A Study on the Mixed Solution of OBS and SDS Surfactants
Fluorocarbon surfactant has "high surface activity, high chemical stability, high thermal stability" three high characteristics, is widely used in oil volatility inhibitor, fluoroprotein foam extinguishing agent, aqueous film foam extinguishing agent. Its principle is to use its aqueous solution to form a water film on the oil surface to inhibit oil volatilization or isolate the oil layer from the air so as to complete the fire extinguishing process. However, the oil-repellent property of fluorocarbon surfactant makes its aqueous solution and oil have high oil-water interfacial tension, which affects the spreading of its aqueous solution on the oil surface, and most of the fluorocarbon surfactants are expensive and their application is limited, and they are often compounded with hydrocarbon surfactants in industry to improve their solution performance and reduce production costs. Xiao Jinxin and Chen Yanlin have studied some fluorocarbon-hydrocarbon surfactant compounding systems and found that the spreading performance and foaming performance of the aqueous solution were significantly improved after compounding with hydrocarbon surfactants. However, not much research has been reported on the compounding system of Sodium perfluoro nonyloxybenzene sulfonate with hydrocarbon surfactants. Sodium perfluoro nonyloxybenzene sulfonate is a powdered anionic fluorocarbon surfactant, which is relatively inexpensive compared with most fluorocarbon surfactants, but its surface activity is average. In this paper, it was compounded with sodium dodecyl sulfate(SDS) in different proportions, and the surface tension, oil-water interfacial tension, spreading performance, and foam performance of the aqueous solution of surfactants after compounding were systematically studied
1 Experimental part
1.1 Method and process
1.1.1 Preparation of solutions
(1) Prepare 1.6 mmol/L of OBS solution (3:1 mixture of water and ethanol by mass as solvent) and 3.47 mmol/L of SDS solution (distilled water as solvent), respectively;
(2) The OBS and SDS were compounded in molar ratios of 1:0, 2.3:1, 1.38:1, 0.46:1, 0.15:1, 0.09:1, and 0:1, respectively, to obtain the mixture;
(3) The mixture was mixed with secondary distilled water to form different concentrations of surfactant aqueous solutions, mixed evenly and left to stand for 2h before starting the test. The concentration of the compounded surfactant aqueous solution is the total concentration of surfactant (OBS+SDS).
1.1.2 Surface (boundary) performance test
The surface tension and oil-water interfacial tension of the surfactant aqueous solution were measured at (20.0±0.5)℃ by the hanging ring method. The oil phase used in this paper was cyclohexane, and the surface tension of cyclohexane was 24.3 mN/m.
1.1.3 Spreading experiment??
The spreading experiment was carried out, and the spreading time ts and spreading volume Vs were recorded. spreading on the surface of cyclohexane, the spreading time less than 0.5s was taken as the standard for rapid spreading.
1.1.4 Foaming performance test
Test the foaming performance of the compounded surfactant aqueous solution at the optimum spreading concentration. The foaming properties of the solution were characterized by the starting foam height H0 (mm) with 40 times/min oscillation for 1 min. Then, the foam height H1 (mm) was recorded after 2h, and the foam break-up speed V (mm/h) was calculated by the following formula as a measure of foam stability. Each solution was measured 3 times to take the average value.
