Hydrophobic forces in thin water films stabilized by dodecylammonium chloride

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.