Foams: basic properties with application to porous media - Langmuir

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Langmuir 1986, 2, 673-677

672

Foams: Basic Properties with Application to Porous Media? Daniel D. Huang,t Alexander Nikolov, and Darsh T. Wasan* Department of Chemical Engineering, Illinois Institute of Technology, Chicago, Illinois 60616 Received February 28, 1986. I n Final Form: June 20, 1986 The use of a foam as a permeability blocking agent in enhanced oil recovery processes to enhance sweep efficiency by improving mobility control has been developed in recent years. The aim of our study is to investigate the effects of dynamic surface properties, such as dynamic surface tension, dilational modulus of elasticity, surface viscosity, and thin-film drainage rate, on foam stability and to illustrate a correlation between dynamic surface properties and fluid displacement efficiency in porous media. Our experimental observations have confirmed the previous studies which showed that the capability of producing foam is directly related to the dynamic surface tension and that the foam stability is significantly influenced by the dilational modulus of elasticity at gas-liquid surfaces. However, our porous media experiments using a y-ray absorption technique to measure the dynamic gas-liquid saturation profiles showed for the first time a striking correlation between the dilational modulus and the breakthrough time, effective gas mobility, and fluid displacement efficiency of foaming solutions.

Introduction Foams play an important role in a number of industrial, biological, and household app1ications.l The utility of foams often depends upon their ability to resist a change in volume or liquid content. Therefore, the main objective is to control the stability of a given foam system so that it is suitable for a particular operation.2 Liquid foams are coarse colloid systems in which a relatively large volume of gas is dispersed in a small volume of liquid. In general, they are polyhedral in shape and the gas phase is separated by thin films or lamellae. Like all other dispersed systems, foams are unstable because the free energy associated with the large interfacial area between the liquid and the gas decreases by coalescence of the bubbles. It is important to note that the term "foam stability" refers to the intrinsic resistance of the lamella to a decrease in interfacial area and does not imply its stability in a thermodynamic sense. A quantitative measure of foam stability can be obtained by measuring the period that lapses between the foam formation and foam breaking. In a foam, individual bubbles are separated from one another by liquid films and plateau borders. Many researchers have used the thin liquid film as a model tQ study foam stability. This model allows the investigation of all kinds of foaming solutions under controlled conditions. Although thin film stability has in general been related to the colloidal stability of foam, it should be pointed out that the effects from plateau border and the gas diffusion from a smaller bubble to an adjacent larger bubble have been neglected. In order to study the lifetime of a thin liquid film it is essential to know both the rate of drainage and the transition phenomenon when the film reaches a critical thickness. The forces of interaction that govern the drainage of thin liquid films are the capillary pressure and the disjoining p r e ~ s u r e .The ~ resultant pressure causes the liquid in the film to flow out toward the meniscus. This flow carries with it the surfactant molecules producing uneven distribution of surfactant along the surface. The nonuniform surfactant concentration on the surface leads to a local variation of surface tension, which produces a stress op-

* To whom correspondence should be addressed. 'Presented a t the symposium on "Fluid-Fluid

Interfaces:

Foams",190th National Meeting of the American Chemical Society, Chicago, IL, Sept 8-13, 1985. *Present address: Polaroid Corp., New Bedford, MA 02745.

