Surface Dilatational Elasticity Research Articles

Adsorption Layer Properties and Foam Behavior of Aqueous Solutions of Whey Protein Isolate (WPI) Modified by Vacuum Cold Plasma (VCP)

For years, cold plasma processing has been used as a non-thermal technology in industries such as food. As interfacial properties of protein play a remarkable role in many processes, this study investigates the effect of cold plasma on the foaming and interfacial behavior of WPI. The objective of this study is to evaluate the effect of different gases (air, 1:1 argon–air mixture, and sulfur hexafluoride (SF6)) used in low-pressure cold plasma (VCP) treatments of whey protein isolate (WPI) on the surface and foaming behavior of aqueous WPI solutions. Dynamic surface dilational elasticity, surface tension isotherms, surface layer thickness, and the foamability and foam stability were investigated in this study. VCP treatment did not significantly affect the adsorption layer thickness. However, an increase in induction time, surface pressure equilibrium value, and aggregated size is observed after SF6VCP treatment, which can be attributed to the reaction of WPI with the reactive SF6 species of the cold plasma. The surface dilational elastic modulus increased after VCP treatment, which can be related to the increased mechanical strength of the protein layer via sulfonation and aggregate formation. VCP treatment of WPI increases the foam stability, while the average diameter of foam bubbles and liquid drainage in the foam depends on the gas used for the cold plasma.

Secondary Bubble Entrainment via Primary Bubble Bursting at a Viscoelastic Surface.

Bubble bursting at liquid surfaces is ubiquitous and plays a key role for the mass transfer across interfaces, impacting global climate and human health. Here, we document an unexpected phenomenon that when a bubble bursts at a viscoelastic surface of a bovine serum albumin solution, a secondary (daughter) bubble is entrapped with no subsequent jet drop ejection, contrary to the counterpart experimentally observed at a Newtonian surface. We show that the strong surface dilatational elastic stress from the viscoelastic surface retards the cavity collapse and efficiently damps out the precursor waves, thus facilitating the dominant wave focusing above the cavity nadir. The onset of daughter bubble entrainment is well predicted by an interfacial elastocapillary number comparing the effects of surface dilatational elasticity and surface tension. Our Letter highlights the important role of surface rheology on free surface flows and may find important implications in bubble dynamics with a contaminated interface exhibiting complex surface rheology.

Effects of Various Types of Vacuum Cold Plasma Treatment on the Chemical and Functional Properties of Whey Protein Isolate with a Focus on Interfacial Properties

Vacuum cold plasma (VCP), a novel non-thermal processing technology used to modify the physicochemical properties and functionalities of food materials, was applied to whey protein isolate (WPI). The treatment affects the protein chemistry and, as a result, leads to differences in the behavior in solution and at interfaces. To minimize the undesirable effects of high oxidation and to increase the effectiveness of reactive species, the VCP treatment was applied at low pressure using different types of gases (air, combination of argon and air, and sulfur hexafluoride (SF6)). The treatment led to a decrease in the sulfur content and an increase in the carbonyl content, evidenced by oxidation reactions and enhanced disulfide bond formation, as well as cross-linking of protein molecules. Fluorescence-based indicators suggest that the hydrophobicity of the proteins as well as their aggregation increase after VCP treatment with an argon–air gas mixture; however, it decreases after VCP treatments with air and SF6. The chemical modifications further lead to changes in the pH of aqueous WPI solutions, as well as the average size and ζ-potential of WPI aggregates. Moreover, the dynamic surface tension, surface dilational elasticity, and the thickness of the WPI adsorption layers at the air/water interface depend on the VCP type. SF6 plasma treatment leads to a significant decrease in pH and an increase in the ζ-potential, and consequently to a significant increase in the aggregate size. The dynamic surface tension as well as the adsorption rates increase after SF6VCP treatment, but decrease after air–VCP and argon–air–VCP treatments. The adsorbed WPI aggregates form strong viscoelastic interfacial layers, the thickness of which depends on the type of VCP treatment.

Surface Properties of Protein-Polyelectrolyte Solutions. Impact of Polyelectrolyte Hydrophobicity.

