pubs.acs.org/Langmuir © 2009 American Chemical Society
Novel Method and Parameters for Testing and Characterization of Foam Stability K. Lunkenheimer,*,† K. Malysa,‡ K. Winsel,† K. Geggel,† and St. Siegel† †
Max-Planck-Institut f€ ur Kolloid- und Grenzfl€ achenforschung, D-14424 Potsdam, Germany, and ‡Institute of Catalysis and Surface Chemistry, ul. Niezapominajek No. 8, PL-30239 Cracow, Poland Received September 16, 2009. Revised Manuscript Received November 2, 2009
The paper presents novel parameters which can be used for a swift characterization of all kinds of liquid foams. The procedure of the automated method developed consists of introducing a predefined volume of gas into the test solution contained in a cylindrical glass column at constant flow rate. The levels of the foam and of the solution are recorded simultaneously in dependence on time using a photosensor system. Two novel parameters, called time of deviation and time of transition, have been derived on the basis of simultaneous measurements of the changes in the foam volume (ΔVF) and the corresponding volume of the drained solution (ΔVS). These parameters enable one to distinguish three different stages of the foam decay, and on their basis the foam stability can be predicted, irrespective of whether constituting an unstable (wet) or a (meta)stable (dry) foam system. The validity of the method elaborated is demonstrated by applying various unstable and stable foam systems, including biological surfactants such as sugar and lung surfactants.
1. Introduction Foams of different stability are widely met in multiple applications. Liquid foams are quite complicated systems of a very wide stability spectrum. They may survive for seconds up to days or even weeks. Thus, in spite of the numerous attempts performed so far, there is neither a general comprehensive theory on foam nor is there a fundamental experimental procedure by means of which any foam system could be tested reliably by a standard method.1-6 The main reason for this state of affairs is presumably also due to the big versatility of foam application spanning from more “classical” processes such as cleaning, washing, flotation, fire fighting, to production of food, 7-9 beverages, and cosmetics, and to protection of the environment10 up to rather exceptional applications such as favorable drilling medium in oil production,11 evaluation of beer12 or in life science.13-15 Because of it, various methods of measurements and tests are available, which serve the (1) Bikerman, J. J. Foams; Springer-Verlag: Berlin, Heidelberg, New York, 1973 (2) Exerowa, D.; Kruglyakov, P. Foam and foam films; Elsevier: Amsterdam, 1998 (3) Pugh, R. J. Adv. Colloid Interface Sci. 1996, 64, 67–142. (4) Malysa, K.; Lunkenheimer, K. Curr. Opin. Colloid Interface Sci. 2008, 13, 150–162. (5) Ivanov, I. B.; Danov, K. D.; Ananthapadmanabhan, K. P.; Lips, A. Adv. Colloid Interface Sci. 2005, 114-115, 61–92. (6) Rusanov, A. I.; Krotov, V. V. Gibbs elasticity of liquid films, threads, and foams; Cadenhead, D. A., Danielli, J. F., Eds.; Progress in Surface Membrane Science; Academic Press: 1979; Vol. 13, pp 415-524. (7) Sceni, P.; Wagner, R. Food Sci. Technol. Int. 2007, 13, 461. (8) Rodriguez Patino, J. M.; Carrera Sanchez, C.; Rodriguez Nino, Ma. R. Adv. Colloid Interface Sci. 2008, 140, 95–113. (9) Faines, A.; Bertrand, D.; Baniel, A.; Popineau, Y. Food Hydrocolloids 1997, 11, 63–69. (10) Watcharasing, S.; Chavadej, S.; Scamehorn, J. F. Sep. Sci. Technol. 2008, 43, 2048. (11) Rovig, J. APPEA J. 1996, 36, 557–561. (12) Kordialik-Bogacka, E. Przemysl Fermentacyjny i Owocowo-Warzywny 2002, 46, 11–13. (13) Cowett, R. M.; Unsworth, E. J.; Hakanson, D. O.; Williams, J. R.; Oh, W. N. Engl. J. Med. 1975, 293, 413–416. (14) Ianniruberto, A.; Destro, F.; Capozzi, A.; Zisa, F.; Cubesi, G.; Parisi, S. J. Perinat. Med. 1975, 3, 105–114. (15) Winsel, K.; Lunkenheimer, K.; Geggel, K.; Witt, Ch Tenside, Surfactants, Deterg. 2004, 41, 10–18.
