The Structure of Monododecyl Pentaethylene Glycol Monolayers with and without Added Dodecane at the Air/Solution Interface:  A Neutron Reflection Study

Abstract

Neutron reflectometry and surface tensiometry have been used to study the composition and structure of a mixed monolayer of dodecane and the nonionic surfactant pentaethylene glycol monododecyl ether (C 12 E 5 ) and the structure of the C 12 E 5 on its own at the surface of aqueous solutions of C 12 E 5 . Partially labeling of the surfactant with deuterium gave the surface normal distributions of the ethylene glycol and hydrocarbon chains of the surfactant and of the water, and these results are compared with those from other members of the C 12 E m series. The C 12 E 5 layer has a higher capacity for oil than that of the comparable hydrocarbon chain length cationic surfactant, dodecyl trimethylammonium bromide (C 12 TAB), and the structure of the composite layer is also different. The separation of the centers of the surfactant chain and dodecane distributions is only 3 Å in the C 12 E 5 system in comparison with 6.5 Å for C 12 TAB. In this respect, the C 12 E 5 /dodecane system more closely resembles the sodium dodecyl sulfate/dodecanol system, and this is attributed to a favorable interaction between the dodecane and the ethylene glycol chain, which encourages a closer approach of the oil to the water surface. The hydrocarbon chain in the C 12 E m series where m is 6 or less is tilted at about 40°a way from the surface normal. Upon incorporation of dodecane into the C 12 E 5 layer, the hydrocarbon chain changes to an almost perpendicular orientation.

Computer simulation studies of surfactant monolayer mixtures at the water/oil interface: charge distribution effects

“…The last results agree with previous simulations of the same system at different concentrations [34]. In neutron reflection experiments at 300 K on monolayer mixtures of SDS/dodecanol at the water/air interface it is observed that the SDS volume fraction profile is displaced toward the water from the dodecanol profile [18,22]; i.e., the SDS seems to be closer to the solvent [36]. For the SDSC/SDSE mixture (middle) it is observed that the profiles are more spread at the interface, even though the head group profile of the SDSE (nonionic) molecules seems to be deeper into the water than the SDSC (cationic) molecules.…”

“…Also, experiments on C 16 TAB and C 12 E 6 show similar positions of the volume fraction distribution at the air/water interface for both surfactants [38]. In other experiments on dodecane and C 12 TAB, that the C 12 TAB is slightly deeper into the solvent [36]. However, for systems richer in C 16 TAB than in C 12 E 6 the distribution of the nonionic molecules seem to be slightly shifted to the solvent [18].…”

Adsorption and structure of the adsorbed layer of ionic surfactants

“…8.b), which form in fact a second adsorbed layer. roughness was experimentally observed by Lu et al [36] by neutron scattering. In such bilayer the decreased repulsion will lead to smaller area per molecule, α, and increased adsorption Γ.…”

Effects of surface pressure on the structure of the monolayer formed at the air/water interface by a non-ionic surfactant

“…The increased ordering of the adsorbed layer is accompanied by a decrease in the effective molecular interfacial area, with a 0 decreasing from 218 Å 2 at 15 mN/m down to 102 Å 2 at 40 mN/m. The value of a 0 obtained for the most compressed monolayer (held at 40 mN/m) is consistent with the trend in values reported for the (closely-packed monolayers of) single chain (C n E m ) surfactants (at their cmc), viz., 55 Å 2 for C 12 E 6 , 62 Å 2 for C 12 E 8 and 72 Å 2 for C 12 E 12 [26].…”