Alkaline Systems

Journal of Colloid and Interface Science 239, 226–229 (2001) doi:10.1006/jcis.2001.7547, available online at http://www.idealibrary.com on

Effects of Added Surfactant on the Dynamic Interfacial Tension Behavior of Acidic Oil/Alkaline Systems Youssef Touhami, Dipak Rana, Vladimir Hornof, and Graham H. Neale1 Department of Chemical Engineering, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada Received March 3, 2001; accepted March 17, 2001

The effects of a ready-made surfactant (sodium dodecyl sulfate) on the dynamic interfacial tension between a model acidic oil (linoleic acid dissolved in paraffin oil) and various aqueous alkaline (NaOH) systems have been studied using pendant drop tensiometry at surfactant concentrations both below and above the critical micelle concentration (CMC). Below the CMC the added surfactant contributes significantly to a further reduction of interfacial tension of the reacting acid/alkaline system, whereas above the CMC the added surfactant plays an important role in damping the dynamic trends observed for the reactive system alone. °C 2001 Academic Press Key Words: dynamic interfacial tension; critical micelle concentration; surfactant; acidic oil; alkali.

INTRODUCTION

The reduction of interfacial tension plays an important role in several practical situations, for example, during enhanced oil recovery processes in petroleum reservoirs and during the remediation of soils contaminated with organic solvents. Both fundamental and applied work in these areas is continuing (1–4). One of the more promising oil recovery techniques is known as alkaline flooding (2), in which the alkali in the aqueous phase and the acid naturally present in the oil phase comprise an interfacially reactive system. Interfacial reaction generates surfactants in situ, leading to a reduction in interfacial tension (IFT) between the two phases, to an improved mobilization of oil droplets, and hence to an increased oil recovery. Surfactants, whether they are generated in situ by chemical reaction or added externally, tend to accumulate at the oil/aqueous interfaces where the hydrophilic and hydrophobic ends of the molecules can be in a minimum energy state. This increases the surface pressure and decreases both the interfacial energy and the interfacial tension. If the concentration of surfactants at the interface is very low, the molecules will lie flat on the surface. As their concentration increases, the surfactant molecules begin to orient themselves at the interface, forming a unimolecular layer (monolayer). At a solution concentration about the same as the solution concentration when this occurs, surfactant monomers in solution begin

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to spontaneously associate into clusters (micelles). The solution concentration at which this occurs is known as the critical micelle concentration (CMC). At the CMC the interface will change physically because of the adsorbed surfactants, the contact angle between the immiscible liquids will decrease, and the spherically shaped interface will tend to become more planar. For successful oil recovery by alkaline flooding in practice the interfacial tension should be maintained at low values for a sufficiently long duration to permit both micro-displacement of the trapped oil and emulsion inversion, which together lead to the formation of a macroscopic oil bank that can be displaced to the producing wells. For this reason, the laboratory study of the fundamental interaction mechanisms between acidic oil and surfactant-enhanced alkaline solutions is of great significance to an improved understanding of the lowering of the interfacial tension of these systems. It is well established that the interfacial tension behavior between acidic oils and surfactant-enhanced alkaline solutions exhibits a marked synergistic effect. This effect is a function of the added surfactant type and concentration and can lead to the generation of ultra-low interfacial tensions at the oil–water interface. The acid, when present in the aqueous phase, contributes to the synergistic process taking place between the added surfactant and the ionized species (5). A comprehensive review of recent literature in this area may be consulted elsewhere (7, 8). In earlier work in this laboratory regarding the IFT behavior between synthetic acidified oils and surfactant-enhanced alkaline solutions, the added surfactant (as opposed to the in situ generated surfactant) reduced the IFT to values lower than those observed for the acid alone and the surfactant alone (6–9). This further reduction in IFT is due to the simultaneous adsorption of unionized acid from the oil side and the surfactant from the aqueous side of the interface. In the present communication we report the results of dynamic interfacial tension behavior, as observed by the pendant drop technique, for the addition of a ready-made surfactant to model acidified oil/alkaline systems. EXPERIMENTAL

Light paraffin oil (BDH Chemicals Ltd) was employed as the oil phase in this study. The density of the paraffin oil was 0.8423 g/mL, and its viscosity was 19.25 mPa · s at 25◦ C. A 226

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Fisher purified grade linoleic acid was dissolved in the light paraffin oil to simulate acidic oil. The acid concentration was fixed at 1 mmol/L. The alkaline reagent employed was sodium hydroxide (ACS grade, BDH Chemicals Ltd). Sodium dodecyl sulfate (SDS) of high purity grade (Analar Biochemical) was obtained from BDH Chemicals Ltd and was used as received. Double-distilled, deionized, and deaerated water was used in the preparation of all of the aqueous solutions. The interfacial tensions (IFT) of the immiscible oil/aqueous systems were measured by using the modified pendant drop apparatus developed previously in this laboratory and described in detail elsewhere (9). All experiments were carried out at atmospheric pressure and at a temperature of 25◦ C. RESULTS AND DISCUSSION

