Ind. Eng. Chem. Res. 1992,31, 1899-1906 at the National Fuel Cell Seminar, San Diego, CA, 1980. Krebs, S.; Littbarski, R. Preparation and Crystal Growth. Current Topics in Material Science; Kaldia, E., Ed.; North-Holland Publishing: New York, 1981;pp 170-198. Lew, S. High-Temperature Regenerative H A Removal by ZnO-Ti02 Systems. M.S. Thesis, Massachusetts Institute of Technology, Cambridge, 1987. Lew, S. The Reduction and Sflidation of Zinc Titanate and Zinc Oxide Solids. Ph.D. Dissertation, Massachusetts Institute of Technology, Cambridge, 1990. Lew, S.; Jothimurugesan, K.; Flytzani-Stephanopoulos,M. HighTemperature H A Removal from Fuel Gases by Regnerable Zinc Oxide-Titanium Dioxide Sorbenta. Znd. Eng. Chem. Res. 1989, 28,535-541. Lew, S.;Sarorm, A. F.; Flytzani-Stephanopoulos, M. The Reduction of Zinc Titanate and Zinc Oxide Solids. Chem. Eng. Sci. 1992a, 47 (6),1421-1431. Lew, S.;Sarofim, A. F.; Flytzani-Stephanopoulos,M. Modeling of the Sflidation of Zinc-Titanium Oxide Sorbents with Hydrogen Sulfide. AIChE J. 199213,in press.
1899
Marcilly, C.; Courty, P.; Delmon, B. Preparation of Highly Dispersed Mixed Oxides and Oxide Solid Solutions by Pyrolysis and Amorphous Organic Precursors. J. Am. Ceram. SOC.1970,53(l), 56-57. Ranade, P. V.; Harrison, D. P. The Variable Property Grain Model Applied to the Zinc Oxide-Hydrogen Sulfide Reaction. Chem. Eng. Sci. 1981,36,1079-1089. Sainamthip, P.; Amarakoon, V. R. W. Role of Zinc Volatilization on the Microstructure Development of Manganese Zinc Ferritee. J. Am. Ceram. SOC.1988, 71 (a),644-648. Westmoreland, P. R.; Gibson, J. B.; Harrison, D. P. Comparative Kinetics of High-Temperature Reaction Between H2S and Selected Metal Oxides. Enuiron. Sci. Technol. 1977,l l , 488-491. Woods, M. C.; Gangwal, S. K.; Jothimurugesan, K.; Harrison, D. P. Reaction between H&3 and Zinc Oxide-Titanium Oxide Sorbents. 1. Single-Pellet Kinetic Studies. Znd. Eng. Chem. Res. 1990,29, 1160-1167.
Received for reuiew March 9, 1992 Accepted May 15,1992
MATERIALS AND INTERFACES Mechanisms for Lowering of Interfacial Tension in Alkali/Acidic Oil Systems: Effect of Added Surfactant Jeff Rudin and Darsh T. Wasan* Department of Chemical Engineering, Illinois Institute of Technology, Chicago, Illinois 60616
Experimental studies are conducted in order to determine the physicochemical mechanisms responsible for lowering of interfacial tension in alkali, surfactant, and surfactanbenhanced alkali/acidic oil systems. A well-defined model oil is chosen to examine the influence of various surfactants and surfactant mixtures, such as oleic acid and ita ionic counterpart, sodium dodecyl sulfate, petroleum sulfonate, and isobutanol, on equilibrium interfacial tension. With added surfactant alone, the interfacial tension goes through an ultralow minimum with increasing acid concentration. This proves for the first time that the un-ionized acid species plays a major role in affecting interfacial tension, and appears to be the key element in the synergistic process taking place between the added surfactant and the ionized acid species. The un-ionized acid species partitions the added surfactant out of the aqueous phase, and the minimum in interfacial tension occurs when the partition coefficient is about unity. When alkali is added, the low interfacial tension is not lost, but actually shifts to different acid concentrations in a systematic way.
