Effects of water and alcohol on the formation of inclusion complexes of d-limonene and cyclodextrins

Systematic studies have been carried out on the role of water and alcohol in the formation of inclusion complexes between d-limonene and alpha-, beta-and gamma-cyclodextrin (CD) by a micro-aqueous method. The inclusion complex was barely formed at zero water content for all CDs. Above the specific water content for each CD, formation of the inclusion complex correlated well with an equation which was derived on the autocatalytic assumption for the inclusion phenomenon. The inclusion complex correlated well with an equation which was derived on the autocatalytic assumption for the inclusion phrnomenon. The minimum water content, which was defined as 1 % of the maximum concentration of the inclusion complex formed, coincided with the number of water molecules inside the cavity of the CD. In the presence of ethanol, a significant amount of the inclusion complex was formed for beta-and gamma-CD/ limonene systems, particularly at lower moisture content. However, for alpha-CD the inclusion fraction decreased significantly in the presence of ethanol. This means that ethanol inhibits the formation of the inclusion complex between alpha-CD and d-limonene. For other linear alcohols, the formation of the inclusion complex between d-limonene and beta-CD increased with decreasing alkyl chain length. This suggests that the more hydrophitic and the smaller (in molecular size) the alcohol is, the more enhanced is the inclusion of d-limonene to beta-CD.