Langmuir 1986,2,649-653 Aqueous foams in which stability has common characteristics independent of the eventual foam application are therefore more easily defiied in terms of their stereological parameters or by a parameter such as a.
Summary This paper has proposed a method for the characterization of foams which depends primarily on the physical chemical processes that operate during the growth and stabilization periods of foam cell evolution. Quite separately, the paper presents the idea that stereological techniques offer a means for experimentally characterizing a foam irrespective of the type of foam or its application. I t does not address the detail by which the physical chemical processes or attributes lead directly to the stereological features. Exploration of this will involve a comparative analysis of many different types of foams and hence will be more appropriate in a future paper. The stereological analysis of a simple foam system can provide a general model for foam cell morphology from which an extension can be made into more complicated cellular systems including the dynamic evolution of such systems.
649
In this manner the applications of stereology for foams will move from the initial research domains in which the basic relationships of the chemical, physical, and mechanical parameters of the foam material become established with those of relevant stereological parameters into the industrial domain. In this latter situation, stereology will function in either a quality-control capacity or in a realtime control manner via computer feedback for processing condition control. There is little difficulty in establishing a characterization method for a foam system of standardized composition and preparation, but a broad selection of foams must be compared in order to establish a valid nonspecific comprehensive characterization method. This type of an investigation will provide the basic information to ensure that future foam design and production provide the optimum cellular performance in any area of application.
Acknowledgment. We express our appreciation to the following for their assistance in the areas of instrument development, sample acquisition, preparation, and testing: R. P. Nathhorst, Carl Zeiss, Inc., Dow Chemical of Freeport, TX, and Nowsco.
Stability of Mechanically Generated Foamst G . M. Nishioka Technical Center, Owens-Corning Fiberglas, Granville, Ohio 43023 Received February 28, 1986. In Final Form: July 10, 1986 A device for measuring the stability of mechanically generated foams is described. Provisions for the control of foam density and the use of different gases are incorporated in the device. The reproducibility of decay properties of foams of different density and produced under different shears in the generator was measured. The decay in area of foams with densities of 0.08 g/cm3is most consistent. Foams at other densities give reproducible results only if they are produced under certain conditions of shear in the generator. The average area, A , and average film life, A, of a foam during its initial decay are proposed as fundamental measures of foam stability.
Introduction Foams are desired in a variety of applications, such as in food manufacture, in production of fire fighting foams, or in the sizing of texti1es.l In most of these applications, foams are generated mechanically. Little is known, however, about the properties of mechanically generated foams produced under a variety of conditions. The influence of factors such as gaslliquid ratio and shear conditions on the stability of foam are herein reported. The stability of foam is measured, most fundamentally, by the decrease in area of extended liquid surface. A device is available2 that measures the decrease in area of foams. The principle of the measurement exploits the relationship existing between the state variables of external pressure P, gas volume V , temperature T , moles of gas n, surface tension u, and area A, for a foam? nfRT = P,Vf + 2uA/3 (1) where nf = number of moles of gas contained in the foam, P, = pressure external to the foam V f = volume of gas in the foam, u = surface tension, and A = area of liquid
surface in the foam. Consider a closed system at constant volume containing a foam. The total amount of gas in the system is ne + nf,where ne = moles of gas external to the foam, or PeVe (PeVf+ 2cA/3) ne + nf = (2) RT RT where V , = volume external to the foam. Let the foam decay a t constant temperature, then P',Vrf P',V'f + 2 ~ A ' / 3 n', n'f = (3) RT RT where the primes indicate later values after decay. Since ne nf = n', n'f PeVe P,Vf + 2 ~ A / 3 Pr,Vl, P',Vrf + 2 ~ A ' / 3 =(4) RT RT RT RT or, after some rearrangement, 3VAP + 2uAA = 0 (5) where V = V , - V , = V e+ V , = V', + V;, AP, = Pre- P,, and AA = A ' - A; V , = total volume of the vessel; V , = +
+
+
+
-+
+ +
~~~
'Presented at the symposium on Tluid-TGd Interfaces: Foams", 190th National Meeting of the American Chemical Society. Chicago, IL, Sept 8-13, 1985.
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(1) Bikerman, J . J . Foams; Springer-Verlag: New York, 1973. (2) Nishioka, G.; Ross, S . J. Colloid Interface Sci. 1981, 81, 1. (3) Ross, S . Ind. Eng. Chem. 1969, 61, 101.
