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Frothing Behavior of Nonionic Surfactant Solutions in the Presence of Organic and Inorganic Electrolytes
Journal of Colloid and Interface Science 235, 194–196 (2001) doi:10.1006/jcis.2000.7288, available online at http://www.idealibrary.com on
NOTE Frothing Behavior of Nonionic Surfactant Solutions in the Presence of Organic and Inorganic Electrolytes series of inorganic salts on the foam stability of froth prepared from solution of SDS. These and other studies have elucidated in general the influence of the electrolyte on the stability of the foam. However, there is a little data available in the literature about the individual effects of the different ions, and scarcely anything about the influence of organic ions on the foam stability. The present investigation of the influence of several inorganic and organic salts on the stability of foam, produced by aqueous solutions of a nonionic surfactant, is aimed in this direction.
The influence of inorganic salts (KCl, KBr, NaI, KI) and organic salts, tetrapentyl ammonium bromide (TPeAB) and tetrabutyl ammonium bromide (TBuAB), on the decay of a foam column from aqueous solutions of octaethyleneglycol mono-n-decylether (C10 E8 ) has been investigated. The salt concentration in all cases was maintained constant (0.01 M). The results from the measurements of the foam decay rates indicate that, of the inorganic electrolytes, KBr is a foam destabilizer, KCl does not influence the froth stability, and NaI and KI act as foam stabilizers. Both TBuAB and TPeAB generate greater initial quantities of foam. Concurrently, both organic salts accelerate the destruction of the foam in the initial stage of drainage, the effect of TBuAB being stronger. It was established also that in the later stage of drainage, where black films form (here C10 E8 bulk concentration is 5 × 10−4 M and its C bl = 10−4 M), TPeAB acts as foam destabilizer, while TBuAB does not influence the foam stability. °C 2001 Academic Press Key Words: foam stability; thin liquid film; froth drainage; electrolyte.
MATERIALS AND METHODS Frothing studies were carried out with aqueous solutions of octaethyleneglycol mono-n-decylether (C10 E8 ). The high-purity-grade surfactant (99.9%) has been supplied by Nikko Chemicals, Japan. Suprapure KCl and NaI (from Merck), KBr (Sigma), KI (Renald), TPeAB (Aldrich), and TBuAB (Fluka) were used as electrolytes. Frothing experiments were conducted at 22◦ C in 250-ml closed cylindrical flasks containing 100 ml of solution. The froth was obtained by standard (10 times) shaking of the flask and then allowed to stand undisturbed. The height of the froth column was recorded as a function of time and the decay rate was established. Because of the fact that the stability of the foam is a random magnitude, and the experimental dependencies are only experimental tendencies, the test for each solution was repeated five times and all the points are included in the statistical processing. This is the first step of a series of systematic experiments which will be further performed with a specially designed foam cell of the same dimensions, with a porous bottom. Surface tension isotherms of C10 E8 in the presence of 0.01 M of one of the above-mentioned salts were determined by the Wilhelmi plate method at 22◦ C. The observed tendencies are given as statistical curves with pointed error bars. All the experimental readings were included in the obtained dependencies.
INTRODUCTION The stability of a foam column in a closed cell can be estimated, keeping in mind that the foam destruction can be considered as consisting of two timeconsuming stages: 1. drainage of liquid from the foam films and plateau channels of the foam body; 2. destruction of the thin films, before or after formation of balck films. Many works have been dedicated to investigating the behavior of foam films and establishing the factors governing foam stability. Foam is a complex system of liquid films, formed between bubbles with different sizes and connected by plateau channels. Nevertheless, there are certain correlations between the behavior of a single foam film and the foam system. Exerowa et al. (1) studied the behavior of thin foam films with several ionic and nonionic surfactants in the presence of electrolyte and have established the effect of electrostatic interactions on the film thickness. Manev and Pugh have studied the effect of the nonionic surfactant concentration [2] and organic electrolyte (3) on the electrostatic interactions in foam films and have related the results to the onset of black film formation (Cbl ) and high foam stability. They have also found (4) an effect of KCl concentration on the stability of a foam column obtained with such a nonionic surfactant. Ross et al. (5) have established the influence of alkali metal chlorides on the stability of foam generated from aqueous solution of SDS. Barneveld et al. (6) investigated the effect of inorganic electrolyte concentration on the behavior of thin foam films formed from solutions of several nonionic surfactants. Angarska et al. (7) investigated the effect of 0021-9797/01 $35.00
C 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.
