ROLE OF CHAIN ORDER IN THE LONG-RANGE ATTRACTIVE FORCE BETWEEN HYDROPHOBIC SURFACES

The dependence of the hydrophobic force parameters on the chain order parameter (S) has been investigated for mica surfaces deposited with monolayer films of dimethyldioctadecylammonium (DDOA) ions. A linear relationship between the force parameters and S was found, suggesting that the hydrophobic force is related to the hydrocarbon chains ordering, which in turn may affect the ordering of vicinal water molecules. It appears that there exists a threshold value of S (almost-equal-to 0.3), below which no significant hydrophobic force is discernible. This observation offers an explanation of the long-range hydrophobic forces measured with mica surfaces coated by double-chain surfactants.