0 1986 American Chemical Society
Nishioka
650 Langmuir, Vol. 2, No. 5 , 1986 LIOUID
GAS COMPUTER
PUMP
l-7
HUMIDIFIER
I 1 d3-YSDUCER +--J
PRESSURE
FLOW CONTROLLERS
I D
I HEAT EXCHANGER
Figure 1. Schematic of the device, including the foam generator and measuring system.
volume of solution in the vessel. Equation 5 is the theoretical basis of the device; it relates pressure changes external to a foam to changes in its internal area. I t is analogous to the constant-pressure relation derived for bubbles by Tait and generalized by Ross for foams with polyhedral or spherical cells, indifferently: 3PAV + 2uAA = 0 (6) Values of absolute area of foam can be obtained in the following manner. The area of liquid surface in foam is A ( t ) = A0 + AA (7) where A ( t ) = area at time t and A. = initial area of the foam. When the foam completely collapses, A ( m ) = 0 = A0 - 3VAl',/2~ (8) and therefore A. = 3VAPm/2u
(9)
A ( t ) = (3V/2a)(AP, - AP(t))
(10)
and Interfacial area of a foam can be measured, therefore, simply by monitoring the change in pressure external to a foam in a container of constant volume and temperature, if the total volume of the system and the surface tension of the foamed liquid are known. The value AP, can be obtained by letting the foam decay for a sufficiently long time or by injecting a small quantity of antifoam through a septum in the container. Values for the area of a decaying foam by use of eq 10 were found to correlate with photographic estimates of the area.2 The device described in ref 2 creates foams by means of a pressurized glass frit; the increase in head space pressure is recorded and the area decay curve is then calculated. Several difficulties arise, however, when a frit is used to generate foams. For example, viscous liquids cannot be foamed using a frit. In addition, glass frits are difficult to clean and are subject to degradation by alkaline solutions. It is also difficult to control the density of foams generated with a frit. These difficulties can be avoided by generating foam mechanically. An improved device for measuring foam stability, using a mechanical foam generator, has been constructed. A detailed description of the apparatus, procedures for its use, and properties of foams generated
Table I. Data Collection Schedule points period, s time interval, h 120 5 0.0-0.166 200 15 0.166-1.0 120 60 1.0-3.0 210 120 3.0-10.0
and measured by the improved apparatus are presented in this paper. Apparatus Figure 1 is a schematic diagram of the experimental device. It consists of controllers to regulate the flow of liquid and gas into the generator, the foam generator (Oakes Foamer with 2-in. mixing head), the foam-measuring system, and a data collection system. Prior to a measurement, the system is purged with the appropriate gas by opening valves C, D, E, and F. The sample chambers are also thermally equilibrated to 20 "C. Foam is generated by metering the liquid and gas at controlled rates into the generator. The rotor speed of the mixing head is also adjusted. Foam is initially bypassed into a vent, by opening valve A and closing B. A measurement is begun when the generated foam appears uniform, usually 1min after the generator is started. Purging is stopped by closing valve C, and foam is loaded into the sample chamber by closing valve A and opening B. After 1 L of foam enters the 1.3-L sample chamber, valves D, F, and E are closed in that order and valve A is opened. Valves F and E are respectively closed 5 and 10 s after valve D is closed. The increase in head-space pressure is then recorded by the microcomputer. Typically 650 data points are collected over a 10-h period, following the schedule listed in Table I. Once a measurement is completed, the liquid from the collapsed foam is collected with a pipet connected to an aspirator and weighed. Typically, between 40 and 100 mL of liquid are collected, with less than 0.2 mL remaining in the sample chamber. The volume of liquid and original foam density are calculated. Equation 2 is then used to compute the decay in area with time. The following points are important in the operation of the apparatus: (1)The pressure of the incoming gas must be sufficient to drive the foam into the device. Usually 30 psi is adequate. (2) The volume of foam inserted into the sample chamber is controlled simply by visual observation of the 1 L mark on the glass sample chamber. As
Stability of Mechanically Generated Foams
Langmuir, Vol. 2, No. 5, 1986 651
Table 11. Foam Generation Conditions condtn rotor speed, rpm foam density, g/mL 750 750 750 1500 1500 1500
1 2 3 4 5 6
0.044 0.080 0.104 0.044 0.080 0.104
E0 ?-:
t:.
...................................