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posing liquid flow. This effect is referred to as the "Marangoni-Gibbs effect". Another stress is the shearing of the surfactant monolayer which opposes the liquid flow. Thus, the rate of drainage of thin liquid films is controlled by the rheological and transport properties at the surface. The film strength factor becomes important when drainage ceases and the lamella reaches its critical thickness. According to De Vries? a black spot is formed in the thinnest portion of the lamella. Expansion of the black film causes a deformation of the surface and the local surface tension increases. This surface tension gradient strengthens the film which may allow it to overcome the disturbance at the surface. Accordingly, the factors that influence foam stability and thin-film lifetime are primarily governed by the dynamic surface properties in the system, such as dynamic surface tension and viscoelasticity of the surfaces. In enhanced oil recovery, the potential for using the foam is based on its unusual physical structure and unique surface properties which produce high flow impedance to improve the sweep effi~iency.~,~ During the process, foam is formed when gas and a solution of a surface-active agent are injected into a porous medium either simultaneously or intermittently. Previous W O ~ ~ S ~ -show ~ O that the amount of foam which can be produced from an aqueous surfactant solution depends on the manner of foam formation and on the amount and nature of the surfactant. Also, the flow rate of liquid and gas have direct relation to the size of the bubble and capillary and to the number and strength of the films in the porous media.l' This reveals that the surface properties are the dominant factors for the determination of mobility control processes.lo,ll Many researchers have attempted to explain foaming behavior in terms of equilibrium surface properties but have not obained a consistent conclusion. As the generation and (1) Bikerman, J. J. Foams; Springer-Verlag: New York, 1973. (2) Ross, S.; Suzin, Y. Langmuir 1985, 1 , 145. (3) Scheludko, A. Adu. Colloid Interface Sci. 1967, I , 391. (4) De Vries, A. J. Recl. Trau. Chim. Pays-Bas 1958, 77, 209. (5) Hu, P. C.; Tuvell, M. E.; Bonner, G. A. Society of Petroleum Engineers, Paper No. 12660, 1984. (6) Hirasaki, G. J.; Lawson, J. B. Society of Petroleum Engineers, Paper No. 12129, 1983. (7) Sharma, M. K.; Shah, D. 0.; Brigham, W. E. Ind. Eng. Chem. Fundam. 1984,23, 213. ( 8 ) Sharma, M. K.; Shah, D. 0.;Brigham, W. E. AIChE J. 1985, 31, 222. (9) Bond, G. G.; Holbrook, 0. C. US.Patent 2866507, 1958. (IO) Holcomb, D. L.; Callaway,E.; Curry, L. L. SOC.Pet. Eng. J. 1981, 21, 410. (11) Ali, J.; Burley, R. W.; Nutt, C. W. Chem. Eng. Res. Des. 1985,63, 101.

0 1986 American Chemical Society

Langmuir, Vol. 2, No. 5, 1986 673

Basic Properties of Foams

Tube

Battery

a

b s s u r @ Indicator Transducer

h n D

Figure 1. Schematic diagram of maximum bubble pressure apparatus.

propagation of foam in porous media is a dynamic process, the scope of this work is to study the effect of dynamic surface properties of different chain length surfactants on foam stability and relate them to the displacement of fluids by foams in porous media. The dynamic surface properties under this investigation include dynamic surface tension, dilational modulus of elasticity, and surface shear viscosity. Also investigated is the dynamic behavior of thin liquid surfactant films. Experimental Section Chemicals. Three different chain lengths (C12, C14,and C16) of a-olefin sulfonates (AOS) were suppled by Ethyl Corp., all of which have been used in foam-enhanced oil recovery tests? These surfactants were a mixture of about 65% by weight of alkenesulfonates and 35% by weight of hydroxyalkanesulfonates (mainly 3- and 4-hydroxy). The average molecular weights were 276,304, and 332 g/mol. All foaming agents were used as such without further treatment. Sodium chloride was obtained from Fisher Scientific Co., Chicago, IL. Deionized water was used in all experiments. Surface Tension. Static surface tension was measured by using the du Nouy ring method. Dynamic surface tension is the surface tension of a freshly created surface measured at a given time and represents a nonequilibrium state. The technique used in this study was a refinement of the maximum bubble pressure method.12J3 The maximum bubble pressure method for the determination of surface tension of liquids is based on the measurement of the maximum pressure necessary to blow a bubble in a liquid from the tip of a capillary. In practice, the pressure increases within the capillary at constant gas flow rate until a bubble appears at the tip of a calibrated capillary orifice (r) which has been placed in the experimental liquid at a predetermined depth (h). The pressure inside and outside of the bubble are related by the Laplace equation:

