Solution pH and Oligoamine Molecular Weight Dependence of the Transition from Monolayer to Multilayer Adsorption at the Air–Water Interface from Sodium Dodecyl Sulfate/Oligoamine Mixtures

Abstract

Neutron reflectivity and surface tension have been used to investigate the solution pH and oligoamine molecular weight dependence of the adsorption of sodium dodecyl sulfate (SDS)/oligoamine mixtures at the air-water interface. For diethylenetriamine, triamine, or triethylenetetramine, tetramine mixed with SDS, there is monolayer adsorption at pH 7 and 10, and multilayer adsorption at pH 3. For the slightly higher molecular weight tetraethylenepentamine, pentamine, and pentaethylenehexamine, hexamine, the adsorption is in the form of a monolayer at pH 3 and multilayers at pH 7 and 10. Hence, there is a pH driven transition from monolayer to multilayer adsorption, which shifts from low pH to higher pH as the oligoamine molecular weight increases from tetramine to pentamine. This results from the relative balance between the electrostatic attraction between the SDS and amine nitrogen group which decreases as the charge density decreases with increasing pH, the ion-dipole interaction between the amine nitrogen and SDS sulfate group which is dominant at higher pH, and the hydrophobic interalkyl chain interaction between bound SDS molecules which changes with oligoamine molecular weight.

Anionic surfactant – Biogenic amine interactions: The role of surfactant headgroup geometry

“…The results are compared with those for SDS (19), and show a greater shift in the pattern of adsorption to lower concentrations with the addition of SLES than for SDS. The enhanced adsorption is attributed to the strong surface interaction between the anionic surfactant and biogenic amine, as previously observed and discussed for biogenic amines (19,23) and in other oligoamine / surfactant mixtures (16,(20)(21)(22). At low pH when the oligoamines are highly charged the interaction arises from the electrostatic attraction between the amine groups and the sulfate groups of the anionic surfactant.…”

“…The strong interaction at high pH has been attributed to the combination of an ion-dipole interaction between the surfactant sulfate headgroup and the amine nitrogen group and the inter-alkyl chain interaction between neighbouring attached surfactant molecules (45). This description of the nature of the interaction is reinforced by a wider range of studies (16,(19)(20)(21)(22)(23)(24)(25) and corroborated by the evidence from other studies (14,15,(46)(47)(48). The impact of an anionic surfactant with a different structure on its interaction and surface properties with PEI was investigated through a comparison between sodium dodecyl benzene sulfonate, LAS and SLES (27).…”

“…The NR data for 5mM spermine / SLES at pH 3 and SLES concentrations of 10 -3 , 5x10 -3 and The NR data corresponding to a surface multilayer structure, as shown in figure 3, are analysed quantitatively using a surface multilayer model based on the kinematic approximation (43,44) and used extensively in related studies (19)(20)(21)(22)(23)41). The key model parameters are the number of bilayers, N, the bilayer thickness dt, where dt=d1+d2 and d1 and d2 are thicknesses of the alkyl chain and hydrated headgroup regions of the bilayer structure, and the scattering length densities of those two regions, ρ1, ρ2.…”

“…The particular focus of this paper is the surface adsorption behaviour of biogenic aminesurfactant mixtures at the air-water interface. It has been demonstrated that for the aliphatic biogenic amines (putrescine, spermidine and spermine) -SDS mixtures (19) and the oligoamine-SDS mixtures (20)(21)(22) the strong interaction of the biogenic amine with the SDS results in enhanced adsorption of the SDS at low SDS concentrations and regions of SDS concentration and solution pH where surface multilayer formation occurs. These studies illustrated how the oligoamine molecular weight (number of amine groups) and the structure (linear or branched) as well as the surfactant and oligoamine concentrations and solution pH affect the surface behaviour.…”

Assembly of small molecule surfactants at highly dynamic air–water interfaces

“…Neutron and X-ray reflectivity have been used extensively to quantify the gas-liquid interface, but on planar interfaces, and not under dynamic conditions relevant to the foam. [22][23][24][25][26][27][28] There also have been a number of small angle scattering studies on foams, [29][30][31][32][33][34] illustrating the viability of the technique, but the conclusions have been largely qualitative and are yet to improve the understanding of the assembly of stabiliser at the air-water interface.…”