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need of special requirements within a limited field of applications. However, just for this reason, a generally applicable method of foam characterization is needed. The various methods of foam characterization differ from each other mainly in the manner of dispersing the gas phase, usually air, in the foaming solution. Thus, foam may be produced mechanically by agitating, stirring, pouring, or dropping the solution and so on. The main disadvantage of these miscellaneous methods is that their boundary conditions cannot be controlled satisfactorily. Even the problem of selecting a proper “zero time” of foam decay is still under discussion.16 In the famous Ross-Miles method, applied to stable foams, a definite amount of the foaming solution is poured from a well-defined height through a small hole of defined diameter onto the said solution,17 but the amount of gas dispersed is not controlled. The amount of foam formed and the lifetime of either the entire or half the height of the foam are then measured. The main advantage and the reason of the widespread application of this method lie in its simplicity. Diverse modifications and standardization were applied2,18-20 to improve the method’s reproducibility. An interesting modification has been put forward recently by Pinazo et al.21 The analogously popular and even simpler shaking test (Bartsch method) has a similar drawback like that one of Ross-Miles; the amount of gas introduced into the system is not controlled at all. Certainly, there are procedures in which the amount and the velocity of the gas introduced into the system are well controlled, for example, the pneumatic methods.1-3,22 However, these methods are more complicated and laborious, and they (16) Iglesias, E.; Anderez, J.; Forgiarini, A.; Salagar, J.-L. Colloids Surf., A 1995, 98, 167–174. (17) Gohlke, F. S. Parfuem. Kosmet. 1964, 45, 59. (18) Kelly, W. R.; Borza, P. F. J. Am. Oil Chem. Soc. 1966, 43, 364. (19) Lemlich, R. J. Colloid Interface Sci. 1971, 37, 497. (20) Lai, K.-Y.; McCandlish, E. F. K.; Aszman, H. In Liquid Detergents; Lai, K.-Y., Ed.; Surfactant Science Series; Marcel Dekker: New York, 1997; Vol. 67, Chapter 7: Light-Duty Liquid Detergents. (21) Pinazo, A. P.; Infante, M. R.; Frances, E. I. Colloids Surf., A 2001, 189, 225– 235. (22) Garrett, P. R. Chem. Eng. Sci. 1993, 48, 367.
Published on Web 11/19/2009
DOI: 10.1021/la9035002
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can hardly be applied in a standard procedure for foams of extremely different stability.23 Recently, we elaborated and patented24 a simple measuring device and a novel approach for the characterization of foamability and foam stability, which can be applied to all kinds of fluid foam systems irrespective of whether they belong to the so-called wet (unstable) or dry (stable) foams. The paper presents briefly the method of measurement and describes in detail the novel parameters (time of deviation, tdev, and time of transition, ttr) which were derived from the rheological behavior of the foam column produced under well-defined conditions. The time of deviation, tdev, determines the initial time interval of the foam decay, when there is only drainage of solution from the foam column but not yet any foam rupture. The time of transition, ttr, refers to that stage of foam decay when the efflux of the solution from the foam column becomes negligible and the rate of foam film rupture is at maximum.
