Surfactant-induced surface freezing at the alkane-water interface

Area of Science:

• Physical Chemistry
• Surface Science
• Materials Science

Background:

• Long-chain alkanes typically exhibit surface freezing at the alkane-air interface, but not the alkane-water interface.
• Understanding interfacial behavior is critical in various chemical and material applications.

Purpose of the Study:

• To investigate the induction of surface freezing at the tetradecane-water interface using a cationic surfactant.
• To characterize the properties and conditions of this induced surface freezing.

Main Methods:

• Ellipsometry was employed to detect and analyze the formation of interfacial layers.
• Surface tensiometry was used to measure interfacial tension and related properties.
• Varying concentrations of hexadecyltrimethylammonium bromide (CTAB) were studied.

Main Results:

• Hexadecyltrimethylammonium bromide (CTAB) induced surface freezing at the tetradecane-water interface, even at low mole fractions (0.1).
• The surface-freezing temperature (Ts) showed a linear dependence on the interfacial excess of CTAB.
• Excess surface entropy below Ts was measured at -0.76+/-0.02 mJ K-1 m(-2), indicative of a rotator phase.
• Ellipsometry confirmed the formation of a frozen monolayer with chains oriented near the surface normal.

Conclusions:

• Cationic surfactants can effectively induce surface freezing at alkane-water interfaces.
• The observed phenomenon involves the formation of an ordered, frozen monolayer with specific chain orientations.
• The findings contribute to the understanding of interfacial phase transitions and surfactant behavior.