Search

Neutron reflection and surface tension have been used to study the adsorption of the nonionic surfactant monododecyl hexaethylene glycol (C 12 E 6 ) and the mixed nonionic–anionic surfactants n -dodecyl triethylene glycol (C 12 E 3 ) and Sodium dodecyl sulfate (SDS) at the air–liquid interface. Water and a sorbitol ( d -glucitol, CH 2 OH(HCOH) 4 CH 2 OH)/water mixture were selected as the solvents. The addition of 300 g/liter of sorbitol to an aqueous solution of C 12 E 6 reduces the critical micellar concentration, cmc, from ∼7 × 10 −5 to ∼3 × 10 −5 M, and increases the surface pressure at the cmc; the effect on the C 12 E 3 /SDS mixture is less. Despite these changes, the pattern of adsorption at the air–liquid interface is essentially unaltered by the addition of sorbitol, and the surface tension and neutron reflectivity data are in good agreement. The consequences of the sorbitol addition are however seen directly in the structure of the C 12 E 6 monolayer. The ethylene oxide (EO) chain is more extended than in water, and more displaced from the solvent, consistent with dehydration of the ethylene oxide group.

The structure of mixed monolayers of the zwitterionic phospholipid dipalmitoylglycerophosphocholine (DPPC) containing nonionic surfactants of the polyethylene glycol monododecyl ether variety (C12En with n = 2, 4, 6, 8) is studied at the air/water interface using a combination of neutron reflectivity with film balance techniques. The two methods applied simultaneously gave good agreement of the molecular area data using the most simple single-layer model for fitting the reflectivity curves. Isotopic substitution of both lipids and surfactants has been employed in order to determine structural parameters of both components separately. All measurements were done at lipid to surfactant molar ratios rL/S > 2 and at surfactant concentrations well below their critical micelle concentration. Measurements were performed in the liquid-condensed (LC) and in the liquid-expanded (LE) phase states of the monolayer. The results show that all surfactants studied readily associate with the DPPC in the LE phase to a mixed monolayer but get completely transferred from the monolayer into the subphase at the transition to the LC phase. A determination of the surfactant partition between the mixed monolayer and subphase at the LE phase state provides evidence that virtually all surfactant is associated with the monolayer. The area per surfactant molecule in the mixed monolayer is found to increase rather linearly from 45 ± 5 Å2 (n = 2) to 165 ± 5 Å2 (n = 8) by an increment of 22 ± 2.5 Å2 per ethylene glycol group (film pressure 5 mN/m). The thickness of the surfactant alkyl chain was found to be 14 ± 2 Å for C12E2 and 8 ± 2 Å for C12E4 which indicates a very high degree of chain disorder. The thickness of the fatty acyl region of the DPPC is not affected by the surfactant for n = 2 and 4. The headgroup region thickness of the mixed monolayer is 14 ± 2 Å for C12E2 and 11 ± 2 Å for C12E4. While the diethylene glycol group exhibits a rather extended conformation, longer surfactant headgroups are prone to a significant incidence of gauche conformations. Moreover, the data indicate that the interaction of C12En with the monolayer is markedly different for n = 2 compared to all other ethylene glycol groups (n = 4, 6, 8) studied. © 1994 American Chemical Society.

Small-angle neutron scattering has been used to study the structure and composition of mixed ionic-nonionic surfactant micelles. A comparison between two different mixed surfactant systems, sodium dodecyl sulfate (SDS) and hexaethylene glycol monododecyl ether (C12EO6) and hexadecyltrimethylammonium bromide (C16TAB) and C12EO6, both in 0.1 M NaCl, has been made. In the latter system, ideal mixing is observed, and in the former, departure from ideality, broadly consistent with regular solution theory, RST, is observed. The deviations from the predictions of RST are attributed to subtle changes in the packing of the two surfactants in the mixed micelles. For the SDS/C12EO6 mixture, the micellar aggregation number is essentially constant with composition and concentration, whereas for the C16TAB/C12EO6 mixture there is a marked micellar growth with increasing concentration and mole fraction of C12EO6 in solution. The SANS results on structure and composition are compared with the results reported for other mixed surfactant systems and are discussed in the context of recent theoretical developments. © 1999 American Chemical Society.

The interactions between poly(ethylene oxide) (PEO) and dodecyl sulfate with different monovalent metal cations (MDS, M = Li+, Na+, Cs+) and dodecyltrimethylammonium bromide (C12TAB) have been stud...

The compositions of the air/solution interface of aqueous mixtures of lithium dodecyl sulfate (LiDS) and cesium dodecyl sulfate (CsDS) with poly(ethylene oxide) (PEO) have been studied as a function of surfactant concentration using neutron specular reflection. Comparison of the results with those for NaDS shows that for all three ionic species the polymer is progressively displaced from the surface on the addition of surfactant until it can no longer be directly detected at the critical aggregation concentration (area per segment > 80 Å2). Comparison of the behavior of the three surfactants with each other and with measurements on the noninteracting combination dodecyltrimethylammonium bromide (C12TAB)/PEO shows that the anionic surfactants interact favorably with PEO at the surface. The additional free energies of binding of surfactant to the surface in the presence of PEO exactly parallel the polymer/micelle binding free energies. Of the three anionics LiDS interacts most strongly, and this is also manifested in adsorption of the PEO at concentrations above the critical micelle concentration of LiDS.

