Measurement of the dynamic interfacial tension and interfacial dilatational viscosity at high rates of interfacial expansion using the maximum bubble pressure method. II. Liquid—liquid interface

Measurement of the Dynamic Interfacial Tension and Interfacial Dilatational Viscosity at High Rates of Interfacial Expansion Using the Maximum Bubble Pressure Method. I1. Liquid-Liquid Interface R. L. KAO, D. A. EDWARDS, D. T. WASAN, ~ AND E. CHEN Department of Chemical Engineering, Illinois' Institute of Technology, Chicago, Illinois 60616 Received April 10, 1990; accepted June 12, 1991 A technique proposed previously for determining the surface dilatational viscosity at a gas-liquid surface is modified to examine liquid-liquid interfaces. Interfaeial dilatational viscosities for aqueous solutions of sodium dodecyl sulfate in contact with several oil phases are reported. © 1992 Academic Press, Inc. INTRODUCTION

In the companion paper ( 1) a technique was presented for measuring the surface dilatational viscosity at large rates of surface expansion from dynamic surface tension measurements obtained by the classical m a x i m u m bubble pressure method (MBPM). Presently we modify this technique to measure the interfacial dilatational viscosity at a liquid-liquid interface. As discussed in Part I ( 1 ), at large rates of surface expansion, bubbles produced at the tip of a circular cylindrical capillary expand as insoluble monolayers owing to relatively small rates of surfactant adsorption. Using the MBPM, the dynamic surface tension of the expanding bubbles is determined at the moment when the bubbles possess a hemispherical shape. This dynamic surface tension may be further decomposed into thermodynamic and dissipative contributions. The essential principle of the MBPM surface dilatational viscometer is then that the surface dilatational viscosity may be determined at high rates of surface expansion from the gradient of dynamic surface tension versus rate of surface expansion. Extension of the MBPM to liquidliquid interfaces requires certain modifications t To w h o m correspondence should be addressed.

to the basic experimental design. Nonetheless, the fundamental experimental principle remains unchanged. MODIFICATIONS TO THE MBPM APPARATUS

A schematic diagram of the modified MBPM apparatus is shown in Fig. 1. The apparatus, while similar in most respects to the apparatus described in Part I, possessed the following two essential differences: (i) The capillary used for liquid-liquid measurements was larger in radius (re = 160.43 + 0.09 t~m) and smaller in length than the MBPM capillary used for the gas-liquid surface (1) to minimize pressure drop clue to viscous losses between the manometer and the capillary tip. Because a finite pressure drop is unavoidable between the manometer and the capillary tip, and in order to ascertain the actual pressure in the liquid droplet, bubbles were produced with a pure water-oil system to determine the capillary tube pressure drop as a function of the liquid flow rate. During experimental runs with surfactant present, the pressure drop value corresponding to the appropriate flow rate was then subtracted from the manometermeasured pressure. (ii) The detection system for droplet frequency differed from the detection system of Part I ( 1 ) in that the spacing of the glass rods which direct the light beam

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&mrnal~fColloidand lnterJhceScience.Vol. 148, No. 1, January 1992

0021-9797/92 $3.00 Copyright © 1992by AcademicPress.Inc. All rightsof reproductionin any formreserved.

258

KAO

E T AL.

MANOMETER AND PIPET I1 c iI

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FIG. 1. The m o d i f i e d M D P M apparatus.

through the path of the produced bubbles was approximately one-half the distance, and the glass rods themselves were of a smaller diameter. These modifications were necessary to obtain a clear frequency detection since the difference in the refraction index for the liquid-liquid system is less than that of the gasliquid system. An additional modification may be desirable for highly viscous oils, such as crude oil. To avoid large viscous pressure drops in the capillary, the aqueous phase may be injected through a Teflon capillary (oriented vertically downward rather than upward) into the continuous fluid. Figure 1 depicts both the waterwet glass capillary tube arrangement and the inverted oil-wet Teflon capillary tube arrangement. SURFACTANT