of colinear glass rods direct the light beam through the tip of the capillary. A photodiode at one end of the glass rod converts the light energy to an electrical signal which is indicated on the oscilloscope. When the bubble passes through the light beam it reduces the intensity of light. The frequency counter automati c d y records each cycle of this process and presents the frequency of bubble formation. Before each experiment the glass cylinder was cleaned with chromic acid, rinsed with deionized water, and dried in an oven. The capillary was calibrated by measuring the surface tension of methyl alcohol, chloroform,and deionized water. Surface Shear Viscosity. The deep channel shear viscometer was used to measure the surfactant solution surface vis~osity.'~ Surfactant solution (100 mL) was poured in the channel, and sufficient time was allowed for the surfactant molecules to diffuse into the surface layer. The surface shear viscosity can be calculated by measuring the angular velocity a t the center line of the phase interface. Film Drainage. The drainage time of foam films was studied by using the interference microscopic technique described in ref 15. Thin films of any desired radius were formed in a specially designed glass cell incorporating a double-concave meniscus. Monochromatic light (wavelengthof 546.1 nm) is incident on the film, and a fiber optic probe scans the film surface. As the films thin, the thickness changes, producing interference patterns. The fiber optic probe transmits the responding signal onto a photomultiplier which is recorded on a strip chart recorder. The values of the photocurrent as a function of time enabled an estimation of both the thinning rate and thickness of the f i s . The drainage time is calculated from the recorded output by noting the chart speed and the distance moved by the chart from the time the f i b were formed to the point of rupture or the appearance of the f i s t black spot. In order to prevent evaporation,the films were formed in a completely closed cell, and sufficient time was always allowed for the atmosphere in the cell to become saturated with the vapors of the studied liquid solution. Foam Stability. Foam stability could be determined by two ways. If it is measured while the foam is being formed, the method of determination is dynamic. If the foam is prepared first and its decay is then measured, the method is static. During the investigation we have used both methods. In static foam stability measurements, a predetermined volume of surfactant solution was placed in a graduated cylinder. The foam was generated by shaking the cylinder. Following foam generations, the height of the foam column was monitored with time. The experimental data were analyzed by assuming that first-order kinetics described the foam volume decay. Thus,

where Vf is the foam volume at time t and Kf is the rate constant. The half-life of the foam can be calculated from the rate constant by using (3)

where a is the surface tension, p the density of liquid, Po the atmospheric pressure, and g the gravitational acceleration. P Pois a maximum where the radius of the bubble equals the radius of the capillary. A schematic diagram of the apparatus is given in Figure 1. The apparatus consists of a precision pump, a capillary tube, a pressure transducer, an indicator, an oscilloscope, and a frequency counter. During the measurement, the bubble is generated a t the tip of a capillary by injecting air from the pump. As the bubble is formed and released from the capillary, the pressure indicator records the pressure history inside the bubble. According to the Laplace equation, the pressure is maximum when the bubble is semispherical (three-phase contact angle is zero). The frequency of formation of the bubble is obtained by an optical method. A pair

The apparatus used for determining the dynamic foam stability is the same graduated cylinder used for the dynamic surface tension measurement. During the experiment we recorded the height of the foam on the top of the surfactant solution with time until it obtained a steady state. The equilibrium foam height occurs when the generation of foam equals to breaking of foam. Fluid Displacement in Porous Media. The Berea cores (4 in. X 3 / 4 in. X 12 in.) used as porous media were vacuumed to displace interstitial air, and then the 1% NaCl solution was used to saturate the cores and the pore volume was determined. About seven pore volumes of the same solution were injected at various flow rates to stabilize the clay as well as to determine the absolute permeability. Then, the surfactant solution (3.16 X M + 1% NaC1) of known surface properties was pumped in the porous medium at a constant flow rate. This injection was followed by nitrogen flow to produce in situ foam. The dynamic fluid saturation distributions of the core were measured by the y-ray absorption technique. The radiation source used in our laboratory

(12) Adamson, A. W. Physical Chemistry of Surfaces, 4th ed.; Wiley-Interscience: New York, 1982; Chapter 2. (13) Bendure, R. L.J . Colloid Interface Sci. 1971,35, 238.

(14) Wasan, D.T.;Gupta, L.; Vora, M. K. AIChE J. 1971,17,1287. (15) Rao, A. A.;Wasan, D. T.; Manev, E. D. Chem. Eng. Commun. 1982, 15, 63.