2. Experimental Section 2.1. Apparatus and Procedure of Measurements. The apparatus is shown schematically in Figure 1 and consists of the following main elements: (i) a cylindrical glass column of 42 mm inner diameter and 25 cm length with a sintered glass G3 at the bottom, (ii) an opto-electric sensor system (consisting of a vertically fixed array of photodiodes) for detecting the heights of the foam and of the solution in the glass column, (iii) a stopcock below the frit to prevent the solution’s leakage from the column, and (iv) a gas inlet connected with a small pump for metering the air supply, the piston of which is driven automatically at a preset gas flow rate. The opto-electric sensor and pumping system, originally developed by the firm Anton Paar, Graz, Austria (FTS - Foam Test System; Paar Physica), was used in the experimental setup. The amount of air dispersed, the volume of the solution, and the gas flow rate are precisely controlled in this setup. In the experiments a total air volume of 90 mL was introduced through the frit into 50 mL of the test solution with a gas flow rate of 18 dm3/h. Then, the changes of the levels of the foam/air (hF) and of the solution/foam (hS) boundaries were measured simultaneously as a function of time. The measuring accuracy of the optoelectric sensor system was (2.5 mm. The principle of measuring the variables of hF(t) and hS(t) is outlined in Figure 2, where the time t = þ0 denotes that moment when the gas supply was finished. 2.2. Foaming Solutions. Solutions of very different surfaceactive compounds were used in the experiments to check the method’s versatility. The samples tested were prepared from aqueous stock solutions of either as-received (ar) or surfacechemically pure surfactant solutions (scp). In the latter case, this peculiar grade of surfactant purity guaranteeing the absence of any intermingling effects of surface-active trace impurity components was prepared especially.25-27 The surfactants sodium dodecyl sulfate, n-nonyl-R-D-glucopyranoside, and n-octanoic acid were obtained from Fluka and/or Sigma. As an important example of biological application, we also investigated foam properties of a sample of the so-called lung surfactants. It is the very complex composite surface-active material of the lungs consisting of various amphiphilic components such as phospholipids and surfactant proteins. The surface-active material of the (23) Khristov, Khr.; Malysa, K.; Exerowa, D. Colloids Surf. 1984, 11, 39. (24) Lunkenheimer, K.; Malysa, K.; Wienskol, G.; Baranska, B. European Patent Bulletin of 16.01.2008 (Art. 97(3) EPC), European Patent No. 1 416 261 (03 024 885.0) (Application EPA 02024377, 31.10.02). (25) Lunkenheimer, K.; Miller, R. Tenside Deterg. 1979, 16, 312. (26) Lunkenheimer, K. Surface-Chemical Purity of Surfactants: Phenomena, Analysis, Results, and Consequences. In Encyclopedia of Surface and Colloid Science, 2nd ed.; Taylor & Francis: New York, 2006; Vol. 8, pp 5879-5906. (27) Pizakowska-Pietras, D.; Lunkenheimer, K.; Piasecki, A. J. Colloid Interface Sci. 2006, 294, 423–428.
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Figure 1. Vertical sectional view of the foam test apparatus used in the measurements. lungs was isolated by bronchoalveolar lavage in the framework of the clinic diagnosis of lung diseases. Briefly, isoosmotic saline was instilled via a bronchoscope into the patients’ lungs and aspirated immediately after instillation as described in detail in ref 15. The experiments were performed at room temperature (22 ( 1 C).
