A thin film balance of Scheludko-Exerowa type was used to determine equilib
rium film thicknesses of dodecylammonium chloride (RNH3Cl) solutions. The d
ata were analyzed in view of the extended DLVO theory, which considers elec
trostatic, van der Waals, and hydrophobic forces. The hydrophobic force was
represented as a power law which is of the same form as for the van der Wa
als force, so that its constant K-232 can be directly compared with the Ham
aker constant, A(232). The results showed that at low surfactant concentrat
ions, K-232 is positive and decreases with increasing surfactant concentrat
ion, suggesting that hydrophobic force plays an important role in thin film
s. When the K-232 versus concentration plot was extrapolated to very dilute
solutions, K-232 approaches 10(-17) J, which is approximately 270 times la
rger than A(232). When the surfactant concentration was increased above 2 x
10(-3) M, however, K-232 becomes negative, indicating that hydration force
appears at high surfactant concentrations. These results suggest that air
bubbles are hydrophobic and the hydrophobicity decreases with increasing su
rfactant concentration. A TFB was used to obtain a disjoining pressure isot
herm at 10(-3) M RNH3Cl in the presence of 10(-4) M NaCl. The results can b
e fitted to the extended DLVO theory with K-232 = 6 x 10(-19) J. Considerat
ion of hydrophobic force predicted a rupture thickness larger than predicte
d using the DLVO theory, but is substantially smaller than the experimental
result. This discrepancy may be ascribed to the hydrodynamic force operati
ng in the film thinning process, (C) 1999 Academic Press.