From stability in aqueous solutions of nonionic surfactant and inorganic electrolyte

Froth Stability in Aqueous Solutions of Nonionic Surfactant and Inorganic Electrolyte The froth stability of aqueous solutions of nonionic surfactant was found to increase on addition of low concentrations of KC1 but to decrease at higher concentrations. From earlier reported studies on free aqueous films, it could be suggested that two different types of stabilizing mechanisms were operating in the froth system. At low electrolyte, the increase in froth stability was explained by the decrease in interfacial electrostatic repulsion and Newton's black films formation while at high electrolyte, the lower stability was accounted for by weaker steric forces. © 1992AcademicPress,Inc. INTRODUCTION In recent years, many fundamental studies have been reported concerning the influence of inorganic electrolyte on the stability of single free aqueous froth films stabilized by nonionic surfactant (1-3). In several of these investigations, the stability and sensitivity of the equilibrium thickness of the films to the electrolyte were reported from experimental studies using the microinterferometric methods developed by Scheludko and co-workers (4, 5 ). In general, in dilute inorganic electrolyte, a reasonable description of the interaction behavior between the interfacial layers could be achieved from DLVO theory by combining the Poisson-Boltzmann treatment for electrostatic repulsive double layers (d.1.) with the Lifshitz treatment for London-van der Waals attractive forces. However, in some cases at increasing electrolyte concentrations, the d.l. force was found to be much less sensitive to ionic strength and although some unstable films were observed which ruptured (usually at a so-called "critical thickness"), in other cases black spots were formed in the film, which led to stable thinner black films. The characteristic bulk surfactant concentration at which black spots appear in the film immediately before rupture occurs has been designated "c-black" (Cb). On increasing the concentration above Cb, the black spots appeared to have sufficient time to coalesce without rupture and form a continuous stable black film. The results of such studies are important for understanding film characteristics and the various phenomena occurring in froth systems such as stability, coalescence, and drainage. In addition, free aqueous film traits are useful for estimating the lifetime of froths. In the present study, the stability of froths was measured using pure nonionic surfactants in double distilled water and electrolyte and the results were compared to previously reported stability behavior on free aqueous froth films. EXPERIMENTAL Frothing studies were carried out from aqueous solutions of pentaethylene glycol n-decyl ether C Io(EO )5 and pentaethylene glycol n -dodecyl ether CIZ(EO)5. The high-pu-

rity-grade surfactants (99.9%) were supplied by Nikko Chemicals, Japan. Suprapure KC1 (Merck) was used as the electrolyte. All solutions were prepared using doubledistilled water. Surface tension was determined by the Du Nofiy ring method. Simple frothing experiments were carried out in 250 ml conical flasks with ground glass stoppers using 125 ml of frothing solution in each test. The flasks were gently shaken by hand (× 10) and allowed to stand undisturbed. The decay time of the froth was recorded as the time (in seconds) before the initial appearance of a periphery of clear surface in the center of the froth. The periphery rapidly expanded exposing a whole froth-free interface. The test was shown to be reproducible to within + 1 s. RESULTS Figure 1 shows the surface tension of the two nonionic surfactants determined in the concentration range from 10 -7 to 10 -2 M at 20°C. The surface tension isotherms indicate the quality of the surfactants used; the critical micelle concentration obtained from these measurements confirms the manufacturer's claim. There are no indications of a minimum or any other irregularities in the surface tension isotherm which would suggest the presence of impurities in the surfactant sample used for the present experimental studies. In addition, the surfactant with the same head group but with the shorter hydrocarbon tail clearly shows a shift in CMC to higher concentrations. It must be noted that the surface tension measurements were made in the absence of electrolyte (i.e., KC1). This, however, is of no importance to the conclusions derived later in the present study. As has been proven by other researchers (6, 7), the effect of such additives on the surface tension of the nonionic surfactants is not even detectable below bulk ionic concentrations as high as 10 -2 M. Therefore, the experimental results are in all cases within the safe concentration range. The foam lifetime was determined experimentally, as a function of the nonionic surfactant concentration, in the range from 10 -6 to 5 × 10 -4 M at 20°C. These studies were made in double-distilled water and in 10 -4 , 10 -3 , and 10 -z M KC1. The froth stability plots (Fig. 2) indicate

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LETTERS TO THE EDITOR several features of interest. In the absence of the electrolyte, the C,2(EO)5 is shown to be the stronger frother with a concentration of about 5 × 10 -6 M required to give a froth lifetime of about 20 s, which corresponds to a surface tension of about 50 m N m -1. At higher surfactant concentrations, the lifetime of the froth is shown to increase drastically to about 80 s in 10-5 M solution with a surface tension of about 45 m N m -~ . For the less active frother, C~0(EO)5, a similar pattern of behavior is observed with higher concentration required to give equivalent surface tensions and froth lifetimes. On addition of electrolyte to the systems, both nonionies again follow a similar pattern of behavior. At low KC1 concentrations ( 10 -4 M), the lifetime of the froth can be seen to increase compared to double-distilled water. This appeared to suggest that a froth restabilizing mechanism is operating. However, on further increasing the electrolyte concentration to 10 -3 M, a destabilizing effect appeared to occur, increasing the froth lifetime to values equivalent to that of double-distilled water at equivalent surfactant concentrations, Also, on further additions of KC1 to 10-z M, the froth clearly showed an additional decrease in stability.