v=(H0-H1)/2
2 Results and discussion
2.1 Surface activity of aqueous solutions of compound surfactants
The surface tension of aqueous solutions of different concentrations of compound surfactants was measured by the method in section 1.1.2 as γ-lgc curves, and the critical micelle concentration was sought from the intersection of tangent lines near the curve turning point. Figure 1 shows the surface tension versus concentration curves of aqueous solutions of different compounding systems of OBS/SDS. In order to observe the turning point of the curves more clearly, Figure 1 was partially enlarged and the range of vertical coordinates was reduced, as shown in Figure 2. From Figs. 1 and 2, it can be seen that the surface tension increased with the increase of the molar fraction of SDS for the mixture with the same concentration below 4.86 mmol/L, but the increase of n(OBS):n(SDS) = 2.3:1, 1.38:1, 0.46:1 compounding system was not significant, which is consistent with the conclusion of the literature; for the mixture with the concentration above 4.86 mmol/L, the surface tension increased with the increase of the molar fraction of The surface tension decreased and then increased with the increase of SDS molar fraction. However, the surface tensions of all the complex systems were much lower than those of the aqueous solutions of the single component of SDS. The analysis suggests that the addition of SDS competes with OBS for adsorption, which decreases the adsorption of fluorocarbon chains and increases the adsorption of hydrocarbon chains on the surface, and therefore, the surface tension of the aqueous solution of the complexed system is increased. When the molar fraction of SDS was low, because the fluorocarbon chain of OBS was more hydrophobic than the hydrocarbon chain of SDS, OBS was preferentially adsorbed on the surface, and its surface adsorption was much larger than that of SDS, so the surface tension of the aqueous solution of the compound system was higher than that of the single component of OBS, but the increase was not significant. As the molar fraction of SDS continued to increase, the proportion of SDS adsorbed on the surface increased, and the surface tension increased and gradually approached the surface properties of SDS single component. The surface tension of the aqueous solution of OBS single system was higher than that of the aqueous solution of the partially compounded system when the concentration of surfactant aqueous solution was higher, which was caused by the presence of ethanol and other impurities in the OBS solution. The OBS used was an industrial product with low purity. In order to increase the solubility of OBS, ethanol with a mass fraction of 25% was added in the preparation of 1.6 mmol/LOBS solution. As the concentration of the aqueous surfactant solution increases, the content of ethanol and impurities increases to a certain extent, which causes the surface tension of the aqueous surfactant solution to rise rapidly. When OBS is compounded with SDS, the amount of OBS decreases, and the content of ethanol and impurities decreases, reducing their influence on the surface properties of the aqueous surfactant solution
picture 1
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Picture 2
he CMC and ultimate surface tension of each surfactant aqueous solution can be found from the turning point of the curve in Fig. 1, and the relevant data are listed in Table 1. Fig. 3 shows the variation curve of the critical micelle concentration with the molar fraction of SDS for the aqueous solution of OBS/SDS complex system and the variation curve of CMC with the molar fraction of SDS calculated by the ideal mixed micelle formula (2).
1/CMCT=x1/CMC01+x2/CMC02 (2)
CMCT, CMC01, CMC02 are the critical micelle concentrations of the mixture, single component 1 and 2, respectively. Table 1 and Figure 3 show that the critical micelle concentration CMC and the limiting surface tension γCMC of the aqueous solution of the compound system increased with the increase of the molar fraction of SDS, and gradually approached the properties of the aqueous solution of the single component of SDS. Figure 3 further shows that the experimental values of critical micelle concentration CMC do not match with the theoretical values and have obvious positive deviations, and it can be clearly observed from Figure 2 that each compounding system has a double fold point, indicating that the OBS/SDS compounding system forms a non-ideal mixed micelle. This is mainly because: after OBS and SDS compounding, the phase separation occurs in the surface layer and inside the solution due to the mutual abhorrence of hydrophobic chains of both, i.e., fluorocarbon chains aggregate with fluorocarbon chains and hydrocarbon chains aggregate with hydrocarbon chains, and each component basically forms micelles independently, and the interaction between them is limited to the homoionic effect of common counter ions.
3 Conclusion
(1) Compared with the single component aqueous solution of OBS, the surface tension of the aqueous solution of OBS/SDS compounding system increases with the increase of the molar fraction of SDS; the surface tension of the aqueous solution of the compounding system decreases and then increases with the increase of the concentration of the aqueous solution of the compounding system in the high concentration area, which is due to the impurities and ethanol contained in the aqueous solution of OBS.
(2) With the increase of SDS molar fraction, the oil-water interfacial tension of OBS/SDS compounding system decreases first and then increases, and the best compounding ratio exists, that is, n(OBS):n(SDS) = 0.15:1 when the system oil-water interfacial performance synergistic effect is the best.
(3) The reduction of oil-water interfacial tension of some of the compounded systems is much larger than the increase of surface tension of their aqueous solutions, and therefore, the spreading performance of their aqueous solutions is significantly improved. In particular, the spreading performance of the aqueous solution of the compounded surfactant with n(OBS):n(SDS) = 0.46:1 and a concentration of 6.17 mmol/L was the best, and it could spread rapidly on cyclohexane, and the amount of OBS fluorocarbon surfactant was significantly reduced by about 46%.
(4) n(OBS):n(SDS) = 0.15:1, and the best foaming performance was obtained for the aqueous solution of the compounded surfactant at a concentration of 6.34 mmol/L.
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