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Influence of pH on the stability of oil-in-water emulsions


Colloids and Surfaces A: Physicochemical and Engineering Aspects 170 (2000) 173 – 179 www.elsevier.nl/locate/colsurfa

In?uence of pH on the stability of oil-in-water emulsions stab

ilized by a splittable surfactant
Chin-Ming Chen, Chun-Hsiung Lu, Chien-Hsiang Chang *, Yu-Min Yang, Jer-Ru Maa
Department of Chemical Engineering, National Cheng Kung Uni6ersity, Tainan 70101, Taiwan, ROC Received 10 October 1999; received in revised form 31 January 2000; accepted 1 February 2000

Abstract A laboratory study was conducted to evaluate the effect of pH on the stability of oil-in-water emulsions stabilized by a commercial splittable surfactant Triton SP-190 by comparison with the results obtained by a common surfactant Triton X-100. The emulsion stability was explored by measuring the volume of oil phase separated and the size of the dispersed droplets. It was found that the addition of inorganic acids did not signi?cantly affect the stability of emulsions stabilized by Triton X-100, but had a profound in?uence on the stability of emulsions stabilized by Triton SP-190. Moreover, the droplet size of a Triton X-100-stabilized emulsion and its dynamic interfacial activity were insensitive to acids. However, at lower pH the droplet size of the emulsions stabilized by Triton SP-190 was considerably increased. From the dynamic interfacial tension measurements the dynamic interfacial activity of Triton SP-190 at the oil/water interface was found to be strongly inhibited by the addition of acids, resulting in a slower decreasing rate of dynamic interfacial tension. The results demonstrate that the dramatic destabilization of Triton SP-190-stabilized emulsions could be realized by the use of acids, which evidently changed the interfacial properties of the surfactant and resulted in a higher coalescence rate of oil droplets. ? 2000 Elsevier Science B.V. All rights reserved.
Keywords: Demulsi?cation; Dynamic interfacial tension; Emulsion; Emulsion stability; Splittable surfactant

1. Introduction Demulsi?cation is an important operation in many industrial processes, such as oil recovery and liquid– liquid extraction, and has a signi?cant effect on the quantity and quality of the ?nal
* Corresponding author. Tel.: + 886-6-2757575, ext. 62671; fax: +886-6-2344496. E-mail address: changch@mail.ncku.edu.tw (C.-H. Chang)

products. Recent developments in oil-contaminated soil remediation by using a surfactant-solution ?ooding approach [1–7] requires an ef?cient emulsion-breaking technique, since a large amount of oil-in-water emulsions must be demulsi?ed for oil separation. In the surfactant-enhanced remediation of oil-contaminated soils, the excellent solubilization and emulsi?cation properties of surfactants are applied to allow ?uids to remove oils ef?ciently from the soils, resulting in a

0927-7757/00/$ - see front matter ? 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 0 0 ) 0 0 4 8 0 - 5

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wastewater ef?uent containing large amounts of oil-in-water emulsions. One would expect that the high ef?ciency of surfactants to recover oils would produce high stability of emulsions in the wastewater ef?uent and prevent the oils from being easily separated from the ef?uent in the latter treatment. It is well known that emulsions are generally stabilized by repulsive charges on the surface of the dispersed phase and by adsorbed layers that act as an interfacial barrier to prevent the close contact or coalescence of the dispersed droplets. Demulsi?cation is usually achieved by physical and/or chemical methods. Physical methods, such as the use of heat [8,9] or an electrical ?eld [10–12], increase the contact frequency of dispersed droplets. Chemical methods, such as the use of demulsi?ers [13 – 17] or acids/bases [18,19], affect the interfacial properties of the adsorbed layers on the droplet surfaces and increase the coalescene rate of dispersed droplets. Among these methods pH adjustment by using acids/ bases in destroying the emulsion stability has certain advantages over other methods since it is usually cheaper and easier to be applied in a process. Recently several types of destructible or splittable surfactants have been reported in the literature and patents [20 – 24]. A common property of the surfactants is that their surfactant characteristics can be easily destroyed by adjusting the pH, which is convenient for demulsi?cation treatments. The pH change was expected to in?uence the interfacial properties of emulsion droplets stabilized by the splittable surfactants and, thus, to affect the droplet size and the emulsion stability. The present work aims to compare the effect of pH on the stability of oil-in-water emulsions stabilized by a common surfactant Triton X-100 and a commercial splittable surfactant Triton SP-190. The goal is to clarify the principles involved in using pH adjustment to break emulsions stabilized by a splittable surfactant. These principles should be helpful in the applications of splittable surfactants in wastewater treatments or other industrial operations where emulsion separation is desired.