3. Results and Discussion 3.1. Time of Deviation: Determination and Physical Meaning. As the column’s diameter was constant, only the heights of foam and solution were measured (Figure 2) to determine the corresponding variations of the foam’s and of the solution’s volume with time: ΔhF ðtÞ ¼ hF ð þ 0Þ -hF ðtÞ
ð1Þ
ΔhS ðtÞ ¼ hS ðtÞ -hS ð þ 0Þ
ð2Þ
The novel derived parameters, time of deviation (tdev) and time of transition (ttr), were defined on the basis of the analysis of the dependence of ΔhF(t) versus ΔhS(t). Evaluating carefully the changes of these dynamic foam variables, we discovered some simple and rather obvious relations. Immediately after having inserted the air into the surfactant solution, that is, when the height of the foam column is at maximum and the related level of the solution is at minimum (t = þ0; cf. Figure 2), the subsequent initial time interval of the foam decay is characterized by the behavior that the decrease of the foam height (ΔhF) remains identical to the increase of the level of the solution (ΔhS). This behavior is illustrated in Figure 3 for an aqueous surfacechemically pure solution of 2 10-4 M n-nonyl-R-D-glucopyranoside. As can be observed in Figure 3, within this initial period of the foam decay, the ratio ðΔhF =ΔhS ÞðtÞ 1
ð3aÞ
that is, the decrease in the foam height (ΔhF), was identical to the increase of the solution level (ΔhS), and it lasted until the Langmuir 2010, 26(6), 3883–3888
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Figure 2. Sketch of the relevant variables required for determining the novel foam stability parameters. The times denote the following states: t = -0, conditions before starting the measurement; t = þ0, height of foam and of solution immediately after having introduced the defined volume of air; t = t, foam and solution height during the foam decay.
Figure 3. Height (volume) of foam decayed (ΔhF) versus related height (volume) of solution drained out (ΔhS) in dependence on measuring time t for foam generated from an aqueous surface-chemically pure solution of 2 10-4 M n-nonyl-R-D-glucopyranoside.
corresponding height of the draining solution reached þ25 mm. The physical meaning of this finding is clear, and it is rather surprising that, as far as we are aware, nobody else has noticed this relation before. A numerical value of exactly 1 of the ratio (ΔhF/ΔhS)(t) means simply that there has not yet been any rupture of the foam films and the volume of the entire system has remained constant, because no air had escaped from the foam system during this interval. Thus, the initial stage of foam decay during the interval 0 e t e tdev can be well characterized24 by the Langmuir 2010, 26(6), 3883–3888
Figure 4. Heights of foam decayed (ΔhF) versus related height of solution drained out (ΔhS) for three different aqueous surfactant solutions: (black triangle) 1.5 10-3 M n-octanoic acid in 0.005 M hydrochloric acid; (gray circle) 3 10-4 M n-decyl-β-D-glucopyranoside; (black square) 3 10-4 M sodium n-dodecyl sulfate.
numerical value of tdev. The moment when the foam films started to rupture is marked in Figure 4. This parameter (time of deviation, tdev) is very sensitive to the properties of the foaming agent, and its numerical values may vary up to 6 orders of magnitude.24 Figure 4 shows a comparison of the ΔhF(t) versus ΔhS(t) relationships for three aqueous surfactant solutions forming foams of considerably varying stability. The examples presented in Figure 4 stand for solutions of foams forming low, medium, and high stability. The data of Figure 4 illustrate that any decaying foam system does survive a certain initial interval that strictly obeys the relationship described by eq 3a and/or 3b (see DOI: 10.1021/la9035002
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Figure 5. Measuring values of the difference (ΔhF - ΔhS) as a function of time (logarithmic time scale) for various surfactant solutions forming foams of very different stability: (black square) 3 10-4 M sodium n-dodecyl sulfate; (black up triangle) 1.5 10-3 M n-octanoic acid in 0.005 M hydrochloric acid (wet foam); (gray circle) 3 10-4 M n-decyl-β-D-glucopyranoside (dry foam; medium foam stability); (black down triangle) 2 10-3 M lithium n-dodecyl sulfate (dry foam; high foam stability).
