**2.4 Drop volume/weight method**

50 Toxicity and Drug Testing

made up of platinum or iridium is submerged into liquid and then pulled through the liquid-air interface. Maximum pull exerted on the ring by the surface is measured which is directly proportional to the surface tension value at equilibrium (Bodour and Miller-Maier, 1998). The ring is submerged into the solution and then slowly pulled through the liquid-air interface to detach from the interface with a force that is correlated to the surface tension. With this method, it is possible to measure the interfacial tension at both liquid-air and

> cos *<sup>F</sup> <sup>f</sup>*

where *p* is the perimeter of the three-phase contact line, *f* is the correction factor (because additional volume of liquid is lifted during the detachment of the ring from the interface) between each measurement. The platinum wire ring was rinsed three times with water, later

One major difference between the Du Noüy ring method and Wilhelmy's plate is the way in which the surface tension measurement is carried out. The ring moves through the interface whereas the plate is static at the interface, therefore there is no disturbance at the interface and this method is the recommended geometry for studying time dependent characteristics. Both ring and plate geometries can be used with the force balance type of tensiometer. A single instrument is normally capable of performing either Wilherlmy plate or du Noüy ring

The surface tension measured by du Noüy method has been utilized in pharmaceutical research, for example, in the measurement of emulsion stability (Ishii et al., 1988; Takamura et al., 1984) and development of a dissolution media to simulate the physiological environment of the gastric region (Luner et al., 2001). This method can be used for characterization of pharmaceutical formulations such as plasticizer and polymer coating and surface tension calculation of various surfactant solutions and their CMC values (Palma et al., 2002) at the point of intersection of the interfacial tension value versus surfactant

The maximum bubble pressure method involves flow of a gas bubble (typically air or nitrogen) at a constant rate and blows them through a capillary with a known diameter which is submerged in the sample liquid. The pressure inside of the gas bubble increases until the bubble becomes hemispherical and its radius corresponds to the radius of the capillary. Beyond this the bubble is unstable and grows explosively until it detaches itself from the capillary and a new bubble is formed. The method is based on the continuous measurement of the applied pressure versus bubble rate formed at the end of the capillary. Figure 1 shows each step of bubble formation and corresponding change of bubble radius.

The dynamic surface tension can be directly calculated by Young-LaPlace equation:

max 2 *P R*

where Δ*P*max is the maximum pressure difference and *R* is the capillary radius (Drelich et al., 2002; Hallowell and Hirt, 1994; Fainerman et al., 2006). This method is one of the most popular

(3)

concentration plot (de la Maza, 1998; Korhonen et al., 2004; Zelkó et al., 2002).

(2)

*p* 

liquid-liquid interfaces. Surface tension can be calculated using the equation below:

with acetone as was blow dried (Drelich et al., 2002).

measurements (Thiessen and Man, 1999).

**2.3 Maximum bubble pressure method** 

Among the conventional methods of surface tension measurement, drop shape techniques have proven to be reliable and easy to handle. This method weighs the mass of the liquid drop or the volume of the drop that falls off a capillary tip of known diameter when pumped very slowly. The weight of the drop falling off the capillary correlates with the interfacial tension and is measured by balancing it against a known gravitational force through the following equation:

$$\mathcal{W} = V \Delta \rho \text{g} = 2 \pi r f \tag{4}$$

where is the difference in the density of the heavy phase and the light phase, *g* is the gravitational constant (*g*=9.81652 m/s2), *r* is the radius of the capillary tip and *f* denotes the empirical drop correction factor introduced by Harkins and Brown. The correction factor is required because only a portion of the drop falls from the capillary tip during detachment and this corrects the deviation of the drop volume from its ideal value (Drelich et al., 2002; Gunde et al., 2001). Impurity of active pharmaceutical solutions (Al-Maaieh and Aburub 2007), emulsion stability (Rangsansarid and Fukada, 2007), potency of local anesthetics (Matsuki et al., 1998), stability of biphasic aqueous systems (Mishima et al., 1998) and surface active properties of drugs (Deo et al., 2004) have been evaluated using this technique.
