O/W emulsion의 효율적 처리를 위한 계면화학적 특성 연구

신성혜, 김동수

이화여자대학교 환경공학과

Characterization of the Interfacial Properties of O/W Emulsion for its Effective Handling

Sung-Hye Shin, Dong-Su Kim

Dept. of Environmental Engineering, Ewha Womans University, Seoul 120-750, Korea

Introduction

Oily wastewater can be discharged from many industrial activities, and the oily wastewater usually has the form of o/w emulsion. The physicochemical characteristics of o/w emulsion is very complex and, therefore, more difficult to treat compared with other types of wastewater. Many studies have been conducted by many investigators as the treatment for emulsion wastewater. However, most of these processes are known to be partly effective and have its own drawbacks in the application. The reason for this may be found in the fact that the physicochemical phenomena of emulsion wastewater have not been studied yet systematically. Therefore, the basic understanding of emulsion behavior in the several conditions which can be encountered in the wastewater treatment processes may be said to be necessary.

The aim of this study is focused on the characterization of emulsion under various conditions for the purpose of better understanding and management of control process of emulsion wastewater. Surface tension is found to be decreased as the increase in oil content and contact angle is influenced by the hydrophobicity of substrate. The work of adhesion of emulsion for substrates with different degree of hydrophobicity has been estimated from the contact angle and surface tension. Temperature and pH have a strong influence on the turbidity and surface tension and it is observed that electrophoretic characteristics of oil droplets is closely related with turbidity. Also, effects of the content of inorganic salts and surfactant on the electrokinetic property of emulsion have been considered. The influence and coagulant on stability is discussed through the calculation of potential energy of interaction between oil droplets, but this results cannot shown here.

Experimental

The test o/w emulsion was prepared by mixing the paraffin oil, triethanolamine,  and oleic acid in  the ratio of 35:20:45 as the weight percent. Aluminium sulfate (Al2(SO4)3·18H2O) and calcium nitrate (Ca(NO3)2·4H2O), both obtained from Duksan Inc., were employed for the preparation of inorganic coagulants. Sodium dodecyl sulfate (SDS, CH3(CH2)11OSO3Na) was purchased from Aldrich Co. The surface tension of all test emulsions under various conditions was measured by a surface tensiometer (Fisher Scientific Co., Surface Tensiomat Model-21) based upon the ring method, and the contact angle of the lens was measured swiftly by goniometer (Rame-Hart, Inc., Model 100) for glass and PET(Polyethylene Terephthalate), which were selected as the hydrophilic and hydrophobic one, respectively. The electrophoretic mobility of oil droplets in the emulsion was observed by electrophoresis measurement apparatus (Zeta Meter Inc., Model 3.0+). The turbidity of emulsion according to variables was observed using turbidimeter (HACH Chemical Co., Model 2100A). The emulsion for the turbidity measurement was conditioned by jar tester (Shinil Science, JS-660).

Results and Discussion

1. Variation of Emulsion Properties according to Oil Conten

Fig. 1 represents that the surface tension of pure water decreased sharply when small amount of oil was added to the water. The decrease in □ of water when oil is added to it can be explained by the fact that the intermolecular force of water, which is mainly contributed to the hydrogne bonding between the molecules, becomes weaker in the existence of other matters. In the present system, the presence of small oil particles between the water molecules seemed to diminish the attractive hydrogen bonding force. It also shows the change of the contact angle of emulsion for the PET and glass substrate according to the oil content. It can be seen that the contact angle of pure water decreases gradually on PET when the oil content in the emulsion is raised. On the contrary, it is shown to be increased steadily for the glass substrate. The reason is that the increased oily property results in the increase in the hydrophobicity of emulsion and the higher affinity of emulsion with PET since the surface nature of PET is originally hydrophobic. The change of Wa of the emulsion for PET and glass substrate computed by equation (1) is shown in Fig. 2.

Wa Ａ □lg ( 1 ／ cos□ )      (1)

Fig. 1. Change of the surface tension of emulsion (●) and contact angle on PET (■), glass (□) according to the oil content at 25℃.

Fig. 2. Variation of work of adhesion of emulsion according to its surface tension for PET and glass.

