Self-assembly of ionic and non-ionic surfactants in type IV cerium nitrate and urea based deep eutectic solvent

Abstract

Understanding and manipulating micelle morphology are key to exploiting surfactants in various applications. Recent studies have shown surfactant self-assembly in a variety of Deep Eutectic Solvents (DESs) where both the nature of surfactants and the interaction of the surfactant molecule with the solvent components influence the size, shape, and morphology of the micelles formed. So far, micelle formation has only been reported in type III DESs, consisting solely of organic species. In this work, we have explored the self-assembly of cationic surfactant dodecyl trimethylammonium nitrate/bromide (C12TANO3/C12TAB), anionic surfactant sodium dodecyl sulfate (SDS), and non-ionic surfactants hexaethylene glycol monododecyl ether (C12EO6) and octaethylene glycol monohexadecyl ether (C16EO8) in a type IV DES comprising metal salt, cerium (III) nitrate hexahydrate, and a hydrogen bond donor, urea, in the molar ratio 1:3.5. C12TANO3, C12TAB, C12EO6, and C16EO8 form spherical micelles in the DES with the micelle size dependent on both the surfactant alkyl chain length and the head group, whereas SDS forms cylindrical micelles. We hypothesize that the difference in the micelle shape can be explained by counterion stabilization of the SDS headgroup by polycations in the DES compared to the nitrate/bromide anion interaction in the case of cationic surfactants or molecular interaction of the urea and the salting out effect of (CeNO3)3 in the DES on the alkyl chains/polyethoxy headgroup for non-ionic surfactants. These studies deepen our understanding of amphiphile self-assembly in this novel, ionic, and hydrogen-bonding solvent, raising the opportunity to use these structures as liquid crystalline templates to generate porosity in metal oxides (ceria) that can be synthesized using these DESs.

Surfactant effects on the synthesis of porous cerium oxide from a type IV deep eutectic solvent

“…Generally, spheroidal particles are formed in all cases; however, it appears that C 12 chain length cationic surfactants lead to the formation of more spherical particles compared to C 16 chain length surfactants or BrijC 10 and C x TANO 3 surfactants appear to induce more uniform particle shapes. This possibly arises due to the formation of uniform spherical micelles in the case of C 12 chain length cationic surfactants, 45 whereas the longer chain length could result in more elongated micelles as is observed for C 16 chain cationic surfactants in other DESs. 59,60…”

“…‡Anionic surfactants were not studied in this work as they form highly elongated micelles, which result in extremely viscous liquids at concentrations as low as 1% w/w making them difficult to handle. 45 …”

“…The results show that cationic ‡ Anionic surfactants were not studied in this work as they form highly elongated micelles which result in extremely viscous liquids at concentrations as low as 1% w/w making them difficult to handle. 45 Calculated from the adsorption isotherm using the HK method b Apparent activation Energy ±95% confidence bounds estimated from at least 6 measurements surfactants are more effective in increasing the porosity compared to the non-ionic surfactant and longer chain length cationic surfactants result in the highest surface area; the highest BET surface area for templated ceria (134.3 m 2 /g for C 16 TAB) is 3× the values obtained for non-templated ceria (46.6 m 2 /g) and the micropore volume for C 16 TAB templated ceria (0.053 cm 3 /g) is thrice that of non-templated ceria (0.017 cm 3 /g), a similar effect as that observed for hydrothermal synthesis of ceria with surfactant templating. 23,61,64 .…”

On the Role of Water in the Formation of a Deep Eutectic Solvent Based on NiCl2·6H2O and Urea

“… 5 , 13 Nonetheless, many MDESs besides exhibiting intrinsic DES qualities such as easy preparation, no need for purification, and tunable physical–chemical properties have shown interesting properties addressing a wide horizon of technological applications. 6 − 11 , 14 − 18 The high concentration of ionic species can indeed provide eutectic solvents with high polarity and conductivity, making these mixtures ideal candidates as new electrolytes and media for electrodepositions and catalytic processes. For example, the ChCl:CrCl 3 ·6H 2 O 1:2 MDES was one of the first to be proposed, and it was demonstrated to provide black chromium deposits with excellent yields after electrodeposition, offering an alternative to Cr(VI) baths.…”

“… 16 , 17 The MDES made by cerium nitrate and urea has been also observed to promote the self-assembly of different types of surfactants. 18 …”