4,
2.0
3.0
4.0
5.0
7.0
6.0
8.0
10.0
9.0
TIME (HRS.1 Figure 3. Latter stages of the decay of foams shown in Figure 2.
o.m
0.25
0.9
TINE
0.75
1.m
(HRS.1
Figure 2. Increase in head-spacefor aqueous foams generated with a rotor speed of 750 rpm with a density of 0.080 g/cm3. Three measurements are shown. will be shown later, this technique gives sufficient reproducibility in the decay curve. (3) Because rapid decay occurs in the initial stages of the foam life, valves D, E, and F should be closed in a reproducible manner. Allowing 5 s between valve ciosings is sufficient to ensure a reproducible start. (4) The density of the foam must be much less than the density of the liquid, to avoid producing a foam containing spherical bubbles (“kugelschaum”). Equation 2 is not valid for a “kugelschaum” (except a t zero gravity), since hydrostatic pressure becomes an additional complication. As aqueous foam consisting of uniform gas spheres cannot have a density less than 0.26 g/mL, in these studies, the initial foam density was kept below 0.10; it was assumed that the foam generated was sufficiently uniform to guarantee that a negligibly small number of bubbles were present as spheres.
I
0.m
0.50
0.25
i.m
0.75
TIME (HRS.1 Figure 4. Decay in area of the three foams shown in Figure 2.
Procedure An aqueous solution consisting of 0.5% sodium dodecyl sulfate (99%, Fisher Scientific) and 0.5% dodecylamine oxide (Stepan Chemicals) was selected for initial study. This solution produces very stable foams in which decay occurs primarily by gas diffusion, since breakage of bubbles does not appear to occur. In other studies, foams were created from 0.5% sodium dodecyl sulfate containing varying amounts of dodecylamine oxide. The surface tension of these solutions were measured at 20 “C using a Wilhelmy plate. These solutions were all well above their critical micelle concentration, hence their surface tension would not be dependent on area. Humidified nitrogen is used as the gas phase in all measurements. Different foams are created by varying the gaslliquid ratio (density), or by varying the rotor speed of the mixer. Each type of foam was measured 3 or 4 times. These conditions are summarized in Table 11.
Results and Discussion In Figure 2 the first 2 h of the raw data are plotted for three measurements of a foam produced under condition
w m
3 E
.......... ...................
-----
.
.
o m
,
0.25
.
.
a
8
I
0.50
.
’
’
.
,
0.75
,
,
“
1
1.m
TIME [HRS.] Figure 5. Decay in area of three foams generated with a rotor speed of 750 rpm with a density of 0.104 g/cm3.
2 (see Table 11). The latter stages of decay are shown in Figure 3. An accurate measurement of AI% requires that these foams decay for a t least 7 h. The decay in area for these foams is plotted in Figure 4. The reproducibility of both the area and pressure decay curves is good. Figure 5 plots the stability of three foams produced under condition 3. The reproducibility of these foams is
Nishioka
652 Langmuir, Vol. 2, No. 5, 1986 Table 111. Deviation of Various Foam Lifetime Parameters conditn S(AI0) S(A2) S(crl0) S(cr2) S(hl0) S(X2) 0.035 0.004 0.071 0.106 0.040 1 0.102 0.027 0.029 0.036 0.024 0.030 2 0.017 0.198 0.130 0.149 0.088 3 0.194 0.127 4 0.214 0.140 0.258 0.183 0.150 0.081 0.030 0.035 0.012 0.052 0.042 5 0.052 0.025 0.078 0.027 0.078 0.010 6 0.085
poor. The poorer reproducibility is not due to the increased initial density of this foam per se; rather it is due to the increased effect of drainage present in the more dense foam. Drainage is an effect that occurs during foam formation, loading, and decay and contributes to the variation in the nature of the foam measured. The reproducibility of several convenient measures of foam stability derived from decay curves can be tested from these data. The average area A of the foam over a time T is
A = 1 / 7 S 0' A ( t ) dt The average specific area CY
=
CY
over a time
o.m
0.25
0.w
0.75
ROTOR-758
1.m
TIME IHRS.)
Figure 6. Effect of rotor speed on the decay of foam: (-) generated with a rotor speed of 750 rpm; (- - -) generated with a rotor speed of 1500 rpm.
(3) T
is
~/SL'CY(~) dT
------_.
