Effect of the ω-Dipoles of Neutral Langmuir Monolayers on the pK of an Embedded Amphiphilic Polarity Probe

Abstract

Our previous investigations have shown that the pKi of the interfacial pH and polarity probe 4-heptadecyl-7-hydroxycoumarin (HHC) embedded in neutral monolayers is different at the air−water and the solid−water interfaces. This difference questions the universality of the reference pKi,o value defined for each interfacial pH and polarity probe at a chosen neutral interface. The shift pKi,os/w − pKi,oa/w was interpreted as due to oriented dipole layers located in the solid substrate at about 6 nm from the monolayer−water boundary. In the present study we check this hypothesis by investigating the effect of the ω-dipoles of heptadecanol and 16-bromohexadecanol monolayers at the air−water interface on the acid−base equilibrium of embedded HHC. Emission and excitation spectra of the amphiphilic dye were recorded at different pH values, and pKi values were evaluated from both emission and excitation titration dependences. The negligible difference between the pKi values of HHC in the two matrices implies that remote dipoles have no significant effect. Emission and excitation spectra of the water soluble 4-methyl-7-hydroxycoumarin (MHC) in the same phosphate solutions applied as liquid substrates for the spread monolayers were recorded and used to obtain the value of the bulk pKb. From the difference pKi − pKb the interfacial dielectric constant at the monolayer−water boundary was evaluated. The values εi = 60 ± 5 for heptadecanol and 55 ± 5 for 16-bromohexadecanol monolayers coincide very well with literature data for εi for sugar micelles. Because of the important role the saccharide moieties play in biological processes, this coincidence underlines the importance of further investigation of spread and deposited monolayers of long chain alcohols as the simplest models of the more complex di- or polysaccharides.