Effect of Surfactants on the Molecular Structure of the Buried Oil/Water Interface

Hosseinpour, Saman; Götz, Vanessa; Peukert, Wolfgang

doi:10.1002/anie.202110091

Cited by 37 publications

(24 citation statements)

References 50 publications

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“…28 The signal contribution of the dispersive term in the organic regime is significantly smaller than that of the adsorptive term, and hence, the dispersive term has been usually ignored in the literature. 31 The coincidence of the input IR frequency with one of the molecular vibrations results in an apparent increase in χ eff,R (2) and the SFG signal. Hence, probing the IR through a frequency range produces a vibrational spectrum of the interfacial molecules.…”

Section: Ir Vis Vismentioning

confidence: 99%

“…As the surfactant concentration at the interface is much higher than that in the diffuse layer, the χ (3) contributions to the SFG signals obtained in the organic range (from 2800 to 3000 cm –1 ) can be assumed purely real . The signal contribution of the dispersive term in the organic regime is significantly smaller than that of the adsorptive term, and hence, the dispersive term has been usually ignored in the literature …”

Section: Sum-frequency Generation Spectroscopymentioning

confidence: 99%

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Salting-Up of Surfactants at the Surface of Saline Water as Detected by Tensiometry and SFG and Supported by Molecular Dynamics Simulation

et al. 2022

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Surfactant adsorption at the air–water interface is critical to many industrial processes but its dependence on salt ions is still poorly understood. Here, we investigate the adsorption of sodium dodecanoate onto the air–water interface using model saline waters of Li+ or Cs+ at pH values 8 and 11. Both cations enhance the surfactant adsorption, as expected, but their largest effects on the adsorption also depend on pH. Specifically, surface tension measurements, sum-frequency generation spectroscopy, and microelectrophoresis show that small (hard) Li+ enhances the surfactant adsorption more than large (soft) Cs+ at pH 11. This effect is fully reversed at pH 8. We argue that this salting-up (increasing adsorption) reversal is attributable to the conversion of the neutralized carboxylic (−COOH) headgroup at pH 8 into the charged carboxylate (−COO–) headgroup at pH 11, which, respectively, interact with Cs+ and Li+ favorably. Molecular dynamics simulation shows that the affinity of Cs+ to the interface is decreased and eventually overtaken by Li+ as the carboxylic groups are deprotonated. This study highlights the importance of the charge and size of salt ions in selecting surfactants and electrolytes for industrial applications.

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Section: Ir Vis Vismentioning

confidence: 99%

Section: Sum-frequency Generation Spectroscopymentioning

confidence: 99%

Salting-Up of Surfactants at the Surface of Saline Water as Detected by Tensiometry and SFG and Supported by Molecular Dynamics Simulation

et al. 2022

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“…The SDS–water interface has also been probed previously using spectroscopic techniques over a broad frequency range. 30 − 32 These studies showed that the negative charge of the amphiphilic DS – ion induces an extended orientation of the water molecules in the vicinity of the water surface. However, how the mixed surfactant systems alter the surface electric field remains quite an unexplored topic.…”

mentioning

confidence: 99%

“…In this study, it was found that C 12 E 6 at CMC (70 μm) generates a strong electric field of 1 V/nm arising from the orientational structure of both the hydrophobic and hydrophilic parts of the surfactant and water molecules. The SDS–water interface has also been probed previously using spectroscopic techniques over a broad frequency range. − These studies showed that the negative charge of the amphiphilic DS – ion induces an extended orientation of the water molecules in the vicinity of the water surface. However, how the mixed surfactant systems alter the surface electric field remains quite an unexplored topic.…”

mentioning

confidence: 99%

Observation of Strong Synergy in the Interfacial Water Response of Binary Ionic and Nonionic Surfactant Mixtures

Sengupta

Gera

Egan

et al. 2022

J. Phys. Chem. Lett.

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Interfacial vibrational footprints of the binary mixture of sodium dodecyl sulfate (SDS) and hexaethylene glycol monododecyl ether (C12E6) were probed using heterodyne detected vibrational sum frequency generation (HDVSFG). Our results show that in the presence of C12E6 at CMC (70 μM) the effect of SDS on the orientation of interfacial water molecules is enhanced 10 times compared to just pure surfactants. The experimental results contest the traditional Langmuir adsorption model predictions. This is also evidenced by our molecular dynamics simulations that show a remarkable restructuring and enhanced orientation of the interfacial water molecules upon DS– adsorption to the C12E6 surface. The simulations show that the adsorption free energy of DS– ions to a water surface covered with C12E6 is an enthalpy-driven process and more attractive by ∼10 k B T compared to the adsorption energy of DS– to the surface of pure water.

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“…Although the knowledge of the interfacial molecular structure and the charging state in emulsions is of great importance, most of the existing knowledge arises from experimental data of extended o/w interfaces because they can be more easily accessed with a large variety of complementary methods such as surface tensiometry, , ellipsometry, and nonlinear optical spectroscopy − as well as with theoretical simulations. ,− The results from extended o/w can explain reasonably well the behavior of macroemulsions since the curvature of the droplet is large enough compared to molecular dimensions and to the relevant Debye lengths of interfacial double layers when charged oil droplets are considered. That is because in the large particle limit, the electric double layer can be still treated as if an extended interface would be present.…”

mentioning

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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Effect of Surfactants on the Molecular Structure of the Buried Oil/Water Interface

Cited by 37 publications

References 50 publications

Salting-Up of Surfactants at the Surface of Saline Water as Detected by Tensiometry and SFG and Supported by Molecular Dynamics Simulation

Salting-Up of Surfactants at the Surface of Saline Water as Detected by Tensiometry and SFG and Supported by Molecular Dynamics Simulation

Observation of Strong Synergy in the Interfacial Water Response of Binary Ionic and Nonionic Surfactant Mixtures

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

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