Equilibrium and dynamic surface tension of quaternary ammonium salts with different hydrocarbon chain length of counterions

Interaction, Surface Activity, and Application of Mixed Systems of Alcohol Ether Sulfate Anionic Surfactants with Multiple Ethylene Oxide Groups and Gemini Quaternary Ammonium Surfactant

The surface activities and application properties for the mixtures of cationic surfactants tetramethylene-1,4-bis[N,N-bis(hydroxypropyl)-hexa/decyloxypropylammonium] bromide (GC10-P) and tetramethylene-1,4-bis[N,N-bis(hydroxyethyl)-hexa/decyloxypropylammonium] bromide (GC10-E) and anionic surfactant isomeric sodium fatty alcohol ether sulfates (iso-AE9S) were investigated using both the tensiometry and the conductometry. The interaction parameters and thermodynamic micellization parameters of GC10-P/iso-AE9S and GC10-E/iso-AE9S mixtures were evaluated by Clint-Rubingh and Motomura theoretical models. When the mole fraction of α1 for GC10-P/iso-AE9S mixed system was 0.2, the critical micelle concentration (CMC) reached a minimum of 1.61 × 10-4 mol/L, and the minimum critical micelle concentration of the GC10-E/iso-AE9S mixed system is 2.67 × 10-5 mol/L at α1 = 0.6. The CMC value of the mixed system is 1-2 orders of magnitude lower than that of any single component. The results indicate that the synergistic effects of the investigated mixed systems (evaluated by βm) are in order of GC10-P/iso-AE9S < GC10-E/iso-AE9S, with maximum βm values of -17.98 and -9.78, respectively. The change in zeta potential indicates that the poly(ethylene oxide) chain has weakened the charge density of the hydrophilic headgroup of the anionic surfactant. The interfacial tension at the oil-water interface in the mixed system of anionic/cationic surfactants is lower than that of any single component, exhibiting a higher interfacial activity. The mixed system exhibits a decreased contact angle and superior wetting ability over any single component, and it also enhances foam performance, emulsification performance, and degreasing performance.

Modeling Dynamic Surface Tension on Surfactant-Enhanced Polydimethylsiloxane

Surfactants are often added to aqueous solutions to induce spreading on otherwise unwettable hydrophobic surfaces. Alternatively, they can be introduced directly into solid hydrophobic materials─such as the soft elastomer, polydimethylsiloxane─to induce autonomous wetting without requiring additional surface or liquid modifications. Given the similarity between mechanisms of these two approaches, models that describe wetting by aqueous surfactant solutions should also characterize wetting on surfactant-solid systems. To investigate this theory, multiple surfactants of varying size and chemical composition were added to prepolymerized PDMS samples. After cross-linking, water droplets were placed on the surfaces at set time points, and their contact angles were recorded to track the temporal evolution of the interfacial tension. Multiple nonlinear models were fitted to this data, their parameters were analyzed, and each goodness of fit was compared. An empirical model of dynamic surface tension was found to describe the wetting process better than the single established model found in the literature. The proposed model adapted better to the longer time scales induced by slow molecular diffusivity in PDMS. Siloxane ethoxylate surfactants induced faster and more complete wetting of PDMS by water than oxyoctylphenol ethoxylates did. The generalizability of this model for characterizing nonionic surfactants of a wide range of physiochemical properties was demonstrated.

Effect of Oxyethylene Groups on Adsorption Properties of Fatty Alcohol Polyoxyethylene Ether Sodium Sulfate with Narrow Ethylene Oxide Distribution

The effect of oxyethylene groups on the adsorption properties of fatty alcohol polyoxyethylene ether sodium sulfate with narrow ethylene oxide distribution (AEmS, m = 3, 5, 7, 9) was characterized by means of the equilibrium surface tension, the dynamic surface tension and the contact angle. The results show that at a concentration lower than the critical micelle concentration (CMC) the oxyethylene groups act like hydrophobic groups to change the absorption properties. If the concentration is higher than the critical micelle concentration, the oxyethylene groups act as hydrophilic groups to change the absorption properties. The long polyoxyethylene chain of AE9S is very curly, which means that AE9S does not strictly follow the rule derived from AEmS (m = 3, 5, 7). The adsorption process of AEmS is a mixed diffusion-kinetic adsorption mechanism.

Lyophobicity may not be the main driving force for long chain surfactants from the bulk phase to the interface

According to the Traube rule, a surfactant with a longer alkane chain is more hydrophobic so its tendency to be driven from a polar solvent to a less polar interface is higher. In this work, we revisited this topic by studying the adsorption of quaternary ammonium salts and carboxylic acids with various alkane chain lengths at the hexadecane-water interface. The adsorption free energies of the surfactants at this oil-water interface from the polar (aqueous solution) or nonpolar phase (hexadecane) were estimated from second harmonic generation measurements. The variation of the free energies per methylene group in the bulk phase, at the oil-water interface and at the air-water interface revealed that there are different interactions between the alkane chains of the surfactants in different environments. The chain-chain interaction at the hexadecane-water interface is lower than that at the air-water interface. The driving force for the alkane chains to adsorb at the oil-water interface from the oil phase is close to that from the aqueous phase. This observation reveals that the chain-chain interaction rather than the lyophobicity of the solute with respect to the solvent is the main contributor to the adsorption free energy. This is the first experimental comparison of the free energies of the alkane chains in oil, in water, at the air-water interface and at the oil-water interface. These results provide information for studying the interactions of hydrophobic species in different environments. This work also provides a method for estimating the solvation energy of some head groups in surfactants.