92168.pdf

Liquid-gas and liquid-liquid interfaces 95

AW

Figure 4. 16 A drop of non-spreading oil on a water surface

^ Saturated solution of

n-hexanol in water

Figure 4.17 Spreading of n-hexanol on a water surface

If the area occupied by the oil drop shown in Figure 4.16 is

increased by d/t, the change in the surface free energy of the system

will be approximately (-y 0 A + 7ow ~ ?WA) d/i. If this quantity is

negative, the process of spreading will take place spontaneously.

Harkins defined the term initial spreading coefficient (for the case

of oil on water) as

S = ~OxoA + Tow) (4.31)

where the various interfacial tensions are measured before mutual

saturation of the liquids in question has occurred. The condition for

initial spreading is therefore that 5 be positive or zero (Table 4.6).

Table 4.6 Initial spreading coefficients (in mN m-^1 ) for liquids on water at 20°C^54

(By courtesy of Academic Press Inc.)

Liquid

rt-Hexadecane

H-Octane

H-Octanol

7wA - (JOA + 7ow) = S

72.8-(30.0+52.1) = -9.3

72.8-(21. 8+50.8) = +0.2

72.8-(27.5+ 8.5) = +36.8

Conclusion

will not spread on water

will just spread on pure water

will spread against contamination