Dissociation constants of long-chain hydroxy fatty acids in Langmuir–Blodgett films

Using contact angle titrations, the effect of –OH group on the pK values of 2-hydroxy stearic acid (2-HSA) and 16-hydroxy stearic acid (16-HSA) in Langmuir–Blodgett films (LB films) on hydrophilic and hydrophobic substrates have been studied. The surface pressure–molecular area (π–A) isotherms and Brewster angle microscopy (BAM) show a large degree of disorder in 2-HSA compared with 16-HSA films. The proximity of the –OH group to carboxyl groups in 2-HSA leading to intramolecular H-bonding results in irregular morphology in BAM. Large differences in the morphology of the condensed phase domains in the two compounds indicate considerable differences in the long-range orientational order. In the case of OH-substitution near the COOH-head-group for 2-HSA, irregular domain growth occurs. For 16-HSA with the OH-substitution at the end of the tail (in 16-HSA) regular domains indicate a high long-range order.Both films of 2-HSA and 16-HSA show increase in pK values (by at least 1 unit) compared to the bulk suggesting increase in stability of the films. Further both the films transferred onto hydrophilic substrates show higher pK compared to those on hydrophobic surfaces. This is possibly due to the partial change compensation in the LB films arising from the hydrophilic glass substrates.