Probing the Molecular Structure of Coadsorbed Polyethylenimine and Charged Surfactants at the Nanoemulsion Droplet Surface

Tran, Emma; Carpenter, Andrew P.; Richmond, Geraldine L.

doi:10.1021/acs.langmuir.0c01095

Cited by 17 publications

(19 citation statements)

References 53 publications

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“…The black, orange, and pink traces represent samples containing SDS in conjunction with 0, 2.5, and 5 mM PNVA, respectively. It can be seen that nanoemulsions stabilized by only SDS (black ●) measure between 300 and 500 nm and possess a highly negative ZP, which is characteristic of SDS-stabilized nanoemulsions. , As PNVA is added to the sample and adsorbs to the droplet surface, two notable phenomena are observed: the nanoemulsion size increases significantly (Figure b) and the magnitude of the ZP decreases to ∼0 mV (Figure c). It is noteworthy that these droplets have roughly zero ZP values when coated with the polymer.…”

Section: Results and Discussionmentioning

confidence: 96%

Interfacial Steric and Molecular Bonding Effects Contributing to the Stability of Neutrally Charged Nanoemulsions

Tran

Richmond

2021

Langmuir

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In cosmetic, pharmaceutical, and food applications, many active ingredients have limited bioavailability in an aqueous environment, and in that context, nanoemulsions provide a mechanism for encapsulation, protection, and transport. These dispersed oil droplets are on the order of 100s of nanometers in diameter and owe their long-term stability to emulsifiers that are commonly charged. More recently, applications have been utilizing nonionic species as stabilizing agents due to their enhanced biosafety. DLVO (named after Derjaguin, Landau, Verwey, and Overbeek) theory has been central in the description of colloid stability, which emphasizes repulsive electrostatic forces, while extended DLVO theory also accounts for steric effects. Past studies of nanoemulsions have largely employed charged surfactants and polyelectrolytes, making it difficult to decouple electrostatic and steric effects as they relate to droplet stability. To better understand steric and molecular factors contributing to the stability of "uncharged" droplets, we have created nanoemulsions with sodium dodecyl sulfate (SDS) and poly(N-vinylacetamide) (PNVA). Though SDS is anionic, with PNVA coating the droplet surfaces, the ζ-potentials of these nanoemulsions are ∼0 mV. Despite minimizing electrostatic contributions, these nanoemulsions are stable for upward of a month with interesting dynamics. By employing dynamic light scattering, vibrational sum frequency scattering spectroscopy, and calculating interaction pair potentials using extended DLVO theory, we learn that the thickness of the PNVA layer plays a critical role in stabilizing these "uncharged" nanoemulsions. Beyond the sterics, the molecular conformation of the PNVA strands also contributes to the droplet stability. The adsorbed PNVA strands are shown to form stratified, rigid polymer networks that prevent the nanoemulsions from rapid destabilization.

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Section: Results and Discussionmentioning

confidence: 96%

Interfacial Steric and Molecular Bonding Effects Contributing to the Stability of Neutrally Charged Nanoemulsions

Tran

Richmond

2021

Langmuir

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“…Measurement of a sum-frequency signal from nanoemulsion surfaces requires interfacially adsorbed molecules to also have a net ordering of their dipole moments . The experimental system used in this work has been described elsewhere; ,, however, in brief, a Ti:Sapphire regenerative amplifier (Libra, Coherent, USA) was used to generate sub-100 femtosecond pulses of 800 nm light at a 1 kHz repetition rate. This 800 nm beam is split, with the majority being used to pump an OPA (OPERA-SOLO 2, Light Conversion) to produce horizontally polarized broadband IR pulses and the rest being sent to the sample cuvette for use as the visible beam in the sum-frequency experiments.…”

