Binary Surfactant Microemulsion Systems: Microstructure,physicochemical Properties, And Applications In Agrochemical Delivery

Microemulsion (ME) is a kind of ordered multi-system, consisting of oil (O), water (W), surfactant (S) and co-surfactant (CS). Because of large interfacial area, ultra-low interfacial tension, and thermodynamic stability, microemulsion has been widely used as drug carrier in agrochemical and other fields. With water as the dispersion medium, microemulsion of agrochemicals can form a thermodynamically stable system. In the microemulsion of agrochemicals, with help of the surfactants, the oil-soluble original drug can be completely disp ersed into water to form ultra-fine particles with size of 0.01-0.1μm. There is few organic solvent in microemulsion of agrochemicals. Usually, agrochemical emulsion includes a lot of aromatic sovent, such as toluene and xylene, causing serious environmental pollution. Nowadays, microemulsion of agrochemicals becomes a worldwide hot topic for the agrochemical new-formulation .Surfactant plays a key role in the formation of microemulsion. However, there is little report about the fundmental investigation of surfactant in the applications of agrochemicals drug delivery.In this paper, a binary surfactants microemulsion system was prepared by mixing an anionic surfactant and a non-ionic surfactant, and its microstructure,physicochemical properties, and application in agrochemicals drug delivery has been intensively studied. The main achievements are described as follows.Firstly, two types of alkylbenzene sulfonate, SDBA and SDBS, were selected as anionic surfactants, and two types of alkylphenol ethoxylate, NP-10 and TX-100, were selected as non-ionic surfactants. And then an anionic surfactant was mixed with a non-ionic surfactan to form a binary surfactants system. The mixed critical micelle concentration (CMC) of the binary surfactants system was determined by surface tension measurement. CMC was used to calculate the molecular interaction parameters and molecule-exchanging energy according to regular solution theory of the binary surfactant mixtures. The interaction parameterβvalues in the mixed micelles and air/water interface were measured to be all negative values, indicating that a synergistic effect was produced in all of the mixtures. Thermodynamics investigation of mixed micelles showed that the Gibbs free energy values of all the mixed systems were negative, and |βM | increased with the increasing temperature. It suggested that there existed favorable interactions that improve the micelle formation. When NP-10 was used to mix with SDBA or SDBS, two binary mixtures, SDBA/NP-10 and SDBS/ NP-10, were formed. The |βaMve| of SDBA/NP-10 is larger than that of SDBS/NP-10, which suggested that the synergism interaction of SDBA and NP-10 was stronger than that of SDBS and NP-10. When SDBA was used to mix with NP-10 or TX-100, two binary mixtures, SDBA/NP-10 and SDBS/ TX-100, were formed. The |βaMve| of SDBA/NP-10 is larger than that of SDBS/TX-100. It suggested that the value of |β| was greater when the carbon atom number of the hydrophobic chain of nonionic surfactant was bigger. This synergy phenomenon was caused mailnly by the electrostatic attraction between the polar head groups of the ionic component and the interaction between the hydrophobic molecules.The critical micelle concentration (cmc) of mixed surfactants was decreased by adding electrolyte, such as NaCl. With increasing concentration of NaCl, the cmc andγcmc were decreased gradually. It was more significant to improve the capacity and efficiency of mixed surfactants in reducing surface tension by adding n-butanol into the mixed system than ethanol.The micro-environmental parameters of mixed surfactant micelle were determined by electron spin resonance (ESR). When the concentration of the SDBA/NP-10 mixture reached cmc, its microviscosity increased suddently, and then formed mixed micelles. The micropolar parameters AN became bigger with the increasing concentration of non-ionic surfactant. It indicated that more non-ionic surfactant entered into the anionic surfactant micelle, which was helpful for the formation of micelles. At the same time, the aggregation numbers of micelles for four pure surfactants SDBA, SDBS, NP-10 and TX-100 were determined to be 38, 34.9, 40.4, and 55.3 by steady-state fluorescence probe method with pyrene as fluorescence probe and benzophenone as quencher. The aggregation numbers of micelles for the mixed systems (SDBA/NP-10, SDBA/TX-100, SDBS/NP-10, SDBS/TX-100) were all bigger than that of the pure anionic surfactants, but smaller than that of pure non-ionic surfactants. The order of aggregation numbers of the four mixed