Ethanol Foams Stabilized by Isobutyl-Based POSS-Organosilica Dual-Particle Assemblies

Abstract

Nonaqueous foams in low-surface tension solvents (<25 mN·m^-1) are highly desired for applications in fire extinguishers and detoxification gels. However, their formation is a Holy Grail of the chemical industry due to the need for stabilizers with low surface energy and high recyclability. Herein, we disclose a new strategy to generate abundant foams in ethanol and a variety of low-surface tension solvents relying on the interfacial coadsorption of two different particles. The particles consist of surface-active fluorinated silica particles, used as a stabilizer, and a novel amphiphilic polyhedral oligomeric silsesquioxane (POSS) decorated with isobutyl cage substituents, used as a frother. The interaction between POSS and fluorinated particles at the ethanol-air interface was thoroughly investigated by combining physicochemical methods (contact angle, dynamic surface tension, and dynamic light scattering methods) and catalytic tests using the model aerobic oxidation reaction of benzyl alcohol. Both particles could be conveniently recycled for at least 5 consecutive runs with high foamability and catalytic activity.

Keywords: POSS; dual particle; ethanol; foam; organosilica.

Figure 1

(a) Molecular structure of IBu₇-POSS and IBu₇/F₁₃–POSS. (b) ¹³C NMR MAS spectra of IBu₇/F₁₃–POSS. (c) FT-IR spectra of IBu₇-POSS and IBu₇/F₁₃–POSS. (d) TGA of IBu₇/F₁₃–POSS.

Figure 2

Foaming tests in ethanol using (a) 1–8 wt % SiNP_F₁₇(1–3), (b) 1–4 wt % IBu₇/F₁₃–POSS, and (c) 1–8 wt % of SiNP_F₁₇(1–3) at constant IBu₇/F₁₃–POSS concentration (1 wt %). (d) Microscope images of the foams pressed by the coverslip. Foaming conditions: 25 °C, stirring at 1500 rpm for 30 min, stabilization for 1 min before visualization. The scale bar in (d) is 1000 μm.

Figure 3

(a) Optical images of foams in ethanol using 4 wt % SiNP_F₁₇(1–3) at variable IBu₇/F₁₃–POSS concentration (0.2–2 wt %). (b) Microscopic images of foams in ethanol using 4 wt % SiNP_F₁₇(1–3) at variable IBu₇/F₁₃–POSS concentration (0.2–2 wt %). (c) Microscopic images of foams in ethanol using 4 wt % SiNP_F₁₇(1–3) at variable IBu₇/F₁₃–POSS concentration (0.2–2 wt %) pressed by a coverslip. Foaming conditions: 25 °C, stirring for 30 min at 1500 rpm, and stabilization for 5 min before visualization. The scale bar in the optical images is 1000 μm.

Figure 4

(a) Evolution of the foamability of the Pd@SiNP_F₁₇(1–3) and IBu₇/F₁₃–POSS biparticle system in ethanol as a function of the IBu₇/F₁₃–POSS concentration. Foaming conditions: 60 °C, 4 wt % Pd@ SiNP_F₁₇(1–3), stirring for 30 min at 1500 rpm, and stabilization for 1 h (reaction time) before visualization. (b) Aerobic oxidation of BnOH over Pd@SiNP_F₁₇(1–3) at variable IBu₇/F₁₃–POSS concentration (0–1.0 wt %). Reaction conditions: 1 g of ethanol, 50 mg of BnOH (5 wt %. 0.40 mM), 4 wt % Pd@SiNP_F₁₇(1–3), 60 °C, ambient O₂ pressure (balloon), and stirring at 1500 rpm for 1 h. (c) Schematic representation of the synergy between IBu₇/F₁₃–POSS and Pd@SiNP_F₁₇(1–3) particles as a function of the IBu₇/F₁₃–POSS concentration.

Figure 5

Foaming tests in different low-surface tension solvents (polar and hydrocarbons) using combined 1 wt % IBu₇/F₁₃–POSS and 5 wt %SiNP_F₁₇(1–3) particles. Foaming conditions: 25 °C, stirring at 1500 rpm for 30 min, stabilization for 1 min, and 2 h before visualization.

Figure 6

(a) Optical images of ethanol foams after each catalytic run. (b) Recyclability and reuse of Pd@SiNP_F₁₇(1–3) and IBu₇/F₁₃–POSS dual-particle systems for the aerobic oxidation of BnOH over five consecutive runs. Reaction conditions: 1 g of ethanol, 50 mg of BnOH, 4 wt % Pd@SiNP_F₁₇(1–3), 0.2% IBu₇/F₁₃–POSS, 60 °C, O₂ balloon, and stirring at 1500 rpm for 1 h.