The strong influence of an amphiphilic polyelectrolyte, poly(N,N-diallyl-N-hexyl-N-methylammonium chloride), on the surface properties of solutions of globular proteins (lysozyme, β-lactoglobulin, bovine serum albumin, and green fluorescent protein) depends on the protein structure and allows elucidation of the contribution of hydrophobic interactions in the protein-polyelectrolyte complex formation at the liquid-gas interface. At the beginning of adsorption, the surface properties are determined by the unbound amphiphilic component, but the influence of the protein-polyelectrolyte complexes of high surface activity increases at the approach to equilibrium. The kinetic dependencies of the dilational dynamic surface elasticity with one or two local maxima give a possibility to distinguish clearly between different steps of the adsorption process and to trace the formation of the distal region of the adsorption layer. The conclusions from the surface rheological data are corroborated by ellipsometric and tensiometric results.

Stability of high-salinity-enhanced foam: Surface behavior and thin-film drainage

Cocamidopropyl hydroxyl sulfobetaine (CHSB) is one of the most promising foaming agents for high-salinity reservoirs because the salt in place facilitates its foam stability, even with salinity as high as 2 × 105 mg/L. However, the synergistic effects between CHSB and salt have not been fully understood. This study utilized bulk foam tests and thin-film interferometry to comprehensively investigate the macroscopic and microscopic decay processes of CHSB foams with NaCl concentrations ranging from 2.3 × 104 to 2.1 × 105 mg/L. We focused on the dilatational viscoelasticity and dynamic thin-film thickness to elucidate the high-salinity-enhanced foam stability. The increase in dilatational viscoelasticity and supramolecular oscillating structural force (ΠOS) with salinity dominated the superior stability of CHSB foam. With increasing salinity, more CHSB molecules accumulated on the surface with a lower diffusion rate, leading to high dilatational moduli and surface elasticity, thus decelerating coarsening and coalescence. Meanwhile, the number density of micelles in the thin film increased with salinity, resulting in increased ΠOS. Consequently, the energy barrier for stepwise thinning intensified, and the thin-film drainage slowed. This work conduces to understand the mechanisms behind the pronounced stability of betaine foam and can promote the widespread application of foam in harsh reservoirs.

Surface Properties of Saponin—Chitosan Mixtures

The surface properties of saponin and saponin-chitosan mixtures were analysed as a function of their bulk mixing ratio using vibrational sum-frequency generation (SFG), surface tensiometry and dilational rheology measurements. Our experiments show that saponin-chitosan mixtures present some remarkable properties, such as a strong amphiphilicity of the saponin and high dilational viscoelasticity. We believe this points to the presence of chitosan in the adsorption layer, despite its complete lack of surface activity. We explain this phenomenon by electrostatic interactions between the saponin as an anionic surfactant and chitosan as a polycation, leading to surface-active saponin-chitosan complexes and aggregates. Analysing the SFG intensity of the O-H stretching bands from interfacial water molecules, we found that in the case of pH 3.4 for a mixture consisting of 0.1 g/L saponin and 0.001 g/L chitosan, the adsorption layer was electrically neutral. This conclusion from SFG spectra is corroborated by results from surface tensiometry showing a significant reduction in surface tension and effects on the dilational surface elasticity strictly at saponin/chitosan ratios, where SFG spectra indicate zero net charge at the air-water interface.

DNA Penetration into a Lysozyme Layer at the Surface of Aqueous Solutions.

The interactions of DNA with lysozyme in the surface layer were studied by performing infrared reflection–absorption spectroscopy (IRRAS), ellipsometry, surface tensiometry, surface dilational rheology, and atomic force microscopy (AFM). A concentrated DNA solution was injected into an aqueous subphase underneath a spread lysozyme layer. While the optical properties of the surface layer changed fast after DNA injection, the dynamic dilational surface elasticity almost did not change, thereby indicating no continuous network formation of DNA/lysozyme complexes, unlike the case of DNA interactions with a monolayer of a cationic synthetic polyelectrolyte. A relatively fast increase in optical signals after a DNA injection under a lysozyme layer indicates that DNA penetration is controlled by diffusion. At low surface pressures, the AFM images show the formation of long strands in the surface layer. Increased surface compression does not lead to the formation of a network of DNA/lysozyme aggregates as in the case of a mixed layer of DNA and synthetic polyelectrolytes, but to the appearance of some folds and ridges in the layer. The formation of more disordered aggregates is presumably a consequence of weaker interactions of lysozyme with duplex DNA and the stabilization, at the same time, of loops of unpaired nucleotides at high local lysozyme concentrations in the surface layer.