RESULTS The influence of different inorganic salts at constant ionic strength (0.01 M) on the stability of foam from 5 × 10−4 M C10 E8 solution is shown in Fig. 1. It can be seen from the figure that NaI and KI act as stabilizers, while KBr has a destabilizing effect and KCl does not influence the stability of the foam. The influence of TBuAB and TPeAB on the stability of the C10 E8 solution with the same surfactant concentration (above Cbl ) is presented in Fig. 2. It can be seen from the figure that in the initial stage of liquid drainage both salts may be considered to enhance the froth stability, because when added to C10 E8 solution, they generate a greater initial amount of foam and a higher foam column. However, the foam formed, especially in the presence of TBuAB, is less stable than that generated by C10 E8 alone, and its decay is faster (Fig. 2). In the later stage, where the durability of the black films starts to play a decisive role in the behavior of the foam, it can be seen that TBuAB does not influence
194
195
NOTE
TABLE 1 Comparison of the Influence of Different Electrolytes on the HalfDecay Time of a Foam Column, τ1/2 , the Initial Foam Volume, Vin , Foam Volume after 500 s, V(t = 500 s), CMC of C10 E8 , and Surface Tension at CMC Additive Parameter
FIG. 1. Comparison of the stability of foams, obtained from the following solutions: 5 × 10−4 M C10 E8 ; 5 × 10−4 M C10 E8 + 0.01 M KBr; 5 × 10−4 M C10 E8 + 0.01 M KCl; 5 × 10−4 M C10 E8 + 0.01 M NaI; 5 × 10−4 M C10 E8 + 0.01 M KI.
the C10 E8 foam stability in this region, while the destructive effect of TPeAB persists. The surface tension of the surfactant solutions was measured in the absence and in the presence of constant electrolyte concentration (Cel = 0.01 M). These measurements allowed us to estimate the influence of the salts on the CMC of C10 E8 (CMC = 0.001 M). The experimental error is 0.2 mN/m. The basic tendencies observed in all experiments are summed up in Table 1, where the influence of different salts on several important parameters, describing the properties of the foam system, is presented: 1. 2. 3. 4. 5.
the half-decay time of a foam column, τ1/2 ; the initial volume of the foam column, Vin ; the volume of the foam column after 500 s, V (t = 500 s); CMC of the C10 E8 surfactant in the presence/absence of electrolytes; surface tension at CMC.
DISCUSSION We chose the concentration of C10 E8 (5 × 10−4 M) to be below CMC (CMC = 10−3 ; see Table 1) but above the so-called “concentration of black
τ1/2 (s) Vin (ml) V (t = 500 s) (ml) CMC (mM) Surface tension at CMC (mN/m)
C10 E8 no additives +TBuAB +TPeAB +KCl +KBr +KI +NaI 202 114 32 1.0 35.6
138 144 27 1.0 35.8
187 144 14 1.0 34.8
188 97 307 306 110 81 116 116 29 10 49 53 1.0 1.0 1.0 1.0 35.8 35.8 35.9 35.1
spot formation” Cbl . The latter quantity represents the minimum surfactant concentration, above which black foam films can be obtained and, as a rule, the transition to high foam stability takes place (1, 2). For C10 E8 we established Cbl = 10−4 M. The black films are known to be stable and their durability depends on the interactions between the surfactant molecules in the adsorption layers on the film surfaces. Different effects are observed when inorganic electrolytes are added to the C10 E8 solution. It can be noticed from Fig. 1 and Table 1 that KBr acts as a defoamer, but KCl does not influence the froth stability in either stage of foam destruction. In general the reason can be sought in the difference in anion size between the two species, although the result is not decisive. The radius of bare Br− is 0.195 nm, but the radius of bare Cl− is 0.181 nm, the hydrated radii of both ions being the same (9), but a little lower than that of TPeAB. That is why their defoaming action is weaker than that for TPeAB. The measurements of the surface tension in the absence and in the presence of additional electrolytes allowed us to conclude that TPeAB diminishes the surface tension noticeably, thus promoting the generation of a greater initial quantity of foam, in comparison with the other salts. The effect of TBuAB on surface tension is within the limits of the experimental error; nevertheless, its effect on the initial foam is similar. The influence of other salts on surface tension is within the limits of the experimental error, with exception of NaI; the effect of the latter is comparable to that of TPeAB. This may be due to specific interactions between the electrolyte ions and the molecules of the surfactant. The presence of electrolyte of the studied concentration (10 mM) does not influence the CMC of C10 E8 noticeably (CMC = 1.0 ± 0.04 mM; see Table 1). The experimental error was ±0.20 mN/m. Here we have established the variation of the effect on foam decay, depending on the kind of electrolytes (ions), however similar they may be. We hope that this and other investigations in progress will contribute to elucidating the individual effects of the different ions on foam stability, whether through their size, surface activity, or other properties.
