Temperature-Switchable Surfactant-Free Microemulsion

Abstract

Stimuli-responsive microemulsions have recently attracted significant interest due to their unique properties. Here, we developed a novel surfactant-free microemulsion (SFME) in a nontoxic ternary mixture, in which dimethyl sulfoxide (DMSO) was used as an amphisolvent, n-butanol was used as a nonpolar phase, and water was used as a polar phase. The DLS results confirmed the presence of the preouzo zone, and the polarity experiment revealed that the single-phase region can be further divided into oil-in-water, bicontinuous, and water-in-oil subregions. The size of droplets increased upon increasing the water or n-butanol content but decreased with increasing DMSO content. With increasing temperature, the area of the single-phase region increased, accompanied by a decrease in the size of the droplets, and the critical point moved to the corner of n-butanol. No matter in what subregion the formulation was found, decreasing temperature to below the phase-transition temperature (PTT) will induce a transition from monophasic MEs to complete phase separation and vice versa. This is mainly attributed to the effect of temperature on the hydrogen-bond interaction. Ag nanoparticles (Ag NPs) can be prepared above the PTT and facilely separated below PTT. The Ag NPs obtained from the current SFME showed higher catalytic activity than that obtained from a common surfactant-based ME.

CO2 and Temperature Control over Nanoaggregates in Surfactant-Free Microemulsion

“…Additionally, temperatureinduced expansion in the single-phase region also occurred in the octanol−MDEA−water and octanol−TEA−water SFMEs (Figure S13). This is similar to octanol−ethanol−water, 32 nbutanol−DMSO−water, 31 and ionic liquids−ethanol−n-hexane SFMEs. 42 Formulation e, consisting of 25.58 wt % octanol, 33.38 wt % water, and 41.04 wt % DMEA (marked by a blue star in Figure 6A), was chosen as a model to further evaluate the influence of temperature on the size of nanoaggregates.…”

“…32,36−38 In the current research, DLS technique was employed to detect the size of nanoaggregates to determine the entire boundary of the pre-ouzo zone, which is generally interpreted as the formation of microemulsion. 31 For example, with increasing the DMEA content at a fixed mass ratio of water and octanol (R W/O = 6:4) following the path shown by a green imaginary line in the Figure 2A, the scattering signal of autocorrelation function became less and less pronounced (Figure S2), reflecting a decrease in the density fluctuation stemmed from nanosized aggregates. That is to say, the droplets size decreased gradually until it exceeded the detection limitation (Figure 2B).…”

“…Thereby, octanol−DMEA−water SFME can be also divided into three subregions according to the type of MEs (Figure 3B). 31 It is similar to conventional surfactant-based MEs. The result was in accordance with that reflected by the fluorescence emission spectrum (Figure S4).…”

“…Another one near the top corner is usually regarded as a true solution area. 31,37 Similarly, if we increase the water content (blue imaginary line indicated in Figure 2A) or octanol content (red imaginary line indicated in Figure 2A) in the single-phase region at a fixed mass ratio of octanol and DMEA (R O/D = 3:7) or DMEA and water (R W/D = 4:6), the scattering signal of autocorrelation function would became more and more evident (Figure S2). Meanwhile, DLS began to detect the presence of nanoaggregates as the composition of formulation entered the preouzo zone.…”

“…It has been demonstrated that MO is extremely sensitive to the changes of microenvironment's polarity. 31 A decrease in the polarity of the microenvironment would lead to a blue shift in the maximum absorption wavelength of MO (λ max ). Moreover, the water bound in SFMEs generally has a lower polarity than that of bulk water.…”