(4)
where a ( t ) = A ( t ) / w ; w = foam weight. The average specific film life X is
X = l / a ( 0 ) J07 a ( t )dt
(5)
where a ( 0 ) = initial specific area of the foam. The standard deviation, S, divided by the appropriate mean value is listed in Table 111, for each of these foamstability parameters. The fractional deviation of each parameter was calculated from the first 2 h of the data as well as over the entire 10-h curve. Thus, S(A10) represents the fractional deviation in average area calculated over a 10-h span from several foam decay curves. For example, foams produced by condition 1gave average areas varying by about 10% over a 10-h decay (S(A10)). It is immediately apparent that foams produced under conditions 3 or 4 are harder to reproduce than those produced under the other conditions. A necessary balance seems to be required between the rotor speed and the gaslliquid ratio that conditions 3 and 4 do not meet. It is not surprising that foam stability parameters derived from the first 2 h of life are more reproducible than those derived from the entire 10 h of life. In the latter stages of decay, the minute increases in pressure that occur are the same order as fluctuations in pressure caused by temperature fluctuations (see Figure 3). That the average specific area CY is not more reproducible than the average area A is revealed by the results included in Table 111. Table I11 reveals that the average area, specific area, or film life of a foam over a 2-h period, can be measured with an error of 3-470. Figure 6 compares decay curves for two foams created at different rotor speeds. Not surprisingly, the foam created at the higher rotor speed has a higher initial area, presumably due to finer cell sizes. However, the higher area foam decays more rapidly, and after 15 min its area is less than that of the foam created a t a low rotor speed. The crossing of these curves demonstrates that foam stability is influenced by many factors, such as the increased drainage that probably occurs in the foam created at higher rotor speed. The effect of variations in liquid composition are revealed in Figure 7. The addition of 0.1% dodecylamine
0% A . C .
2X A . O . 0.1X A.O.
Cl
om
'
0'25
0:s
075
1.M
TINE (HRS.1
Figure 7. Stability of 0.5% sodium dodecyl sulfate foams generated with a rotor speed of 1500 rpm and density of 0.080 g/cm3. Each curve represents the average of three measurements: (7) no added amine oxide; (- - -) 0.1 % amine oxide; (-) 0.2% amine oxide, (---) 0.3% amine oxide; (---) 0.4% amine oxide.
oxide to a 0.5% sodium dodecyl sulfate foam is shown to increase the initial area of the foam. However, after 1 / 2 h both foams are of about the same area. Since addition of small amounts of amine oxide retard the drainage rate of liquid from foam, we speculate that the decrease in drainage manifests itself in the first half hour of the decay curve. Addition of successively more amine oxide causes small increases in the initial area of foam, but these foams are always of higher area than the sodium dodecyl sulfate foam a t equivalent times. Rosen4ts has shown that amine oxide forms complexes with anionic surfactants a t an aqueous interface, with optimum packing occurring at an equimolar ratio. Therefore, it is reasonable to believe that the effect of increasing amounts of amine oxide decreases the gas permeability through the interface, and this affects foam stability over the entire decay curve. The largest source of error with this technique is associated with the insertion of foam into the sample chamber. The back pressure the generator encounters is not controlled and may affect the structure of the foam. In addition, the foam ages for about a minute between the time it is generated and the time it is loaded into the sample chamber. Part of the decay curve is therefore not mea(4)Rosen, M. J.; Friedman, D.; Gross, M. J. Phys. Chem. 1964, 68, 3219. ( 5 ) h e n , M. J. Surfactants and Interfacial Phenomena;Wiley: New York, 1978,p 17.
Langmuir 1986,2,653-659 sured. Finally, the quantity of foam inserted into the sample chamber varies. However, the cumulative result of these errors has been shown to cause minimal variations between decay curves measured on a single device. Time averages of various foam stability parameters were used as measures of reproducibility. Although an analytical treatment of the experimental results is preferred, no simple analytical function was found to adequately model these data. The best approach toward use of these data is to compare them with existing models of foam decay. Interbubble gas diffusion always occurs in foams and can be modeled by Lemlich’s approach,&8which is
653
similar to classical Ostwald ripening theory. Foam drainage also occurs, an effect that must be accounted for in models of foam stability. There is at present no model that accurately predicts the foam decay curves reported here. Area decay curves may provide the information needed for development of more sophisticated models of foam decay. (6) Lemlich, R. Ind. Eng. Chem. Fundam. 1978,17,89. (7) Ranadive, A. Y.; Lemlich, R. J. Colloid Interface Sci. 1979,70,392. (8) Nishioka, G.;Ross, S.; Whitworth, M. J. Colloid Interface Sci. 1983, 95, 435.