Probing high-salinity-enhanced stability of betaine foam for foam application in harsh reservoirs

The stability of betaine foam can be enhanced by salts in reservoirs, even with salinity as high as 2 × 105 mg/L, which offers great potential for cost-effectively improving the foam performance in high-salinity reservoirs. There is, however, a lack of understanding of the mechanisms behind this behavior. This study focused on the surface and bulk phase properties of three betaines with alkyl chain lengths of C12–C21 to probe the mechanisms leading to the high-salinity-enhanced foam stability. The dilatational viscoelasticity, adsorption behavior, surface relaxation, rheology, and thin-film drainage were examined in a wide NaCl range of 2.3 × 104 to 2.1 × 105 mg/L. With increasing salinity, betaine molecules adsorbed on the gas–water surface increased, and molecule diffusion–exchange between the surface and bulk phase decelerated, resulting in increased dilatational moduli and surface elasticity. The enhanced dilatational viscoelasticity slowed coarsening and coalescence, thus promoting the stability of betaine foam. The foam generated by oleicyl dimethyl amidopropyl carboxybetaine exhibited much stronger stability than the other betaine foams, especially when the NaCl concentration approached 2.1 × 105 mg/L. The viscoelastic micelle induced by high salinity dominated its superior stability.

A model for bubble dynamics in a protein solution

This paper presents a model for bubble dynamics in a protein solution, including varying surface tension and dynamic adsorption/desorption of protein onto the bubble surface. We model the protein-coated bubble surface as a linear viscoelastic interface. Two cases are studied to understand the importance of the surface excess stress: (1) bubble dissolution in a protein solution due to gas diffusion; and (2) bubble response to an imposed fluctuating pressure field. In the first case study, the surface excess stress stabilizes the bubble against dissolution. Initially, the surface excess stress is negligible, and the dissolution rate is governed by the Weber number, which compares the gas inertial force and the surface tension force. As the bubble shrinks, the surface excess stress grows and eventually balances the surface tension. After that, the dissolution rate is governed by the protein desorption rate and the elasto-capillary number, which compares the surface tension and the surface dilatational elasticity. Our model predictions for the dissolution process agree with experiments before the bubble buckles. In the second case study, the surface dilatational viscosity and dilatational elasticity add resistance and stiffness to the system, respectively. Including the surface excess stress increases (or reduces) the amplitude of the bubble radius if the frequency of the imposed fluctuating pressure is greater (or less) than a critical value. These results highlight the importance of the surface rheology on the protein-coated bubble dynamics, which has applications in drug delivery and ultrasound contrast agents.

A natural source of saponin: Comprehensive study on interfacial properties of Chubak (Acanthophyllum Glandulosum) root extract and related saponins

Chubak (Acanthophyllum Caryophyllaceae) is a shrubby plant with saponin-rich roots. This feature makes this plant to a new source of natural surfactant. This article presents some basic information about the surface activity of Chubak root extract (CRE) and the corresponding purified saponin. The Calendasaponin B and Calendasaponin D were characterized in the purified saponin using Liquid Chromatography-Mass Spectrometry as the two main saponins. The dynamic surface tension and dilational surface visco-elasticity of CRE and the purified saponin were studied using a profile analysis tensiometer. The equilibrium surface tensions of CRE and the purified saponin in the concentration range up to the CMC show that the used purification method is not efficient to separate the complete saponin content of CRE. The surface activity of CRE started at 0.005 g/l, while the minimum saponin concentration for a first visible surface tension decrease was 0.0012 g/l. The uniform trend in surface dilational elasticity and viscosity of CRE and the purified saponin is the evidence for the similarity of dominant surface-active compounds in both samples. Comparing the short time surface tension of oscillating and steady drops showed the impact of adsorption layer oscillations on the surface tension variations.

