruction

o

10

20

30

40

after rv 180 mJ~

50

60

t[minJ

Fig. 2. Dependence of the water co~tent W on the time t for foams with NBF at L1Po =5 x 103 Pa, curve 1, and in gravitational field, curve 2.

applied L1Po, i.e. L1Po-P" = 0 (see plateau in curve 1) [1,4,20,21]. The scattering of the experimental results in this figure as well as in Figs. 3-6 lay within the scope of the points drawn on them. These results show clearly that applying increased pressure in the Plateau borders of the foam leads to a significant acceleration of the drainage process. The hydrostatic equilibrium is reached at much shorter time (3-5 times) compared with the foam lifetime. That is why the processes

3. Results and discussion

Fig. 2 depicts the dependence of the water content W as a function of time t of a foam obtained from a solution of 10- 3 mol dm -3 NaDoS in the presence of 5 x 10 -1 mol dm - 3 NaCl, i.e. foam with NBF. The height H of the foam column studied was 3 cm. It is seen that the rate of drainage at L1P o = 5 x 10- 3 Pa (curve 1) is much greater than that in the gravitational field (curve 2). For example at the tenth minute the water content W at L1Po = 5 x 10 - 3 Pa (curve 1) is three orders lower than W in a gravitational field (curve 2). The process of foam drainage ceases, i.e. a hydrostatic equilibrium is reached, when the capillary pressure P" in the Plateau borders becomes equal to the

2.0 3

1.0 2

o -1.0

L.-_ _-I.-_ _---l..-_ _---l.

o

400

800

1200

L.-

___

1600 t[sec]

Fig. 3. Dependence of the water content W on the tIme t for foam with CBF in gravitational field. Curve 1, level 3-2 cm; curve 2, level 2-1 cm and curve 3, level 1-0 cm.

Khr. Khristov, D. Exerowa/Colloids Surfaces A: Physicochem. Eng. Aspects 94 (1995) 303-309

306

lJ'l

1.5

10

0.5

-0.5

-15

-2.5

0.1

a

200

400

600

800

t[secJ

Fig. 4. Dependence of the water content W on the time t for foam with CBF at L1Po =5 x 103 Pa. Curve 1, level 3-2cm; curve 2, level 2-1 cm and curve 3, level 1-0 em.

lJ'l

a

100

200

300

400

500

600

t

[sec]

FIg. 6. Dependence of the water content W on the time t (initial slope) at L1Po =5 x 103 Pa for foams from NaDoS solution wIth concentration 10- 3 mol dm -3 and various concentrations of NaCl: 10- 1 mol dm -3 (CBF), filled squares on curve 1; 0.32 mol dm -3 (CBF), open squares fitting on curve 1; 5 x 10- 1 mol dm- 3 (NBF), filled circles on curve 2; 0.33 mol dm -3 (NBF), open circles on curve 2.

2.0

1.0

a -1.0

- 2.0 L-_----'-_ _---'--=::!~-=~=_==-=..:.::..::.--=-_ a 200 400 600 800 t [sec] Fig. 5. Dependence of the water content W on the time t at L1Po =5 x 103 Pa for foam with thin films, curve 1; NBF, curve 2 and CBF, curve 3.

of coalescence and drainage occurring in a foam can be studied independently (see, for example, Refs. [1,4,8,13]). Evidence has been presented (see, for example, Refs. [1,2,22]) that in the gravitational field the kinetics of foam drainage is a function of the level

of the foam column at which it is determined. However, the application of increased pressure on the foam liquid phase leads to changes in the kinetics of the foam drainage (see Fig. 2). That is clearly shown also in Figs. 3 and 4 which depict the results obtained from parallel studies of foam drainage occurring in the gravitational field and under increased pressure, applied on the foam liquid phase. The Wit curves in Fig. 3 present the rate of foam drainage in the gravitational field recorded at three different levels in the foam column. At the lowest level (1-0 cm, curve 3), a clearly pronounced maximum is observed, i.e. at the beginning W increases due to the flow of liquid from the upper foam layers. Curve 2 (2-1 cm) is running in a similar way, while at the highest level (3-2 cm, curve 1) W decreases continuously. Fig. 4 presents the data of the Wit dependence recorded at the same levels of the foam column when increased pressure (LlP 0 = 5 X 103 Pal is applied. Here the differences in the development of the curves 1, 2 and 3 are not pronounced but

Khr. Khnstov, D. Exerowa/Colloids Surfaces A: Physicochem. Eng Aspects 94 ( 1995) 303-309

are beyond the scattering of the experimental results. That is why the investigations to follow were carried out at a fixed level (2~ 1 cm) in the foam column. In order to account only for the influence of the type of foam films on the rate of foam drainage and to eliminate all other factors affecting it, studies were performed to estimate the role offoam column height H, pressure applied iJP o, initial water content W, initial dispersity of the foam, temperature, etc. on the process [23]. Thus we succeeded in defining the following optimum conditions: H = 3 cm, iJP 0= 5 X 103 Pa, constant initial dispersity and W, fixed level, 2-1 cm, in the foam column and T = 23 C. All the experiments considered in the present study were performed under these conditions. The results obtained for foams with thin film (curve 1), common black film (curve 3) and Newtonian black film (curve 2) under applied pressure iJP 0= 5 X 103 Pa, are presented in Fig. 5. The slowest rate of foam drainage is observed in foams with thin films (curve 1) while in foams with NBF (curve 2) and CBF (curve 3) it increases significantly, being the highest for CBF. Here the Wit dependences are considered down to the plateau, i.e. when iJP o = P(J and a hydrostatic equilibrium is reached (see, for example, Refs. [1,4,20,21]). The values of W at which the plateau begins are different for each of the three curves. A possible explanation of this fact is that the thicknesses h of thin films, CBF and NBF are different. Besides, in contrast to NBF, the thickness of thin films and CBF depends on the applied pressure (see, for example, Refs. [1,4,24,25]). In the case considered they are: for thin films h ~ 16 nm, for CBF h ~ 8 nm and for NBF h~4.2 nm [8,24,25]. Fig. 6 depicts the initial slopes d W/dt (linear parts) of the curves describing the process of drainage of a foam with CBF, filled squares on curve 1, and NBF, the filled circles on curve 2. Since all experimental conditions are kept the same then the obvious difference in the initial slopes, i.e. the rate of foam drainage, can be attributed to the different type of foam films constituting the foam. In support of that statement are the results shown in Fig. 6 about the kinetics of drainage of foams obtained from solutions having the same 0

