Covalent Capture of Nanoparticle-Stabilized Oil Droplets via Acetal Chemistry Using a Hydrophilic Polymer Brush

Abstract

We report the capture of nanosized oil droplets using a hydrophilic aldehyde-functional polymer brush. The brush was obtained via aqueous ARGET ATRP of a cis-diol-functional methacrylic monomer from a planar silicon wafer. This precursor was then selectively oxidized using an aqueous solution of NaIO₄ to introduce aldehyde groups. The oil droplets were prepared by using excess sterically stabilized diblock copolymer nanoparticles to prepare a relatively coarse squalane-in-water Pickering emulsion (mean droplet diameter = 20 μm). This precursor was then further processed via high-pressure microfluidization to produce ∼200 nm squalane droplets. We demonstrate that adsorption of these nanosized oil droplets involves acetal bond formation between the cis-diol groups located on the steric stabilizer chains and the aldehyde groups on the brush. This interaction occurs under relatively mild conditions and can be tuned by adjusting the solution pH. Hence this is a useful model system for understanding oil droplet interactions with soft surfaces.

Scheme 1. Covalent Attachment of Hydrophobic Nanoparticle-Stabilized Oil Droplets to a Hydrophilic Aldehyde-Functional Brush

An oil-in-water Pickering nanoemulsion was prepared via high-pressure microfluidization using squalane and PGMA₅₀-PTFEMA₅₂ nanoparticles. The aldehyde-functional brush was obtained by growing a cis-diol-functional brush from a planar silicon wafer followed by NaIO₄ oxidation in aqueous solution.

Figure 1

(A) Synthetic route for the preparation of cis-diol-functional PGMA₅₂-PTFEMA₅₀ nanoparticles via RAFT aqueous emulsion polymerization of TFEMA at 70 °C. (B) Representative TEM image (uranyl formate stain) and (C) DLS size distribution obtained for PGMA₅₂-PTFEMA₅₀ nanoparticles.

Figure 2

(A) Schematic representation for the formation of O/W Pickering nanoemulsions using PGMA₅₂–PTFEMA₅₀ nanoparticles as a Pickering emulsifier. First, high-shear homogenization of 2.5–10% w/w aqueous dispersions of such nanoparticles with 20% v/v squalane resulted in the formation of a Pickering macroemulsion with a mean droplet diameter of 20 μm. Then this precursor macroemulsion was passed ten times through a high-pressure microfluidizer at 20,000 psi to produce the desired O/W Pickering nanoemulsion with a mean droplet diameter of approximately 200 nm. (B) DLS size distribution, (C) cumulative size distribution via analytical centrifugation (LUMiSizer instrument), and (D) representative TEM image for the O/W Pickering nanoemulsion obtained using this protocol.

Figure 3

(A) (i) Preparation of a cis-diol functional PGEO5MA precursor brush via ARGET ATRP from a initiator-functionalized planar silicon wafer, (ii) selective oxidation to produce the corresponding aldehyde-functional PAGEO5MA brush, and (iii) its subsequent exposure to a squalane-in-water Pickering nanoemulsion prepared using PGMA₅₂-PTFEMA₅₀ nanoparticles (with Nile Red dye dissolved in the oil phase prior to nanoemulsion formation). (B) AFM image of a patterned PGEO5MA brush with (C) corresponding brush height profile, as indicated through the line profile [see white box shown in (B)]. (D) Confocal fluorescence micrograph recorded for a surface-patterned PAGEO5MA brush after exposure to the dilute nanoemulsion [N.B. This image was recorded for the water-swollen brush to minimize droplet evaporation].

Figure 4

Change in frequency of the third overtone, Δf₃, over time at 25 °C for QCM analysis of silica sensors coated with (a) a 41 nm aldehyde-functional PAGEO5MA brush and the corresponding cis-diol-functional PGEO5MA precursor brush on exposure to a 1% w/w squalane-in-water Pickering nanoemulsion at pH 4 or (b) a 41 nm aldehyde-functional PAGEO5MA brush on exposure to a 1% w/w squalane-in-water Pickering nanoemulsion at pH 4, 7, or 10. In each case, the vertical dashed line indicates the time at which flow was switched to a purely aqueous solution (i.e., attempted wash-off of any weakly adhering adsorbed material).