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

Abstract

Self-assembling behavior of hydrophobic molecules in water confirms that the hydrophobic effect (HE) of alkyl groups stabilizes the aqueous colloids. Fe(CO){CO(CH)CH}(Cp)(PPh) molecules (FpC) with a Fp head and alkyl tail are water-insoluble. FpC aggregates in water precipitated in hours, whereas the hydrophobic interaction (HI) of long alkyl groups drove FpC (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.

Active Role of Water in the Hydration of Macromolecules with Ionic End Group for Hydrophobic Effect-Caused Assembly

“…This investigation started from the FT-IR analysis of P­(FpP) 7 in tetrahydrofuran (THF) (1 mL, 40 mg/mL) in the presence of various amounts of water (up to 20 μL to avoid HE-caused aggregation). As shown in Figure a, upon the addition of water, the absorption peak for the terminal CO groups (CO–Fe– in the repeating unit) shifts by 14 cm –1 with a shoulder remaining at 1914 cm –1 , indicating the interaction of the CO groups with water. ,,− This interaction also occurs for the solution of P + [BPh 4 ] − , as indicated by the FT-IR spectra (Figure b), suggesting that the ionic end group with a hydrophobic [BPh 4 ] − does not disturb the hydration. However, there is no detectable water carbonyl interaction for the solution of P + I – because no shift of the CO absorption bands is observed before and after the addition of water (Figure c).…”

“…The migration insertion polymerization (MIP) of FpP (Fp = (Cp)­Fe­(CO) 2 , P = alkylphosphine group) produces P­(FpP), − which is able to assemble in water into vesicles via a lateral association of the macromolecular chains into a vesicular membrane. , The interaction of water with CO groups is detectable by Fourier transform infrared (FT-IR). − It suggests that hydration is responsible for the assembly, but the role of water is still a matter of research. By converting the phosphine end group to phosphonium cation (Scheme ), we create P + X – (P + = P­(FpP) phosphonium cation, X – = I – or [BPh 4 ] − ) to study the dependence of water carbonyl interaction on the water structure perturbated by an ionic end group.…”

Hierarchical Self‐Assembly Induced by Dilution‐Enhanced Hydrophobic Hydration

“…We have reported aqueous self‐assembly of hydrophobic molecules, FpX (Fp head=(PPh 3 )(Cp)Fe(CO)(CO‐); X=hydrocarbon tail) . FpX molecules are non‐surface active and not amphiphilic, but able to assemble in water into Metal Carbonyl bi‐layer vesicles (MCsome) . HH of the hydrophobic Fp heads is responsible for the dispersion of MCsomes in water .…”

“…FpX molecules are non‐surface active and not amphiphilic, but able to assemble in water into Metal Carbonyl bi‐layer vesicles (MCsome) . HH of the hydrophobic Fp heads is responsible for the dispersion of MCsomes in water . The zeta potentials (ζ) of MCsomes is negative due to the presence of water carbonyl interactions (WCIs) .…”

Vesicular Membrane with Structured Interstitial Water

“…8,9 We have explored aqueous assemblies of FpR (FpR = (Cp)Fe(PPh 3 )(CO)(COR), Cp = cyclopentane, Ph = benzene, R = hydrocarbons) molecules. FpR molecules are not traditional amphiphiles, as both Fp head and R groups are not water-soluble 10 and the molecules are not surface active. 11,12 The assembly has created metal carbonyl vesicles (MCsome) with nonpolar R groups associated within the membrane and the polar Fp groups exposed to water.…”