307

NaDoS concentration but containing different quantities of NaCI: 0.32 mol dm -3, open squares fitting on curve 1 for foams with CBF, and 0.33 mol dm - 3, open circles fitting on curve 2 for foam with NBF. The kinetics of drainage for foams with CBF and NBF was followed in a range of Cel> 0.1 ~0.32 mol dm - 3, and 0.33-0.5 mol dm - 3, respectively. Only the limiting value of NaCI concentration of the CBF/NBF transition appears in the figure because all the data obtained about the other Cel lay on both curves, i.e. on curve 1 for foams with CBF and on curve 2 for foams with NBF. This permitted us to determine the critical electrolyte concentration Cel •cr of the CBF/NBF transition which was found to be 0.33 + 0.05 mol dm~3. It is worthwhile to compare this value of Cel •cr with those evaluated by other foam methods, for example the method of foam destruction by rxparticle irradiation. Fig. 7 illustrates the 'R/Z/(Cel ) dependence for foams from 5 x 10- 4 mol dm- 3 NaDoS solutions. Here 'R/Z is the time of destruction of the foam column by the rx-particle source (1 /lCi) at a distance R/2 (R is the free path of the rx-particles in air). Under rx-particle irradiation the stability of CBF decreases with the increase in Cel while the stability

10

B

6

4

2'-----'---'--'-J..-'------'----'----:.-'_---",...-

0.25

to.35 Cel,cr

J

0.45 eel [mol/dm3

Fig. 7. Dependence of time CRf2 of destruction of the foam column on Cel under a-particle irradiation.

Khr. Khristov, D. Exeroll'a/CollOlds Surfaces A: Physicochem. Eng. Aspects 94 (1995)

308

of NBF

increases

(see,

for

example,

Refs.

[1,11,26,27]). As seen from Fig. 7 a clearly pronounced minimum in the 'R/Z/(Cel ) curve is

observed which corresponds to the critical electrolyte concentration of the CBF/NBF transition which is found to be Cel. er = 0.325 + 0.005 mol dm- 3 . Its value coincides with Cel ,er=0.33+ 0.05 mol dm- 3 discussed above (see Fig. 6). Furthermore it should be emphasized that the above mentioned Cel,er values obtained by both foam methods are very close to those obtained by other methods involving single microscopic films. For example, the "contact angles" method gives C el ,er=0.334 mol dm -3 (see, for example, Refs. [1,28,29 ]), while the method based on the study of the electrical properties of films gives Cel,er = 0.3 mol dm- 3 (see, for example, Refs. [30,31]). All this favors the assumption that the study of single microscopic foam films as a model of the "film structure" of a foam is one of the most reliable approaches for understanding the properties of foams (see, for example, Refs. [1,4,8,10]).

4. Conclusion

The present study is an attempt to estimate the influence that the type of foam films has on the process of foam drainage. An automated apparatus, based on the method of creating an increased pressure in the Plateau borders of the foam was constructed especially for the purpose. This method allows us to regulate the rate of foam drainage thus making it possible to carry out precise hydrodynamic investigations of that process. The experimental results obtained clearly indicate that the electrolyte concentration affects the rate of drainage through the change in the type of foam films in the foam. The difference observed between the rate of drainage of foams with CBF or NBF permitted evaluation of the critical electrolyte concentration Cel. er of the CBF/NBF transition and the value obtained proves to be very close to those determined by other methods. However, further studies of single foam films and foams stabilized with various surfactants are necessary in order to describe quantitatively the process of foam drainage.

303~309

Acknowledgment

We are indebted to the National Fund "Research" of the Ministry of Education and Science for financial support under Contract X-I.

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[26] D. Exerowa, I. Penev and S. Avgarska, Ann. Univ. Sofia, Fac. Chim., 69 (1974/75) 123 [27] D. Exerowa, I. Penev and Khr. Khristov, III E. Wolfram (Ed.), Proc. Int. Conf. on CollOid and Surface Science, Vol. 1, Academia Vlado, Budapest, 1975, p. 575. [28] T. Kolarov, A. Scheludko and D. Exerowa, Trans. Faraday Soc., 64 (1968) 2864.

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[29] D. Exerowa, Khr. Khristov and M. Zaharieva, in B.Y. Derjaguin, Metastable foam films, in Surface Forces III Thin Films, Nauka, Moscow, pp. 186, 1979, in Russian. [30] D. Platikanov and N. Rangelova, C. R. Acad. Bulg. SCI., 21 (1968) 913. [31] D. Platikanov and M. Nedyalkov, Ann. Univ. Sofia, Fac. Chim., 63 (1968/69) 91.