2. Experimental Triton SP-190 surfactant was purchased from Union Carbide, USA, and was used without further puri?cation. The major component of Triton SP-190 is polyethylene glycol and the active content is 100 wt.%, according to the information provided by the manufacturer. The average degree of ethoxylation is 9 mol/mol and the estimated hydrophilic–lipophilic balance (HLB) number is 13. Triton X-100 (approximately 97% pure) was supplied by Aldrich, USA, and was used as received. Research-grade n-pentadecane obtained from Tokyo Kasei Kogyo, Japan, was chosen as a model oil. All experiments were conducted with puri?ed water that was passed through a Milli-Q plus puri?cation system (Millipore, Japan) with a resistivity of 18.2 MV-cm. n-Pentadecane-in-water emulsions were prepared by homogenizing 30 ml of n-pentadecane (20 vol.%) and aqueous phase (80 vol.%) containing 500 ppm surfactant. The emulsion was generated by sonication with an ultrasonic processor (model VCX 600, Sonics and Materials, USA) for 30 s at room temperature before being adjusted to the desired pH using 37 wt.% HCl solution. The pH value of the system was detected by a pH meter (model SP-701, Suntex, Taiwan). The prepared emulsions at different pH conditions were immediately placed into standard titration pipettes to test their stability. The oil droplets had a lower density than the surrounding aqueous phase and, therefore, moved upward due to buoyancy force. The emulsion was then settled and generally formed oil, coarse emulsion, and ?ne emulsion phases from the top. The emulsion stability was evaluated by measuring the volumes of the oil phase separated and the coarse emulsion phase as a function of time at room temperature. The droplet size distribution of the emulsions was determined by using a laser light scattering instrument (model Mastersizer, Malvern, UK). It captures the actual scattering pattern from a ?eld of droplets when a laser is passed through the emulsion. Since each size of droplet has its own characteristic scattering pattern, the droplet size which creates that pattern can be predicted by using the Mie theory [25]. The measurements of

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size distributions were performed on very dilute emulsions prepared by adding 0.25 ml n-pentadecane to 500 ml aqueous phase containing 500 ppm surfactant in a beaker, which was contacted with the pump arm of the particle size analyzer. Serving as a pump and stirrer, the pump arm also contained an ultrasonic probe that could be used to aid sample dispersion by breaking up sample agglomerates. As the emulsion was dispersed and stirred at 1520 rpm for 10 min, it was pumped to pass through a measurement cell and the size distribution was then monitored as a function of time. After 20 min of measurements, the emulsion was adjusted to the desired pH using HCl solutions if the pH effect was to be investigated. The results are reported as droplet volume distribu-

tions and volume mean diameters. The dynamic interfacial tension-lowering activity of a surfactant at an oil/aqueous phase interface was obtained with a drop volume tensiometer (model TVT1, Lauda, Germany). At ?rst an aqueous drop containing 500 ppm surfactant with a de?nite volume, which is smaller than the critical volume corresponding to the equilibrium interfacial tension of the n-pentadecane/aqueous phase interface, was formed very fast at the tip of a capillary surrounded by n-pentadecane. The interfacial tension decreased with time due to the dynamic adsorption behavior of surfactant molecules at the interface. The time necessary for the interfacial tension to decrease to a value that the drop detached from the capillary tip due to gravitational force was measured, and the interfacial tension was calculated from the known volume of the drop and the capillary diameter. After the drop detached from the capillary tip, the next drop was formed with a slightly smaller volume and the procedure started again. By repeating this procedure, the dynamic interfacial tension behavior at a n-pentadecane/aqueous phase interface was constructed [26].

3. Results and discussion The effect of pH on the emulsion stability was investigated for n-pentadecane/water emulsions stabilized by two surfactants, Triton X-100 and Triton SP-190. In Fig. 1(a) the volume fraction of the oil phase separated in an emulsion is plotted versus time at different pH conditions for n-pentadecane emulsi?ed with an aqueous phase containing 500 ppm Triton X-100. It can be seen from this plot that the emulsion was very stable with no acids added since there was no separation of the oil phase after being settled for 3 h. It is well known that surfactant molecules have the ability to adsorb at the oil/water interface and form a protective ?lm around the dispersed oil droplets preventing their coalescence, thus leading to an enhanced stability of the emulsion. When the pH of the aqueous phase was adjusted to 3.5 or as low as 1.1, the emulsion stability was only slightly affected. It has been reported that a selec-

Fig. 1. Time dependence of the volume fraction of oil phase separated in n-pentadecane-in-water (1/4, v/v) emulsions stabilized by (a) 500 ppm Triton X-100 and (b) 500 ppm Triton SP-190 under different pH conditions.

