•Interfacial dilational rheology of lysozyme and BSA with [C12mim]Br were studied.•Lysozyme and [C12mim]Br molecules have competitive adsorption at the interface.•BSA and [C12mim]Br molecules have co-adsorption at the interface.This contribution is concerned with the imidazolium-based ionic liquid surfactant being considered potential amphiphilic molecules. The interfacial dilational rheology of solutions of globular proteins (lysozyme and bovine serum albumin (BSA)) with imidazolium-based ionic liquid surfactant (1-dodecyl-3-methyl imidazolium bromide ([C12mim]Br)) have been measured as a function of the surfactant concentration, interface lifetime and interfacial pressure. The dynamic interfacial dilational modulus of lysozyme/[C12mim]Br solutions are always monotonous, but that of the BSA/[C12mim]Br solutions become nonmonotonous indicating the destruction of the protein structure. One can assume that some lysozyme molecules desorb from interface due to competitive of free [C12mim]Br molecules for lysozyme/[C12mim]Br solution with [C12mim]Br concentration increasing, however, BSA molecules unfolding of compact globule have been subject to conformational changes so that it can give space for more [C12mim]Br molecules to co-adsorb. This aimed to provide more theoretical information for practical applications of imidazolium-based ionic liquid surfactant.Download high-res image (262KB)Download full-size image

In this research the wetting behavior of agro-surfactant solutions (Triton X-100, SDS, DTAB) on wheat leaf surfaces have been investigated based on the surface free energy, surface tension, and the contact angle. The results show that the contact angle of those surfactant solutions keeps constant with low adsorption at interfaces below 1 × 10−5 mol L−1. With the increase in concentration, the contact angles of Triton X-100 decrease sharply because the adsorption of molecules at solid–liquid interfaces (ΓSL′) is several times greater than that at liquid–air interfaces (ΓLV). With regards to SDS and DTAB, the contact angle also decreases but is even larger than 90° above the CMC, while the ratio of ΓSL′ to ΓLV is about 1.20, demonstrating that the Gibbs surface excess is related to the structure of surfactant molecules. Obviously, besides the properties of wheat leaf surfaces and surfactant solutions, the wetting behavior mainly depends on their noncovalent interactions. Among these, the hydrophobic interaction is the main force promoting molecules to adsorb on the surface, with the assistance of the Lifshitz–van der Waals interactions and the electrostatic interactions. According to the mechanism of their wetting behavior on plant surfaces, the recipe of pesticide formulation can be adjusted with better wettability to reduce its loss, consequently improving pesticide utilization and decreasing environmental contamination.

The deposition of pesticide liquid droplets on plant surfaces is of great importance in agricultural application. More than 50% of agrochemicals are lost and wasted during the spraying and wetting processes. Because the surfaces of plant leaves are negatively charged, we produced a kind of positive charge lambda-cyhalothrin loaded oil-in-water nanoemulsion in the system water/EL-80-Span 60/solvent 150# via the phase inversion composition (PIC) method with addition of Ionic Liquids (ILs). The electrostatic interaction between the negative charge surface and positive charge droplet would highly increase the wetting property. The results show that the positive zeta potential of nanoemulsion droplets gained along with the increase concentration of P14444Br, but with the increase concentration of P4444Br, the negative zeta potential of nanoemulsion droplets changed slightly and stayed around −17.8 mV (without any ILs). Besides, we explored the rheology characteristic of prepared nanoemulsions. Additionally, the contact angle experiments show that the positively charged nanoemulsions have a stronger adsorption on the negative surface and the contact angle decreased along with the increase of positive charge. Significantly, with addition of ILs, the nanoemulsions showed an excellent long-term stability due to electrostatic repulsion between droplets and homogeneous nanoscale droplet size. The highly stable pesticide nanoemulsions are of great importance in practical application and the nanoemulsions prepared with ILs offered a good potential as a carrier for drug delivery.

The dynamic interfacial properties of mixed solutions of bovine serum albumin (BSA) and the ionic liquid-type imidazolium surfactant ([C16mim]Br) were measured as a function of the interface age, surfactant concentration and solution pH. Three BSA conformers were investigated: the normal N form as well as the fast F and aged A forms, corresponding to the different solution pH, respectively. The interfacial tension and the interfacial dilational elasticity isotherms for the mixed adsorption layers indicate that the addition of [C16mim]Br to the different structures of BSA isomers influences the properties of the adsorption layer at the decane/water interface. The addition of [C16mim]Br does not influence the structure of the protein at pH below the isoelectric point of BSA, but at higher solution pH, the addition of surfactants significantly influences the dynamic interfacial properties of BSA solutions due to the electrostatic interaction between the components.

