In contrast, exposure to Li + , Mg 2+ or Ca 2+ decreased the spectral intensity considerably (by factors of 5, 3 or 1.7, respectively) and yielded broadened spectra with less defined peaks. The irreversibility of the change in absorbance (Figure ) points to cation-induced removal of EB from the membrane core towards the membrane/bulk interface or even into the bulk solution. These scenarios are further supported by the difference spectra upon ion exchange, in which the negative peaks resemble lost signal of the spectral shapes observed in Na + . As such, we observe the lowest intensities of EB absorption peaks for Li + , and the least defined spectrum with weakly identifiable peaks in the presence of Mg 2+ . For Li + and Ca 2+ peaks are identified at ca. 1745/(1720)/1659 cm -1 and 1740/(1722)/1668 cm -1 , while the position of the "middle" peak may be inaccurate due its weak intensity and broad background. With the bivalent Mg and Ca 2+ an additional broad band at 1550 -1560 cm -1 is observed, which is not present with monovalent cations.