Interaction of fullerene C60 with bovine serum albumin at the water – air interface

This work is directed to the elucidation of the peculiarities of fullerene C60 interactions with bovine serum albumin (BSA) in mixed layers at the water-air interface. To this aim, the dilational surface elasticity was measured as a function of surface pressure and surface age and optical methods together with the atomic force microscopy (AFM) were applied. The dependencies of the dilational dynamic surface elasticity of the mixed C60/BSA layers on the surface pressure have two local maxima indicating a conformational transition in the layer. At low surface pressures (region of the first maximum) the surface properties are determined mainly by the protein, while in the second region of relatively high surface pressures the contribution of the fullerene is more important. The AFM images show that both C60 layers and mixed C60/BSA layers contain separate large fullerene aggregates with the length in Z-directions of up to 100 nm but the morphology of the regions between these aggregates differs for the two systems. The mixed layer contains some patches of a network of almost merged fullerene/protein aggregates with a length in Z-direction not longer than about 20 nm. The formation and the subsequent reorganization of the network lead to non-monotonic kinetic dependences of the dynamic surface elasticity in the course of protein penetration into the fullerene layer. The obtained results show strong interactions between the components in the surface layer and thereby can change the toxicity of the fullerene.

A bactericide peptide changing the static and dilatational surface elasticity properties of zwitterionic lipids at the air-water interface: Relationship with the thermodynamic, structural and morphological properties

In this paper, we studied how different hydrophilicity degrees of the polar groups of the lipids dipalmitoylphosphatidylcholine (DPPC) and dipalmitoyl phosphatidylethanolamine (DPPE) influence the interaction of the antibiotic peptide vancomycin (VC), affecting the physicochemical properties of the monolayers, including thermodynamic, rheological, structural and morphological ones. Lipid Langmuir monolayers were prepared at air-water interfaces with VC aqueous solution as subphase and characterized with tensiometry, Brewster angle microscopy, infrared spectroscopy, dilatational, and interfacial shear rheology. The presence of PC or PE groups as polar head groups of the phospholipid monolayers modulated the interaction of VC adsorbing from the aqueous subphase since for DPPC, vancomycin condenses the monolayer, making it less stable, fluid, and more disordered. In contrast, for DPPE, vancomycin expands the monolayer, making it more stable, keeping the compressibility, and leading to the formation of interfacial aggregates, which are not observed for DPPC. We concluded thatelectrostatic interactions induced the insertion of the peptide into the polar heads of the monolayers (DPPE), while hydrophobic interactions, in addition to ion-dipole interactions, induced the adsorption of the peptide onto the polar head of the monolayers (DPPC).

Effects of cation salinity on the dynamic interfacial tension and viscoelasticity of a water-oil system

This experimental study aims to obtain insights into the effect of brine salinity on the dynamic interfacial tension (IFT) and interfacial behavior of a water-oil system. The effects of monovalent and divalent cations (Na+, Ca2+, and Mg2+) in a salinity range of 0.01–0.1 mol/l on the equilibrium and dynamic interfacial tension of the water-oil system are studied using pendant drop experiments. To understand the adsorbed layer of surface active components (asphaltene and resin) at the interface, surface dilational elasticity is also studied. The results reveal that the types of the ions, salinity concentration, and type of the surface-active components in the crude oil, are dominant parameters that affect the interfacial properties. Divalent cations especially Mg2+ show a much stronger interaction with surface-active components at the brine-oil interface while monovalent cations have a negligible effect. The IFT of the water-oil system decreases in the presence of salts up to a minimum point and a further increase in the salinity leads to an increment in the IFT. The effect of salinity on the formation of asphaltene and resin components at the drop surface is studied by employing surface area compression experiments. When asphaltene components are at the interface, the drop profile shows a sharp increase in standard deviation from Laplacian form at the closely-packed state, while for maltene (deasphalted oil), no rise is observed in the standard deviation values. The closely-packed state and the asphaltene layer collapse happen at an earlier time with increasing salt concentration up to an optimum point due to the presence of the higher amount of complexes at the interface.

Influence of interfacial characteristics and dielectric properties on foam structure preservation during microwave-assisted vacuum drying of whey protein isolate-maltodextrin dispersions

This study aimed at assessing the correlation between interfacial and dielectric properties of whey protein isolate (WPI)-maltodextrin (MD) dispersions and their foam structure stability during conventional vacuum drying (CVD) and microwave-assisted vacuum drying (MWVD). Interestingly, the presence of MD clearly increased the surface activity of WPI, whereas the surface dilatational properties were rather unaffected. Besides, loss factor and resonant frequency linearly increased with MD concentration, while the dielectric constant decreased. Regarding drying, foam structure preservation during CVD was mainly correlated with the dilatational elasticity. Thereby, all samples remained stable throughout the drying process. Compared to CVD, MWVD required ≥20% MD to avoid foam collapse. In addition, MWVD-dried samples exhibited larger bubble sizes, which was linked to the volumetric heating and higher mechanical stress as compared to CVD. Overall, high surface dilatational elasticity and bulk viscosity were identified as key parameters to ensure foam structure preservation during vacuum drying. • Maltodextrin content ≥30% affects interfacial properties of WPI. • Dielectric properties of WPI depend on maltodextrin content and temperature. • Foam decay is related to interfacial properties. • Foam structure stability during MWVD is linked to surface elasticity.