Introduction It is well-known that interfacial tension between a surfactant solution and a hydrocarbon can become ultralow. A number of variables, such as salinity (Chan and Shah, 19801, oil chain length (Chan and Shah, 1980), alcohol concentration and type (Miller and Neogi, 1985), surfactant concentration (Chan and Shah,1980) and type (Doeet al., l977,1978a,b),and temperature (Miller and Neogi, 198!5),have been found to affect the position of the interfacial tension minimum. Chan and Shah (1981) observed two regions of ultralow interfacial tension as surfactant concentration is increased. One region is at low surfactant concentrations of about 0.1 wt ?% (a two-phase region), and the other is at high surfactant concentrations of about 4 wt 3' % (a three-phase region). In the two-phase
* To whom correspondence should be addressed.
region, the ultralow interfacial tensions are due to the presence of a saturated monolayer at the interface (Chan and Shah,1980,Pouchelon et d,1981). In the three-phase region, the ultralow interfacial tensions are due to a critical behavior (Fleming and Vinatieri, 1979, 1981; Fleming et al., 1980). Chan and Shah (1980) also found that an increase in salinity causes partitioning of the surfactant from the aqueous to the oil phase, and interfacial tension is ultralow when the partition coefficient is about unity. Chan and Shah (1981) also showed that the interfacial tension goes through an ultralow minimum when the oil chain length is increased, and the minimum again occurs when the partition coefficient is about unity. With the addition of a small amount of surfactant to the alkaline solution, the interfacial tension can become lower than either surfactant or alkali alone (Schuler et al., 1986). The reason for this synergism is not well-known. In this paper, we investigate the interaction of the un-ionized acid
0888-5885/92/2631-1899$03.00/00 1992 American Chemical Society
1900 Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992
8 0
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Figure 1. Interfacial tension aa a function of the acid concentration in the oil.
with the added surfactant and ionized acid to better understand this synergism.
Materials and Methods In this study, an alkane containing an organic acid was contacted with an aqueous solution of alkali and/or surfactant. The oil used in this study is n-decane (99% pure) obtained from Sigma Chemical Company. The solutions contained sodium hydroxide (NaOH), sodium chloride (NaCl), and isobutanol (IBA) obtained from Fisher Scientific Company with ACS purity. Three surface-active compounds, an organic acid and two surfactants, are used. The organic acid is oleic acid (99+% pure), which was obtained from Sigma Chemical Company. The preformed surfactant, a petroleum sulfonate, Petrostep B-100 (58.7% active) was obtained from Stepan Chemical Company. The equivalent weight of Petrostep B-100 was determined to be 417 using the two-phase hyamine titration technique, and 1.0% (100% active) Petrostep B-100 equals 24.0 mol/m3. The other surfactant,sodium dodecyl sulfate (SDS),specially purified for biological work, was obtained from BDH Chemicals Ltd. Preequilibrated samples were made by periodically shaking equal volumes of aqueous and oil phases for 4 weeks at 25 "C. The interfacial tensions were measured for these equilibrated samples using the Wilhelmy plate and the spinning drop technique. Readings were made when the interfacial tension had reached a steady value. The pH measurements were made using an Orion microprocessor ionalyzer/901 with a Roes combination electrode designed for low sodium error. Ionized acid concentrations in the water were determined using the dichlorofluorescein titration technique, and un-ionized acid concentrationsin the oil were determined using ASTM p r d u r e D-664.All solutions were made on a weight/volume basis with respect to the initial phase, and the ionic strength was kept constant with sodium chloride. The surfactant solutions were prepared on a 100% active basis. All experiments were performed at 25 O C . Throughout this paper, added surfactant refers to Petrostep B-100, acid refers to un-ionized oleic acid, and soap refers to the ionized form of the acid. The petroleum sulfonate Petrostep B-100 used in this study is a blend of surfactants. When referring to the critical micelle concentration (crnc) of it, the effective cmc for the system is meant. Results and Discussion Effect of Alkali. Figure 1shows the interfacial tension as a function of acid concentration against 1?4 NaCl. This figure shows that there is a continual decrease in interfacial
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in1 HiOH
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Figure 2. Ionization and interfacial tension curves aa a function of equilibrium pH and sodium hydroxide concentration.
