Linking molecular/ion structure, solvent mesostructure, the solvophobic effect and the ability of amphiphiles to self-assemble in non-aqueous liquids

Abstract

Sixteen non-ionic molecular solvents have been found to exhibit the solvophobic effect and to support the formation of amphiphile self-assembly mesophases. The solvents were low molecular weight polar solvents which contained various combinations of amine, hydroxyl or ether moieties with relatively small proportions of hydrocarbon unit constituents. The studied amphiphiles were hexadecyltrimethylammonium bromide (CTAB), hexadecylpyridinium bromide (C16PyrBr) and tetraethylene glycol monohexadecyl ether (C16E4). Lyotropic liquid crystal mesophases with lamellar, normal hexagonal and normal bicontinuous cubic, with ordered one-, two- and three-dimensional periodic structure respectively, were identified in CTAB and C16PyrBr systems by using cross-polarised optical microscopy (CPOM). Mesophase diversity and thermal stability ranges correlated to the Gordon parameter (G) value, a proxy for the solvent cohesive energy density. Infrared spectroscopy confirmed that all the studied molecular solvents were associative liquids. Solvent mesostructure was studied by synchrotron small angle X-ray scattering. The small sub-set of neat solvents which were mesostructured, with polar and non-polar domain segregation, displayed the lowest G values, and amongst the lowest mesophase diversity and thermal stability ranges. It has been established that the G value is a good indicator of whether or not a molecular solvent is likely to behave as a co-surfactant, residing within the amphiphile-solvent interfacial region of self-assembled objects, thereby influencing specific mesophase structure formation. Structure-property behaviour has been explored and shows that beneficial solvent features for serving as amphiphile-self assembly media, with the potential for rich mesophase diversity, include the presence of hydroxyl > amine > ether moieties, while methyl moieties have an adverse effect larger than that of methylene moieties.

Micelle formation of a non-ionic surfactant in non-aqueous molecular solvents and protic ionic liquids (PILs)

“…Cohesive energy density = cg LV /V m 1/3 (1) where c is a scalar quantity. The Gordon parameter, G, is obtained from eqn (1) when c is set to 1, as shown in eqn (2).…”

“…The strength of the solvophobic effect varies between solvents and hence the aggregation of amphiphiles within them varies, such as which lyotropic liquid crystal (LLC) mesophases are supported and what is the thermal stability range of the phases. 1 Studying micelle formation in solvents provides insight into the interactions between amphiphiles and solvents, and a comparison between solvents for their ability to support amphiphile self-assembly. An important micellisation parameter is the critical micelle concentration (CMC), which is defined as the surfactant concentration above which micelles spontaneously form.…”

“…These solvents were all selected because they have previously been shown to support LLC phases. 1 A protic ionic liquid, EAN, has been previously shown to support micelle formation. 21,22 The chemical structure of the amphiphile hexa-ethyleneglycol mono n-dodecyl ether (C 12 E 6 ) is provided in Fig.…”

Exploring the bulk-phase structure of ionic liquid mixtures using small-angle neutron scattering

“…20,30,31 Since then, the structure of ILs has been an intense research area with several reviews on their bulk structure having being written. 7,8,[32][33][34][35] Early molecular dynamics (MD) simulations postulated the formation of nanostructure within several different systems of [Cnmim][X]. 21,[23][24][25] When alkyl chains were sufficiently long the chains were observed to aggregate to form alkyl-chain domains.…”

Effect of cosolvents on the self-assembly of a non-ionic polyethylene oxide–polypropylene oxide–polyethylene oxide block copolymer in the protic ionic liquid ethylammonium nitrate

“…[1][2][3] This is driven by a solvophobic effect that is similar to the hydrophobic effect in water. 4 The lyotropic liquid crystal structures formed in PILs vary depending on both the amphiphile and the solvent properties of the PILs. 3,5,6 The self-assembled structures formed in the solvents, such as discrete cubic, hexagonal, bicontinuous cubic, and lamellar phases, have attracted significant interest in the templated synthesis of porous inorganic structures.…”