Insights into characteristic motions and negative chemotaxis of the inanimate motor sensitive to sodium chloride

Inanimate motors, driven by the difference in surface tension, provide platforms for studying the physics of characteristic motion and mimicking the complex behaviors of biological systems. However, it is challenging to endow inanimate motors with high autonomy, with an emphasis on simulating the behavior of living organisms in response to external stimuli. Herein, by applying sodium chloride (NaCl) as an external stimulus, we achieve the regulation of motion mode and chemotaxis in a self-propelled camphor system. We present a comprehensive surface/interface understanding of motion bifurcation with the increase of concentration NaCl, i.e., continuous motion to no motion via oscillatory motion. The features of motions (the speed and frequency) and the mechanisms are elucidated depending on the concentrations of NaCl and sodium dodecyl sulfate (SDS). Furthermore, the characteristic motion and chemotaxis to the salt stimulus are correlated to the dynamic breaking/reforming of the surface tension balance and gradient-type distribution phenomenon triggered by dynamic camphor dissolution, surfactant adsorption /diffusion and camphor-surfactant interaction. This work sheds light on the typical motions of inanimate motors and scrutinizes the synergy between dual additives, which will boost the design of advanced self-propelled systems with nonlinear characteristic motion.

Synergistic Effect of Salt and Anionic Surfactants on Interfacial Tension Reduction: Insights from Molecular Dynamics Simulations

Surfactants are commonly utilized in chemical flooding processes alongside salt to effectively decrease interfacial tension (IFT). However, the underlying microscopic mechanism for the synergistic effect of salt and surfactants on oil displacement remains ambiguous. Herein, the structure and properties of the interface between water and n-dodecane are studied by means of molecular dynamics simulations, considering three types of anionic surfactants and two types of salts. As the salt concentration (ρsalt) increases, the IFT first decreases to a minimum value, followed by a subsequent increase to higher values. The salt ions reduce the IFT only at low ρsalt due to the salt screening effect and ion bridging effect, both of which contribute to a decrease in the nearest head-to-head distance of surfactants. By incorporating salt doping, the IFTs can be reduced by at most 5%. Notably, the IFTs of different surfactants are mainly determined by the hydrogen bond interactions between oxygen atoms in the headgroup and water molecules. The presence of a greater number of oxygen atoms corresponds to lower IFT values. Specifically, for alkyl ethoxylate sulfate, the ethoxy groups play a crucial role in reducing the IFTs. This study provides valuable insights into formulating anionic surfactants that are applicable to oil recovery processes in petroleum reservoirs using saline water.

Unraveling driving regimes for destabilizing concentrated emulsions within microchannels

Coalescence is the most widely demonstrated mechanism for destabilizing emulsion droplets in microfluidic chambers. However, we find that depending on the channel wall surface functionalization, surface zeta potential, type of surfactant, characteristics of the oil as a dispersed phase, or even the presence of externally-induced stress, other different destabilization mechanisms can occur in subtle ways. In general, we observe four regimes leading to destabilization of concentrated emulsions: (i) coalescence, (ii) emulsion bursts, (iii) a combination of the two first mechanisms, attributed to the simultaneous occurrence of coalescence and emulsion bursts; and (iv) compaction of the droplet network that eventually destabilizes to fracture-like behavior. We correlate various physico-chemical properties (zeta potential, contact angle, interfacial tension) to understand their respective influence on the destabilization mechanisms. This work provides insights into possible ways to control or inflict emulsion droplet destabilization for different applications.

Properties of Binary Mixture of Cetyl Diphenyl Ether Disulfonate and Linear Alkylbenzene Sulfonate

The surface properties of the binary mixture of cetyl diphenyl ether disulfonate (C16-MADS) and linear alkylbenzene sulfonate (LAS) have been investigated by salinity and hardness tolerance, equilibrium surface tension, wettability, foam and emulsification measurements. The results show that with increasing mass ratio of LAS in the C16-MADS/LAS mixed system, the ability to reduce the surface tension of the solution is improved. The wetting performance is also improved. The critical micelle concentration (CMC) values are increased first, then decreased, and are higher than the ideal CMC values, which indicates that there is an antagonistic effect that is not conducive to the formation of mixed micelles between C16-MADS and LAS. In addition, the C16-MADS/LAS system improves the hard water resistance of LAS and reduces the foam properties of LAS.

The influence of negatively charged silica nanoparticles on the surface properties of anionic surfactants: electrostatic repulsion or the effect of ionic strength?