The interfacial rheological property is closely related to the stabilities of foams and emulsions, yet there have been limited studies on the interaction between proteins with ionic liquid-type imidazolium surfactants at the decane–water interface as well as in the bulk. Herein, we investigated the interfacial and bulk properties of pepsin (PEP) and an ionic liquid (IL), 1-hexadecyl-3-methylimidazolium bromide, [C16mim]Br. The interfacial pressure and dilational rheology studies were performed to describe the formation of [C16mim]Br–pepsin complexes. The influence of the oscillating frequency and the bulk concentration of [C16mim]Br on the dilational properties were explored. The conformational changes were studied by monitoring the fluorescence and far UV-CD spectra. The results reveal that the globular structure of pepsin is one of the decisive factors controlling the nature of the interfacial film. The monotonous increase in the dilational elastic modulus of pepsin–[C16mim]Br solutions with the surface age indicates that no loops and tails had formed. Interestingly, with an increase in the concentration of [C16mim]Br, the εd–c curve first passes through a plateau value due to steric hindrance and the electrostatic barrier of already absorbed tenacious pepsin–[C16mim]Br complexes. With the further addition of [C16mim]Br, the remarkable decrease in dilational elastic modulus indicates that the compact structure is destroyed gradually. The results of the fluorescence spectra and far UV-CD spectra confirm that [C16mim]Br did not produce perceptible changes in pepsin at the concentrations studied in the dilational experiment. Possible schematic programs of the pepsin–[C16mim]Br interaction model at the interface and in bulk phase are proposed.

The interfacial behavior of β-casein and lysozyme solutions has been investigated in the presence of an ionic liquid-type imidazolium surfactant ([C16mim]Br) at the decane/water interface. The dynamic dilational properties of the protein/surfactant solutions are investigated by the oscillating drop method and interfacial tension relaxation method. The interfacial tension isotherms for the mixed adsorption layers indicate that the increased addition of [C16mim]Br to a pure protein changes the properties of the complex formed at the decane/water interface. Whereas the interfacial tension data of the protein/surfactant mixed layers do not clearly show differences with changing bulk composition, the dilational rheology provides undoubted evidence that the structure and, in particular, the dynamics of the adsorbed layers depend on the bulk surfactant concentration. The experiment data for β-casein/[C16mim]Br solutions indicate that at higher bulk [C16mim]Br concentrations, β-casein in the interfacial layer is subject to conformational changes, where it gives space to [C16mim]Br molecules in the form of coadsorb rather than replacement; in contrast, in lysozyme/[C16mim]Br solutions some lysozyme molecules desorb from the interface due to the competitive adsorption of free [C16mim]Br molecules. Experimental results related to the interfacial dilational properties of the protein/surfactant solutions show that the dilational modulus turns out to be more sensitive to the conformation of protein/surfactant mixture at the liquid interface than the interfacial tension.

•The dilational rheology of imidazolium surfactants [Cnmim] Br were studied.•There is a shift of elasticity of [Cnmim]Br as increasing concentration.•Interface elasticity of [Cnmim]Br increases to a maximum as concentration increases.•The interface dilational modulus of [Cnmim]Br were affected by the steric effect.The interface tension and interface dilational properties of a series of ionic liquid-type imidazolium surfactants solutions, [Cnmim]Br, at the decane–water interface are reported. The dilational rheology properties are measured by means of oscillating drop method. The influences of dilational frequency, bulk concentration and structure on dilational rheology properties have been investigated. The results reveal that the interface dilational elasticity of the imidazolium surfactants with longer hydrophobic chain is larger than that with shorter hydrophobic chain at lower concentration, but the opposite is observed at high [Cnmim]Br surfactants solution concentrations. At lower concentrations, with the length of hydrophobic chain increasing, the dilational modulus increase resulting from the greater hydrophobic interaction. However, at higher concentration, [Cnmim]Br surfactants with longer hydrophobic chain desorbs from the interface more easily, but more difficult to resist the interface deformation.The interesting thing is that the interfacial dilational modulus of the three ionic liquid-type imidazolium surfactants ([Cnmim]Br) with long hydrophobic chain is larger than that with short hydrophobic chain at lower concentration, whereas the opposite is observed at higher surfactant concentration. At the lower concentration, the interaction of surfactant molecules is loose because of the few amount of surfactant molecules adsorbed at the interfacial. With the length of hydrophobic chain increasing, the dilational modulus increases resulting from the greater hydrophobic interaction. With increasing surfactant concentration, the rate of molecular diffusing between bulk and interface increases, also, surfactants which have longer hydrophobic chain have more compacting state at the interfacial. In other words, at higher concentration, surfactant with longer hydrophobic chain desorbs from the interface more easily, but more difficult to resist the interface deformation. Thus the longer the surfactant hydrophobic chain is, the easier the diffusion exchange becomes, resulting in more obvious decrease on the dilational modulus.