Neutron Reflection Study of a Double-Chained Sugar Surfactant

Abstract

Neutron reflection and surface tension have been used to investigate the structural features exhibited at the air-liquid interface by an adsorbed layer of the sugar-based nonionic surfactant 7,7-bis [(1,2,3,4,5pentahydroxyhexanamido)methyl]-n-tridecane. It consists of two glucamide headgroups and two C 6 H 13 alkyl chains, i.e., (C 6 H 13 ) 2 C(CH 2 NHCO(CHOH) 4 CH 2 OH) 2 , abbreviated to di(C 6 -Glu). Isotopic substitution was used to distinguish the chains of the surfactant from the headgroup and solvent distributions. The critical micelle concentration (CMC) was found to be (1.9 ( 0.1) × 10 -3 M using surface tension measurements, and the latter also indicated that the surface properties were not affected by isotopic substitution. The specular reflection of neutrons has been used to determine the surface concentrations of skeletally deuterated di(C 6 -Glu) over the concentration range CMC/400 to 2CMC. The neutron measurements showed the area per molecule A at the CMC to be 66 ( 3 Å 2 , as compared with 60 ( 10 Å 2 from surface tension measurements. The layer structure has been determined using three different isotopic contrasts at concentrations of CMC and CMC/400. The fit of a two-layer model shows that the total monolayer thickness decreases from 17.5 ( 1.5 Å at the CMC to 14.5 ( 1.5 Å at CMC/400, respectively. The kinematic approximation was used to fit the neutron data and an X-ray reflection profile. The most remarkable feature of the structure obtained is the apparent smoothness of the layer. The calculated roughness of the layer at the CMC is 9.7 Å 2 which corresponds exactly to the value expected for capillary waves using the model of a pure liquid. At the CMC the molecule as a whole appears to be tilted about 35-40°away from the surface normal, but at CMC/400 the hydrocarbon chains are approximately normal to the surface while the gluconamide groups are strongly tilted.

Interfacial tension of reactive, liquid interfaces and its consequences

“…where R is the gas constant, and m depends on the nature of the surfactant. For non-ionic surfactants, m = 1 is in very good agreement with several experimental studies [34][35][36][37][38][39], as well as m = 2 for 1:1 ionic surfactants 1 assuming electrical neutrality of the interface [40,41]. For 2:1 surfactants, however, discrepancies in the experimental results motivated the use of an adjustable parameter α caused by ion impurities at the interface [41,42], and led to m = 2 − α.…”

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

“…Neutron reflection has been used in determining the detailed structure of the monolayers formed at the air/water interface by single-chained non-ionic surfactants (see for example, [1][2][3][4][5]) but there have been comparatively few studies of this type dealing with the monolayers formed by doublechained non-ionic surfactants [6][7][8]. Here, we report on the application of neutron specular reflection measurements (together with isotopic substitution) to determine the molecular architecture of the monolayers formed at the air-water interface by a synthetic (vesicle-forming) non-ionic surfactant, 1,2-di-O-octadecyl-rac-glyceryl-3-(ω-methoxydodecakis (ethylene glycol)) (2C 18 E 12 ) (Fig.…”

Synthesis and Properties of X‐Type Alkyl Sulfonate Gemini Surfactants Derived from Cyanuric Chloride

“…The efficiency can be characterized by the value of logarithm of the surfactant concentration C 20 at which the surface tension of water is reduced by 20 mN/m. The pC 20 (-logC 20 ) value measures the efficiency of adsorption of surfactant at the air-water interface; the larger the value of pC 20 , the greater the tendency of the surfactant to adsorb at the air-water interface, relative to its tendency to form micelles, and the more efficiently it reduces the surface tension [28]. The values of pC 20 of XC n are also listed in Table 1.…”

“…In recent years, new classes of Gemini surfactants have emerged, such as di-(C8-Glu) [17][18][19][20], Glu(8)-2-Glu(8) [21] as shown in Fig. 2a, b.…”