tension with acid concentration, indicating that un-ionized acid is, by itself, surface active. If the acid species has the ability to form micelles, it does not form micelles in the range investigated here. Figure 2 shows the acid concentration of the oil phase, the soap concentration of the aqueous phase, and the interfacial tension as a function of NaOH. Since the maximum concentration of generated soap in the aqueous phase (at high pH) equals the initial concentration of acid, the soap does not partition into the decane. It can also be seen that the acid is completely ionized at 0.1 9% NaOH and greater, which corresponds to the constant value of interfacial tension. The concentration of soap in the aqueous solution equals the concentration of acid in the decane at the minimum in interfacial tension, occurring at 0.025% NaOH. The sodium ion was kept constant at 171 mol/m3 to eliminate the effect of ionic strength, and the interfacial tension still went through a minimum without partitioning of the soap, indicating that the ionic strength does not cause the minimum in interfacial tension. Some other mechanism must be operative in alkali/acidic oil systems. The interfacial tension behavior as a function of NaOH can be rationalized in the following manner. In the limit of no added alkali, the interfacial tension corresponds to only adsorption of acid on the interface. Here, the tension depends on the acid concentration and acid type. In the limit of high concentrations of alkali, all of the acid is ionized, and the tension corresponds to only adsorption of ionized acid on the interface. As seen in Figure 2, increasing NaOH past 0.1% does not affect the interfacial tension. We would expect that as NaOH increases, the acid would become ionized, interfacial tension would lower, and there would be a point where the cmc of the soap is reached, resulting in a leveling off of interfacial tension. However, in between the limits of low and high concentrations of NaOH, the interfacial tension does not level off as expected, but instead it goes through a minimum. What happens in between is a synergisticeffect between the acid and ita generated soap. In other words, the acid behaves like an impurity. It is just that interfacial tension is more sensitive to impurities than surface tension, causing a deeper minimum (i.e., a change of just 1mN/m or less in interfacial tension can make it ultralow, since the tension is already low). Rudin and Wasan (1992b) have shown that the un-ionized acid forms mixed micelles with the ionized acid, causing a reduction in the cmc of the ionized acid. As NaOH is increased, the concentration of un-ionized acid decreases, resulting in an increase in cmc (higher ionized acid monomer concentration) and a consequent decrease in interfacial tension. The minimum in interfacial tension is reached when the concentration of un-ionized acid is too low to affect the cmc. Incidentally, Rudin and Wasan (1992a) have measured the cmc of so-
Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992 1901 1
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uo
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101 Na011.
D=mKnya 171 m Vm Nafl
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-Urn1
Figure 3. Effect of initial acid concentration on interfacial tension as a function of sodium hydroxide.
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Figure 5. Effect of acid on interfacial tension for different added surfactant concentrations.
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Figure 4. Effect of initial acid concentration on equilibrium pH as a function of sodium hydroxide concentration.
dium oleate to be 0.023 mol/m3 and they (1992b) also theoretically predicted the same value. The simultaneous adsorption of ionized and un-ionized acid upon the interface results in the ultralow interfacial tension. With further increase in NaOH past the minimum, the interfacial tension increases because the un-ionized acid becomes more ionized contributing less and less until it is completely ionized at 0.1% NaOH, resulting in a leveling off of interfacial tension. This shows that the ionizable surfactant species should be equipartitioned between phases to minimize interfacial tension, which extends the mechanism of Chan and Shah (1980). Figure 3 shows the effect of alkali on interfacial tension for three different initial acid concentrations. As the concentration of acid in the oil increases, the minimum in interfacial tension shifts to higher NaOH concentrations and the width of the minimum becomes narrower. These phenomena can be explained by the examination of Figure 4, which shows the equilibrium pH as a function of alkali at different acid concentrations. The interfacial tension is shown in Figure 2 to be a strong function of the equilibrium pH. The shifting of the interfacial tension curves in Figure 3 is connected with a shifting of the equilibrium pH. Although the minimum does not occur at exactly the same pH, it does occur at the same relative position on the pH curve. Effect of Added Surfactant. Figure 5 shows the variation of interfacial tension with acid concentration at various levels of added surfactant. Since the interfacial tension goes through an ultralow minimum with acid, a synergism exists between the acid and the added surfactant, similar to the synergism between the acid and the
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Figure 6. Partition coefficient of added surfactant as a function of acid and added surfactant concentrations.
soap that was observed in the previous section. The great difference in interfacial tension of curves 0.01% and 0.025% Petrostep B-100 indicates that the cmc of Petrostep B-100 is near these concentrations. When unionized acid is increased, it will adsorb on the interface along with the surfactant and reduce interfacial tension. However, the acid will form mixed micelles with the surfactant, which reduces the cmc (and therefore surfactant monomer), resulting in increased interfacial tension with additional acid. The addition of acid also has the effect of partitioning the surfactant out of the aqueous phase as shown in Figure 6. The acid is changing the partition coefficient of the surfactant, which is defined as the ratio of the added surfactant concentrationin the aqueous phase to that in the oil. The minimum in interfacial tension occurs at a partition coefficient of about unity. When surfactant is partitioned into the oil, it can adsorb onto the interface from the oil side, independently of that adsorbing from the aqueous side, resulting in increased low-
1902 Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992
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Figure 8. Effect of acid on interfacial tension at different concentrations of sodium dodecyl sulfate.