Influence of Hydrophobic Particles on the Foaming of Aqueous Surfactant Solutions+ Michael P.Aronson Lever Research, Inc., Edgewater, New Jersey 07020 Received March 27, 1986. In Final Form: June 1 1 , 1986 Garett (1979) showed that inert Teflon particles can act as antifoams and proposed they function by forming a hole in the foam film via a bridging mechanism. This work has been extended to various other hydrophobic materials (paraffiis, alkanes, and fatty acids) dispersed in representative surfactant solutions. Their influence on the foaming of dilute solutions was determined and their effect on the stability of single foam films was also studied. Bridging appears to be the principal mode of antifoaming, and solid particles were found to be much more effective in reducing foamability than liquid particles of the same size and chemical composition. Solid particles are more effective than pure liquids probably because of their greater ability to rupture the film that separates the particles from the surface of the foam bubble. This appears to be related to surface roughness, and the difference in behavior is particularly striking in anionic surfactant solutions which form stable films on hydrophobic surfaces over a range of surfactant concentrations. The situation is more complex in surfactant systems that are soluble in the dispersed phase, e.g., alcohol ethoxylates. Here the particles can deplete the surfactant which also contributes to a reduction in foaming.
Introduction Numerous factors control the formation and stability of foams even in relatively simple In many practical applications the situation is further complicated by the presence of dispersed particles that are incorporated in the foam by accident or design. In some applications such as in foods and cosmetics, dispersed particles, frequently liquid crystalline, are used to increase the stability or alter the texture of the In other cases, foam is undesirable and oil dispersions are added as antifoam^.^,^,^ In still other cases, particles confer a “controlled” degree of stability to the foam, an example being polyurethane foams.g A number of studies have attempted to unravel how particles destabilize aqueous foams. Ross and co-workerslO,” have clearly demonstrated the importance of spreading of a water-soluble agent particularly for the destruction of labile foams,12 although other processes appear to operate in “filled” antifoams, e.g., dispersions of solids in an ~ i l . ’ ~ ,Garrett,15 ’~ Dippenaar,16and others17 demonstrated that chemically inert dispersed solids also act as antifoams and proposed that the particles rupture the foam film by a bridging-dewetting mechanism. The following study was undertaken to better define the conditions under which these antifoam mechanisms opt Presented at t h e symposium on ’Fluid-Fluid Interfaces: Foams”, 190th National Meeting of the American Chemical Society, Chicago, IL, Sept 8-13, 1985.
0743-7463/86/2402-0653$01.50/0
erate in relatively simple model systems. In particular, the influence of various chemically similar hydrophobic particles on the foaming of aqueous surfactant solutions has been investigated. In contrast with most studies reported (1) Lucassen, J. In Anionic Surfactants; Lucassen-Reynders, E. H., Ed.; Marcel Decker: New York, 1981, Vol 2, p 217. (2) Prins, A. In Foams; Akers, R. J., Ed.; Academic Press: New York, 1976; p 51. (3) Bikerman, J. J. Foams; Springer Verlag: New York, 1973. (4) Berger, K. G. In Foods Emulsions; Friberg, S., Ed.; Marcel Decker: New York, 1981; p 141. (5) Handleman, A. R.; Conn, J. F.; Lyons, J. W. Cereal Chem. 1961, 38, 294. (6) Saito, H.; Friberg, S. Pramona Suppl. No. 1 537, 1975. See also:
Manev, B. E. D.; Sazdonova, S. V.; h a , A. A.; Wasan, F. T.J. Dispersion Sci. Technol. 1982, 3, 435. (7) Foam and Emulsion Control Agents; Colbert, J. C . , Ed.; Noyes Data Corporation: New Jersey, 1981. (8) Pattle, R. E. J.S.CI, 1950,69,363. See also: Sinka, J. V. Lichtman, I. A. Int. Dyer Text. Printer Bleacher Finish. 1970,489. (9) Roasmy, G.; Kollmeier, H. J.; Schator, H.; Wiemann, M. J. Cell. Plast. 1981, Nov, 319. (10) Ross. S. J. Phvs. Chem. 1950. 54. 429. (11) Ross; S.; Hugh&, A. F.; Kennedy, M. L.; Mardoian, A. R. J. Phys. Chem. 1953,57, 684. (12) Ross, S.; Nishioka, G. Colloid Polym. Sci. 1977, 255 (6), 560. (13) Kulkarni. R. D.: Goddard. E. D.: Kanner, B. Ind. Eng. Chem. Fundam. 1977, 16, 472. (14) Van Boekel, M. A. J. S.; Walstra, P. Colloids Surf. 1981,3, 109. (15) Garrett, P. R., J. Colloid Interface Sci. 1979, 69, 107. (16) Dippenaar, A. Int. J. Miner. Process. 1982, 9, 1, 15. (17) Kurzendoerfer, C. P. Tr.-Mezhdunar. Kongr. Pouerehn.-Akt. Veschestuam, 7th 1976,2, 537.
0 1986 American Chemical Society
