Surface Freezing of Cetyltrimethylammonium Chloride–Hexadecanol Mixed Adsorbed Film at Dodecane–Water Interface

Abstract

The surface freezing transition of a mixed adsorbed film containing cetyltrimethylammonium chloride (CTAC) and n-hexadecanol (C16OH) was utilized at the dodecane–water interface to control the stability of oil-in-water (O/W) emulsions. The corresponding surface frozen and surface liquid mixed adsorbed films were characterized using interfacial tensiometry and X-ray reflectometry. The emulsion samples prepared in the temperature range of the surface frozen and surface liquid phases showed a clear difference in their stability: the emulsion volume decreased continuously right after the emulsification in the surface liquid region, while it remained constant or decreased at a much slower rate in the surface frozen region. Compared to the previously examined CTAC–tetradecane mixed adsorbed film, the surface freezing temperature increased from 9.5 to 25.0 °C due to the better chain matching between CTAC and C16OH and higher surface activity of C16OH. This then renders such systems much more attractive for practical applications.

Anomalous Temperature-Controlled Concave–Convex Switching of Curved Oil–Water Menisci

“…We demonstrate here that such temperature-controlled curvature inversion is possible in a capillary-contained macroscopic water–oil interface. The inversion occurs under the joint, interrelated, action of two nanoscale effects: one is the interfacial freezing (IF), whereby a crystalline monolayer spontaneously forms at the interface at a temperature T s . − The freezing switches the temperature-slope of the interfacial tension, γ ow , from slightly negative at T > T s to strongly positive at T < T s , causing γ ow to decrease upon cooling and vanish at some T SE , above the bulk freezing point of the oil (cf. Supporting Information (SI) Figure S1).…”

“…Indeed, T u is systematically lower than T s by a few °C. Since the interfacially frozen γ ow decreases dramatically − ,,,, upon cooling below T s (see SI Figure S1), γ ow ( T u ) ≪ γ ow ( T s ). For the presently studied C 16 :C 18 TAB:C 16 OH combinations, γ ow tends to zero at T SE , a few °C below T s .…”

“Magic Numbers” in Self‐Faceting of Alcohol‐Doped Emulsion Droplets

“…This structure also explains the T d reduction in the C 17 OH‐doped system: the different OH orientation in odd alcohols [ 33 ] renders the bilayer‐stabilizing hydrogen bonds less favorable, yielding a reduction of the corresponding T s and T d . The high entropy loss upon the interfacial freezing, reported for the alcohol‐doped systems, [ 28,30 ] is similarly rationalized by the formation of a bilayer. As the number of conformational degrees of freedom per unit area is (roughly) twice larger for a molecular bilayer compared to a monolayer, [ 31 ] the larger entropy loss reported in the literature [ 28,30 ] strongly supports our conclusions.…”

“…Our recent studies [ 28,29 ] revealed a dramatic increase in T s and T d upon doping of C 18 TAB‐stabilized C 16 alkane droplets by sub‐percent concentrations of hexadecanol [CH 3 (CH 2 ) m − 1 OH, denoted: C m OH, with m = 16]. This increase, along with the unusually large interfacial entropy loss measured [ 28,30 ] at T s , suggests that the structure of the interfacial crystal is radically altered by the incorporation of the long‐chain alcohol molecules. Yet, no structural studies of alcohol‐doped emulsion interfaces have been hitherto reported, let alone systematic investigations of alcohol doping effects on self‐faceting.…”

Effect of Surface Freezing of a Cationic Surfactant and n-Alkane Mixed Adsorbed Film on Counterion Distribution and Surface Dilational Viscoelasticity Studied by Total Reflection XAFS and Surface Quasi-Elastic Light Scattering

“…Mixed adsorbed film of a surfactant and n -alkane shows a first-order transition from a two-dimensional liquid to solid states at the air–water − and the oil–water − interfaces upon cooling. These surface freezing transitions were driven by the enhanced lateral van der Waals interaction between surfactant chains and incorporated n -alkanes and lead to a stable oil-in-water emulsions. , Furthermore, it was also reported that the surface-frozen layer at the oil–water interface induced the self-shaping or faceting of oil droplets placed in surfactant solution. − Mixing of n -alkanes in surfactant-adsorbed films therefore attracts widespread attention in emulsion science and morphology control of related surfactant systems.…”