# 18.7: Surface Tension

Surface tension is a measure of the surface free energy of liquids, i.e., the extent of energy stored at the surface of liquids. Although it is also known as interfacial force or interfacial tension, the name surface tension is usually used in systems where the liquid is in contact with gas.

Qualitatively, it is described as the force acting on the surface of a liquid that tends to minimize the area of its surface, resulting in liquids forming droplets with spherical shape, for instance. Quantitatively, since its dimension is of force over length (lbf/ft in English units), it is expressed as the force (in lbf) required to break a film of 1 ft of length. Equivalently, it may be restated as being the amount of surface energy (in lbf-ft) per square feet.

Katz et al. (1959) presented the Macleod-Sudgen equation for surface tension ($$\sigma$$) calculations in dynes/cm for hydrocarbon mixtures:

$\sigma^{1 / 4}=\sum_{i=1}^{n} \operatorname{Pch}_{i}\left[\frac{\rho_{i}}{62.4\left(M W_{l}\right)} x_{i}-\frac{\rho_{g}}{62.4\left(M W_{g}\right)} y_{i}\right] \label{18.20}$

where:

• $$Pch_i$$ is the Parachor of component “i”,
• $$x_i$$ is the mole fraction of component “i” in the liquid phase,
• $$y_i$$ is the mole fraction of component “i” in the gas phase.

In order to express surface tension in field units (lbf/ft), multiply the surface tension in (dynes/cm) by $$6.852177 \times 10^{-3}$$. The parachor is a temperature independent parameter that is calculated experimentally. Parachors for pure substances have been presented by Weinaug and Katz (1943) and are listed in Table 18.1.

Table 18.1: Parachors for pure substances (Weinaug and Katz, 1943)
Component Parachor

CO2

78.0

N2

41.0

C1

77.0

C2

108.0

C3

150.3

iC4

181.5

nC4

189.9

iC5

225.0

nC5

231.5

nC6

271.0

nC7

312.5

nC8

351.5

Weinaug and Katz (1943) also presented the following empirical relationship for the parachor of hydrocarbons in terms of their molecular weight:

$P c h_{i}=-4.6148734+2.558855 M W_{i}+3.404065 \cdot 10^{-4} M W_{i}^{2}+\frac{3.767396 \cdot 10^{3}}{M W_{i}} \label{18.21}$

• This correlation may be used for pseudo-components or for those hydrocarbons not shown in Table 18.1.