The changes in interfacial tension with respect to time for 7 mmol/L SDS, 1.25 mmol/L NaOH, and a mixture of both 7 mmol/L SDS and 1.25 mmol/L NaOH, against 1 mmol/L linoleic acid in the light paraffin oil are shown in Fig. 1. In earlier work performed in this laboratory based upon IFT measurements (6–9), the CMC of the SDS solution in contact with pure paraffin oil was about 8 mmol/L whereas in the presence of acid it was about 6 mmol/L. For SDS solution, the IFT decreases slightly with respect to time. For NaOH solution and for mixtures of NaOH and SDS, the IFT initially decreases and after a certain time period the IFT value becomes almost constant, following which it increases slightly with time. The important observation is that in the presence of both NaOH and SDS, the IFT value decreases more rapidly as compared to the NaOH solution alone. This indicates that the addition of ready-made

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surfactant produces more favorable results than in situ surfactant formation alone. It is seen that a minimum IFT value is reached after 1050 s at steady state for NaOH solution, whereas for NaOH/SDS mixtures, the minimum occurs at about 600 s. Figure 2 depicts the IFT behavior of the acidified oil in contact with 3 mmol/L SDS, 12.5 mmol/L NaOH, and combined 3 mmol/L SDS and 12.5 mmol/L NaOH. The initial IFT values for these solutions are approximately 30, 14, and 8 mN/m, respectively. As expected, with increasing surfactant concentration the IFT value decreases. For the 3 mmol/L SDS solution, the IFT value decreases slightly with time. This behavior is similar to the trend seen in Fig. 2. In the presence of 12.5 mmol/L NaOH solution, the IFT value decreases and reaches a minimum value of about 2.8 mN/m and then increases slightly as time goes on. The minimum IFT values for the 12.5 mmol/L NaOH and for the combined 12.5 mmol/L NaOH and 3 mmol/L SDS mixture occur at about 50 and 300 s, respectively. Figure 3 shows the IFT behavior of the acidified oil with respect to 6 mmol/L SDS, 12.5 mmol/L NaOH, and mixtures of 6 mmol/L SDS and 12.5 mmol/L NaOH. For 6 mmol/L SDS solution, the IFT value decreases with time, which is similar to the previous two concentrations. For the mixed 6 mmol/L SDS and 12.5 mmol/L NaOH solutions, the IFT value is initially constant at about 4.5 mN/m (lower than for the 12.5 mmol/L NaOH solution, which is 8 mN/m), and drops rapidly to about 0.7 mN/m, and finally attains an almost constant value. The initial IFT value for the 12.5 mmol/L NaOH and 3 mmol/L SDS mixtures is about 14 mN/m, which is higher than that for the 12.5 mmol/L NaOH solution alone. The minimum IFT value for the 12.5 mmol/L NaOH/6 mmol/L SDS mixtures is reached at about 60 s. Comparing this with Fig. 2, a two-fold change

FIG. 1. Interfacial tension for 1 mmol/L linoleic acid in paraffin oil against 1.25 mmol/L NaOH (d), 7 mmol/L SDS (m), and mixtures of 1.25 mmol/L NaOH and 7 mmol/L SDS (j).

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FIG. 2. Interfacial tension for 1 mmol/L linoleic acid in paraffin oil against 12.5 mmol/L NaOH (d), 3 mmol/L SDS (m), and mixtures of 12.5 mmol/L NaOH and 3 mmol/L SDS (j).

of the SDS concentration changes significantly the relaxation behavior with respect to time. The effect of surfactant concentration on IFT in the presence of high-concentration alkaline solution (125 mmol/L NaOH) is given in Fig. 4. Here three different surfactant concentrations (3, 4, and 5 mmol/L SDS) are used. In the presence of 125 mmol/L NaOH solution, the IFT initially drops rapidly dur-

ing the period 0–10 s and then increases slowly within the time period 10–100 s, following which a sigmoid type of behavior is observed. It is interesting to observe that the IFT value decreases by about three times with a ten-fold increase of NaOH concentration. For example, IFT values are 28, 8, and 2.8 mN/m for 1.25, 12.5, 125 mmol/L NaOH solutions, respectively. On the other hand, in the presence of SDS the IFT values initially drop

FIG. 3. Interfacial tension for 1 mmol/L linoleic acid in paraffin oil against 12.5 mmol/L NaOH (d), 6 mmol/L SDS (m), and mixtures of 12.5 mmol/L NaOH and 6 mmol/L SDS (j).

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DYNAMIC INTERFACIAL TENSION BEHAVIOR

FIG. 4. Interfacial tension for 1 mmol/L linoleic acid in paraffin oil against 125 mmol/L NaOH (d), mixtures of 125 mmol/L NaOH and 3 mmol/L SDS (m), mixtures of 125 mmol/L NaOH and 4 mmol/L SDS (j), and mixtures of 125 mmol/L NaOH and 5 mmol/L SDS (r).

rapidly within the time period 0–10 s and then remain almost constant. As expected, a decrease in IFT value is obtained as the SDS concentration increases. In summary, it is apparent that for surfactant concentrations below the CMC the addition of a ready-made surfactant can lead to IFT values significantly lower than those obtainable with a chemical reaction alone. Conversely, above the CMC, the addition of a ready-made surfactant can play a significant role in damping the dynamic IFT behavior arising from a chemical reaction alone. It is hoped that the results presented here will prove useful in those practical situations in which a decreased oil/aqueous IFT is desirable. ACKNOWLEDGMENT The authors are grateful to the Natural Sciences and Engineering Research Council of Canada (NSERC) for continuing financial support.

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