Structure of an Adsorbed Layer of n-Dodecyl-N,N-dimethylamino Acetate at the Air/Solution Interface As Determined by Neutron Reflection

Abstract

The composition and structure of the interfacial layer formed at the air/water interface by a zwitterionic surfactant, n-dodecyl-N,N-dimethylamino acetate, has been determined by neutron reflection. The compositions determined by neutron reflection and by surface tension measurements in conjunction with the Gibbs equation are in excellent agreement if the prefactor in the Gibbs equation is one, i.e., the surfactant behaves as though it is completely uncharged. At the critical micelle concentration (cmc) the structure most consistent with the neutron data is one where the head group is vertically oriented with respect to the surface normal, but the chain is strongly tilted away from the surface normal with a value of 〈cos θ〉 ≈ 0.45. This is a larger angle of tilt than observed with other surfactants at a comparable surface density and is attributed to the vertical orientation of the head group forcing the chain to be tilted away from the surface normal. Unlike most other surfactants, there is little change in the tilt of the chain as the coverage is decreased, probably because the tilt is already nearly a maximum at the cmc. The coverage of the layer increases markedly with temperature, and analysis of the thermodynamic parameters of the adsorption suggests the possibility that, at higher coverages, this is driven by a large gain in entropy from loss of water of hydration from adsorbed surfactant.

Surface adsorption of zwitterionic surfactants: n-alkyl phosphocholines characterised by surface tensiometry and neutron reflection

“…The difference shows that their Langmuir trough method is technically inappropriate for studying soluble surfactant monolayer. For the zwitterionic C12 betaine, a value of 50 Å 2 was however reported by Hines et al [2]. For a similar zwitterionic C12 sulfobetaine, A cmc was reported to be around 47 Å 2 from computer modelling [39].…”

“…However, it would be more reliable to believe that the small CMC variation with temperature elevation is the balance between the degree of increase in dehydration at the hydrophilic group which favours micellisation and the extent of disruption of the structured water molecules around the hydrophobic group which disfavours micellisation and would increase the CMC. Others also found that the CMC decrease for C 12 H 25 N + (CH 2 C 6 H 5 )(CH 3 )CH 2 COO − with increasing temperature [46], but for the C12 betaines, little variation in the CMC was detected [2].…”

“…This mixed water did not contribute to the specular reflection at the air/water interface and was termed null reflecting water (NRW). When a deuterated surfactant layer was adsorbed, the specular signal arose from the surfactant layer only [2,[26][27][28]. Fig.…”

“…These studies have revealed interesting differences in their surface adsorption arising from different types of surfactant head groups [1]. In spite of a number of recent studies on the zwitterionic type (e.g., the work by Hines et al [2]), zwitterionic surfactants are less studied and their adsorption characteristics are less understood. We have recently examined the surface adsorption from a group of zwitterionic alkyl phosphocholines (PC) by a combined measurement of surface tension and neutron reflectivity.…”

Study of an Amphoteric Surfactant in a Soil Decontamination Process Using ANS Enhanced Fluorescence: Micellar Behavior and Dosing in Synthetic and Soil Solutions

“…They have shown that the adsorption of surfactant on solid hydrophobic compounds is approximately the same as liquid/liquid or liquid/air interfaces (Hines et al 1997). …”