ACKNOWLEDGMENT We thank Sofia University Scientific Research Fund for the financial support we received for this work.
REFERENCES FIG. 2. Comparison of the stability of foams, formed from the following solutions: 5 × 10−4 M C10 E8 ; 5 × 10−4 M C10 E8 + 0.01 M TBuAB; 5 × 10−4 M C10 E8 + 0.01 M TPeAB.
1. Exerowa, D., Zaharieva, M., Cohen, R., and Platikanov, D., Colloid Polym. Sci. 257, 1089 (1979). 2. Manev, E. D., and Pugh, R. J., Langmuir 8, 2253 (1992).
196
NOTE
3. Manev, E. D., and Pugh, R. J., Surf. Interface Anal. 17, 681 (1991). 4. Pugh, R. J., and Manev, E. D., J. Colloid Interface Sci. 152, 582 (1992). 5. Ross, S., and Bramfitt, J. H., J. Phys. Chem. 61, 1261 (1957). 6. Barneveld, P. A., Scheutjens, J. M. H. K., and Lyklema, J., Colloids Surf. 52, 107 (1991). 7. Angarska, J. K., Tachev, K. D., Kralchevsky, P. A., Mehereteab, A., and Broze, G., J. Colloid Interface Sci. 199, 001 (1998). 8. Manev, E. D., Karakashev, St. I., and Milushev, A. M., Bulgarian Chem. Commun., to be published. 9. Israelachvili, J. N., “Intermolecular and Surface Forces.” Academic Press, London, 1991.
Stoyan Ivanov Karakashev1 Emil Deyanov Manev Department of Physical Chemistry St. Kliment Ohridski University of Sofia 1 James Bourchier Boulevard 1126 Sofia, Bulgaria Received May 22, 2000 1 To
whom correspondence should be addressed. E-mail: [email protected].
NOTE Frothing Behavior of Nonionic Surfactant Solutions in the Presence of Organic and Inorganic Electrolytes series of inorganic salts on the foam stability of froth prepared from solution of SDS. These and other studies have elucidated in general the influence of the electrolyte on the stability of the foam. However, there is a little data available in the literature about the individual effects of the different ions, and scarcely anything about the influence of organic ions on the foam stability. The present investigation of the influence of several inorganic and organic salts on the stability of foam, produced by aqueous solutions of a nonionic surfactant, is aimed in this direction.
The influence of inorganic salts (KCl, KBr, NaI, KI) and organic salts, tetrapentyl ammonium bromide (TPeAB) and tetrabutyl ammonium bromide (TBuAB), on the decay of a foam column from aqueous solutions of octaethyleneglycol mono-n-decylether (C10 E8 ) has been investigated. The salt concentration in all cases was maintained constant (0.01 M). The results from the measurements of the foam decay rates indicate that, of the inorganic electrolytes, KBr is a foam destabilizer, KCl does not influence the froth stability, and NaI and KI act as foam stabilizers. Both TBuAB and TPeAB generate greater initial quantities of foam. Concurrently, both organic salts accelerate the destruction of the foam in the initial stage of drainage, the effect of TBuAB being stronger. It was established also that in the later stage of drainage, where black films form (here C10 E8 bulk concentration is 5 × 10−4 M and its C bl = 10−4 M), TPeAB acts as foam destabilizer, while TBuAB does not influence the foam stability. °C 2001 Academic Press Key Words: foam stability; thin liquid film; froth drainage; electrolyte.