Evolution of adsorbed layers of asphaltenes at oil-water interfaces: A novel experimental protocol

Hypothesis: Asphaltenes can form rigid interfacial films surrounding water droplets rendering water separation from crude oil sluggish. Therefore, the quantitative characterization of such complex film formation is of great importance. As the adsorbed layers of asphaltene illustrate crumpling under compression at certain conditions, the evolution process from soft to rigid states of the film can be evaluated considering standard deviations from Young-Laplace shape fitting. ExperimentalIn this study, novel experimental protocols are introduced to investigate the evolution of adsorbed asphaltene layer to a film of aggregates at model oil/water interface by means of dynamic interfacial tension (IFT) and dilational surface rheology measurements. In particular, the surface elasticity and standard deviation from the Young-Laplace shape fitting (YL-SD) are introduced as important indicators for the transformation of a regular asphaltene adsorbed layer to a film of aggregates. Different parameters affecting the film formation and stability, such as aging time, asphaltene concentration, and history of interfacial dynamics, are discussed and linked to emulsion stability.Findings: It is shown for the first time that the standard deviation of drop profile fitting from the Young-Laplace equation can be used as a rigorous parameter to reveal the properties of the interfacial asphaltene film, which cannot be recognized by regular IFT measurements. Via this novel technique, it is revealed that the transformation of an asphaltene adsorbed layer to a rigid film depends not only on the asphaltene concentration but also on the aging time and the interfacial area perturbations. The results of this new method are supported by measurements of the dilational surface elasticity, which is known as an important parameter for the characterization of complex adsorbed layers, and further verified by an emulsion stability analysis.

Dynamics of interfacial layers for sodium dodecylbenzene sulfonate solutions at different salinities

This study investigates the adsorption mechanisms and the dilational surface rheology of the anionic surfactant sodium dodecylbenzene sulfonate (SDBS) at the air–liquid interface, in the presence and absence of NaCl over a wide range of SDBS concentrations. We also evaluate the Langmuir and Frumkin models in order to predict the dynamic adsorption of SDBS solutions. Our results reveal that the equilibrium surface tension of SDBS solutions is a monotonically decreasing function of NaCl concentration. However, the dilational viscoelastic properties of SDBS solutions are found to be a non-monotonic function of NaCl concentration. NaCl manifests opposing effects on the dilational viscoelastic moduli of the gas–liquid interface, depending on the frequency of surface oscillation and surfactant bulk concentration. Our measured dilational surface elasticity and viscosity data show a shift in the surfactant transition concentration, at which the viscoelastic moduli reach their maximum values, towards smaller surfactant bulk concentrations with increasing salt concentration. It is attributed to suppressing the electrostatic repulsion between the hydrophilic head groups of surfactant molecules in the presence of electrolytes. The maximum viscoelastic moduli at different NaCl concentrations is found to be the reflection of the relative impact of enhanced adsorption and increased diffusion exchange between bulk and interface.

Molecular and air-water interfacial properties of potato protein upon modification via laccase-catalyzed cross-linking and conjugation with sugar beet pectin

Foaming-related functionalities of potato proteins are important for the development of innovative food products. The main purpose of current study was to investigate the effects of laccase-catalyzed modifications on the tertiary structural property, the thermal property, the air-water interfacial adsorption kinetics and the film rheological characteristics of potato protein at selected pH values. The cross-linking of potato protein (PPT) with or without the presence of ferulic acid (FA) as mediator and its conjugation with sugar beet pectin were biocatalyzed by fungal laccases (Trametes versicolor or Coriolus hirsutus). As compared to the native potato protein, all modified proteins were unfolded, and a decrease in the structural compactness was observed. The conjugation of potato protein with sugar beet pectin (PPT-SBP-H/-L) exhibited stabilization effect against unfolding and heating, which also promoted the adsorption of potato protein, while the cross-linked potato proteins (MPPT24h, MPPT24hFA) contributed to high surface dilatational elasticity of the interface, at neutral pH condition. The extended laccase-cross-linking treatment (MPPT48h, MPPT48hFA) resulted in a higher structural stability of the proteins than the other modified ones. At acidic pH conditions, these protein samples exhibited relatively high surface dilatational moduli. This study provided insights into the impact of laccase-catalyzed modifications on the functionalities of potato protein from molecular and mesoscopic aspects.

