Hydrophobic Effect of Alkyl Groups Stabilizing Self-Assembled Colloids in Water

Self-assembling behavior of hydrophobic molecules in water confirms that the hydrophobic effect (HE) of alkyl groups stabilizes the aqueous colloids. Fe(CO){CO(CH₂)_n-1CH₃}(Cp)(PPh₃) molecules (FpC_n) with a Fp head and alkyl tail are water-insoluble. FpC₁ aggregates in water precipitated in hours, whereas the hydrophobic interaction (HI) of long alkyl groups drove FpC_n (n = 6-18) molecules assemble into colloids in water. The alkyl tails interacted and stretched as indicated by IR analysis, while the separation of Fp head groups at the surface of colloids was indicated by the redox behavior of Fe elements. The molecules therefore arranged into a proposed "Y" shape with hydration cavities. The IR and cyclic voltammetry (CV) analyses indicated that the longer the alkyl tail, the larger the hydration cavity and the stronger the HI. Both HI and hydrophobic hydration (HH) were identified as the HE stabilizing the colloids in water. This knowledge is applicable for a broad range of disciplines, including biology, chemistry, and material science.