Phase diagrams and microstructures of aqueous short alkyl chain polyethylene glycol ether carboxylate and carboxylic acid triblock surfactant solutions

Biliquid oil-in-water nanofoams and spontaneous emulsification obtained with a surfactant resistant to curvature changes

HYPOTHESIS

Due to its huge polar headgroup, octaoxyethylene octyl ether carboxylic acid (C8E8CH2COOH = Akypo LF2™) is supposed not to be able to change its curvature sufficiently to form bicontinuous microemulsions. Instead, upon adding an oil to the binary water - surfactant system, excess oil could be squeezed out or a biliquid foam could form.

EXPERIMENTS

An auto-dilution setup was used to record small-angle X-ray scattering data along six dilution lines in the newly established phase diagram of the ternary system 2-ethylhexanol - C8E8CH2COOH - water.

RESULTS

Evaluation of the data in combination with the recorded phase diagram revealed that the ternary microemulsions with a slightly amphiphilic oil indeed do not show a classical structural inversion via a bicontinuous structure with increasing oil content, but instead the sequence: O/W micelles - O/W biliquid nanofoam - molecular co-solubilization in the oil phase. The biliquid nanofoam structure with 102-104 oil molecules enclosed by locally flat layers of interdigitated hydrated headgroups exists in the middle of the phase diagram. We may speculate that this phase can be used as a multitude of nanocontainers, e.g., for chemical reactions in an aqueous environment, but with negligible water chemical potential. In the vicinity of the critical point, spontaneous formation of stable mesoscale droplets (an "Onuki-like" structure, as known with antagonistic salts) is detected in a region showing a pronounced Tyndall effect.

Temperature-Insensitive Nonpolar Suspensions of Polyoxyethylene Alkyl Ether-Grafted Silica Nanoparticles

Nonpolar suspensions of organically modified particles exhibit a strong temperature sensitivity owing to the high-temperature-induced desorption/decomposition and the low-temperature-induced disorder/order conformational transition of the modifiers. This strong temperature sensitivity limits their applications, such as lubricants and oil-based drilling fluids, which require the suspensions to operate over a wide temperature range (e.g., 0-200 °C). We hypothesize that the introduction of a flexible ethylene oxide (EO) chain into the modifiers can disrupt the low-temperature-induced ordered conformation to improve the stability of the nonpolar suspensions. In this article, nonpolar suspensions with temperature insensitivity in the range of 5-160 °C were obtained via the covalent modification of silica NPs and the introduction of EO chains into the modifier molecules. Here, octadecyl-grafted silica NPs (C18-SiO2) and polyoxyethylene alkyl ether-grafted silica NPs (AEOn-SiO2) were synthesized and subsequently dispersed in mineral oil. The rheological properties of each suspension at different temperatures were evaluated, and the thermal stability of AEOn-SiO2 in mineral oil was investigated along with the conformational changes of the grafted chains. In the temperature range of 5-160 °C, the apparent viscosity and gel strength of the C18-SiO2 suspension changed dramatically, whereas the AEOn-SiO2 suspensions exhibited constant rheological properties over this temperature range. This temperature insensitivity of AEOn-SiO2 suspensions is attributed to the excellent thermal stability of AEOn-SiO2 in mineral oil and the disordered conformation of the EO chains upon cooling. This study provides a novel approach to preparing temperature-insensitive nonpolar suspensions, which have potential applications in the petroleum and lubricant industries.

A dilute nematic gel produced by intramicellar segregation of two polyoxyethylene alkyl ether carboxylic acids

MOTIVATION

Surfactants like C8E8CH2COOH have such bulky headgroups that they cannot show the common sphere-to-cylinder transition, while surfactants like C18:1E2CH2COOH are mimicking lipids and form only bilayers. Mixing these two types of surfactants allows one to investigate the competition between intramicellar segregation leading to disc-like bicelles and the temperature dependent curvature constraints imposed by the mismatch between heads and tails.

EXPERIMENTS

We establish phase diagrams as a function of temperature, surfactant mole ratio, and active matter content. We locate the isotropic liquid-isotropic liquid phase separation common to all nonionic surfactant systems, as well as nematic and lamellar phases. The stability and rheology of the nematic phase is investigated. Texture determination by polarizing microscopy allows us to distinguish between the different phases. Finally, SANS and SAXS give intermicellar distances as well as micellar sizes and shapes present for different compositions in the phase diagrams.

FINDINGS

In a defined mole ratio between the two components, intramicellar segregation wins and a viscoelastic discotic nematic phase is present at low temperature. Partial intramicellar mixing upon heating leads to disc growth and eventually to a pseudo-lamellar phase. Further heating leads to complete random mixing and an isotropic phase, showing the common liquid-liquid miscibility gap. This uncommon phase sequence, bicelles, lamellar phase, micelles, and water-poor packed micelles, is due to temperature induced mixing combined with dehydration of the headgroups. This general molecular mechanism explains also why a metastable water-poor lamellar phase quenched by cooling can be easily and reproducibly transformed into a nematic phase by gentle hand shaking at room temperature, as well as the entrapment of air bubbles of any size without encapsulation by bilayers or polymers.

