Pattern Formation in a Self-Assembled Soap Monolayer on the Surface of Water:  A Computer Simulation Study

Shelley, Mee; Sprik, and M.; Shelley, John C.

doi:10.1021/la990704q

Cited by 8 publications

(8 citation statements)

References 20 publications

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“…Indeed, deuterium order parameter profiles calculated from the simulations (see Figure S2 in the Supporting Information) show considerably lower degrees of order in comparison to experimental profiles obtained from CTAC micelles 63 . Although surface aggregates composed of conventional surfactants have not been observed at air/water interfaces, prior simulations 35,64 and theoretical calculations 65 suggest that disordered CTAB hemiaggregates could exist at the interface. However, we do not have evidence of the formation of surface micelles, the aggregates are disordered structures and no characteristic size is measured.…”

Section: Resultsmentioning

confidence: 99%

Surfactant Interactions and Organization at the Gas–Water Interface (CTAB with Added Salt)

et al. 2018

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We have studied adsorbed layers of cetyltrimethylammonium bromide (CTAB) at air-water interfaces in the presence of added electrolyte. Fast bubble compression/expansion measurements were used to obtain the surface equation of state, i.e., the surface tension vs CTAB surface concentration dependence. We show that while a simple model where the surfactant molecules are assumed to be noninteracting is insufficient to describe the measured response of the surfactant layer, a modified Frumkin equation where the local interactions between the molecular components depend on their surface concentration captures the response. The variation of the effective interactions in the surfactant layer in the model shows that the interactions in the surfactant layer change from effectively repulsive to attractive with increasing surface concentration. Molecular dynamics simulations are performed to probe the origins of the change in the interactions. The simulations indicate that already at low surface concentrations the surfactants aggregate as highly dynamic rafts with surfactant orientation parallel to the interface. Increasing the concentration leads to a change in the assembly morphology at the interface: the surfactant layer thickens and the surfactants sample a range of tilted orientations with respect to the interfacial plane. The change from transient raftlike assemblies to dynamical aggregates at the interface involves a clear increase in the degree of counterion binding: we speculate that the flip of the effective interaction parameter in the model used to interpret the experimental results could result from this. The work here presents basic steps toward a proper understanding of the molecular organization and interactions of surfactants at an air-water interface. This is crucially important in understanding macroscopic properties of surfactant-stabilized systems such as foams, emulsions, and colloidal dispersions.

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Section: Resultsmentioning

confidence: 99%

Surfactant Interactions and Organization at the Gas–Water Interface (CTAB with Added Salt)

et al. 2018

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“…The area of a laurate ion in a surface monolayer has been estimated to be 50 Å 2 = 5 × 10 −19 m 2 (Shelley et al, 2000) and the other homologous fatty acid carboxylate ions studied can be expected to have comparable sizes in such monolayers. Modelling approach (1) predicts a surface excess at activation around 1–2 × 10 18 molecules m –2 for the fatty acid sodium salts and particle diameters studied and thus the fractional droplet surface coverage by surfactant molecules is of the order of 0.5–1.0.…”

Section: Resultsmentioning

confidence: 99%

Surfactant partitioning in cloud droplet activation: a study of C8, C10, C12 and C14 normal fatty acid sodium salts

Prisle

Raatikainen

Sorjamaa

et al. 2008

Tellus B: Chemical and Physical Meteorology

149

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A B S T R A C T 12 COONa] in the diameter range 33-140 nm at 296 K using a static thermal gradient diffusion cloud condensation nucleus counter. These fatty acid sodium salts are surface active molecules which have all been identified in atmospheric aerosol particles. Experimental critical supersaturations increased systematically with increasing carbon chain length and were in the range 0.96-1.34% for particles with a dry diameter of 40 nm. The experimental data were modelled using Köhler theory modified to account for partitioning of the surface active fatty acid sodium salts between the droplet bulk and surface as well as Köhler theory including surface tension reduction without accounting for surfactant partitioning and Köhler theory using the surface tension of pure water. It was found that Köhler theory using the reduced surface tension with no account for surfactant partitioning underpredicts experimental critical supersaturations significantly, whereas Köhler theory modified to account for surfactant partitioning and Köhler theory using the surface tension of pure water reproduced the experimental data well.

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“…Computer simulations have been used for the last few decades to study molecular-level aspects of both ionic and nonionic surfactants at the water–vapor interface − as well at interfaces between water and hydrophobic surfaces such as graphene and trichloroethylene . Despite the useful structural properties provided by these and other simulations, to the best of our knowledge, the recent work of Nguyen et al is the only study providing a detailed picture of the electrostatic potential across the water–vapor interface in the presence of a dilute surfactant monolayer.…”

Section: Introductionmentioning

confidence: 99%

Surface Electrostatic Potential and Water Orientation in the presence of Sodium Octanoate Dilute Monolayers Studied by Means of Molecular Dynamics Simulations

Bernardino

Moura

2015

Langmuir

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A series of atomistic molecular dynamics simulations were performed in the present investigation to assess the spontaneous formation of surfactant monolayers of sodium octanoate at the water-vacuum interface. The surfactant surface coverage increased until a saturation threshold was achieved, after which any further surfactant addition led to the formation of micellar aggregates within the solution. The saturated films were not densely packed, as might be expected for short-chained surfactants, and all films regardless of the surface coverage presented surfactant molecules with the same ordering pattern, namely, with the ionic heads toward the aqueous solution and the tails lying nearly parallel to the interface. The major contributions to the electrostatic surface potential came from the charged heads and the counterion distribution, which nearly canceled out each other. The balance between the oppositely charged ions rendered the electrostatic contributions from water meaningful, amounting to ca. 10% of the contributions arising from the ionic species. And even the aliphatic tails, whose atoms bear relatively small partial atomic charges as compared to the polar molecules and molecular fragments, contributed with ca. 20% of the total electrostatic surface potential of the systems under investigation. Although the aliphatic tails were not so orderly arranged as in a compact film, the C-H bonds assumed a preferential orientation, leading to an increased contribution to the electrostatic properties of the interface. The most prominent feature arising from the partitioning of the electrostatic potential into individual contributions was the long-range ordering of the water molecules. This ordering of the water molecules produced a repulsive dipole-dipole interaction between the two interfaces, which increased with the surface coverage. Only for a water layer wider than 10 nm was true bulk behavior observed, and the repulsive dipole-dipole interaction faded away.

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Pattern Formation in a Self-Assembled Soap Monolayer on the Surface of Water: A Computer Simulation Study

Cited by 8 publications

References 20 publications

Surfactant Interactions and Organization at the Gas–Water Interface (CTAB with Added Salt)

Surfactant Interactions and Organization at the Gas–Water Interface (CTAB with Added Salt)

Surfactant partitioning in cloud droplet activation: a study of C8, C10, C12 and C14 normal fatty acid sodium salts

Surface Electrostatic Potential and Water Orientation in the presence of Sodium Octanoate Dilute Monolayers Studied by Means of Molecular Dynamics Simulations

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