Automated Interfacial Tensiometer

Abstract

The interfacial tension in a liquid-liquid system is determined by estimating the flow rate of the heavier liquid as it is discharged under constant head through a capillary immersed in a body of the lighter liquid. A known voltage is applied to each drop which forms at and detaches from the capillary tip and an electrometer is used to measure the total electrical charge per drop and the frequency of drop formation/detachment. The determination is made under conditions in which a plot of interfacial tension versus the square of the applied voltages yields two straight lines whose slopes of ±C/2 where C is the capacitance of the interface per unit area and the positive value is generated from the positive polarity voltage and the negative from the negative polarity. Based upon the value of C determined from the slope, the known voltage and the measured values of dropping frequency and of total charge per drop, the correctness of the estimated flow rate is determined and, in an iterative process, a new flow rate estimation and consequent plots are made until the estimated flow rate equals the flow rate as determined from measurements. The value of the interfacial tension at zero charge is then obtained from the intersection of the two lines.