MATERIALS AND METHODS Frothing studies were carried out with aqueous solutions of octaethyleneglycol mono-n-decylether (C10 E8 ). The high-purity-grade surfactant (99.9%) has been supplied by Nikko Chemicals, Japan. Suprapure KCl and NaI (from Merck), KBr (Sigma), KI (Renald), TPeAB (Aldrich), and TBuAB (Fluka) were used as electrolytes. Frothing experiments were conducted at 22◦ C in 250-ml closed cylindrical flasks containing 100 ml of solution. The froth was obtained by standard (10 times) shaking of the flask and then allowed to stand undisturbed. The height of the froth column was recorded as a function of time and the decay rate was established. Because of the fact that the stability of the foam is a random magnitude, and the experimental dependencies are only experimental tendencies, the test for each solution was repeated five times and all the points are included in the statistical processing. This is the first step of a series of systematic experiments which will be further performed with a specially designed foam cell of the same dimensions, with a porous bottom. Surface tension isotherms of C10 E8 in the presence of 0.01 M of one of the above-mentioned salts were determined by the Wilhelmi plate method at 22◦ C. The observed tendencies are given as statistical curves with pointed error bars. All the experimental readings were included in the obtained dependencies.
INTRODUCTION The stability of a foam column in a closed cell can be estimated, keeping in mind that the foam destruction can be considered as consisting of two timeconsuming stages: 1. drainage of liquid from the foam films and plateau channels of the foam body; 2. destruction of the thin films, before or after formation of balck films. Many works have been dedicated to investigating the behavior of foam films and establishing the factors governing foam stability. Foam is a complex system of liquid films, formed between bubbles with different sizes and connected by plateau channels. Nevertheless, there are certain correlations between the behavior of a single foam film and the foam system. Exerowa et al. (1) studied the behavior of thin foam films with several ionic and nonionic surfactants in the presence of electrolyte and have established the effect of electrostatic interactions on the film thickness. Manev and Pugh have studied the effect of the nonionic surfactant concentration [2] and organic electrolyte (3) on the electrostatic interactions in foam films and have related the results to the onset of black film formation (Cbl ) and high foam stability. They have also found (4) an effect of KCl concentration on the stability of a foam column obtained with such a nonionic surfactant. Ross et al. (5) have established the influence of alkali metal chlorides on the stability of foam generated from aqueous solution of SDS. Barneveld et al. (6) investigated the effect of inorganic electrolyte concentration on the behavior of thin foam films formed from solutions of several nonionic surfactants. Angarska et al. (7) investigated the effect of 0021-9797/01 $35.00
C 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.
RESULTS The influence of different inorganic salts at constant ionic strength (0.01 M) on the stability of foam from 5 × 10−4 M C10 E8 solution is shown in Fig. 1. It can be seen from the figure that NaI and KI act as stabilizers, while KBr has a destabilizing effect and KCl does not influence the stability of the foam. The influence of TBuAB and TPeAB on the stability of the C10 E8 solution with the same surfactant concentration (above Cbl ) is presented in Fig. 2. It can be seen from the figure that in the initial stage of liquid drainage both salts may be considered to enhance the froth stability, because when added to C10 E8 solution, they generate a greater initial amount of foam and a higher foam column. However, the foam formed, especially in the presence of TBuAB, is less stable than that generated by C10 E8 alone, and its decay is faster (Fig. 2). In the later stage, where the durability of the black films starts to play a decisive role in the behavior of the foam, it can be seen that TBuAB does not influence
194
195
NOTE
TABLE 1 Comparison of the Influence of Different Electrolytes on the HalfDecay Time of a Foam Column, τ1/2 , the Initial Foam Volume, Vin , Foam Volume after 500 s, V(t = 500 s), CMC of C10 E8 , and Surface Tension at CMC Additive Parameter
FIG. 1. Comparison of the stability of foams, obtained from the following solutions: 5 × 10−4 M C10 E8 ; 5 × 10−4 M C10 E8 + 0.01 M KBr; 5 × 10−4 M C10 E8 + 0.01 M KCl; 5 × 10−4 M C10 E8 + 0.01 M NaI; 5 × 10−4 M C10 E8 + 0.01 M KI.
the C10 E8 foam stability in this region, while the destructive effect of TPeAB persists. The surface tension of the surfactant solutions was measured in the absence and in the presence of constant electrolyte concentration (Cel = 0.01 M). These measurements allowed us to estimate the influence of the salts on the CMC of C10 E8 (CMC = 0.001 M). The experimental error is 0.2 mN/m. The basic tendencies observed in all experiments are summed up in Table 1, where the influence of different salts on several important parameters, describing the properties of the foam system, is presented: 1. 2. 3. 4. 5.
the half-decay time of a foam column, τ1/2 ; the initial volume of the foam column, Vin ; the volume of the foam column after 500 s, V (t = 500 s); CMC of the C10 E8 surfactant in the presence/absence of electrolytes; surface tension at CMC.
