Cationization versus surface activity - the effect on electrospray ionization

Ethoxylated fatty alcohols, C12E1, C12E2, C18E1 and C18E2, were studied by electrospray ionization mass spectrometry (E is the ethoxylene unit -OCH2CH2-). For compounds containing two ethoxylene units, which form quite stable adducts with sodium cation, the abundances of [M+Na](+) ions were not affected by alkyl chain, so the hydrophobic effect was not observed. For the compounds containing one ethoxylene unit, forming rather unstable adducts with sodium, the hydrophobic effect was clearly seen since the [C18E1+Na](+) ion was more abundant than the [C12E1+Na](+) ion. Two ethoxylene units are not able to form stable adducts with potassium cations, therefore the hydrophobic effect was observed for the [C12E2+K](+) and [C18E2+Na](+) ions, the latter being more abundant than the former. For lithium cation adducts with C12E1 and C18E1, the hydrophobic effect was observed, but was less manifested than for sodium cations since lithium adducts are more stable than sodium ones. C18E1 and C18E2 gave more intense signals at higher cone voltage values than C12E1 and C12E1, respectively. However, this is not related to the hydrophobic effect but to the collisions being less effective for the former.