2. Effect of Temperature on the Emulsion Properties

The influence of temperature on the surface tension and turbidity of emulsion system (1000 ppm) has been investigated. Fig. 3 shows the change of surface tension of emulsion as the temperature varies. Examining the result of Fig. 3, the value of □ is shown to be decreased gradually as the increase in temperature. This may be due to the decrease of surface tension of water, which is the dispersion medium for the present system, when the temperature is raised. The variation of □ of pure water at different temperatures is also represented in Fig. 3. In comparison of the two, the trend in the change of the surface tension of emulsion according to the temperatures is presumably analogous to that for the dispersion medium.

Fig. 3. Variation of surface tension of emulsion at different temperatures.

Fig. 4. Effect of temperature on the turbidity of emulsion.

Fig. 4 represents the variation of turbidity according to the different temperatures. The decreasing tendency in the turbidity of emulsion as an increase in the temperature is possibly explained by the coagulation theory of colloids. This can be explained by using equation (2);

-(dn/dt) Ａ 4□Drn2           (2)

where, D Ａ diffusivity, r Ａ collision radius, n Ａ no.of particles per unit volume.

In the above equation, every collision between particles is assumed to accomplish the coalescence and contribute to the decrease in the number of particles. For a disperse system, the diffusivitiy (D) is known to be proportional to the temperature. Therefore, the increase in the temperature of a disperse system results in the increased collision rate between particles and coalescence. The coalescence between particles implies the decrease in the turbidity of disperse system. If lowering down the turbidity of emulsion, not the decrease in the oil content, is just intended, heating of the emulsion system can be considered to be an applicable method.

3. Effect of pH

The present o/w emulsion can be said to belong to the aquatic systems because the major constituent is water. For the present study, the change in turbidity, surface tension, and contact angle has been investigated according to the pH of emulsion system (1000 ppm). In addition, the influence on the electrophoretic mobility of oil droplets was observed and its relationship with the turbidity has been considered.

The turbidity of emulsion is about 36 NTU at pH 2 and the turbidity increases sharply as the increase of pH. The change in the turbidity of the present system may be explained by the variation of electrophoretic characteristics of oil droplets according to pH. The value of potential is observed to be negative at all this system of the pH range surveyed except at the extremely high pH, and granually decreased until pH reaches 12 point. Beyond this point, the value of potential decreases abruptly down to almost zero.  The reason for the increase of potential in the negative direction as the rise of pH may be explained by the fact that specific adsorption of hydroxide ion onto the surface of oil droplets occurs. The drastic change in the electrokinetic potential at very high pH range is probably explained by the change in the thickness of electrical double layer. .

4. Influence of Inorganic Salts and Surfactant on the Emulsion Behavior.

In the present system, aluminium and calcium salts are employed as inorganic coagulants and their effects on the stability of emulsion (1000 ppm) are examined. Fig. 5 shows the effect of salt concentration on the electrokinetic potential of oil droplets and turbidity of emulsion. It can be seen that the potential shifts to the positive direction according to the addition of both coagulants, while the result for its turbidity also represents that Al3+ is better for the induction of coagulation in emulsion than Ca2+ and shows a good agreement with the results of electrokinetic measurement and thermodynamic calculation. Related with the result in Fig. 5, it may be possible to consider the change in the interaction energy between oil droplets according to the addition of coagulants.  The interaction force between particles in a disperse system may be explained by the DLVO theory.

The effect of the concentration of SDS on the electrophoretic property and turbidity of emulsion is represented in Fig. 6. It can be seen from the result that electrokinetic potential of oil droplets continues to be increased in the negative direction. The negative increase of electrokinetic potential may be explained by the fact that SDS molecule contributes negative charge to disperse particles through its adsorption onto them.  Although SDS is an anionic surfactant, it has a long hydrocarbon tail in its structure, hence, some amount of adsorption of SDS on the surface of oil particles presumably takes place through hydrophobic interaction.  As a result, surface charge density of oil droplets becomes to be increased and so does the electrokinetic potential.  However, when SDS molecules start to form micelles, there won't be no more adsorption of SDS taking place on particles and the electrokinetic potential remains to be constant.

Fig. 5. Change of the electrokinetic potential of

oil dropltes (●:Al3+,■:Ca2+) and turbidity of

emulsion (○:Al3+,□:Ca2+) according to salt

concentration.

Fig. 6. Effect of SDS on the electrokinetic of

oil droplets (●) and turbidity of emulsion (○).

References

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