Water/oil interfacial tension reduction - an interfacial entropy driven process

The interfacial tension (IFT) of a fluid-fluid interface plays an important role in a wide range of applications and processes. When low IFT is desired, surface active compounds (e.g. surfactants) can be added to the system. Numerous attempts have been made to relate changes in IFT arising from such...

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Published inPhysical chemistry chemical physics : PCCP Vol. 23; no. 44; pp. 2575 - 2585
Main Authors Bui, Tai, Frampton, Harry, Huang, Shanshan, Collins, Ian R, Striolo, Alberto, Michaelides, Angelos
Format Journal Article
LanguageEnglish
Published Cambridge Royal Society of Chemistry 17.11.2021
Subjects
Cosmetics
Detergents
Entropy
Industrial applications
Molecular dynamics
Molecular structure
Surface tension
Surfactants
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Summary:The interfacial tension (IFT) of a fluid-fluid interface plays an important role in a wide range of applications and processes. When low IFT is desired, surface active compounds (e.g. surfactants) can be added to the system. Numerous attempts have been made to relate changes in IFT arising from such compounds to the specific nature of the interface. However, the IFT is controlled by an interplay of factors such as temperature and molecular structure of surface-active compounds, which make it difficult to predict IFT as those conditions change. In this study, we present the results from molecular dynamics simulations revealing the specific role surfactants play in IFT. We find that, in addition to reducing direct contact between the two fluids, surfactants serve to increase the disorder at the interface (related to interfacial entropy) and consequently reduce the water/oil IFT, especially when surfactants are present at high surface density. Our results suggest that surfactants that yield more disordered interfacial films ( e.g. with flexible and/or unsaturated tails) reduce the water/oil IFT more effectively than surfactants which yield highly ordered interfacial films. Our results shed light on some of the factors that control IFT and could have important practical implications in industrial applications such as the design of cosmetics, food products, and detergents. Interfacial disordering/interfacial entropy plays a critical role in determining the interfacial tension (IFT) of a fluid-fluid interface.
Bibliography:ab initio
NP
dodecane-water as a function of temperature, the evolution of the water/dodecane IFT and the system's potential energy, distribution of interfacial water, dodecane molecules using the ITIM algorithm, representative simulation snapshots showing the coexitence of dodecane and SDS at the interfacial film, a comparison between water/air/SDS and water/dodecane/SDS IFT, principal motions of saturated
PCA approach, C-C-C-C torsional potentials of SDS alkyl tail implementing the original TraPPE-UA force field and the refitted to the
Electronic supplementary information (ESI) available: A comparison of the water/dodecane/SDS IFT calculated using the
NVT
n
via
vs.
unsaturated SDS molecules
AT
10.1039/d1cp03971g
data, order parameter for SDS alkyl tail implementing two different sets of torsional angle potentials, simulation equilibration evaluation. See DOI
and
ensembles, maximum surface excess concentration of SDS gathered from multiple studies in the literature, results of the IFT of
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ISSN:1463-9076
1463-9084
DOI:10.1039/d1cp03971g