Abstract
Two long alkyl chain cyclic carbonates, 1,2-hexadecene carbonate (HDC) and 1,2-dodecene carbonate (DDC), were synthesised via a green catalytic approach from CO2 and the corresponding epoxide, achieving nearly complete conversion with full selectivity. After purification, the two cyclic carbonate products were studied for application as non-ionic surfactants through different tests, including interfacial tension, emulsion stability and droplet size measurements. HDC demonstrated to be suitable for application as a non-ionic surfactant, as it was able to reduce the interfacial tension between water and hexane in an inverse emulsion (i.e. water-in-oil, w/o) at different HDC concentrations (0.5–2.5 wt.%). HDC allowed reaching a similar surface activity compared to a benchmark surfactant as sodium dodecyl sulphate (SDS), although the latter for an oil-in-water (o/w) emulsion, in the concentration range from 1.5 to 2.5 wt.%. On the other hand, DDC was not an efficient emulsion stabiliser, yielding only a slight decrease in the interfacial tension when compared to the results obtained with HDC under the same conditions. This underlined the role of the hydrophobicity of the longer alkyl chain in HDC on the performance of these carbonates as surfactants. For HDC, the optimum ratio of water to hexane for preparing an inverse emulsion was 50/50 vol %, which showed the highest colloidal stability at 2.0 % (w/v) concentration of the surfactant. Increasing the HDC concentration from 1.0 to 2.0 % (w/v) resulted in a significant decrease in average droplet size from 170 to 72 nm, in addition to a decrease in the droplet polydispersity of the dispersion. A further decrease in droplet size and a narrowing in the size distribution, leading to nearly monodisperse nanoemulsions, was achieved upon the addition of CaCl2. The combination of the results obtained in this study reveals a new, promising class of sustainable surfactants consisting of long alkyl chain cyclic carbonates synthesised from CO2.
Highlights
The utilisation of carbon dioxide as a building block for the synthesis of valuable chemicals is a highly attractive topic from the point of view of green chemistry and sustainability, since CO2 is a non-toxic, abundant and renewable raw material [1,2,3]
The results showed that when 1,2-epoxyhexadecane was used as a substrate, full conversion of this epoxide into 1,2-hexadecene carbonate was reached, as indicated by 1H-NMR (Fig. 2): three new signals were observed after the reaction in the characteristic region for the protons on the carbonate ring at chemical shifts of 4.1, 4.5 and 4.7 ppm, whereas the signals of the protons on the epoxide ring in the range 2.6–3.0 ppm disappeared
For this reaction, Fig. 2. 1H-NMR spectrum of the untreated reaction mixture containing 1,2-hexadecene carbonate (HDC) obtained by reaction of 1,2-epoxyhexadecane with CO2 [the peaks characteristic of the 1,2-hexadecanediol impurity are visible in the insert (3.3-3.8 ppm) and partially overlap with those of the -CH2- connected to N in the Bu4NI catalyst]
Summary
The utilisation of carbon dioxide as a building block for the synthesis of valuable chemicals is a highly attractive topic from the point of view of green chemistry and sustainability, since CO2 is a non-toxic, abundant and renewable raw material [1,2,3]. A variety of catalytic systems have been developed to enable the cyclo addition of CO2 to epoxides to be performed under mild conditions [11,12,13,14,15] In this context, it is relevant to design tailored cyclic carbon ates for new, specific potential applications. Inspired by the high po larity of one of the most common cyclic carbonates, i.e. propylene carbonate, we reasoned that preparing cyclic carbonates with a long alkyl chain (Fig. 1) would yield compounds with amphiphilic behaviour, where the carbonate group would provide the hydrophilic head and the alkyl chain would act as hydrophobic tail [16,17] As a consequence, such long alkyl chain cyclic carbonates could act as a new class of non-ionic surfactants, i.e. as compounds that lower the interfacial ten sion between two phases by adsorbing on a liquid–liquid interface or a surface. With the purpose of investigating our idea of using long chain cyclic carbonates as surfactants, two
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