Mechanisms of Alcohol Modifiers

The effect of methanol and ethanol on the separations could possibly be explained by stronger solvation of the sample solutes in the mobile phase. However, this explanation becomes unlikely for an eluent containing only 5 % 1-butanol, the remainder being water. 

Scott and Simpson studied the adsorption of aliphatic alcohols, aldehydes, and carboxylic acids in binary mixtures with water by ODS-2 silica [9]. They found that the 

Adsorption of a layer of alcohol on the polymeric resin surface is believed to explain the dramatic effects observed in our separations of carboxylic acids. Butanol has the highest distribution coefficient of the alcohol moderators studied, and only a low concentration in the aqueous eluent is needed to coat the resin. surface. Partitioning of the various solute acids between the predominately aqueous eluent and the coated resin surface is much different that it is with an uncoated polystyrene surface. 

It was demonstrated that carbon dioxide or carbonic in the I-butanol-water eluent equilibrates with the resin to form an adsorbed layer that is responsible for conversion of carboxylate salts to the molecular form. 

Adsorbed H2CO3 + RCO2 Adsorbed HCO3- + RCO2H 

---