Studying surfactants adsorption on heterogeneous substrates

Abstract

Surface active compounds are continually designed and formulated to advance a large number of modern applications. In the past, the key design principle was the amount of surfactants adsorbed on a surface, because most attention was on changing the substrate wettability. As technology progresses, this is no longer sufficient, and the community is interested in preparing uniform surfactant films on a variety of substrates, typically characterized by surface roughness, both geometric and chemical. Much of our understanding on surfactant films is due to advanced experimental techniques that function best on chemically homogeneous atomically smooth surfaces. Simulations studies have also been conducted on similar substrates, often achieving results in apparent agreement with experimental observations. However, mounting evidence suggests that surface heterogeneity strongly affects the properties of adsorbed surfactant aggregates. To probe using computational approaches systems relevant for experimental investigations, one requires a combination of atomistic and coarse-grained approaches, both implemented using advanced algorithms. We discuss here a few possible approaches that could be used to study surfactant films adsorbed on heterogeneous surfaces, and how they could be combined with experimental investigations. Some recent trends that attempt to predict surfactant performance are also briefly highlighted.

Adsorption of Photoresponsive Surfactants at Solid–Liquid Interfaces

“…The ability to control the adsorption and desorption of surfactant molecules on a solid surface is of fundamental importance in colloids science and surface chemistry. − Among other external stimuli, such as pH, CO 2 , and heat, − light is the most convenient one to achieve the remote triggering of the adsorption process with spatiotemporal control. For this purpose, the surfactant should be rendered photoresponsive, which is usually done by introducing a photosensitive group into the hydrophobic tail.…”

Exploring the Free Energy Landscape To Predict the Surfactant Adsorption Isotherm at the Nanoparticle–Water Interface

“…On the other hand, molecular dynamics (MD) simulations have been successfully exploited to atomistically describe the interfacial environment, highlighting the variety of the free energy landscapes of surfactants approaching solid substrates. , Enhanced-sampling algorithms, specifically well-tempered metadynamics, umbrella sampling, , and steered molecular dynamics, , have attracted considerable attention to explore the equilibrium states of adsorption without simulating the extensive natural progression of surfactant exchange from bulk to the solid interface. Interesting results have unveiled that ion exchange, ion pairing, dispersion forces, and hydrophobic bonding are the main adsorption mechanisms of surface-active molecules onto a solid substrate. , More in general, the quality of solvents on one hand and the chemical, physical, and topographical features of the nanoscale interfaces on the other hand exhibit dominant roles in altering the process of adsorption and the orientation of surfactant molecules. − …”

The Role of Chemical Heterogeneity in Surfactant Adsorption at Solid-Liquid Interfaces

“…A wide variety of experimental techniques have been applied to probe the structure of adsorbed surfactants on solid surfaces. 1,26,27 These include neutron reflectivity, 28 neutron scat-tering, 29 ellipsometry, [30][31][32] nuclear magnetic resonance (NMR), 33 atomic force microscopy (AFM) 1,[34][35][36] and quartz crystal microbalance with dissipation (QCM-D). 37,38 A total of six distinct adsorbate morphologies have been observed on flat solid surfaces.…”

“…Recent reviews by Striolo et al have surveyed the profound effect which surface features can have upon surfactant-surface interactions. 1,27 They are known to significantly impact surfactant adsorption isotherms, 34,[51][52][53][54][55][56] the heat of adsorption, 55,57 the number of adsorbed surfactants 51,55,[58][59][60] and adsorbed micelle morphologies. 34,42,52,54,[58][59][60][61] From a modelling and simulation perspective, several authors have attempted to directly link the topology of chemical surface heterogeneity to the adsorption characteristics of surfactant dispersions.…”