Interactions between Naphthenic Acids: Dependence on Molecular Structure Revealed through Statistical Analysis of Ultra-High-Resolution Electrospray Mass Spectra

Negative-ion electrospray mass spectra of samples of naphthenic acids contain peaks due to monomeric species [M–H]⁻ and dimeric species [2M–H]⁻. Working with a model system, intensities of the dimers were related to the intensities of monomers through linear inverse modelling. The statistical approaches investigated and the details of their applications to naphthenic acids are described here. The statistical analysis gives estimates of the relative probabilities of association of all pairs of monomers, where the monomers are defined by their accurate masses. The trends observed in these calculated probabilities of association exhibit breakpoints in the vicinity of monomers with 16 carbon atoms. These findings are discussed in terms of hydrophobic effects influencing the probability of association of naphthenic acids.