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Fig. 2. Time dependence of the volume fraction of coarse emulsion phase in n-pentadecane-in-water (1/4, v/v) emulsions stabilized by 500 ppm Triton SP-190 under different pH conditions.

tive adsorption of OH? ions from the aqueous phase can cause excess negative charges in the vicinity of the non-polar oil-water interface [27]. Thus, the slightly affected stability of Triton X100-stabilized emulsions was probably due to the decreased negative charges near the interface by the addition of acids. In general, the stability of n-pentadecane-in-water emulsions stabilized by 500 ppm Triton X-100 was quite insensitive to the addition of acids or decreased pH. However, it was found that adjusting the pH signi?cantly affected the stability of Triton SP190-stabilized emulsions. Fig. 1(b) and Fig. 2 show the effect of pH adjustment on the stability of n-pentadecane-in-water emulsions containing 500 ppm Triton SP-190. With no acid added or at pH 3.4, the volume of the oil phase separated was essentially negligible, and the volume of the coarse emulsion phase increased slightly due to the coalescence of ?ne oil droplets. It is obvious from Fig. 1(b) that the emulsion stability of the system was decreased substantially by decreasing the pH from 3.4 to 1.2, resulting in signi?cant oil phase separation. The instantaneous increase of the volume of the coarse emulsion phase occurred

at pH 1.2 indicates that the emulsi?cation ef?ciency of Triton SP-190 was strongly affected by acids, resulting in larger oil droplets in the emulsions (Fig. 2). The following decrease in the volume of the coarse emulsion phase, which corresponded to the increase in the volume of the oil phase separated, suggests that signi?cant coalescence occurred between dispersed oil droplets. The separation of the oil phase involves two processes, coalescence of emulsi?ed oil droplets and creaming of coalesced oil drops. Emulsions stabilized by Triton SP-190 apparently exhibited higher droplet coalescence rates at lower pH. This effect is probably attributed to the in?uence of the decreased pH on the adsorbed Triton SP-190 molecules in the interfacial ?lms, which could then induce ?occulation or coalescence. Since the rate of oil separation in creaming depends on the effective size of the oil droplets, the higher droplet coalescence rate would cause a higher creaming velocity and thus faster separation of the emulsi?ed oil from the water. This would account for the observed decreased volume of the coarse emulsion phase and the corresponding increased volume of the oil phase separated at pH 1.2 (Fig. 1(b) and Fig. 2). The results clearly demonstrate that the stability of Triton SP-190-stabilized emulsions was particularly sensitive to low pH. Emulsions stabilized with Triton SP-190 exhibited a higher droplet coalescence rate than those stabilized with Triton X-100 at an equivalent pH. In the absence of acids Triton SP-190-stabilized emulsions did not show much ?occulation or coalescence in 3 h (Fig. 1(b)). However, decreasing the pH value of the aqueous phase by adding more acids could greatly induce droplet coalescence, leading to signi?cant oil phase separation within 3 h, probably because the surfactant characteristics of Triton SP-190 was destroyed or inhibited at a low pH as reported by Union Carbide [28]. Volume mean diameters and droplet size distributions of n-pentadecane-in-water (1/2000, v/v) emulsions stabilized by 500 ppm Triton X-100 or Triton SP-190 were analyzed as a function of pH using the light scattering technique (Figs. 3 and 4). Fig. 3 shows the volume mean diameter as a function of time for emulsion systems stabilized

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by 500 ppm Triton X-100 at various pH. As can be seen from Fig. 3 the volume mean diameters of the emulsions were not affected by the addition of acids for the pH range 1 – 5. One should note that the progressively decreased droplet size with time was evidently due to the continuous stirring during the measurements and was not related to the emulsion stability. However, a pH dependence was found on the volume mean diameter of the emulsions stabilized by 500 ppm Triton SP-190 (Fig. 3). In the absence of acids the droplet size slightly decreased with time as observed in the case of Triton X-100. At pH 3 the droplet size variation of the emulsion still resembled that of the emulsion without the addition of acids. Nevertheless, a further decrease of the pH value, for example at pH $ 1, there was a pronounced increase in the volume mean diameter after 50 min of measurements. Before that the droplet size was similar to that of the emulsions at higher pH. The considerable increase in droplet size at pH $ 1 might be caused by ?occulation or coalescence of droplets as observed in the emulsion stability tests. Since the emulsions were stirred continuously during the measurements,
Fig. 4. Effects of pH on the droplet size distribution of n-pentadecane-in-water (1/2000, v/v) emulsions stabilized by (a) 500 ppm Triton X-100 and (b) 500 ppm Triton SP-190.

