Interface alkane-water

Surface SHG [4.307] produces frequency-doubled radiation from a single pulsed laser beam. Intensity, polarization dependence, and rotational anisotropy of the SHG provide information about the surface concentration and orientation of adsorbed molecules and on the symmetry of surface structures. SHG has been successfully used for analysis of adsorption kinetics and ordering effects at surfaces and interfaces, reconstruction of solid surfaces and other surface phase transitions, and potential-induced phenomena at electrode surfaces. For example, orientation measurements were used to probe the intermolecular structure at air-methanol, air-water, and alkane-water interfaces and within mono- and multilayer molecular films. Time-resolved investigations have revealed the orientational dynamics at liquid-liquid, liquid-solid, liquid-air, and air-solid interfaces [4.307]. 

A different type of experiments was performed by Van Hunsel Joos (1987b). They studied the steady state of adsorption of various alkanols at the alkane/water interface by means of the drop volume method (Fig. 5.35). The steady states differ remarkably from the equilibrium state. A description of the adsorption process has therefore to allow for a transfer of hexanol molecules across the hexane/water interface. The difference of the two studied steady states is... 

Recently, reliable relaxation experiments at liquid/liquid have been performed by different techniques. Bonfillon Langevin (1993) measured the dilational elasticity of different surfactants (Triton X-100, SDS in water and in 0.1 M NaCl) at water/air as well as alkane/water interfaces by a modified longitudinal wave damping method (cf. Section 6.3.1). While at the water/air interface, the behaviour of Triton X-100 could be described by a... 

The HT voltammetry with gold electrodes was also recently used to measure the surface partitioning constant of a soluble, redox-active surfactant at the air/water interface [25]. Malec and coworkers modified the surface of gold electrodes by self-assembly of short alkane chain thiols in order to mimic the thermodynamic properties of the air/water interface. They relied on the fact that the surface tensions of the air/water interface and of the liquid alkane/water interface are similar [8]. Indeed, the HT measurements of the Gibbs monolayer formation constant were in agreement with their surface tensiometry and Brewster angle microscopic measurements [25]. 

Fig. 3 Schematic of the izomerization process in Malachite Green at the alkane/water interface. The phenyl moiety protrudes into the alkane phase whereas the nitrogen containing groups are on the aqueous side. From the experimental results, the torsional motion is undergone along the two arrows...

M. J. Wirth and J. D. Burbage,/. Phys. Chem., 96, 9022 (1992). Reorientation of Acridine Orange at Liquid Alkane Water Interfaces. 

Patel S, Brooks CL (2006) Revisiting the hexane-water interface via molecular dynamics simulations using nonadditive alkane-water potentials. J Chem Phys 124(20) 204706... 

In n-octane/aqueous systems at 27°C, TRS 10-80 has been shown to form a surfactant-rich third phase, or a thin film of liquid crystals (see Figure 1), with a sharp interfacial tension minimum of about 5x10 mN/m at 15 g/L NaCI concentration f131. Similarly, in this study the bitumen/aqueous tension behavior of TRS 10-80 and Sun Tech IV appeared not to be related to monolayer coverage at the interface (as in the case of Enordet C16 18) but rather was indicative of a surfactant-rich third phase between oil and water. The higher values for minimum interfacial tension observed for a heavy oil compared to a pure n-alkane were probably due to natural surfactants in the crude oil which somewhat hindered the formation of the surfactant-rich phase. This hypothesis needs to be tested, but the effect is not unlike that of the addition of SDS (which does not form liquid crystals) in partially solubilizing the third phase formed by TRS 10-80 or Aerosol OT at the alkane/brine interface Til.121. 

S. Marie Bertilla, J.L. Thomas, P. Marie, M.P. Krafft, Co-surfactant effect of a semi-fluorinated alkane at a fluorocarbon/water interface. Impact on the stabilization of fluorocarbon-in-water emulsions, Langmuir 20 (2004) 3920-3924. 

Silica particles synthesized in nonionic w/o microemulsions (e.g., poly-oxythylene alkyl phenyl ether/alkane/water) typically have a narrow size distribution with the average value between 25 and 75 nm [54,55]. Both water and surfactant are necessary components for the formation of stable silica suspensions in microemulsions. The amounts of each phase present in the micro emulsion system has an influence on the resulting size of the silica nanoparticle. The role of residual water (that is the water that is present in the interface between the silica particle and the surfactant) is considered important in providing stability to the silica nanoparticle in the oil... 

FIGURE 2.1. Experimental geometries. Top CCU/water interfaces showing the polarization scheme ssp and the critical angle c. Bottom alkaneAvater interfaces with adjustable thickness of alkane layer to minimize IR absorption by the alkane. 

The results are compared to those above for the CCI4/H2O interface. Several properties of alkane/water and CCU/water interfaces suggest that their interfacial characteristics should be similar. The measured interfacial tensions are 49.7 mN/m for hexane/water and 45 mN/m for CCU/water [73,74], with molecular dipole polarizabilities of 11.9 and 11.2 X 10 cm respectively [75]. However, IR experiments by Conrad and Strauss [76,77] show that water molecules dissolved in an alkane solvent are free to rotate while water dissolved in CCU is relatively constrained. It is the details of these molecular interactions that dominate interfacial structure and dynamics. 

With cetyl alcohol, there is the complication that the polarity of the molecule may cause it to reside at the surface of the droplet, imparting additional colloidal stability. Here, the surfactant and costabilizer form an ordered structure at the monomer-water interface, which acts as a barrier to coalescence and mass transfer. Support for this theory lies in the method of preparation of the emulsion as well as experimental interfacial tension measurements [79]. It is well known that preparation of a stable emulsion with fatty alcohol costabilizers requires pre-emulsification of the surfactants within the aqueous phase prior to monomer addition. By mixing the fatty alcohol costabilizer in the water prior to monomer addition, it is believed that an ordered structure forms from the two surfactants. Upon addition of the monomer (oil) phase, the monomer diffuses through the aqueous phase to swell these ordered structures. For long chain alkanes that are strictly oil-soluble, homogenization of the oil phase is required to produce a stable emulsion. Although both costabilizers produce re-... 

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