폼(Foam) 및 에멀션에 있어서 필름 및 계면의 동적 장력

김영호, Darsh T. Wasan*

국립 한경대학교 화학공학과

*Chem. Eng. Dept., Illinois Institute of Technology, Chicago, IL 60616, U.S.A.

Dynamic Film and Interfacial Tensions in Emulsion and Foam Systems

Young-Ho Kim and Darsh T. Wasan*

Dept. of Chem. Eng., Han Kyung National University, Ansung 456-749, Korea

*  Dept. of Chem. Eng., Illinois Institute of Technology, Chicago, IL 60616, U.S.A.

Introduction

Coalescence of emulsions and rupture of foam bubbles depend on the stability of their intervening thin liquid films. The action of emulsifiers and demulsifiers, refining processes, defoaming, food industry, and environmental area relating to this subject are common practical examples where the stability of the emulsion and the foam films under static or dynamic conditions has paramount importance. The rheological behavior of foams and emulsions depends on the response of the thin liquid films and the Plateau borders during shear and dilation [1]. Several researchers have determined the static and dynamic interfacial properties of gas/liquid and liquid/liquid interfaces in the presence of surfactants using a number of methods [2,3].

In real emulsions or foams, the thin liquid films are formed between the bubbles or drops and the films formed typically have spherical curved shape due to the polydispersity of the bubbles or drops (Fig. 1). The interfaces of the film hydrodymically interact: the gravity and the capillary forces deform their interfaces and aqueous lamellae form between the bubbles (foam) or drop (emulsion). Note that the thin liquid films have adjoining menisci, or Plateau borders. The Plateau borders have much larger liquid volume than the films due to the very small thickness of a typical liquid film. The surface area of this film, however, is much larger than the area of the meniscus.

The rupture of a film inside the foam or emulsion acts as a local "detonation", resulting in very fast expansion in the nearby lamellae. Moreover, the polyhedral film system quickly rearranges itself to a new mechanical equilibrium after the film rupture. At the same time, surfactant diffusion and adsorption start to restore the adsorption equilibrium in the films which expanded, i.e., film tension relaxation takes place.

During film drainage, the film continuously and slowly expands due to the liquid flow inside the film as driven by the capillary pressure and gravity. Similarly, continuous liquid film expansion takes place in foams or emulsion under continuous shear or dilation.

In this study, we report static and dynamic interfacial tension and film tension measurements with a foam as well as an O/W emulsion system stabilized by an anionic and a nonionic surfactant, respectively. The relaxation process in a foam or emulsion system was modeled by stress relaxation experiments in which the film was first fast expanded and then kept at a constant size while the dynamic film tension was monitored. These experiments made it possible to determine the Gibbs film elasticity as well. The continuous film expansion process accompanying film drainage was modeled by dynamic film tension experiments in which the film were expanded with constant and slow rate. The film tensions and the IFTs of the respective single interfaces measured in dynamic conditions were compared.

Experimental

Device A curved, spherical cap-shaped aqueous film was formed (Fig. 2) at the tip of a glass capillary with its meniscus adhering to the capillary tip. The capillary used had an internal diameter of 0.6 mm and a wall thickness of 0.05 mm. The size of the film was varied using a computer controlled microsyringe attached to the capillary. In the emulsion film (O/W/O) studies, the capillary and outer side of the film were filled with dodecane (oil phase). The rate of surface expansions of one film interface were in the 3.5×10-4∼6.0×10-3 mm2/s range. In the foam film studies, the film was formed in the air (inner and outer sides of the capillary). The rate of surface expansions of one foam film surface used were in the 0.7×10-2∼2.5×10-2 mm2/s range.

The pressure applied across the curved film, i.e. capillary pressure was directly measured by a sensitive pressure transducer connected to the capillary. The film tension was calculated from the measured capillary pressure using the Young-Laplace equation. Single interfaces were also studied by forming a drop at capillary, under the same experimental conditions as in the respective film experiment. Further details of the apparatus and experimental technique have been published elsewhere [4].