Fig. 3. Time dependence of the mean droplet size of n-pentadecane-in-water (1/2000, v/v) emulsions stabilized by 500 ppm Triton X-100 ( , 2, ) or Triton SP-190 (
, ", ) at various pH conditions.

one would expect that ?occulated droplets would be dissociated, whereas coalesced droplets remained intact. Thus, the increase in droplet size was most likely due to the coalescence of the oil droplets. Furthermore, the initial droplet size variation at pH$ 1 during the ?rst 50 min was similar to that obtained for the emulsion without the addition of acids, which suggests that the effect of acids on the stability of Triton SP-190stabilized emulsions was kinetic-controlled. The size distributions of dispersed droplets in emulsions stabilized by 500 ppm Triton X-100 or Triton SP-190 are shown in Fig. 4, which shows the volume distribution curves obtained at 1 h after the addition of the acids. As was previously discussed, the emulsions stabilized by Triton X100 possessed similar volume distributions for the

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pH range 1 –5. As for the emulsions stabilized by Triton SP-190, little change in droplet size distribution occurred as the pH value was decreased from 5.4 to 3.4. However at pH $ 1, the emulsions displayed a signi?cant increase in the size of oil droplets and the droplet size distribution became broader, which implies that considerable droplet coalescence occurred. Furthermore, a broadening of the droplet size distribution could promote oil phase separation by allowing boundary coalescence among many droplets rather than simple binary coalescence [15]. In order to characterize the effects of pH on the interfacial properties of Triton SP-190, the dynamic interfacial activity of surfactant molecules was next investigated. Dynamic interfacial ten-

sions at the interface of n-pentadecane/aqueous phase containing 500 ppm Triton X-100 or Triton SP-190 were determined in both the absence and the presence of acids and are shown in Fig. 5(a) and (b). In the case of Triton X-100 (Fig. 5(a)), no appreciable change in the dynamic interfacial tension curve was observed, which implies that the interfacial property or dynamic interfacial activity of Triton X-100 at the interface was not in?uenced by acids, as expected from the results of emulsion stability and droplet size analysis. Fig. 5(b) shows that the dynamic interfacial tension curve for the case of Triton SP-190 could be considerably affected by acids. As the pH value was decreased to 3.6, a change in the dynamic interfacial tension curve was not detected. However, a further decrease in the pH value to 1.6 appeared to exert a signi?cant increase in dynamic interfacial tensions or decrease in dynamic interfacial activity. The increase of dynamic interfacial tensions when the pH value was low suggests that the dynamic interfacial activity of the surfactant, Triton SP-190, was inhibited by acids. Apparently, when the hydrophobic part of the surfactant was chemically cleaved from the surfactant hydrophile in the presence of acid as reported by Union Carbide [28], the interfacial properties of Triton SP-190 were affected, and its emulsi?cation ef?ciency was greatly reduced.

4. Conclusions This study investigated the effects of pH on the stability of emulsions stabilized by a common surfactant Triton X-100 and a splittable surfactant Triton SP-190. Experimental results suggest that the stability and the droplet size of emulsions stabilized by Triton X-100 were not signi?cantly in?uenced by acids. The dynamic interfacial activity of Triton X-100 was also insensitive to acids. On the contrary, the addition of inorganic acids had a profound effect on the stability of Triton SP-190-stabilized emulsions. The average droplet size and droplet size distribution of the emulsions stabilized by Triton SP-190 were also found to be increased by lowering pH, probably because of the enhanced coalescence rate of the oil droplets.

Fig. 5. Effects of pH on the dynamic interfacial tension behavior at the interfaces between n-pentadecane and 500 ppm of (a) Triton X-100 and (b) Triton SP-190 aqueous solutions at 25°C.

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Moreover, the dynamic interfacial activity of Triton SP-190 at the oil/water interface was greatly inhibited by acids, which implies that the interfacial properties of Triton SP-190 molecules were strongly in?uenced by acids. Apparently, by the addition of acids, the stability of emulsions stabilized by Triton SP-190 was signi?cantly decreased along with the deactivated interfacial activity of Triton SP-190 molecules.

Acknowledgements This work was supported by the Chinese Petroleum Corporation and the National Science Council of the Republic of China through Grants c NSC87-CPC-E-006-007 and NSC88-CPC-E006-010.

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