Organic solvent-free synthesis of calcium sulfate hemihydrate at room temperature

Calcium sulfate hemihydrate, also known as bassanite or Plaster of Paris, is one of the most extensively produced inorganic materials worldwide. Nowadays, bassanite is mainly obtained by thermal dehydration of calcium sulfate dihydrate (gypsum) - a process that consumes considerable amounts of energy and thus leaves a significant carbon footprint. Towards a more sustainable future, alternative technologies for bassanite production at low temperatures are therefore urgently required. While successful approaches involving organic solvents have been reported, we chose precipitation from aqueous solutions as a potentially even "greener" way of synthesis. In a previous work, we have shown that spontaneous formation of bassanite in water (in competition with thermodynamically favoured gypsum) can be achieved at 40 °C by the use of additives that maintain specific interactions with calcium sulfate precursors and modulate the local hydration household during crystallisation. The results of the present study demonstrate that bassanite can be obtained via simple precipitation from aqueous solutions at room temperature by the combination of additives acting through orthogonal mechanisms.

Improvement of a Surfactant Blend for Enhanced Oil Recovery in Carbonate Reservoirs by Means of an Ionic Liquid

The promising experimental performance of surfactant blends encourages their use in recovering the large quantity of crude oil still remaining in carbonate reservoirs. Phase behavior studies were carried out in this work to propose a blend for practical application. To that aim, the surfactants dioctyl sulfosuccinate sodium (AOT) and polyoxyethylene(8) octyl ether carboxylic acid (Akypo LF2) were mixed. A formulation consisting of 1 wt% of AOT_50wt%/LF2_50wt% blend in synthetic sea water (SSW) led to a low value of interfacial tension with crude oil of 1.50·10^-2 mN/m, and 0.42 mg/g_rock of dynamic adsorption. A moderate additional oil recovery (7.3% of the original oil in place) was achieved in a core flooding test. To improve this performance, the surface-active ionic liquid 1-dodecyl-3-methylimidazolium bromide ([C₁₂mim]Br) was added to the system. The electrostatic interactions between the oppositely charged surfactants (AOT and [C₁₂mim]Br) led to a higher surface activity. Thus, a formulation consisting of 0.8 wt% of AOT_20.7wt%/[C₁₂mim]Br_25.3wt%/LF2_54wt% in SSW reduced the interfacial tension and surfactant adsorption achieved with the binary blend to 1.14 × 10^-2 mN/m and 0.21 mg/g_rock, respectively. The additional oil recovery achieved with the blend containing the ionic liquid was 11.5% of the original oil in place, significantly improving the efficiency of the binary blend.

Cloud point, auto-coacervation, and nematic ordering of micelles formed by ethylene oxide containing carboxylate surfactants

HYPOTHESIS

In a recent paper, we determined the phase behavior of an aqueous solution of octyl ether octaethylene oxide carboxylic acid ([H⁺][C8E8c^-], Akypo™ LF2) and with partial replacement of H⁺ by Na⁺ and Ca²⁺. It was found that even the neat surfactants are liquid at room temperature and that they form only direct micelles for any aqueous content and over large temperature ranges. The aim of the present work was to find an explanation for the clouding in these systems as well as for the coacervation observed at very low surfactant content. We expected that very similar phase diagrams would be found for a full replacement of H⁺ by the mentioned ions.

EXPERIMENTS

We established the respective phase diagrams of the above-mentioned salts in water and determined the structures of the occurring phases in detail with small-and wide-angle X-ray scattering, small-angle neutron scattering, dynamic light scattering, heat flux differential scanning calorimetry, as well as surface tension, ESI-MS, and NMR experiments.

FINDINGS

To our surprise, we discovered a new type of nematic phase between an isotropic and a hexagonal phase. Based on the complete description of all occurring phases both in the acidic and the charged surfactant systems, we were able to design a coherent and unified picture of all these phases, including the auto-coacervation at low surfactant concentration, the non-conventional clouding at high temperatures, the unusual liquid crystalline phases in a small domain at high surfactant concentrations, and the L_β phase at low temperatures and at very low water content. It turned out that all phenomena are a consequence of the subtle interplay between a) the packing constraint due to the very large head-group, b) the relatively small hydrocarbon chain and c) the tunable electrostatic interactions versus entropy.

Ionic Liquids Based on the Concept of Melting Point Lowering Due to Ethoxylation

Most of the commonly used Ionic Liquids (ILs) contain bulky organic cations with suitable anions. With our COMPLET (Concept of Melting Point Lowering due to Ethoxylation), we follow a different approach. We use simple, low-toxic, cheap, and commercially available anions of the type C_x(EO)_yCH₂COO^- to liquefy presumably any simple metal ion, independently of its charge. In the simplest case, the cation can be sodium or lithium, but synthesis of Ionic Liquids is also possible with cations of higher valences such as transition or rare earth metals. Anions with longer alkyl chains are surface active and form surface active ionic liquids (SAILs), which combine properties of ionic and nonionic surfactants at room temperature. They show significant structuring even in their pure state, i.e., in the absence of water or any other added solvent. This approach offers new application domains that go far beyond the common real or hypothetical use of classical Ionic Liquids. Possible applications include the separation of rare earth metals, the use as interesting media for metal catalysis, or the synthesis of completely new materials (for example, in analogy to metal organic frameworks).