Materials In the foam film studied, anionic surfactant, sodium decylsulfonate (SDeS) (from Research Plus Inc.) was used at 3.2×10-2 mol/dm3 concentration, which is highly above the CMC (3.0×10-2 mol/dm3) of this surfactant. In the study of emulsion film, a commercial nonionic surfactant, Brij58, hexadecyl polyoxyethylene ether (from Sigma) was used. This surfactant is highly water soluble but practically insoluble in oil and its CMC is around 2.5×10-6 mol/dm3. n-Dodecane (Gold Label, from Aldrich) was used as the oil phase. The oil was purified by treating it with activated charcoal for several days. The water phase used was deionized water.

Results and Discussion

Equilibrium film tension Static film tensions were measured by forming a film at the capillary tip and maintaining it at that size. The equilibrium film tensions for the foam and emulsion films at the given surfactant concentrations are shown in Table 1. For the comparison with the film, equilibrium single IFTs are also presented, which were measured by forming drop at the capillary tip. The table shows that the film tensions are two times the single interfacial or surface tension within experimental error.

Film stress relaxation The film stress relaxation experiments with anionic and nonionic surfactant were carried out for the emulsion and foam films. In Fig.3, the relaxation curves of the foam film formed from 3.2×10-2 mol/dm3 SDeS are shown. The relaxation curves with different expansions have similar shape and all of them approach the same equilibrium film tension. This indicates that there is indeed no measurable depletion in the meniscus. Fig.3 also provides information on the stability of the stressed film. Higher expansion (higher stress) resulted in lower stability indicated by the film rupture time. The easier film rupture at higher stress was probably caused by the lower surface concentration on the film surface after expansion and the larger film area after the film is expanded [5]. The larger film area has a higher probability of rupture at a given mechanical shock produced by self-vibration or mechanical disturbance.

Comparison of film tension and IFT of single interface Fig.4 shows comparison of stress relaxation of the film tension as well as two times the respective single IFT in the emulsion system with 12 CMC Brij58 at 48% initial expansion. The film tension relaxation is significantly slower than the relaxation of the single interface. Thus, in dynamic condition, before the film tension reaches equilibrium, the film tension is greater than two times the single IFT. This difference can be expressed in terms of relaxation time.

Film elasticity The initial (maximum) film tension (fi) after the expansion in the film relaxation experiments can be used to determine the film elasticity. The film elasticity (Ef) can be expressed as

[eqn: E_f = - {df_i} over {d ln (A/A_o)}]

()

where A and Ao are the expanded and the initial film areas, respectively. A plot of the initial film tension vs the logarithm of the relative film expansion is shown in Fig.5 for the foam films stabilized by 3.2×10-2 mol/dm3 SDeS. For comparison, the initial single IFTs obtained in the experiments with the respective single air/water surface are also plotted. The initial film tensions indeed show a linear relationship with the logarithm of relative surface expansion, as predicted by Eq. (1). The film elasticity obtained from the slope of the curve is equal, within experimental error, to twice the surface elasticity (Esf) of the corresponding surface, which was obtained in a similar way as the film elasticity.

Dynamic film tension Fig.6 shows the results of dynamic film tension measurements with emulsion films stabilized by Brij 58 at 12 CMC concentration. The films were slowly and continuously expanded with constant rate from the initial equilibrium state while the film tension were monitored. The dynamic film tension depends on the rate of the film expansion and the rate of surfactant adsorption during the expansion. A higher film expansion rate results in higher dynamic film tensions because the surfactant adsorption gets more and more delayed at increasing expansion rates.

References

1. D. A. Edwards and D. T. Wasan, J. Rheol. 32, 429 (1988).

2. R. Defey and G. Petre, "Dynamic Surface Tension", Surface and Colloid Science, Vol.3 (Matijevic, Ed.), Wiley, New York (1979).

3. D. A. Edwards, H. Brenner, and D. T. Wasan, "Interfacial Transport Process and Rheology", Butterworth-Heinemann (1991).

4. R. Nagarajan and D. T. Wasan, J. Colloid Interface Sci., 159, 164 (1993)

5. A. Prins, in "Foams", Proc. Symp. at Brunel Univ., R. J. Akers ed. (1975).

Table 1. Equilibrium Film Properties and IFTs of Some of              the Systems Studied.

	Foam film	Emulsion film
Surfactant and Concentration (mol/dm3)	SDeS,3.2×10-2	Brij58,2.0×10-2
Film tension, f, (mN/m)	75.2±0.3	27.8±0.3
IFT (mN/m)	37.5±0.2	13.8±0.2
2×IFT (mN/m)	75.0±0.3	27.6±0.3