DISCUSSION It has been well established from previous studies that single aqueous froth films containing dilute nonionie surfactant are stabilized by charge and the surface potential usually reaches values between - 3 5 and - 4 5 mV at natural pH (8-10). It was also shown that the charge is not due to the surfactant but is a property of the air/water interface itself, under the studied experimental conditions (8). This charge remains fairly constant in the region of the dilute surfactant concentration. On addition of KC1, it has also been shown that the thickness and stability of the single froth film is reduced and this has been explained by a decrease in d.1. thickness

80

I

I

l

I

L

7O

~ ~o ~ 40 ~

30

"

-

20 10-7

I0 -6

10.5 10-4 If'" FROTHER CONCENTRATION (M)

10.2

10-I

FIG. 1. Surface tension versus concentration plots for C12(E0)5 ( ~ ) and Clo(EO)s ( • ) surfactants.

100

C12(EO)5 80

Clo(EO)5

A

~ 60

o< 4O

20

10-6

lff 5

10-4

10.3

SURFACTANT C O N C E N T R A T I O N / M

FIG. 2. The relationship between foam lifetime and the concentration of C12(EO)5 and C1o(EO)5 surfactants in double-distilled water (©), 10 -4 M KC1 ( • ), 10-3 M KC1 (A), and 10 -2 MKC1 ( 0 ) . following conventional DLVO theory (8). It would therefore seem unlikely that this effect could also explain the increase in stability in the frothing experiments as observed in the present study, which occurred in the region of low electrolyte concentrations. However, at increasing electrolyte concentrations, earlier researchers (3, 5 ) have reported that single froth films show an increase in stability which begins to occur beyond the region ofcb. This appears to correlate with the frothing experiments and can account for the increase in the lifetime of the froth which begins to occur about 10-4 M KCI. Values of c~ for the two surfactants are indicated in Fig. 2. It was earlier suggested that eb was associated with structural changes within the film and possibly cohesion ofsurfactant in the adsorbed layers (5). In fact, a possible cause of the increase in stability is the interaction of steric forces during the close approach of twin adsorbed monolayers. Naturally, this relative simple approach based on the comparison of the time of froth decay to the stability of an individual thin film can only be treated qualitatively. In fact, it is difficult to develop a precise quantitative relationship, since the froth system cannot be simply considered as the sum of many independent films. As discussed by Kruglyakov ( 11 ), it is clearly a much more complex system, consisting of interrelating elements; films, PlateauGibbs borders, and junctions. Gas diffusion between bubbles and evaporation must also be taken into consideration Journal of Colloid and Interface Science, VoL 152, No. 2, September 1992

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LETTERS TO THE EDITOR

in assessing froth stability, in some cases causing larger froth bubbles to grow at the expense of smaller bubbles. However, so-called "collective effects" (12) can occur in froth systems where the rupture of single films can lead to the destruction of adjacent films and borders because of structural rearrangement and local perturbations. REFERENCES 1. Exerowa, D., Zacharieva, M., Cohen, R., and Platikanov, D., Colloid Polymer Sci. 257, 1089 (1979). 2. Clnnie, J. M., Corkhill, J. M., Goodman, J. F., and Ingram, B. T., Spec. Discuss. Faraday Soc. 1, 30 (1970). 3. Kolarov, T., Cohen, R., and Exerowa, D., Colloids Surf. 42, 49 (1989). 4. Scheludko, A., Kolloid Z. 155, 39 (1957). 5. Scheludko, A., Adv. Colloid Interface Sci. 1, 391 (1967). 6. Barneveld, P., Scheutiens, J. M. H. M., and Lyklema, J., Colloids Surf. 52, 107 (1991). 7. van den Boomgard, A., Ph.D. thesis, Chap. 5. Wageningen University, 1985.

Journal of Colloid and Interface Science, Vol. 152, No. 2, September 1992

8. Manev, E. D., and Pugh, R. J., Langmuir 7, 2253, (1991). 9. Kolarov, T., Cohen, R., and Exerowa, D., Colloids Surf 42, 49 (1989). 10. Yoon, R. H., and Yordan, J. L., J. Colloid Interface Sci. 113, 430, (1986). 11. Kruglyakov, P, M., in "Thin Liquid Films, Fundamentals and Applications" (I. B. Ivanov, Ed.), p. 767, Dekker, New York, 1988. 12. Khristov, Kh., Kruglyakov, P. M., and Exerowa, D., Colloid Polym. Sci. 252, 506 (1979). R. J. P U G H t E. D. MANEV

Institute for Surface Chemistry Box 5607 Stockholm, Sweden S-11486 Received May 11, 1992; accepted June 1, 1992

To whom correspondence should be addressed.