Langmuir, Vol. 2, No. 5, 1986

Huang e t al.

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Figure 2. Variation of surface tension with bulk concentration and frequencies for C12AOS.

Figure 4. Variation of surface tension with bulk concentration and frequencies for c16 AOS.

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Figure 3. Variation of surface tension with bulk concentration and frequencies for C14AOS. is a 25 mCi cesium-137 isotope pellet and the detecting system is composed of a 2 in. X 2 in. cylindrical-shapedsodium-iodine scintillation crystal detector, a preamplifier, a single-channel analyzer,a timer, and a counter. The y-ray works on the principle of mass attenuation coefficient difference between liquid and gas and its energy loss obeys the Beer-Lambert's law. During the core flooding experiment, a computer-controlledtable moves the core to the desired scanning area. The y-ray absorptionmeter automatically logged saturation data for preset periods of time. The produced fluids and breakthrough time were recorded for a constant-pressuredifference (15 psig) across the porous medium.

Results and Discussion Basic Properties. a-Olefin sulfonates (AOS) of different concentrations with 1% NaCl solution were used in surface tension measurements. Figures 2-4 illustrate the variation in surface tension of three different surfactant solutions as a function of concentration and frequency. It is expected that the equilibrium surface tension decreases to a steady value as the concentration increases and the longer the chain length the lower the cmc (critical micelle concentration). Dynamic surface tension vs. concentration curves for increasing frequencies compared to the equilibrium surface tension shift to the right with higher concentration. In order to explain this phenomenon we should consider the formation of the bubble a t the tip of the capillary tube. As the bubble grows the surface expands with time. This results in a surface concentration different from the equilibrium value and thus the surface tension increases. To restore the equilibrium surface concentration the surfactant molecules in the bulk diffuse to the expanding surface. This diffusion-controlled adsorption process depends upon the time scale of the bubble creation.16 Therefore, a t fixed concentration and high frequencies less time is allowed for surfactant molecules (16) Ward, F. H.; Tordai, L. J . Chem. Phys. 1946, 14, 453.

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Figure 5. Variation of dynamic surface tension as a function of frequency for Clz, CI4,and c16 AOS at fixed concentration of 3.16 X M + 1% NaC1. to diffuse to the surface, subsequently the surface tension is higher. On the other hand, to reach the same surface tension at higher frequencies the rate of diffusion must be faster. This can only be achieved by increasing the bulk concentration so that the concentration gradient and diffusivity are larger." As the surfactant concentration increases, the dynamic surface tension gradually falls to an equilibrium value. A possible explanation for this phenomenon is that at high concentrations the rate of diffusion of surfactant molecules is also very high. Thus when any new surface is formed the surfactant molecules will quickly adsorb to the surface. In addition, the data showed that the dynamic surface tension curves move further apart from the equilibrium curve with increasing chain length. The reason is that for longer chain length surfactant systems the equilibrium surface tension curves shift to left due to low cmc, and dynamic surface tension curves shift to the right due to low diffusivity. In the foam flooding process, this information is essential for selecting the concentration of the surfactant since the generation of the foam must depend on the dynamic surface tension. Figure 5 is a plot of dynamic surface tension as a function of the rate of bubble formation. It is observed that the dynamic surface tension of C16 AOS has a substantial change and that of CI4AOS has a small change, while that of C12AOS remains constant in the frequency range. Since the rate of bubble generation is related to the dynamic surface tension, the foaming capability of CI6AOS shows a stronger frequency dependence than that of the other two surfactants. The dilational modulus du/d In A is defined as the surface tension variation with respect to the unit fraction (17) Weihneimer, R. M.; Evans, D. F.; Cussler, E. L. J. Colloid Interface Sci. 1981, 80, 357.

