Effect of Temperature, Oil Type, and Copolymer Concentration on the Long-Term Stability of Oil-in-Water Pickering Nanoemulsions Prepared Using Diblock Copolymer Nanoparticles

A poly(glycerol monomethacrylate) (PGMA) precursor was chain-extended with 2,2,2-trifluoroethyl methacrylate (TFEMA) via reversible addition–fragmentation chain transfer (RAFT) aqueous emulsion polymerization. Transmission electron microscopy (TEM) studies confirmed the formation of well-defined PGMA52–PTFEMA50 spherical nanoparticles, while dynamic light scattering (DLS) studies indicated a z-average diameter of 26 ± 6 nm. These sterically stabilized diblock copolymer nanoparticles were used as emulsifiers to prepare oil-in-water Pickering nanoemulsions: either n-dodecane or squalane was added to an aqueous dispersion of nanoparticles, followed by high-shear homogenization and high-pressure microfluidization. The Pickering nature of such nanoemulsion droplets was confirmed via cryo-transmission electron microscopy (cryo-TEM). The long-term stability of such Pickering nanoemulsions was evaluated by analytical centrifugation over a four-week period. The n-dodecane droplets grew in size significantly faster than squalane droplets: this is attributed to the higher aqueous solubility of the former oil, which promotes Ostwald ripening. The effect of adding various amounts of squalane to the n-dodecane droplet phase prior to emulsification was also explored. The addition of up to 40% (v/v) squalane led to more stable nanoemulsions, as judged by analytical centrifugation. The nanoparticle adsorption efficiency at the n-dodecane–water interface was assessed by gel permeation chromatography when using nanoparticle concentrations of 4.0, 7.0, or 10% w/w. Increasing the nanoparticle concentration not only produced smaller droplets but also reduced the adsorption efficiency, as confirmed by TEM studies. Furthermore, the effect of varying the nanoparticle concentration (2.5, 5.0, or 10% w/w) on the long-term stability of n-dodecane-in-water Pickering nanoemulsions was explored over a four-week period. Nanoemulsions prepared at higher nanoparticle concentrations were more unstable and exhibited a faster rate of Ostwald ripening. The nanoparticle adsorption efficiency was monitored for an aging nanoemulsion prepared at a copolymer concentration of 2.5% w/w. As the droplets ripened over time, the adsorption efficiency remained constant (∼97%). This suggests that nanoparticles desorbed from the shrinking smaller droplets and then readsorbed onto larger droplets over time. Finally, the effect of temperature on the stability of Pickering nanoemulsions was examined. Storing these Pickering nanoemulsions at elevated temperatures led to faster rates of Ostwald ripening, as expected.


Table of Contents
Figure S1 Assigned 1 H NMR spectra recorded for a PGMA 52 precursor.S2

S5
Figure S5 DLS particle size distributions recorded for a series of ndodecane-in-water nanoemulsions prepared using different copolymer concentrations.

Structural Models Used for Small-Angle X-ray Scattering (SAXS) Analysis
Spherical micelle and core-shell scattering models were taken from the literature 1-5 and modified as indicated below.In general, the intensity of X-rays scattered by a dispersion of nanoparticles [usually represented by the scattering cross section per unit sample volume, (q)] can be expressed as: where is the form factor, is a set of k parameters describing the structural morphology, is the distribution function, S(q) is the structure factor and N is the nanoparticle number density per unit volume expressed as: where is the volume of the nanoparticle and is the volume fraction of Given the relatively low nanoparticle concentration, the structure factor term in Equation S1 was assumed to be unity [S(q) = 1].