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Figure 7. Effect of added surfactant on interfacial tension at different acid concentrations.
ering of the interfacial tension. When more surfactant is present, more micellar phase is there, so that more acid is needed to counterbalance its trapping in the micellar phase (or binding with the surfactant), resulting in a shift of the minimum to higher acid concentrations. This is reflected in the partition coefficient as a shift to higher acid concentrations. It should also be noted that no spontaneous emulsificationwas observed in any samples (unlike those with alkali), even in the cases with ultralow interfacial tension, and the system remained two-phase. Wilson and Brandner (1977) also saw no spontaneous emulsification when studying surfactant formulations against a slightly acidic crude oil. Figure 7 shows the inversion of Figure 5 such that we may investigate the effect of added surfactant on interfacial tension at different acid concentrations. For acid concentrationsbetween 0 and 10.0 mol/m3, the interfacial tension goes through a minimum as surfactant concentration is increased. This behavior is similar to that for an impurity, such as lauryl alcohol in sodium lauryl sulfate solutions when sodium lauryl sulfate is increased (Vijayan et al., 1978). The un-ionized acid appears to behave in a way similar to that of the alcohol. For acid concentrations between 0 and 10.0 mol/m3, an increase in acid concentration causes the interfacial tension curve to be lower at all surfactant concentrations, and the critical micelle concentration (cmc),as can be seen from the isotherms in Figure 7, shifts from 1.0 to 0.6 mol/m3 surfactant. Shinoda (1953,1954) showed that an increase in the concentration of alcohol causes a decrease in the cmc of the surfactant by formation of mixed micelles. This further supports that the acid behaves like the alcohol in affecting interfacial tension and micellar properties. Not only is cmc shifting, but also the interfacial tension is lower for all values. This is because the acid adsorbs on the interface independent of the surfactant, and more acid is seen to be on the interface with increase in acid concentration from 0 to 10 mol/m3. From this figure and Figure 6, the acid is seen
to be doing three things: (1)adsorbing onto the interface, (2) partitioning surfactant out of the aqueous phase, and (3) forming mixed micelles. Therefore, an increase in acid will decrease interfacial tension by the simultaneous adsorption of acid and surfactant onto the interface. The formation of mixed micelles resulta in the reduction of the cmc and surfactant monomer concentration. Partitioning of surfactant into the oil allows adsorption of surfactant onto the interface from the oil side, independently of that adsorbing from the aqueous side. With further increase in acid from 10.0 to 40.0 mol/m3, the system shows what appears to be two minima or two cmc's in Figure 7. Isaacs and Smolek (1983) investigated the system of bitumen (acid number 3.6)/Suntech 5 (a synthetic sulfonate), and their results were suprisingly similar to the 40.0mol/m3 acid isotherm in Figure 7. The interfacial tension decreases rapidly, levels off, and then decreases again and goes through a small minimum, spanning 1 order of magnitude in interfacial tension and 2 orders of magnitude in sulfonate concentration. They believe that a change in the microstructure of the amphiphile at cmc is brought about by an increase in the ionic strength of the aqueous phase with increasing surfactant concentration due to the relatively large amount of impurities present in the surfactant sample. In light of our present investigation,this phenomenon is brought about more by the acid species than, if at all, by the ionic strength. Figure 8 shows the effect of added acid on the interfacial tension for various levels of sodium dodecyl sulfate (SDS). Figure 8 has the same behavior as that in Figure 1, except that the interfacial tension is lower because of the simultaneous adsorption of SDS. No ultralow minimum is observed in interfacial tension as acid concentration is increased. This may be partly due to the SDS having a very high solubility in the aqueous phase, and as a result, the partition coefficient of unity is never reached in the concentration range of acid shown. It could ala0 be partly due to the poor effectiveness of the SDS in lowering interfacial tension. In other words, the interfacial tension is not as sensitive to impurities (i.e., the acid) at high values. In general, the acid must be compatible with the added surfactant to achieve an ultralow minimum in interfacial tension. We can also see that the cmc appears
Ind. Eng. Chem. Res., Vol. 31, No. 8,1992 1903 I d 0 OBNIOH 0 O.rnJ%
