Electrochemical Stimulation of Water–Oil Interfaces by Nonionic–Cationic Block Copolymer Systems

Abstract

Variable interfacial tension could be desirable for many applications. Beyond classical stimuli like temperature, we introduce an electrochemical approach employing polymers. Hence, aqueous solutions of the nonionic–cationic block copolymer poly­(ethylene oxide)114-b-poly­{[2-(methacryloyloxy)­ethyl]­diisopropylmethylammonium chloride}171 (i.e., PEO114-b-PDPAEMA171 with a quaternized poly­(diisopropylaminoethyl methacrylate) block) were investigated by emerging drop measurements and dynamic light scattering, analyzing the PEO114-b-qPDPAEMA171 impact on the interfacial tension between water and n-decane and its micellar formation in the aqueous bulk phase. Potassium hexacyanoferrates (HCFs) were used as electroactive complexants for the charged block, which convert the bishydrophilic copolymer into amphiphilic species. Interestingly, ferricyanides ([Fe­(CN)6]3–) act as stronger complexants than ferrocyanides ([Fe­(CN)6]4–), leading to an insoluble qPDPAEMA block in the presence of ferricyanides. Hence, bulk micellization was demonstrated by light scattering. Due to their addressability, in situ redox experiments were performed to trace the interfacial tension under electrochemical control, directly utilizing a drop shape analyzer. Here, the open-circuit potential (OCP) was changed by electrolysis to vary the ratio between ferricyanides and ferrocyanides in the aqueous solution. While a chemical oxidation/reduction is feasible, also an electrochemical oxidation leads to a significant change in the interfacial tension properties. In contrast, a corresponding electrochemical reduction showed only a slight response after converting ferricyanides to ferrocyanides. Atomic force microscopy (AFM) images of the liquid/liquid interface transferred to a solid substrate showed particles that are in accordance with the diameter from light scattering experiments of the bulk phase. In conclusion, the present results could be an important step toward economic switching of interfaces suitable, e.g., for emulsion breakage.

Interfacial Rearrangements of Block Copolymer Micelles Toward Gelled Liquid–Liquid Interfaces with Adjustable Viscoelasticity

“…[34,40] The polymer system (initial charge-to-charge ratio at around 2) was chosen because of its interfacial activity demonstrated in a previous publication. [10] In the spreading solution, the polymer system coand self-assembles into spherical (crew-cut-type) micelles with a core radius (qPDPAEMA 170 /[Fe(CN) 6 ] 3-) of (21.4 ± 5.0) nm and a corona thickness of ≈5.5 nm, as seen by static X-ray scattering experiments (according to a core-shell form factor model with a monodisperse total corona radius and a Gaussian core radius distribution-the errors indicate the standard deviation of the distribution; hard sphere form factor with Gaussian core radius distribution fitting yields R ≈ (23.7 ± 3.5) nm; see Figure S9, Supporting Information). The rather nonstretched corona might indicate a limited aggregation number and therefore a partially swollen core domain (by water and/or isopropanol).…”

“…[41][42][43] In contrast to these systems, there are no microgels in our case, but monomodally distributed micellar aggregates with a hydrodynamic radius of R h ≈ 25.3 ± 0.1 nm were indeed found with dynamic light scattering (DLS) experiments of the spreading solution (see Figure S2 (Supporting Information); the unimers have rather a radius of 8 nm). [10] The micelles might still be present-at least partially-at the interface after the spreading process. Shortly afterward, the micelles might be stuck to the interface consisting of a core of qPDPAEMA chains and ferricyanide ions, while they are surrounded by a corona of PEO ) dissolved in a water/2-propanol (v/v 4:1 containing 0.0075 mol L −1 K 3 [Fe(CN) 6 ]) mixture were spread.…”

“…Regarding their behavior in the bulk phase an aggregation was detected for different polymer architectures depending on the valency of the counterions. [37,38,10] Any additional ionspecific influence of the counterion can be explained by its polarizability and different solvation energies. [39] Previous research on a system containing poly(ethylene oxide) 110 -b-poly{ [2-(methacryloyloxy)ethyl]diisopropylmethylammonium chloride} 170 (PEO 110 -b-qPDPAEMA 170 ) and potassium hexacyanoferrates indicates a higher interfacial activity for the block copolymer in the presence of ferricyanide ions ([Fe(CN) 6 ] 3-) compared to ferrocyanide ions ([Fe(CN) 6 ] 4-).…”

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modified by several types of additives such as low molecular weight surfactants and proteins, [3][4][5] particles, [6][7][8][9] or polymers. [10][11][12][13][14][15][16] Hereby, the adsorption of surface-active entities can occur by adsorption of unimers to form eventually more or less homogeneous monolayers. However, amphiphiles are often present in colloidal form as micelles.

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“…Since other colloidal particles, e.g., nanoparticles and microgels, can adsorb to interfaces in a pickeringlike fashion, intact micelles might also be prone to adsorb to the interface. [10,17,18] However, little is known on the fate of such interfacially attached micelles. In most cases, the kinetics of the rearrangements toward a monolayer are hard to capture.Generally, the interfacial tension is regarded as a primary indicator of adsorption processes, often deemed sufficient to trace changes at liquid interfaces.…”