below). Hence, we can conclude that the characteristic behavior described by eq 3 can be applied for the characterization of foam systems of quite different stability. As can be observed in Figures 3 and 4, the scatter of measurements seems to become greater the longer the decay procedure lasts. However, this finding is actually caused by the altering properties of the foam systems. Thus, during the initial stage of the foam decay, a comparatively large amount of solution has drained out. With increasing time, the stable foam has become drier and drier. As the measuring accuracy of the solution level was about 2.5 mm only, so rising volumes of the drying foam had to rupture to get the subsequent solution level detected. Therefore, as can be noted in Figure 4, a rising number of measuring values of the foam height were recorded at apparently unaltered height of the solution with time. When the ratio ΔhF/ΔhS(t) becomes greater than 1, it means that the foam syneresis is accompanied by rupture of the foam films, and the foam system passes into the next stage of its decay, which we call the intermediate stage of foam decay (stage II). Finally, when we have ΔhF/ΔhS . 1, together with ΔhS ≈ 0, this means that syneresis had practically finished and only film rupture has still been proceeding (final state, stage III). We proposed24 to characterize the moment of transition from stage II to the onset of stage III by an additional characteristic parameter called time of transition, ttr (see below). To determine the numerical values of the parameters of tdev, the difference of the measured variables ΔhF(t) and ΔhS(t) is favorably presented as a function of time according to ðΔhF -ΔhS ÞðtÞ ¼ 0
ð3bÞ
As can be taken from Figure 5 by plotting this difference (ΔhF - ΔhS)(t) as a function of time, you can immediately get the values of the time of deviation (tdev). In addition, the features of the different stages of the decaying foam column are revealed. In the case of the unstable foam formed from the octanoic acid solution, there is only a very short time interval in which stage I was observed (see Figure 5). It is finished already at a time as short as about 0.3 s, and after having survived this interval foam film rupture and syneresis will take place simultaneously. Moreover, 3886 DOI: 10.1021/la9035002
Figure 6. Differences of the measuring values (ΔhF - ΔhS) as a function of time (logarithmic scale). The related inflection points (transition time, ttr) are marked by arrows. (9) 3 10-4 M sodium n-dodecyl sulfate; (1) 2 10-3 M lithium n-dodecyl sulfate.
opposite to the case of stable foams, such unstable foams hardly do reach the final stage III, because the entire foam column ruptures very quickly and the stage of the dry foam is never reached. This example stands for and illustrates a general behavior of wet (unstable) foam systems. In the case of the very stable, dry foam formed by the 2 10-3 M lithium dodecyl sulfate solution, the tdev related value (ca. 70 s) is greater by 2 orders of magnitude. Thus, the parameter of tdev obviously enables, after rather short measurements, one to distinguish easily differences in the stability of foams, which were generated using quite different surfactant solutions. 3.2. Transition Time. The first stage of the decaying foam characteristic, that is, only solution flows out of the foam column, ends when the measuring values of the dependence (ΔhF - ΔhS)(t) begin to get greater than zero. For times t g tdev, eq 3a no longer holds. It means that another process (rupture of the foam films) has set in. Evaluating the (ΔhF - ΔhS)(t) dependences, it can be noted (see Figure 5) that in the case of stable foams these functions have an inflection point. The existence of the inflection point indicates that the (ΔhF - ΔhS)(t) dependences are characterized by two limiting constant values, one at the onset, with (ΔhF ΔhS) = 0 at t e tdev, and the second one at the end with (ΔhF ΔhS) f constant for t f ¥. In physical terms, it means that at t e tdev, there is no foam film rupture, and for t f ¥ the entire foam column had ruptured. Thus, with respect to the time variable, there is another characteristic parameter (the inflection point) of the foam’s decay process, which we call transition time, ttr.24 The parameter of the transition time, ttr, refers to that peculiar state of foam decay when the decrease of the foam volume, which initially was caused exclusively by solution efflux, turns into a behavior, which is mainly governed by rupture of the foam films only (negligible drainage). Thus, at t . ttr, the solution increment ΔΔhS(t) will approximate zero, that is hS ðtÞ f hSmax ¼ hSt ¼ -0 The meaning of the transition time, ttr, is illustrated in Figure 6 for two surfactant solutions forming very stable foam systems. The values of the parameter time of transition are related to that characteristic state of foam decay when the rate of the rupturing foam column passes through a maximum. At t > ttr, the decay process is dominated by rupture of the foam films with negligible drainage only. Langmuir 2010, 26(6), 3883–3888
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Figure 8. Transition time, ttr, versus concentration for some soluFigure 7. Dependence of (ΔhF - ΔhS) versus time (logarithmic time scale) of a bronchoalveolar lavage fluid (lung surfactant).