DISCUSSION We chose the concentration of C10 E8 (5 × 10−4 M) to be below CMC (CMC = 10−3 ; see Table 1) but above the so-called “concentration of black
τ1/2 (s) Vin (ml) V (t = 500 s) (ml) CMC (mM) Surface tension at CMC (mN/m)
C10 E8 no additives +TBuAB +TPeAB +KCl +KBr +KI +NaI 202 114 32 1.0 35.6
138 144 27 1.0 35.8
187 144 14 1.0 34.8
188 97 307 306 110 81 116 116 29 10 49 53 1.0 1.0 1.0 1.0 35.8 35.8 35.9 35.1
spot formation” Cbl . The latter quantity represents the minimum surfactant concentration, above which black foam films can be obtained and, as a rule, the transition to high foam stability takes place (1, 2). For C10 E8 we established Cbl = 10−4 M. The black films are known to be stable and their durability depends on the interactions between the surfactant molecules in the adsorption layers on the film surfaces. Different effects are observed when inorganic electrolytes are added to the C10 E8 solution. It can be noticed from Fig. 1 and Table 1 that KBr acts as a defoamer, but KCl does not influence the froth stability in either stage of foam destruction. In general the reason can be sought in the difference in anion size between the two species, although the result is not decisive. The radius of bare Br− is 0.195 nm, but the radius of bare Cl− is 0.181 nm, the hydrated radii of both ions being the same (9), but a little lower than that of TPeAB. That is why their defoaming action is weaker than that for TPeAB. The measurements of the surface tension in the absence and in the presence of additional electrolytes allowed us to conclude that TPeAB diminishes the surface tension noticeably, thus promoting the generation of a greater initial quantity of foam, in comparison with the other salts. The effect of TBuAB on surface tension is within the limits of the experimental error; nevertheless, its effect on the initial foam is similar. The influence of other salts on surface tension is within the limits of the experimental error, with exception of NaI; the effect of the latter is comparable to that of TPeAB. This may be due to specific interactions between the electrolyte ions and the molecules of the surfactant. The presence of electrolyte of the studied concentration (10 mM) does not influence the CMC of C10 E8 noticeably (CMC = 1.0 ± 0.04 mM; see Table 1). The experimental error was ±0.20 mN/m. Here we have established the variation of the effect on foam decay, depending on the kind of electrolytes (ions), however similar they may be. We hope that this and other investigations in progress will contribute to elucidating the individual effects of the different ions on foam stability, whether through their size, surface activity, or other properties.
ACKNOWLEDGMENT We thank Sofia University Scientific Research Fund for the financial support we received for this work.
REFERENCES FIG. 2. Comparison of the stability of foams, formed from the following solutions: 5 × 10−4 M C10 E8 ; 5 × 10−4 M C10 E8 + 0.01 M TBuAB; 5 × 10−4 M C10 E8 + 0.01 M TPeAB.
1. Exerowa, D., Zaharieva, M., Cohen, R., and Platikanov, D., Colloid Polym. Sci. 257, 1089 (1979). 2. Manev, E. D., and Pugh, R. J., Langmuir 8, 2253 (1992).
196
NOTE
3. Manev, E. D., and Pugh, R. J., Surf. Interface Anal. 17, 681 (1991). 4. Pugh, R. J., and Manev, E. D., J. Colloid Interface Sci. 152, 582 (1992). 5. Ross, S., and Bramfitt, J. H., J. Phys. Chem. 61, 1261 (1957). 6. Barneveld, P. A., Scheutjens, J. M. H. K., and Lyklema, J., Colloids Surf. 52, 107 (1991). 7. Angarska, J. K., Tachev, K. D., Kralchevsky, P. A., Mehereteab, A., and Broze, G., J. Colloid Interface Sci. 199, 001 (1998). 8. Manev, E. D., Karakashev, St. I., and Milushev, A. M., Bulgarian Chem. Commun., to be published. 9. Israelachvili, J. N., “Intermolecular and Surface Forces.” Academic Press, London, 1991.
Stoyan Ivanov Karakashev1 Emil Deyanov Manev Department of Physical Chemistry St. Kliment Ohridski University of Sofia 1 James Bourchier Boulevard 1126 Sofia, Bulgaria Received May 22, 2000 1 To
whom correspondence should be addressed. E-mail: [email protected].