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to be between 0.5 and 1.0 mol/m3 added surfactant. Effect of Alkali and Added Surfactant Combined. Figure 9 shows the variation of the interfacial tension as a function of the initial acid concentration for various levels of alkali. As the concentration of alkali increases, the minimum in interfacial tension shifts to higher acid concentrations, as was observed in Figure 5 with increasing levels of added surfactant. In other words, the ionized acid contributes to the interfacial tension in the same way as the added surfactant. The interfacial tension at low acid concentrations (i.e., 1.0-5.0 mol/m3 acid) increases as the alkali (or ionized acid) concentration increases. This increase of over 2 orders of magnitude in interfacial tension is caused from the ionized acid not being as effective a reducer of interfacial tension as the added surfactant. The effectiveness of the ionized acid can be obtained from adding exceas alkali and measuring the interfacial tension, which corresponds to adsorption of only ionized acid. We saw at low acid concentrations that adding alkali will only increase interfacial tension. At medium to high acid concentrations, though, the interfacial tension goes through a minimum as alkali is increased. This indicates that it is beneficial to use alkali with medium to high acid concentration oils. The ionized acid has a synergistic effect with the added surfactant, and depending upon the species involved and their interaction with the un-ionized acid, the interfacial tension can certainly go ultralow. Since the ionized acid does not partition into the decane, the major effect of the ionized acid is to form mixed micelles. If excess alkali (i.e., 0.5% NaOH) is added, however, the synergistic effect with the acid is totally lost, since all of the acid is ionized. With exceas alkali, the influence of the added surfactant is lost, because the ionized acid is now the dominating species. It can also be seen in Figure 9 that interfacial tension decreases as the acid is reduced for 0.025,0.05,and 0.5% NaOH, giving rise to a maximum (or two minima) in interfacial tension. This resulta from domination of added
Figure 10. Effect of alkali on interfacial tension for different acid concentrations. 7
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surfactant at low concentration of acid where there is not enough acid to generate sufficient quantity of ionized acid, approaching conditions for no added alkali. Figure 10 is the inversion of Figure 9 such that the effect of alkali on interfacial tension for different acid concentrations can be studied. As the initialconcentration of acid in the oil increases, the minimum in interfacial tension shifts to higher alkali concentrations, similar to that without added surfactant, except that the addition of surfactant causes the minimum to occur at lower alkali concentrations. This is reasonable if one considers that the added surfactant behaves like the ionized acid. As a result, adding surfactant means that less ionized acid is needed for a given equilibrium acid concentration, so the minimum in interfacial tension occurs at lower alkali concentrations. This result can also be seen in Figure 11, which shows the effect of alkali on equilibrium pH for different acid concentrations and fixed added surfactant. The minimum point in interfacial tension for 13.0 mol/m3
1904 Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992
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initial acid occurs at a pH of 8.60 with 0.1% added surfactant, and the pH is 9.25 without added surfactant. Therefore, adding surfactant reduces the pH of the minimum in interfacial tension and shifta the minimum to lower alkali concentrations. This likely results from the formation of mixed micelles. It should be noted that these results are for one type of surfactant and may change with another surfactant because of the way that the ionized acid and surfactant interact. Figure 12 illustrates the synergistic effect of added surfactant or soap with the acid. For instance, for an acid concentration of 25.0 mol/m3, 0.025% NaOH has an interfacial tension of 0.05 mN/m and 0.1% added surfactant has an interfacial tension of 0.013 mN/m, but if we combine the two, the interfacial tension is more than an order of magnitude lower (i.e., 0.001 mN/m). Schuler et al. (1986)have reported that there is a beneficial synergistic effect from combining surfactant and alkali. We can now explain this synergistic effect in terms of the contribution of the acid to the formation of a mixed monolayer and mixed micelle formation. Effwt of Alcohol and Added Surfactant Combined. Figure 13 shows the effect of alcohol on the adsorption kinetics at different acid concentrations. The open symbols are without alcohol and the closed symbols are with alcohoL One effect of adding isobutanol (IBA) is to reduce the time needed to achieve equilibrium interfacial tension after a new interface is created. The reduction in time is about 10 min. Figure 14 shows the effect of acid on interfacial tension at different alcohol concentrations. The interfacial tension in the absence of acid is 0.019,0.014,and 0.027mN/m for 1, 3, and 6% isobutanol, respectively. The interfacial tension curve for the case of no added alcohol has a minimum at about the same acid concentrationas the 3% IBA case (see Figure 5). With addition of alcohol, the minimum in interfacial tension shifta to lower acid concentrations, and with further increase in alcohol concentration, the minimum shifta to higher acid concentrations. The shifting of the minimum to lower acid concentrationsindicates that the alcohol is helping the acid in the lowering of interfacial tension, and at higher alcohol concentrations, the alcohol impedes the contribution of the acid to the interfacial tension. Others (Miller and Neogi, 1985) have reported
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Figure 14. Effect of acid on interfacial tension at different acid concentratione.