The presence of negatively charged nanoparticles affects the surface activity of anionic surfactants in an aqueous phase. Recent studies suggest that electrostatic repulsive forces play an important role in increasing the surface activity of surfactants. However, the addition of nanoparticles also increases the ionic strength of the system, which has a significant impact on the surfactant's properties, e.g. its critical micelle concentration (CMC). To investigate how and to what extent electrostatic forces and ionic strength influence the behavior of ionic surfactants, the surface tension and elasticity of different solutions were measured using drop profile tensiometry as a function of the surfactant (SDBS), nanoparticle (silica) and salt (KNO₃) concentration. It is observed that the surface activity of the surfactants is mainly influenced by the change in the system's ionic strength due to the presence of nanoparticles. Several characteristic parameters including the equivalent concentration of the surfactant, the CMC and the apparent partial molar area of the adsorbed surfactant are theoretically calculated and further employed to validate experimental observations. Both the nanoparticles and electrolyte decrease the CMC, while the equivalent concentration of the surfactant remains nearly constant. This paper presents a criterion to estimate the possible influence of such forces for nanoparticles of different sizes and mass fractions.

Synthesis of Fatty Alcohol Ether Carboxylic Ester Surfactant and its Performance in an Anionic/Non-Ionic Mixed System

A salt-type fatty alcohol ether carboxylic ester (AECE-Na) surfactant with both anionic and non-ionic characteristics was synthesised using a fatty alcohol ether (AEO) and succinic anhydride. Compared with traditional synthetic methods, neither irritating substances (chloroacetic acid and chloroacetate) nor precious metals (Pt and Pd) were used in the synthesis, which is a simple, economic, and green method. FT-IR and 1H NMR spectroscopies and MALDI TOF mass spectrometry were used to prove the molecular structure of the target product AECE-Na. In addition, the surface activities, intermolecular interactions, application properties, and aggregation behaviours of the individual systems (AEO and AECE-Na) and the anionic/non-ionic mixed system (AECE-Na/AEO) were investigated. The results showed that AECE-Na/AEO exhibits synergistic effects in terms of surface tension reduction efficiency, wetting, emulsification, foaming, and detergency compared with the individual systems.

Synthesis and Properties of Sodium Salts of Sulfonated Cardanol Polyethoxylates

Sodium salts of sulfonated cardanol polyethoxylates (NSF–nSO, with n = 3 or 6 for the average ethylene oxide number) were synthesized using cardanol polyethylene oxides from renewable cardanol and sodium isethionate as reagents. The surface tension, interfacial tension (IFT), foaming, wetting ability and emulsifying ability of NSF–nSO were investigated. It was found that there was no significant difference between NSF–3SO and NSF–6SO in the critical micelle concentration (CMC), but the surface tension of NSF–3SO at CMC was lower than that of NSF–6SO. The diffusion coefficient (Deff) decreases with the increase of the NSF–nSO concentration. The IFT between NSF–3SO solution and dodecane is lower than that of NSF–6SO at studied salt concentrations. The foaming, wetting and emulsifying abilities of NSF–3SO are better than those of NSF–6SO. The above results show that NSF–3SO has better surface/interface activity than NSF–6SO.

Surface and Interfacial Properties of Mono and Didodecyl Diphenyl Ether Disulfonates

In this paper, monododecyl diphenyl ether disulfonate (C12-MADS) and didodecyl diphenyl ether disulfonate (C12-DADS) are Friedel-Craft reaction product of 1-dodecene and diphenyl oxide using sulfated zirconium as a catalyst, followed by sulfonation with fuming sulfuric acid in 1,2-dichloroethane and neutralization with aqueous sodium hydroxide solution. The relationship between the structures of the surfactants C12-DADS, C12-MADS and linear alkylbenzene sulfonate (C12-LAS) and their surface and interfacial properties was studied by measuring equilibrium surface tensions, dynamic surface tensions and dynamic interfacial tensions (IFT). The results show that the surface and interfacial activity of C12-MADS is better than that of C12-DADS and C12-LAS. The gemini surfactant C12-DADS shows most unfavorable surface and interfacial activity due to the fact that the cross-linked hydrophobic carbon chains decreases the number of exposed methyl in molecule. The dynamic surface tensions results show that the diffusion coefficients values of C12-MADS and C12-DADS are lower than that of C12-LAS, and the adsorption process of surfactants at air/water interface is a mixed diffusion-kinetic adsorption mechanism. The data of dynamic ITF between aqueous surfactants solutions and dodecane indicate that the NaCl and CaCl2 concentration has a weak effect on the stable value of dynamic IFT for C12-DADS. With increasing the NaCl and CaCl2 concentration, the stable values of dynamic ITF for the three surfactants mostly passes through a minimum at an optimum concentration, and the C12-MADS and C12-LAS can reduce the IFT to the 10−2 mN/m order of magnitude.