Langmuir, Vol. 2, No. 5, 1986 675

Basic Properties of Foams

Table 11. Thin-Film Drainage Time of AOS at 25 "C drainage time, min, at film radius R AOS 0.4 mm 0.3 mm 0.2 mm 0.1 mm

50 r

2.1 2.15 2.65

2.6 2.7 3.1

c12 c 1 4

C16

1.6 1.65 2.4

0.85 0.88 1.1

Table 111. Porous Media ExDeriments Clz AOS C,, AOS Cln AOS displacement efficiency: % 51 54 61 gas breakthrough time, min 65 84 133 " % efficiency = (volume of liquid produced at breakthrough

Formation

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Bubble ( s k i )

time)/(total amount of liquid present initially).

Figure 6. Variation of dilational modulus as a function of frequency for C,,, C14,and CIBAOS at fixed concentration of 3.16 x M + 1%NaC1. Table 1. Surface Shear Viscosity of a-Olefin Sulfonates (AOS) at 25 "C AOS surface viscositv CW C1a C,G %w' 4 s 1.1 x 10-4 1.3 X lo4 1.7 X lo4

area change. A is characterized by the tip diameter of the capillary used for dynamic surface tension measurements; thus, dA/A is proportional to the rate of bubble formation. It has been shown elsewherel*that the slope of dynamic surface tension curve as a function of the frequency is directly related to the dilational modulus of the surfactant solution. Data given in Figure 6 demonstrate that the c16 AOS possesses the highest dilational modulus in the frequency range. The values of dilational modulus for C14 AOS vary from 0.35 to 5.2 dyn/cm (which is considered low), and C12AOS has negligible elasticity. As mentioned earlier, since the dilational modulus gives a measure of the ability of a surface to adjust its surface tension in an instant stress, this property produces valuable information for controlling the stability characteristics of foam in the dynamic process. Surface shear viscosities of different surfactant solutions are shown in Table I. We noted the same trend with surface shear viscosity as we did with the dilational modulus. c16 has the largest value, C14 has the next largest, and Clz has the smallest value. It should be pointed out that the surface shear visosity is small in the range of concentration we studied. Hence, the results suggest that the surface shear viscosity plays a minor role in the cy-olefin surfactant systems studied here. The lifetime of foam or a thin film consists of two stages which can be characterized as thinning and rupture of a liquid film. In the first stage, the film drains from an initial thickness to a critical thickness. In the second stage, deformations at the film surface began to grow. The forces that govern the lifetime of a liquid film are capillary pressure (suction at meniscus) and the disjoining pressure.19 Disjoining pressure is composed of the attractive van der Waals forces among all the molecules of the film, the electrostatic repulsive forces between ions on the two surface layers, and the steric forces due to the steric hindrance in closely packed monolayers. In order to study the lifetime of liquid films, it is essential to obtain the information of both the drainage time and the thickness transition phenomenon at which the film ruptures. (18) Huang, D. PhD Thesis, Illinois Institute of Technology, Chicago, IL, 1985. (19) Deryaguin, B. V.; Titijevskaya, A. A. Proc. Int. Congr. Surf. Act. 2nd 1957,211.

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CIL AOS Chain Length

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Figure 7. Variation of thin-film drainage time and dilational modulus with AOS chain length.

The data of the thin-film drainage experiments are presented in Table 11. At any film diameter, C16 AOS always has the highest drainage time compared to the other two surfactants, and C14and Clz AOS have a small variation in the drainage time. During the film drainage, the flow in the film exerts shear stress at the surface which causes the surface to move tangentially. The surfactant molecules which are carried away toward the meniscus result in bulk and surface diffusion, tending to restore the equilibrium surface tension. The nonuniform surfactant distribution along the surface leads to a local variation of surface tension, which produces a surface stress opposing the liquid flow. This phenomenon is known as the Marangoni-Gibbs effect which can be measured by the dilational modulus. Figure 7 shows the correlation between the drainage time ( t ) and dilational modulus of three surfactant solutions. It is observed that the drainage time is proportional to the dilational modulus thus providing evidence of a direct relationship between them. The film sizes have also been tabulated in Table 11. As the film radius decreased, the drainage time of all of the surfactant solutions decreased. As mentioned before, capillary pressure and disjoining pressure are the forces responsible for the thin-film drainage process. The capillary pressure is inversely proportional to the radius of the film. Therefore the capillary suction force increases with decreasing film radius and drainage time is less. This suggests that the foam film in a smaller capillary will have a shorter drainage time. Once the f i i drains to the critical thickness it can either rupture or go through a series of transition steps to a new equilibrium state like a Newton film. Since the concentration of the electrolyte in this study is high (1%NaCI), the film could jump from the common film to the Newton film. However, both processes still depend upon the force balance between capillary pressure and disjoining pressure. In the case of dynamic foam stability tests, the capillary pressure also equals the hydrostatic force which results

Huang et al.