that increasing the concentration of a relatively oil-soluble alcohol such as n-pentanol or n-hexanol causes the minimum in interfacial tension to occur at lower salinities, and increasing the concentration of a relatively water-soluble alcohol such as isopropanol causes the minimum to occur at higher salinities. Since isobutanol is both oil-soluble and water-soluble, increasing the concentration shifta the minimum to lower and then to higher acid concentrations. At low alcohol concentrations, the alcohol will behave like the acid in its ability to partition the added surfactant out of the aqueous phase. That is why the minimum occurs
Ind. Eng. Chem. Res., Vol. 31, No. 8,1992 1908
TIME m m
Figure 16. Effect of acid on the rate of water separation.
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Figure 16. Correlation of partition coefficient with the minimum in interfacial tension.
with less acid. As the amount of alcohol (i.e., low chain length alcohols) becomes greater, the solvent power of the water will alter, resulting in a greater solubilization of the surfactant in the water (Nishikido et al., 1974;Matuura et al., 1961). This altering of the solvent property of the water could be seen visually when, upon addition of the alcohol to the surfactant solution, the appearance went from opaque to translucent. At higher alcohol concentrations, the altering of the solvent property of the water will allow more surfactant in the aqueous phase. As a result, more acid is needed to obtain a partition coefficient of about unity for the minimum in interfacial tension. From this explanation we can see how the minimum in interfacial tension shifts to lower acid concentrations and then to higher acid concentrations with increasing amounts of alcohol. Furthermore, the alcohol does not seem to be adsorbing on the interface or partitioning the added surfactant out of the aqueous phase below the cmc of the surfactant, since the minimum is only shifting and there is not a large increase in interfacial tension at higher alcohol concentrations. However, this behavior is likely to change with longer chain length alcohols. The phase behavior of the isobutanol system was such that there was a middle phase (i.e., a clearly separate phase) present when the interfacial tension was ultralow. However, in the absence of isobutanol, a middle phase was not observed in the region of ultralow interfacial tension, but instead a macroemulsion was present in the upper and/or lower phase. Figure 15 shows the correlation of the partition coefficient with the minimum in interfacial tension for 1% IBA. The minimum in interfacial tension corresponds with a partition coefficient of about unity. As also seen in Figure 15, the acid is partitioning the surfactant out of the aqueous phase. However, by adding alcohol, less acid is needed to get the same partitioning (i.e., the minimum in interfacial tension shifts to lower acid concentrations), supporting the notion that the alcohol helps the acid at low alcohol concentrations. On the left of the minimum in Figure 15,the surfactant is mostly in the aqueous phase, and on the right half of the minimum, the surfactant is mostly in the oil phase. According to Bancroft’s rule, the aqueous phase will try to be the continuous phase on the left half of the minimum, and the oil phase will try to be the continuous phase on the right half of the minimum. Thus, if we shake the
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Figure 17. Effect of acid on the rate of oil separation.
