Synergistic Role of Temperature and Salinity in Aggregation of Nonionic Surfactant-Coated Silica Nanoparticles

Abstract

The adsorption of nonionic surfactants onto hydrophilic nanoparticles (NPs) is anticipated to increase their stability in aqueous medium. While nonionic surfactants show salinity- and temperature-dependent bulk phase behavior in water, the effects of these two solvent parameters on surfactant adsorption and self-assembly onto NPs are poorly understood. In this study, we combine adsorption isotherms, dispersion transmittance, and small-angle neutron scattering (SANS) to investigate the effects of salinity and temperature on the adsorption of pentaethylene glycol monododecyl ether (C₁₂E₅) surfactant on silica NPs. We find an increase in the amount of surfactant adsorbed onto the NPs with increasing temperature and salinity. Based on SANS measurements and corresponding analysis using computational reverse-engineering analysis of scattering experiments (CREASE), we show that the increase in salinity and temperature results in the aggregation of silica NPs. We further demonstrate the non-monotonic changes in viscosity for the C₁₂E₅-silica NP mixture with increasing temperature and salinity and correlate the observations to the aggregated state of NPs. The study provides a fundamental understanding of the configuration and phase transition of the surfactant-coated NPs and presents a strategy to manipulate the viscosity of such dispersion using temperature as a stimulus.

Figure 1

(a) SANS profile of silica NPs in D₂O at 20 °C (circles). Line is the best fit to the experimental data using the form factor of the sphere with log-normal size distribution. Inset: TEM image showing the spherical shape of the silica NPs of diameter 30 nm. (b) BET plot of the N₂ gas adsorption isotherm for silica NPs. Inset: adsorption–desorption isotherm for the N₂ gas on silica NPs.

Figure 2

Isotherms for the adsorption of C₁₂E₅ on silica NPs containing (a) 0, (b) 2, and (c) 5 mM NaCl at 20, 30, and 40 °C. The points are the experimentally measured values, and solid lines represent the best fit using the Gu–Zhu adsorption model given by eq 1. The maximum surface excess of the surfactant bound to the silica surface increases upon simultaneously increasing dispersion salinity and temperature.

Figure 3

(a) Transmittance of C₁₂E₅-coated silica NPs with 0, 2, and 5 mM NaCl upon increasing temperatures from 20 to 60 °C. (b) Photographs of the dispersions highlighting the change in transmittance for the aqueous dispersion upon increasing salinity and temperature.

Figure 4

(a) Schematic representation of the surfactant adsorption and shell formation on silica NPs. Here, the SLD of the H₂O/D₂O solvent matches the SLD of the silica NPs such that the neutrons scatter only from the surfactant shell. (b–g) Experimental SANS data (circles) with increasing temperature and salinity and corresponding best fit computed scattering using CREASE analysis to test four potential scenarios, namely, shell thickness is constant among all NPs (black), shell thickness exhibits dispersity (red), shell thickness is constant but can overlap with other shells (blue), and shell thickness exhibits dispersity and can overlap with other shells (green).

Figure 5

(a) Schematic of the C₁₂E₅-coated silica NP dispersion in the H₂O/D₂O mixture matching the SLD of the surfactant shell. (b–d) SANS profiles for the silica–C₁₂E₅ mixture under surfactant contrast-matched conditions in the presence of (b) 0, (c) 2, and (d) 5 mM NaCl at 30, 35, 40, and 45 °C. The distinction in scattering intensity at q < 0.07 nm^–1 at different temperatures in (c) and (d) can be attributed to the aggregation of silica NPs and corresponding contribution of the structure factor to the overall scattering profile. (e–g) Structure factor, S(q) profiles (discrete points), and corresponding fits using the SWPY model (solid lines) for C₁₂E₅-coated silica NPs in the presence of (e) 0, (f) 2, and (g) 5 mM NaCl at 30, 35, 40, and 45 °C. The S(q) is obtained from the scattering profiles shown in (b–d) using eq 3. The individual S(q) curves are shifted vertically for clarity and better visualization.

Figure 6

SWPY fit parameters obtained by analysis of the structure factor data for silica–C₁₂E₅ complexes, as shown in Figure 5. The fit parameters are (a) packing fraction of silica NPs within aggregates and (b) stickiness of the NPs are plotted as a function of temperature at increasing concentrations of NaCl.

Figure 7

Representative snapshots of the real space 3D structure of C₁₂E₅-coated silica NPs (blue spheres) at various NaCl concentrations at (a–c) 30 °C and (d–f) 45 °C. These 3D structures are obtained as one of the outputs of the CREASE analysis of the experimental SANS data performed in the H₂O/D₂O mixture matching the SLD of the silica NPs. The images show the densification of the aggregates in 5 mM NaCl.

Figure 8

Viscosity of C₁₂E₅-coated silica NPs (10 wt %) as a function of temperature in 0 mM (triangles) and 5 mM NaCl (circles). The measurements were performed at a constant shear rate of 2.64 s^–1. The dispersion becomes significantly more viscous upon the addition of NaCl and shows a non-monotonic change with temperature due to the densification and settling of aggregates upon increasing temperature.