ARTICLE NO. CS974891 NOTE Effect of Neck Formation on the Measurement of Dynamic Interfacial Tension in a Drop Volume Tensiometer so that hydrodynamic effects can be neglected. Conditions under which neck formation may occur are studied, and a model is proposed to relate Dynamic interfacial tension values obtained by drop volume the apparent values of dynamic interfacial tension obtained from drops tensiometry will be affected under certain experimental condipossessing necks to actual values. tions by the formation of a neck between the drop and the capillary tip. This phenomenon must be accounted for to obtain accurate values of interfacial tension. In this work, neck forma-EXPERIMENTAL tion for a water -mineral oil system is studied under conditions The aqueous phases for all experiments were made with deionized where hydrodynamic effects can be neglected. A model origiwater. The oil phase used in the experiments was a light, white mineral nally developed for the determination of the surface tension oil obtained from Sigma Chemical Co. At room temperature, its density of a suspended drop is modified for application to dynamic was measured to be 862 kg m 03 , and its viscosity was 22.7 mPa s. interfacial tensions of surfactant-containing liquids. The model The surfactants used were Span 80 ( sorbitan monooleate ) from Sigma relates apparent values of interfacial tension calculated from Chemical Co. and sodium dodecyl sulfate ( SDS ) from Fisher Scientific drops possessing necks to actual values. Experiments with Span Ltd. Both surfactants were tested as received, without further purifica- 80 ( sorbitan monooleate ) and sodium dodecyl sulfate ( SDS ) tion. The density of Span 80 was measured to be 999 kg m 03 at room surfactants in a mineral oil -water system are used to test the temperature. SDS was supplied as a powder. In all concentrations tested, validity of the developed model. For the small tip diameter used, the Span 80 was readily soluble in the oil phase. SDS was used in good agreement is obtained for Span 80 up to the critical micelle aqueous solutions. concentration, and for low concentrations of SDS, when the surfactant adsorption is diffusion-limited. In both cases, the Apparatus neck diameter of the growing drop can be considered constant All interfacial tension measurements were made with a Kruss drop over the range of dynamic interfacial tensions tested. α§ 1997 volume tensiometer, Model DVT-10. A schematic representation of this Academic Press
apparatus is presented in Fig. 1. One liquid phase is placed in the glass Key Words: drop volume tensiometer ; dynamic interfacial tube. The second liquid is introduced into the tube through a 254-mm tension; neck formation; mineral oil; sorbitan monooleate; socapillary using a syringe pump. The capillary can be attached to the tube dium dodecyl sulfate.
either at the top or at the bottom. This allows two possible capillary orientations: vertically up ( Fig. 1A ) , thus allowing the introduction of the light phase into the heavy phase; or vertically down ( Fig. 1B ) , thus allowing the introduction of the heavy phase into the light phase. In the following sections, the configuration represented by Fig. 1A will be