676 Langmuir, Vol. 2, No. 5, 1986

.

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Dilational Modulus (dyne/cm)

Figure 8. Foam half-life and equilibrium foam height with dilational modulus.

50

100

150

200

250

Time ( m i n )

Figure 10. Variation of water saturation with time and the distance along the length of the core for C14 AOS.

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Figure 9. Variation of water saturation with time and the distance along the length of the core for C12 AOS. from the foam height.20 As the foam reaches the equilibrium height, the hydrostatic force in terms of capillary pressure overcomes the disjoining pressure. Thus, the mechanical energy balance between these two forces no longer exists and the foam breaks. The foam stability tests have been demonstrated in Figure 8. Both dynamic and static measurements show consistent data that the stability of the foam follows the sequence c16, C14, C12. Not surprisingly, the results are in confirmity with the dilational modulus as well as the thin-film drainage measurement. The foam breakage process is similar to the thin-film drainage and involves a deformation of the plateau border due to the change of capillary force. This deformation extends locally and lowers the local concentration. It also causes a local increase in surface tension, and the stronger the surface tension gradient the stronger it resists rupture. Figure 8 demonstrates the stability of foams with the dilational modulus at 10 Hz. It appears that the stability of foam correlates well with the dilational modulus. Foam Displacement in Porous Media. In order to study the effect of dynamic surface properties of different chain length of surfactant solutions on foam performance in porous media, the saturation profiles, produced fluids, and breakthrough times were measured. The saturation distribution data of the surfactant solution, measured by y-ray absorption technique outlined earlier, along the Berea sandstone core are presented in Figures 9-11. In the case of C12AOS, Figure 9 shows that the saturation at the first position dropped below 0.4 in 50 min, while the saturation for the rest of the tappings remained between 0.60 and 1.00. From this point on, saturation dropped at (20) Khugljakov, P.M.;Exerowa, D.R.;Kristov, K. I. J . Colloid Interface Sci. 1981, 79, 584.

50

100 150 Time ( m i n )

200

250

Figure 11. Variation of water saturation with time and the distance along the length of the core for C16 AOS. a relatively slow rate, producing saturation differences less 10% until each position reaches a steady state. By comparison, Figure 10 demonstrates the saturation profile of C14AOS is different than that of C12AOS. Over 160 min, the water saturation dropped to a value of 0.30 at positions 1 and 2 and the rest of the core was above 0.45. In Figure 11, c16 AOS shows a better pistonlike displacement than C12 and C14 AOS. It is observed that the saturation dropped to a low value of about 0.30 toward the middle of the core due to foam formation in situ and subsequently blocked the pore and improved the displacement efficiency. The results in fluid displacement efficiencies and breakthrough times with different surfactant solutions are represented in Table 11. It is observed that the order of increase in fluid displacement efficiency and breakthrough time with respect to different foaming agents was found to be C12, C14, C16. This suggests that the generation of in situ foam reduces the flow rate of nitrogen as well as possible gravity override in the porous medium and, hence, increases the sweep and displacement efficiency. To delineate the effect of surfactant chain length in terms of dynamic surface properties on the foam flooding performance, effective gas mobilities were calculated from the cumulative volume of the fluid collected and gas breakthrough time. The effective gas mobility is defined as19 Kg - =- QL (4) c ~ g At@' where (5)

Langmuir 1986,2, 677-682

- Gas Mobility *

677

has a striking correlation with the effective gas mobility in the porous medium.

