A Study of Wetting Behavior of the Systems of “Water＋Oil＋Surfactant” and “Water＋Surfactant”

For a three-phase (α,β,γ) coexisting equilibrium system, the densities of these three phase are in the order ργ＞ρβ＞ρα and the middle phase (β) may exist in the form of non-wetting, partial wetting or complete wetting between the upper（α）and lower（γ）phases. The transition from a partial wetting region to a non-wetting (or a complete wetting) region is called a wetting transition. Consequently, what we are concerned about is the wetting behavior near the tricritical point and whether there are wetting transitions or not. By adjusting the system parameters, such as temperature, pressure, salt concentration and kinds of surfactants, we can make the system approach to its tricritical point. In a multi-component liquid mixture, a tricritical point (TCP) is defined as a point at which three coexisting liquid phases become simultaneously single phase. Thus, in this study, we try to approach the tricritical point by changing the oil-chain length of the system “water +oil +surfactant”. With the decrease of the oil chain length, we can successfully approach the tricritical point.

In the study, the main three-component systems are “water＋alkane（octane、nonane、decane）＋C4E1” and pendant bubble/drop tensionmetry was applied to measure interfacial/surface tensions and calculate the wetting coefficients to judge the wetting behaviors of the systems. The symbol C4E1（ethylene glycol n-butyl ether） is a nonionic surfactant CiEj and CiEj is the abbreviation of CH3(CH2)i-1(OCH2CH2)jOH. From the conclusion of the study, when rising temperature from the lower critical point to upper critical point, we can find both two wetting transitions: complete wetting→partial wetting→complete wetting. Importantly, these behaviors obey Cahn’s theory（J. Chem. Phys., 66, 3667, 1977）, which asserts that for a three-phase coexisting system, complete wetting should be observed near the critical temperature.

Besides, we also experiment on the two-component systems “water＋CiPj（C3P1、C3P2、C4P1、C4P2、C4P3）”. CiPj is another nonionic surfactant which is the abbreviation of CH3(CH2)i-1(OCH2CH(CH3))jOH. From the systems, we want to understand that whether there are wetting transitions or not by changing the temperature with different surfactants. In the conclusion, we can find that there is a wetting transition (partial wetting→complete wetting ) in the system “water＋C3P2” which also obey Cahn’s critical wetting theory and the system “water＋C3P1” expresses complete wetting in our operating temperature range (10~40℃). However, three systems “water＋C4P3”, “water＋C4P2”and “water＋C4P1” all express partial wetting in the range from 10~40℃. Besides, we also discuss the effects of the hydrocarbon chain length and oxypropylene group length for wetting behaviors.