A N D OIL S Y S T E M S

Sodium dodecyl sulfate (SDS) was purchased from BDH Chemicals (Poole, England) and was used as received. The oils nJournal o/Colloid and lmed2lce Science, Vol. 148, No. 1, January 1992

hexane, n-octane, n-tetradecane, and n-hexadecane were purchased from Sigma Chemical Co. (St. Louis, M O ) , and the oils n-decane and n-dodecane were purchased from Fisher Scientific C o . (Fair Lawn, N J ) . Each alkane oil was used as received. EXPERIMENTAL RESULTS Figures 2 and 3 show dynamic interracial tension values for aqueous solutions of 1.0 and 3.0 m M SDS, respectively, for the glass and Teflon capillary arrangements. The insoluble monolayer region (plateau region) occurs at a rate of interface expansion which is roughly one order of magnitude smaller than what was observed for the a i r / w a t e r / S D S surface (1). We attribute this difference to the smaller rates of surfactant adsorption at the liquid-liquid interface. Interfacial dilatational viscosity values are reported in Table I. These values were ob.tained from the slopes in Figs. 2 a n d 3 within the insoluble monolayer zone and are seen to

259

INTERFACIAL DILATATIONAL VISCOSITY, II 1 a

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1600

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2800

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INTERFACE EXPANSION RATE, Vs-v(Hz)

FIG. 2. Dynamic interfacial tension vs rate of interfacial expansion for glass capillary arrangement.

increase with increasing oil-phase chain length. In addition, the viscosity values are significantly larger than the surface dilatational viscosity values reported for the SDS gas-liquid surface examined in Part I ( 1 ) . We attribute these larger interfacial dilatational viscosities to the comparatively smaller rate of interracial expansion encountered within the insoluble monolayer zone. Figure 4 graphically depicts this non-Newtonian effect. t

t

As discussed in Part I (1), the very large rates of surface expansion encountered in the MBPM device result in a significantly reduced intrinsic surface dilatational viscosity compared to results from longitudinal wave experiments. It may be recalled that this conclusion followed from a comparison of dilatational viscosity values measured for an aqueous solution of 1.0 m M SDS in contact with decane using both the longitudinal wave I

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FIG. 3, Dynamic interracial tension vs rate of interfacial expansion for Teflon capillary arrangement. Journal of Colloid and Interface Science, V o

148, N o . 1, J a n u a r y 1992

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KAO ET AL.

a p p a r a t u s (interface e x p a n s i o n rate o f app r o x i m a t e l y 10 -2 H z ) a n d the m o d i f i e d M B P M (interface e x p a n s i o n rate o f a p p r o x i m a t e l y l 0 4 H z ) . T h e d i l a t a t i o n a l viscosity m e a s u r e d f r o m b o t h devices possessed a similar qualitative b e h a v i o r in relation to surfactant c o n c e n t r a t i o n , b u t differed in m a g n i t u d e b y several orders. As the e x p a n s i o n rate enc o u n t e r e d in the p r e s e n t investigation lies between the e x p a n s i o n rates i m p o s e d b y the longitudinal w a v e a n d M B P M devices, the current results m a y p e r h a p s be seen as further evidence o f n o n - N e w t o n i a n surface behavior.

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CONCLUSIONS The modified MBPM apparatus presented is an e x t e n s i o n o f t h e classical m a x i m u m b u b ble pressure m e t h o d a n d m a y be used to m e a -

TABLE I Interfacial Dilatational Viscosity as a Function of Oil Phase Type lnterfacial dilatational viscosity (sP)

Oil phase

Glass capillary S D S concn: I r a M

Teflon capillary S D S concn: 3 m M

Hexane Octane Decane Dodecane Tetradecane Hexadecane

-0.0007 0.0009 0.0013 0.0015 0.0018

0.0009 0.0015 0.0017 0.0019 0.0023 0.0027

Jo rnal ~?[Colloid and lnterfltee Science Vol, 148, No. l, January 1992

FIG. 4. Interfacial dilatational viscosity vs rate of interfacial expansion. sure d y n a m i c interfacial t e n s i o n s a n d interfacial d i l a t a t i o n a l viscosities o f l i q u i d - l i q u i d interfaces at high rates o f interracial e x p a n s i o n . Both oil-in-water and water-in-oil measurem e n t s m a y be m a d e . ACKNOWLEDGMENTS This work has been supported by the U.S. Department of Energy and by the National Science Foundation. The authors are grateful to Professor P. Joos for his useful comments. REFERENCES 1. Kao, R. L., Edwards, D. A., Wasan, D. T., and Chen, E., J. Colloid Interface Sci. 147, 247 (1991 ). 2. Ting, L., Wasan, D. T., and Miyano, K,, J. Colloid Interface Sei. 107, 345 (1985).