- Dilational Modulus

Conclusions

“

\f

I

I

2

1. Dynamic surface tension, surface viscoelasticity and thin film drainage times and foam stability have been measured for determining the basic foaming Characteristics of a-olefin surfactants. It was found that c16 AOS has the highest surface shear viscosity and dilational modulus followed by C1,and Clz AOS. 2. A good correlation was obtained between surface properties of surfactant solutions and foam stability. The data suggested that the larger the dilational modulus, the more stable the foam. 3. Dynamic gas-liquid saturations in consolidated porous media were measured using the y-ray absorption technique. The saturation distribution revealed that the c16 AOS in situ foam has a pistonlike displacement in the core, while C14and ClzAOS are less effective. 4. The role of dynamic surface properties in foam-enhanced fluid displacement was determined. The results showed that the higher the modulus of elasticity, the slower the movement of the gas, thus increasing the displacement efficiency of the foam in the porous medium.

2ouo 10

5

0

c12 c14 AOS Chain Length

c16

Figure 12. Correlation of gas mobility with dilational modulus with varying AOS chain length. and Q is the cumulative volume of liquid produced at gas breakthrough, L is the length of the core, tb is the breakthrough time, A is the cross-sectional area, P1 and Pz are the upstream and downstream pressures, and P,, is the standard absolute pressure. The lower the effective gas mobility the higher the blocking capability of the foam since the effectiveness of the foam depends on its flow resistance. Figure 12 illustrates the effective gas mobility and dilational modulus with three surfactants. It appears that the chain length of the surfactant influencing the mobility control of foam can be directly explained by the dilational modulus. The results show that the dilational modulus

Acknowledgment. This study was funded by the National Science Foundation and in part by the Department of Energy. Registry No. NaCl, 7647-14-5.

Surface Chemistry of Sodium, Chlorine, and Oxygen on Chromium and Chromium(II1) Oxide J. S. Foord Department of Chemistry, The University, Southampton SO9 5NH, U.K.

R. M. Lambert* Department of Physical Chemistry, University of Cambridge, Cambridge CB2 IEP, U.K. Received October 22, 1985. I n Final Form: June 28, 1986 Chemisorption and coadsorption of sodium, chlorine, and oxygen on Cr(100) and on epitaxially grown Cr203(OOOl)surfaces have been studied by LEED,AES, TDS, and A4 techniques. Na adsorption on Cr(100) results in the formation of positively charged, ordered monolayers, exhibiting strong lateral repuisions and a heat of adsorption of 175 kJ mol-’ at low coverages;subsequent exposure to oxygen brings about oxidation of the Na phase and the underlying Cr(100) lattice. Coadsorption of Na and C1 on Cr(100) causes island formation of (100)-oriented NaCl and implies an appreciable lateral mobility for the surface species at 300 K at high chlorine coverages evidence is presented suggesting formation of CrClZand a mixed CrNaxC12+x corrosion phase. Na adsorption on outgassed Crz03exhibits characteristics similar to the interaction of alkalis with transition metals, although the Na is more strong6 bound than on Cr(100). When heated in the presence of oxygen, reaction of adsorbed Na with the underlying CrzO3 lattice takes place, resulting in formation of a mixed Na-Cr-0 compound. Chlorine adsorption on the outgassed Cr203surface results in the appearance of two TDS states, chlorine being evolved at low temperatures and chromium chloride(s) desorbing at high temperatures. This latter process is suppressed by coadsorbed oxygen and the nature of the reactions occurring is discussed. Electron-stimulated desorption of chlorine is observed in the Cr(100)/Na/C1and Cr20,(0001)/C1chemisorption systems and cross sections for the stimulated desorption processes are presented. 1. Introduction Alkali and halogen species play important roles as promoters in heterogeneous catalysis1 ana as a consequence

* To whom correspondence

should be addressed.

0743-7463/86/2402-0677$01.50/0

their surface chemistry is of particular interest. While the properties of the individual species adsorbed on transition metals has now been studied in some detail (e.g., ref 2-7), (1)Martin, G. A. In Metal-Support and Metal-Additiue E f f e c t s in Catalysis; Imelik, B., et al., Eds.; Elsevier: Amsterdam, 1982; p 315.

0 1986 American Chemical Society