To check further and give evidence for the general suitability of the novel parameters elaborated, the investigations were also carried out using foaming solutions of extraordinary origin, namely, that of the lung surfactant system (bronchoalveolar lavage fluid).15 The foam behavior of this aqueous system containing the extremely complex composite surface-active material of the lungs is shown in Figure 7. It is interesting to note that, although this extraordinary biological foam system forms very stable foams, a pronounced transition region is observed nevertheless. This is indicated by the fact that the parameter of the transition time is still almost 1 order of magnitude greater than the value of the corresponding parameter time of deviation. 3.3. Foam Decay and Characterization of Foam Stability: General Features. Let us summarize the most important features and advantages of the method of analysis elaborated. According to this approach, the duration of the initial interval (stage I of foam decay) is indicated by the time of deviation, tdev, and serves as a direct measure of foam stability. The greater the numerical value of tdev, the more stable the foam will be. The characteristic behavior (ΔhF - ΔhS)(t) as illustrated in the Figures 5-7 for solutions of various surfactants clearly reveals that the parameter tdev may cover a few orders of magnitude. Foams generally survive three characteristic physical stages of their decay by means of which the different types of foam can be discriminated. These different stages can be distinguished on the basis of the following relations:24 (i) initial stage: ΔhF =ΔhS ¼ 1
and=or
ΔhF -ΔhS ¼ 0
ð3a; bÞ
(ii) transitional stage: ΔhF =ΔhS > 1
and=or
ΔhF -ΔhS > 0
ð4Þ
and=or
ΔhF -ΔhS .0
ð5Þ
(iii) final stage: ΔhF =ΔhS .1
The parameter of transition time, ttr, is related to the transition from the initial state of exclusive draining to the final state of the Langmuir 2010, 26(6), 3883–3888
tions of various surfactants: (gray circle) n-decyl-β-D-glucopyranoside; (empty square) n-decyldimethylphosphine oxide; (black square) sodium n-dodecyl sulfate.
dry foam, when there is merely foam film rupture and practically no drainage left (cf. Figures 5-7). The transition time, ttr, is plotted against the concentration of several solutions of various surfactants in Figure 8. For simple surfactant solutions, the corresponding parameters ttr cover about 4 orders of magnitude depending on the type of the surfactant and the concentration of their solutions.24 Thus, the characterization of unstable (wet) and (meta)stable (dry) foam systems can be performed by the parameter of transition time, too, and the following criteria were proposed:24 t tr < 10 s f unstable foam ttr > 10 s f ðmetaÞstable foam The higher the ttr value, the more stable the foam system will be. For very stable foams, ttr g 100 s. The parameter ttr is favorably applied to describe and discriminate stable foam behavior, whereas the unstable foam behavior is preferably characterized by the parameter time of deviation tdev. Thus, the elaborated two novel parameters allow one to characterize the foamability of the solutions and to predict the stability of the foams on the basis of quick measurements. Moreover, the characteristic of the transitional interval reveals further information about the behavior of stable foams, too. For unstable foams, the transitional interval will obviously not be fully established because they usually will have already completely collapsed at times a little longer than ttr. On the other hand, for extremely stable foams, two alternative cases have been observed. The first one refers to conditions when the initial efflux rate is relatively high as compared with that of the foam lamellae rupture. In such cases, the transitional stage is passed over relatively fast, which means that the parameters tdev and ttr do approximately coincide. An example of such behavior is presented in Figure 4 for the solutions of the nonionic surfactant 3 10-4 M n-decyl-β-Dglucopyranoside. For this solution, the slope of the measured dependence ΔhF/ΔhS(t) remains equal to 1 up to t = tdev ≈ ttr (within the limit of measuring error). The slope of the measured function of (ΔhF - ΔhS)(t) becomes very steep then (cf. Figure 5). This feature expresses the peculiarity of this foam system, namely that at t = tdev their foam lamellae have already lost almost all of DOI: 10.1021/la9035002
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the surfactant solutions, which usually are of particular interest in application, the parameter t1/2 approaches sometimes its maximum value at concentrations much lower than the maximum concentration used. For example, the maximum of the dependence t1/2(c) of the sodium and lithium dodecyl sulfate solutions is reached already well below their critical concentrations of micelle formation, cmc, which amounts to 5 to 8 10-3 M. Opposite to it, the dependence ttr(c) still increases steadily up to the highest concentration. Hence, this relationship allows a much better discrimination and description of the corresponding foam behavior.