system and monitor the rate of water breakout (see Figure 161,we see that the points on the left side of the minimum, where the surfactant is soluble in the aqueous phase, have a faster waterbreak, which is to be expected from Bancroft’s rule. The points to the right of the minimum, where the surfactant is soluble is the oil,had a slower waterbreak (see Figure 16),which is also to be expected from Bancroft’s rule. The minimum in interfacial tension corresponds to the maximum rate of breakage of the oil lamella structure, which is a function of the phase in which the surfactant is soluble. As a result, optimal acid concentration or the minimum in interfacial tension can be determined by simply shaking the two phasea and observing the rate of breakage of the oil lamella structure. Figure 17 shows the oil breakout of the same system shown in Figure 16. The oil phase has a relatively higher rate of breakout than the water phaae, so Bancroft’s rule cannot be simply applied to the oil phase as was done with the water phase as shown in Figure 16. However, the phase in which the surfactant is mostly soluble approachea 100% breakout quicker, and this can be seen by comparing Figures 16 and 17,which may be used to determine the minimum in interfacial tension. The condition of least stability occurs at the minimum in interfacial tension, as can be seen in Figures 16 and 17. These results support those of Berger et al. (1987)as well as Vinatieri (1980). That is, low interfacial tension is required for emulsion stability, but too low an interfacial tension resulte in unstable emulsions. As the surfactant is partitioned from lower to middle to upper phase, the water and oil phases are lemt stable when middle phase is preaent. The preaence of the middle phase indicate that the surfactant is concentrated in this phase, leaving the water and oil excess phases relatively free of surfactant. As a result, no surfactant is in the water or oil to stabilize emulsions of these phases, leading to rapid coalescence. Significant Findings 1. Interfacial tension goes through a minimum with pH at constant ionic strength.
1906 Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992
2. The interfacial tension against a solution at low concentrations of added surfactant goes through an ultralow minimum when the concentration of a long-chain fatty acid is increased, and the minimum occurs when the partition coefficient of the surfactant is about unity. 3. An increase in added surfactant concentration causes the minimum in interfacial tension to occur at higher acid concentrations. 4. Interfacial tension shows no ultralow minimum with increasing acid concentration, when the added surfactant is highly water soluble. 5. The un-ionized acid reduces the cmc of the added surfactant. 6. At low acid concentrations, the addition of an alkali to the added surfactant solution will only make interfacial tension increase. At medium to high acid concentrations, addition of an alkali can produce ultralow interfacial tension. 7. An increase in isobutanol concentration shifts the minimum in interfacial tension to lower and then to higher acid concentrations. 8. The addition of isobutanol reduces the time needed to achieve equilibrium interfacial tension after a new interface is created. 9. The minimum in interfacial tension corresponds to the rate of breakage of the oil lamella structure in the aqueous phase, which is a function of the phase in which the surfactant is soluble.
Conclusions 1. The un-ionized acid species plays a major role in controlling the minimum in interfacial tension. It appears to be the key element in the synergistic process taking place between the added surfactant and the ionized acid species. 2. The minimum in interfacialtension is formed, in part, from the un-ionized acid partitioning the added surfactant out of the aqueous phase with the minimum occurring at a partition coefficient of unity. 3. The added surfactant alone should produce tensions of the order of 0.1 mN/m and have a moderate solubility in both phases before the acid will synergisticallyproduce an ultralow minimum. 4. kobutanol affects interfacial tension at low concentrations by aiding in the partitioning of the added surfactant. At higher concentrations, it alters the solvent power of the water, making the added surfactant more water soluble. 5. The minimum in interfacial tension can be determined by observing the breakage of the oil lamella structure in the aqueous phase. Acknowledgment We are grateful for the support provided by the U.S. Department of Energy. Registry No. Oleic acid, 112-80-1;Petrostep B-100,11678871-7;Na dodecyl sulfate, 151-21-3; decane, 12418-5;isobutanol, 78-83-1.