system was NSDBA/NP-10> NSDBS/NP-10>NSDBA/TX-100> NSDBS/TX-100. The micropolarity of mixed micelles (I1/I3) determined by steady-state fluorescence probe method was agreement with the ESR results. The analysis result showed that there existed a synergistic effect both in the efficiency and ability of reducing the surface tension and forming micelles when two surfactants were mixed with a certain proportion. These results would provide some basic reference for the application of mixed surfactants with high efficiency.In this paper dissipative particle dynamics (DPD) was applied to simulate the micelle formation process of SDBA, NP-10, and SDBA/NP-10, and the dynamics process of microemulsion, including the structure of micelles, density distribution and fraction of water in the micelles and interfacial tension.During the micelle formation of mixture of SDBA/NP-10, the SDBA molecules were not evenly arranged, but formed some small clusters through their head groups. The cavities of clusters were filled by TX-100 molecules.For the microemulsion system of SDBA/NP-10/n-butanol (or amyl alcohol)/n-hexane (or n-heptane)/water (or salt water), the effect of cosurfactant alcohol on the interfacial composition, microstructure and its thermodynamic properties was studied by the dilution method. The distribution of cosurfactant (1-butanol and 1-pentanol) between the oil-water interfacial region (nai) and the continuous oil phase (n-hexane and heptane) (nao) were both increased with the increase of oil's molecular chain length. The longer carbon chain of oil would improve the dispersion of alcohol in the interface. For all these systems, ?G<0, so it was spontaneous for 1-butanol and1-pentanol to transfer to the interface from the oil phase. However, the transfer process was easier for n-heptane than for n-decane because of the lower Gibbs free energy. The ionic surfactant would give important effect on the W/O microemulsion with the presence of non-ionic surfactant. Both Re and Rw values increased with increasing water content (ω=10-50) in all these surfactant systems. Rw increased more quickly than Re, so the effective thickness of interface layer of liquid drops (dI) tended to decrease. It suggested that the transition from W/O to O/W microemulsion would occur at high water content (ω).The phase behavior, microstructure and thermodynamic properties of drug-carried microemulsion system (SDBA/ NP-10) /1-butanol / (bifenthrin / cyclohexanone) / water were studied through ternary phase diagram. The results showed that the O/W microemulsion region area of mixed surfactants (NP-10 / SDBA) was larger than that of a single nonionic surfactant, and the temperature had little effect on the phase behavior of microemulsion. The standard free energy change of cosurfactant alcohol transfered into the microemulsion interface layer from the dispersed phase was negative, e.g. ?Gs <0. The absolute value of ?Gs for the process of microemulsion formation increased with the growth of alcohol's carbon chain, which would improve the formation of microemulsion and increase its area of microemulsion. The standard enthalpy change of microemulsion formation was zero, e.g. -?Hs = 0, which revealed that the process is non-thermal. It indicated that the standard free energy change ?Gs were contributed by the entropy change of the alcohol molecules ?SS. The structure change of the formation process of the system was detected by conductivity, viscosity, refractive index and polarizing microscope. It was a W/O microemulsion system when the mass fraction of water is less than 32 %, e.g. w (H2O) <32%. The liquid crystal structure was formed at 32%63%. The allied toxicity measurement and field efficacy trials results showed that 2.5 wt% bifenthrin microemulsions was an environment-friendly agrochemical formula with long duration of the efficacy.The influence of several soluble promotors on the (SDBA/NP-10) / butanol / (flusilazole / cyclohexanone) / water system was tested. The results showed that 0.5 mol?L-1 sodium benzoate and 0.3mol?L-1 sodium salicylate could improve the solubility of flusilazole in water, and the solubility increased with the increase of the two hydrotrope concentration. The effect of urea on the phase behavior of flusilazole microemulsion was insignificant. Resorcinol, glucose and sodium chloride reduced noticably the flusilazole microemulsion zones of the ternary phase diagram. 8wt% flusilazole microemulsion prepared by 5 wt% sodium salicylate showed good heat storage stability. The allied toxicity measurement and field efficacy trials results showed that 8 wt% flusilazole microemulsions had great control effect against pear scab, indicating that it was an environmentally friendly fungicide.