Section: Methodsmentioning

confidence: 99%

Effects of Salt-Induced Charge Screening on AOT Adsorption to the Planar and Nanoemulsion Oil–Water Interfaces

et al. 2021

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Nanoemulsions, nanosized droplets of oil, are easily stabilized by interfacial electric fields from the adsorption of ionic surfactants. While mean-field theories can be used to describe the impact of these interfacial fields on droplet stability, the influence of these fields on the adsorption properties of ionic surfactants is not well-understood. In this work, we study the adsorption of the surfactant sodium dioctyl sulfosuccinate (AOT) at the nanoemulsion and planar oil–water interfaces and investigate how salt-induced charge-screening affects AOT adsorption. In the absence of salt, vibrational sum-frequency scattering spectroscopy measurements reveal the ΔG ads and the maximum surface density is the same for AOT at the hexadecane nanoemulsion surface as at the planar hexadecane–H2O interface. Upon the addition of NaCl, an increase in AOT surface density is detected at both interfaces, indicating that repulsive electrostatic interactions between AOT monomers are the dominant force limiting surfactant adsorption at both interfaces. The bulky alkyl chains of AOT molecules cause our observations in this study to differ from those found in previous studies investigating the adsorption of linear-chain ionic surfactants to the nanoemulsion surface. These results provide necessary information for understanding factors limiting the adsorption of ionic surfactants to nanodroplet surfaces and highlight the need for further studies into the adsorption properties of more complex macromolecules at nanoemulsion surfaces.

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“…This lack of information on a molecular level is due to a lack of experimental methods that can be applied in situ to the internal interfaces of emulsions. Investigations on emulsion droplets and planar o/w interfaces performed with high-resolution methods, e.g ., neutron and X-ray scattering or nonlinear optical light scattering, ,,− are compared to the available information from extended interfaces, in fact, scarce. Often, the experimenter is limited to extended surfaces and has to assume that conclusions from the latter are transferable to droplet surfaces.…”

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confidence: 99%

Nanoscale Effects on the Surfactant Adsorption and Interface Charging in Hexadecane/Water Emulsions

Glikman

Braunschweig

2021

ACS Nano

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Nanoscale properties at interfaces play a key role in the colloidal stability of emulsions and other soft matter materials where physical properties need to be controlled from the nano to macroscopically visible length scales. Our molecular level understanding of oil− water interfaces arises mostly from results at extended interfaces and the common view that emulsions are stabilized by a large number of surfactant molecules at the droplet's interface which, however, has been recently challenged. In this work, we show that the particle size and the curvature of oil droplets at the nanoscale is of great importance for the interface adsorption of dodecyl sulfate surfactants and possible counterion condensation at the charged hexadecane−water interface. Using second-harmonic scattering, we have studied the surface charge of oil droplets in nanoemulsions where we systematically varied the particle size R between 80 and 270 nm and demonstrate that the surface charge density σ changes drastically with size: For sizes >200 nm, σ is similar to what can be expected at flat extended interfaces, while σ is dramatically reduced by almost an order of magnitude when the particle size of the oil droplet is 80 nm. Using a theoretical approach that considers counterion condensation, we quantify the nanoscale effects on the change in surface charge with particle size and find excellent agreement with our experimental result. Modeling of the experimental results also implies that the charge per particle remains constant and depends on a critical balance of surfactant adsorption and ion condensation.

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Probing the Molecular Structure of Coadsorbed Polyethylenimine and Charged Surfactants at the Nanoemulsion Droplet Surface

Cited by 17 publications

References 53 publications

Interfacial Steric and Molecular Bonding Effects Contributing to the Stability of Neutrally Charged Nanoemulsions

Interfacial Steric and Molecular Bonding Effects Contributing to the Stability of Neutrally Charged Nanoemulsions

Effects of Salt-Induced Charge Screening on AOT Adsorption to the Planar and Nanoemulsion Oil–Water Interfaces

Nanoscale Effects on the Surfactant Adsorption and Interface Charging in Hexadecane/Water Emulsions

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