Figure 9. Time of rupture of half of the foam column, t1/2, versus the corresponding transition time, ttr, for several solutions of various surfactants: () n-nonyl-R-D-glucopyranoside; (0) n-decyl-dimethylphosphine oxide; (9) sodium n-dodecylsulfate; (1) lithium n-dodecylsulfate.
their intralamellar solution, resulting in rather “dry” foam films. The second option usually observed (having a broad transitional interval, cf. Figures 5-7) indicates that the duration of the simultaneously proceeding processes of solution drainage and foam film rupture last relatively long. Thus, even from the kind of measured dependence of ΔhF/ΔhS(t) in the intermediate stage, you may obtain additional information about the foam properties. It is worthy to mention here that unstable foams may also be formed at comparatively low concentrations of such surfactants, which generally form foams of high stability only. However, at extremely low surface coverage, these surfactants may obviously reveal typical wet foam behavior, too. 3.4. Comparison of the Novel Foam Parameters with Those Commonly Used. The time of rupture of half of the foam column, t1/2, is the commonly used quantity to characterize foam stability. This parameter is related to the time interval elapsed when the initially produced foam column has decayed down to half of its original height. In Figure 9, the half lifetime, t1/2, is plotted versus the corresponding transition time, ttr, for several solutions of various surfactants. It is seen that there is a linear relationship between the “classical” parameter, t1/2, and the parameter ttr within a broad concentration range, with the exception of the highest concentrations. At very high concentrations of
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4. Conclusions A simple automated method was developed for testing and characterization of foams of extremely varying stability, from wet, unstable to dry, stable foams. Two novel parameters, called time of deviation (tdev) and time of transition (ttr), have been derived on the basis of simultaneous measurements of the changes of the foam volume (ΔVF) and of the corresponding volume of the drained solution (ΔVS). The parameters elaborated (tdev and ttr) are related to the rheological feature of the decaying foam systems and enable one to distinguish the stability of foam systems on the basis of swift and comparatively short measurements. The tdev and ttr parameters characterize different stages of the foam decay, namely, (i) the initial stage (I), during which only the solution’s syneresis from the foam column occurs without any rupture of foam lamellae; (ii) the transitional stage (II), when syneresis and rupture of the foam lamellae occur simultaneously; and (iii) the final stage (III), when the foam lamellae rupture becomes the predominant process because the foam has become so dry that the resulting amount of syneresing liquid becomes negligibly small. The tdev value denotes the end of the initial stage, while the ttr value refers to that moment of transition from the initial to the final stage, which is characterized by maximal rate of rupture. It is shown that the numerical values of the parameters tdev and ttr usually cover a few orders of magnitude, depending on the type of the surfactant and on the concentrations of their solutions. The following criteria were proposed24 to distinguish unstable and stable foams: ttr < 10 s f unstable foam and ttr > 10 s f stable foam. The higher the ttr value, the more stable the foam system will be. For very stable foams, ttr g 100 s. The validity of the method elaborated was demonstrated by applying various unstable and stable foam systems, including biological surfactants such as sugar and lung surfactants.
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