Spherical Micelle Model
The spherical micelle form factor for Equation S1 is given by: 3 where r 1 is the radius of the sphere core and is the radius of gyration of the coronal steric  g stabilizer block (in this case, PGMA 52 ).The X-ray scattering length contrasts for the core and corona blocks are given by and respectively.Here, , and are the X-ray scattering length densities of the core block ( = 12.76 x 10 10  c  sol  PTFEMA cm -2 ), corona block ( = 11.94 x 10 10 cm -2 ) and solvent (water) ( = 9.42 x 10 10 cm -2 ),  PGMA  sol respectively.and are the volumes of the PTFEMA core block and the PGMA corona  s  c block, respectively.These volumes were calculated using where the mass density  =  n, pol  A  of a PTFEMA homopolymer was previously reported ( = 1.47 g cm -3 ) 6 and the  PTFEMA density of PGMA was taken to be 1.31 g cm -3 . 4orresponds to the number-average  n, pol molecular weight of the diblock copolymer chains determined by 1 H NMR spectroscopy.The sphere form factor amplitude is used for the amplitude of the core self-term: 2 ) where .A sigmoidal interface between the two blocks was assumed for the spherical micelle form factor (Equation S4).This is described by the exponent term with a width accounting for a decaying scattering length density at the  micellar interface.This value was fixed at 0.25 nm during fitting.
The form factor amplitude of the spherical micelle corona is: The radial profile, μ c (r), can be expressed by a linear combination of two cubic b splines, with two fitting parameters s and a corresponding to the width of the profile and the weight coefficient, respectively.The self-correlation term for the corona block is given by the Debye function: The structure factor in Equation S2, S 1 (q), is usually expressed for interacting spherical micelles as: Herein the form factor of the average radial scattering length density distribution of micelles is expressed as  av (,  1 ) =  s [ s  s (,  11 ) +  c  c ()] and  PY (,  PY ,  PY ) is a hard-sphere interaction structure factor solved using the Percus-Yevick closure relation, 7 where R PY is the interaction radius and f PY is the hard-sphere volume fraction.For dilute dispersions of micelles it is assumed that  PY (,  PY ,  PY ) = A polydispersity for one parameter (r 1 ) is assumed for the micelle model, which is described by a Gaussian distribution.Thus, the polydispersity function in Equation S1 can be represented as: where R s is the mean spherical micelle core radius and is its standard deviation.In   s accordance with Equation S2, the number density per unit volume for the micelle model is expressed as: where is the total volume fraction of copolymer in the spherical micelles and is the  ( 1 ) total volume of copolymer in a spherical micelle .
( 1 ) = ( s +  c ) s ( 1 ) Assuming that the projected contour length of a PGMA monomer is 0.255 nm (two C-C bonds in all-trans conformation), the total contour length of a PGMA 52 block, L PGMA52 = 52 x 0.255 nm = 13.26nm.Given a mean Kuhn length of 1.53 nm (based on the known literature value for PMMA) an estimated unperturbed radius of gyration, R g = (13.26x 1.53/6)0.5 = 1.83 nm is calculated.The data fit to the SAXS pattern recorded for PGMA 52 -PTFEMA 50 spheres using the spherical micelle suggested that the experimental R g for the corona PGMA block (2.20 nm) is physically reasonable, since it is close to this theoretical estimate.

Core-Shell Model
Following our prior study of the characterisation of core-shell nanocomposite particles comprising polymer latex cores and particulate silica shells, 5 the SAXS data recorded for o/w Pickering nanoemulsions were analysed using a two-population model (n = 2).Population 1 (i = 1) is represented by core-shell spheres, where the cores comprise the oil or water droplets and the adsorbed layer of nanoparticles form the shell.The particulate nature of the shell is described by spherical micelles (see above), which corresponds to population 2 (i = 2).

Core-Shell Particle Model
The following functions and parameters were used for the core-shell particle (i = 1) model: where R s is the mean core radius and σ Rc is the standard deviation of the droplet core radius.
The number density for the first population is expressed as: where φ droplet is the relative volume fraction of the core-shell nanoemulsion droplets.In all cases, a dilute dispersion (1% v/v) of nanoemulsions has been used, so the structure factor is set to unity [S 1 (q)=1].

Figure S4 .
Figure S4.UV GPC calibration plot constructed for varying concentrations of PGMA 52 -PTFEMA 50 diblock copolymer nanoparticles recorded at a wavelength of 298 nm.