Controlling Interfacial Film Formation in Mixed Polymer鈥揝urfactant Systems by Changing the Vapor Phase

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• 作者：Tahereh Mokhtari ; Quoc Dat Pham ; Christopher Hirst ; Benjamin M. D. O鈥橠riscoll ; Tommy Nylander ; Karen J. Edler ; Emma Sparr
• 刊名：Langmuir
• 出版年：2014
• 出版时间：August 26, 2014
• 年：2014
• 卷：30
• 期：33
• 页码：9991-10001
• 全文大小：460K
• ISSN：1520-5827

文摘

Here we show that transport-generated phase separation at the air鈥搇iquid interface in systems containing self-assembling amphiphilic molecules and polymers can be controlled by the relative humidity (RH) of the air. We also show that our observations can be described quantitatively with a theoretical model describing interfacial phase separation in a water gradient that we published previously. These phenomena arises from the fact that the water chemical potential corresponding to the ambient RH will, in general, not match the water chemical potential in the open aqueous solution. This implies nonequilibrium conditions at the air鈥搘ater interface, which in turn can have consequences on the molecular organization in this layer. The experimental setup is such that we can control the boundary conditions in RH and thereby verify the predictions from the theoretical model. The polymer鈥搒urfactant systems studied here are composed of polyethylenimine (PEI) and hexadecyltrimethylammonium bromide (CTAB) or didecyldimethylammonium bromide (DDAB). Grazing-incidence small-angle X-ray scattering results show that interfacial phases with hexagonal or lamellar structure form at the interface of dilute polymer鈥搒urfactant micellar solutions. From spectroscopic ellipsometry data we conclude that variations in RH can be used to control the growth of micrometer-thick interfacial films and that reducing RH leads to thicker films. For the CTAB鈥揚EI system, we compare the phase behavior of the interfacial phase to the equilibrium bulk phase behavior. The interfacial film resembles the bulk phases formed at high surfactant to polymer ratio and reduced water contents, and this can be used to predict the composition of interfacial phase. We also show that convection in the vapor phase strongly reduces film formation, likely due to reduction of the unstirred layer, where diffusive transport is dominating.