Literature Cited Berger, P. D.; Hsu, C.; Arendell, J. P. Designing and Selecting Demulaifiers for Optimum Field Performance Based on Production Fluid Characteristics. Presented at the SPE International Sym-
posium on Oilfield Chemistry, San Antonio, TX, Feb 4-6, 1987; paper SPE 16285. Chan, K. S.; Shah, D. 0. The Molecular Mechanism for Achieving Ultra Low Interfacial Tension Minimum in a Petroleum Sulfonate/Oil/Brine System. J . Dipersion Sci. Technol. 1980,1,55. Chan, K. S.;Shah, D. 0. The Physico-Chemical Conditions Necessary to Produce Ultralow Interfacial Tension at the Oil/Brine Interface. In Surface Phenomena in Enhanced Oil Recovery; Plenum Press: New York, 1981;pp 53-72. Doe, P. H.; El-Emary, M.; Wade, W. H.; Schechter, R. S. Surfactants for Producing Low Interfacial Tensions I Linear Alkyl Benzene Sulfonates. JAOCS, J . Am. Oil Chem. SOC.1977,54,570. Doe, P. H.;El-Emary,M.; Wade, W. H.; Schechter, €2. S. Surfactants for Producing Low Interfacial Tensions: 111. Di and Tri n-Alkylbenzenesulfonates. JAOCS, J. Am. Oil Chem. SOC.1978a,55, 513. Doe, P. H.; El-Elmary, M.; Wade, W. H.; Schechter, R. S. Surfactants for Producing Low Interfacial Tensions: 11. Linear Alkylbenzenesulfonates with Additional Alkyl Substituents. JAOCS, J . Am. Oil Chem. SOC.197813,55,505. Fleming, P. D.; V i t i e r i , J. E.Quantitative Interpretation of Phase Volume Behavior of Multicomponent Systems Near Critical Points. AlChE J. 1979,25,493. Fleming, P. D.; Vinatieri, J. E. The Role of Critical Phenomena in Oil Recovery Systems Employing Surfactants. J. Colloid Znterface Sci. 1981,81,319. Fleming, P. D.; Vinatieri, J. E.; Glinsmann, G. R. Theory of Interfacial Tensions in Multicomponent Systems. J . Phys. Chem. 1980,84,1526. Isaacs, E. E.; Smolek, K. F. Interfacial Tension Behavior of Athabasca Bitumen/Aqueous Surfactant Systems. Can. J. Chem. Eng. 1983,61,233. Matuura, €2.; Furudate, K.; Tsutsumi, H.; Miida, S. The Effect of Alcohols on the Solubilization of Oleic Acid in the Aqueous Solution of Sodium Dodecyl Sulfate. Bull. Chem. SOC.Jpn. 1961, 34, 395. Miller, C. A.; Neogi, P. Interfacial Phenomena: Equilibrium and Dymmic Effects;Surfactant Science Series; Marcel Dekker: New York, 1985; Vol. 17,p 168. Nishikido, N.; Moroi, Y.; Uehara, H.; Matuura, R. Effect of Alcohols on the Micelle Formation of Nonionic Surfactants in Aqueous Solutions. Bull. Chem. SOC.Jpn. 1974,47,2634. Pouchelon, A; Chatenay, D.; Meunier, J.; Lmgevin, D. Origin of Low Interfacial Tensions in Systems Involving Microemulaion Phases. J . Colloid Interface Sci. 1981,82,418. Rudin, J.; Wasan, D. T. Mechanisms for Lowering of Interfacial Tension in Alkali/Acidic Oil Systems, Part I Experimental Studies. Colloids Surf. 1992a,in press. Rudin, J.; Waean, D. T. Mechanisms for Lowering of Interfacial Tension in Alkali/Acidic Oil Systems, Part I 1 Theoretical Studies. Colloids Surf. 19924 in press. Schuler, P. J.; Lerner, R. M.; Kuehne, D. L. Improving Chemical Flood Efficiency with Micellar/ Alkaline/Polymer Processes. Presented at the Fifth Symposium on Enhanced Oil Recovery, Tulsa, OK, April 2&23,1986; paper SPE/DOE 14934. Shinoda, K. The Effect of Chain Length, Salts and Alcohols on the Critical Micelle Concentration. Bull. Chem. SOC.Jpn. 1953,26, 101. Shinoda, K. The Effect of Alcohols on the Critical Micelle Concentrations of Fatty Acid Soaps and the Critical Micelle Concentration of Soap Mixtures. J. Phys. Chem. 1954,58,1136. Vijayan, S.; Woods, D. R.; Vaya, H. Bulk and Interfacial Physical Properties of Aqueous Solutions of Sodium Lauryl Sulphate and Lauryl Alcohol with Air and Benzene Systems: Part 11-Mixed Aqueous Solutions of Sodium Lauryl Sulphate and Lauryl Alcohol. Can. J. Chem. Eng. 1978,56,103. Vinatieri, J. E. Correlation of Emulsion Stability with Phase Behavior in Surfactant Systems for Tertiary Recovery. SOC.Pet. Eng. J . 1980,Oct, 402. Wilson, P. D.; Brandner, C. F. Aqueous Surfactant Solutions which Exhibit Ultralow Tensions at the Oil-Water Interface. J. Colloid Interface Sci. 1977,60, 473.
Received for review January 24, 1992 Accepted May 25, 1992