Surface and foaming properties of potato proteins: Impact of protein concentration, pH value and ionic strength

Using a multiscale approach, potato protein isolate was characterized holistically in terms of its solubility and surface charge, surface activity and surface dilatational properties as well foaming properties and bubble structural attributes. By means of varying protein concentration (0.1–10.0%), pH (3.0–10.0) and NaCl concentration (0–200 mM), it was aimed to establish a better mechanistic understanding of potato protein functionality. Overall, surface activity was found to be rather unaffected by a modification of protein concentration and pH, whereas pH had a clear effect on surface dilatational elasticity. Likewise, foam stability was affected by pH, yielding a maximum around the isoelectric point, which is due to an enhanced network formation at the air/water interface imparting an increased surface film stability. Coarsening exponents were found to reflect the results on foam stability. By comparison, NaCl presence clearly had an enhancing effect on surface activity and led to increased protein-protein interactions within the surface film. More compact surface films in turn resulted in a better foamability and foam stability as well as lower coarsening exponents. Comparison of findings to mechanisms known for β-lactoglobulin-stabilized surfaces revealed their principal applicability also to potato proteins’ functional properties. Overall, by means of the obtained knowledge, foam structures based on potato proteins can be controlled more specifically.

Salt Effects on Formation and Stability of Colloidal Gas Aphrons Produced by Anionic and Zwitterionic Surfactants in Xanthan Gum Solution

This work is devoted to the influence of NaCl salt concentration on the formation and stability of colloidal gas aphrons (CGA) produced by the anionic surfactant sodium dodecyl sulfate (SDS) and zwitterionic surfactant coco amido propyl betaine (CAPB) in the presence of xanthan gum (XG) as a stabilizer. Dynamic surface tension measurements as well as volume and half-life time of the produced foams are considered for stability analysis. A sharp decrease of the half-life time and volume of the CGAs at NaCl concentrations larger than 20,000 ppm was observed, which was attributed to the precipitation of SDS in the solution. The mentioned SDS precipitation altered the dynamic surface tension behavior, dilational surface elasticity, and turbidity of the solution. The main reason for the precipitation of SDS is the increased Krafft point caused by the addition of salt. However, for the zwitterionic surfactant CAPB, the effects of added NaCl on the interfacial properties required for CGAs production was negligible due to the simultaneous effects on the cationic and anionic head groups in the CAPB leading to negligible changes in the net repulsion forces. Yet, an overall reduction in the half-life time of CGAs with increasing salt concentration, even when we have no precipitation, was observed for both surfactants, which could be explained by the reduction in the ability of XG to increase the viscosity with increasing salt concentration.

Adsorption behaviour of surfactant-nanoparticles at the gas-liquid interface: Influence of the alkane chain length

Silica nanoparticles (NPs) have shown tremendous application potential as stabilising agents in foams. The adsorption of cationic surfactants on the surface of NPs through electrostatic interactions forms integrated surfactant-NP compounds and endows the NPs with surfactancy. The aim of this study was to investigate the influence of the alkane chain length of cationic surfactants on NP surfactancy and foam stability. The results demonstrate that the surface tension decreases and dilational elasticity modulus increases with surfactant chain length. Simultaneously, coalescence of bubbles is markedly suppressed and the stability of surfactant-NP foams is enhanced in static and dynamic tests. With a coarse-grained molecular dynamics simulation (CGMD), it was found that the surfactant-NP complexes evolve into an asymmetric Janus structure at the gas-liquid interface from their symmetric configuration in the aqueous phase. In addition, the resistance force during the pulling of a NP away from the interface increases with surfactant chain length. The simulation results imply that surfactant-NPs have a high surfactancy and the adsorption stability is improved with increasing surfactant chain length, which is in good agreement with the experimental results.