EFFECT OF THE OMEGA-DIPOLES OF NEUTRAL LANGMUIR MONOLAYERS ON THE PK OF AN EMBEDDED AMPHIPHILIC POLARITY PROBE

Our previous investigations have shown that the pK(i) of the interfaci al pH and polarity probe 4-heptadecyl-7-hydroxycoumarin (HHC) embedded in neutral monolayers is different at the air-water and the solid-wat er interfaces. This difference questions the universality of the refer ence pK(i,o) value defined for each interfacial pH and polarity probe at a chosen neutral interface. The shift pK(i/o)(s/w)-pK(i/o)(a/w) was interpreted as due to oriented dipole layers located in the solid sub strate at about 6 nm from the monolayer-water boundary. In the present study we check this hypothesis by investigating the effect of the ome ga-dipoles of heptadecanol and 16-bromohexadecanol monolayers at the a ir-water interface on the acid-base equilibrium of embedded HHC. Emiss ion and excitation spectra of the amphiphilic dye were recorded at dif ferent pH values, and pK(i) values were evaluated from both emission a nd excitation titration dependences. The negligible difference between the pK(i) values of HHC in the two matrices implies that remote dipol es have no significant effect. Emission and excitation spectra of the water soluble 4-methyl-7-hydeoxycoumarin (MHC) in the same phosphate s olutions applied as liquid substrates for the spread monolayers were r ecorded and used to obtain the value of the bulk pK(b). From the diffe rence pK(i)-pK(b) the interfacial dielectric constant at the monolayer -water boundary coincide very well with literature data for epsilon(i) for sugar micelles. Because of the important role the saccharide moie ties play in biological processes, this coincidence and 55 +/- 5 for 1 6-bromohexadecanol monolayers coincide very well with literature data for epsilon(i) for sugar micelles. Because of the important role the s accharide moieties play in biological processed, this coincidence unde rlines the importance of further investigation of spread an deposited monolayers of long chain alcohols as the simplest models of the more c omplex di- or polysaccharides.