Neutron reflection has been used to study the composition of layers of sodium dodecyl sulfate (SDS) and dodecanol mixtures adsorbed at the air/water interface. Saturation of the surface by dodecanol at a 6.7 mM SDS concentration occurs when the bulk dodecanol concentration is about four times the solubility of dodecanol in pure water, indicating that even below the cmc dodecanol is solubilized by SDS. At a fixed dodecanol/SDS ratio the adsorption of dodecanol passes through a maximum at about half the cmc as the SDS concentration is changed, dropping sharply, although not to zero, at the cmc. Above the cmc it is estimated that an average of about two dodecanol molecules are incorporated into a single micelle. The adsorption of SDS is affected by the presence of dodecanol. At low SDS concentrations (

Sediment samples from nine culture ponds for freshwater fish at and near waste incineration sites for metal reclamation in Wan-Li, southern Taiwan, Republic of China were analyzed for PCDDs and PCDFs by HRGC-HRMS. Four of nine samples were heavily polluted by PCDDs and PCDFs, indicating the latter to be dominant. The total TEQ concentrations of PCDDs and PCDFs in the four samples were in the range of 257 to 12200 pg/g dry weight. In addition, the levels corresponded to those of surface soil samples from the same waste combustion areas.

The structure of layers of triethylene glycol monododecyl ether (C12E3) adsorbed at the air/water interface at four concentrations from the critical micelle concentration (cmc = 5.5 × 10-5 M) down to 1 × 10-6 M have been determined using neutron specular reflection together with isotopic substitution. The distribution of any part of the layer is described in terms of the full width of the distribution at half height, σ; i.e., σ is a measure of the thickness of that part of the layer. The width, σ, of the alkyl chain part of the layer varied from 14 ± 1 A at an area per molecule (A) of 36 A2 down to 10 ± 1.5 A at an A of 89 A2. The corresponding widths of the ethylene glycol region varied from 13 ± 1 to 5 ± 2 A, and the separation of the two chain regions changed from 8 ± 0.5 to 4.5 ± 1 A. The separation of the alkyl chains from the midpoint of the water distribution was found to be 2 A greater than from the ethylene glycol chains at the cmc but exactly the same at 1 × 10-6 M. Isotopic labeling of the two halves of the alkyl chain was used to establish that the alkyl chain is substantially tilted away from the surface normal at all concentrations and that at least one gauche conformation occurs on average in the alkyl chain at the highest surface concentration. The shape of the overall chain distribution is better described by a Gaussian distribution than by a uniform layer. The static displacement of the surfactant molecules in the direction normal to the surface is shown to be strongly disordered at the cmc, the mean displacement being about 9 A, which is of the order of half the projected length of the molecule in the vertical direction. The ethylene glycol chain is oriented close to the normal direction when A = 36 A2 but seems to lie nearly flat in the surface plane when A = 89 A2.

The composition and structure of soluble monolayers of cationic surfactants of alkyltrimethyl ammonium bromide adsorbed at the air-water interface have been studied by the neutron reflection technique. — At the critical micelle concentration (CMC) the area per surfactant molecule was found to be 55 ± 3 A2 for decyl trimethyl ammonium bromide (C10TAB), 48 ± 3 A2 for tetradecyl trimethyl ammonium bromide (C14TAB), and 43 ± 3 A2 for octadecyl trimethyl ammonium bromide (C18TAB). — The determination of the structure of the surfactant layers at their corresponding CMC was based on three differnt isotopic combinations of water and surfactant. The results were analysed using both optical matrix method and the kinematic approach. The thickness of the whole surfactant monolayer was found to be 19 ± 2 A for C10TAB, 22 ± 2 A for C14TAB and C18TAB. The thickness of alkyl chain layer out of water was found to be 11 ± 1 A for C10TAB and 12.5 ± 1 A for the two other surfactants. The amount of alkyl chain immersed in water was found to be 30 – 40%, with shorter chain surfactants being more immersed.

Neutron reflectivity with isotopic substitution has been used to determine directly the structure of triethylene glycol monododecyl ether adsorbed at the air-water interface. The surface excess has been measured from concentrations ranging from twice the critical micelle concentration (cmc) (=5.5 × 10-5 M) to one fiftieth the cmc. The results are in good agreement with those from surface tension measurements and show that the area per molecule tends to a limiting value of 36 Å2. The structure of the adsorbed layer has been determined at four concentrations: cmc, 0.2 cmc, 0.0545 cmc, and 0.0182 cmc. Two different methods of analysis of the data have been employed. One is the optical matrix method which fits a single structural model to the reflectivity profiles from a set of isotopic species at a given concentration and the other is a more direct approach based on the kinematic theory. The two methods give structures identical within experimental error. The variation of the thicknesses of the hydrocarbon chain region of the adsorbed layer and the region where solvent has a density less than its bulk value (approximately described as the head group region) and the relative locations of chain and water distributions across the surface with bulk concentration have been determined. For the saturated monolayer at the cmc the thickness of the chain region is found to be 20 ± 1 Å, that of the head group region to be 11 ± 1 Å, and that of the mean center to center distribution of chains and water to be 10 ± 1 Å. The number of water molecules associated with each surfactant molecule is 6. At the lowest concentration (0.0182 cmc) where the monolayer is at its most dilute, it is found that the thickness of the whole layer is 15 ± 1 Å, that of the head regions is 7 ± 1 Å, and the separation of the two distributions is 5 ± 1 Å, with the number of water molecules per surfactant now about 10. Comparison of these values with the width of the chain and solvent distributions shows that the immersion of the hydrocarbon chains in the water varies from about 25% at the cmc to 40% at 0.0182 cmc. © 1993 American Chemical Society.

The surface excess of sodium dodecyl sulfate (SDS) in aqueous solutions of SDS and the polymer poly-(vinylpyrrolidone) (PVP) has been measured as a function of SDS and PVP concentrations using neutron reflection. Below the critical aggregation concentration (CAC) the adsorption of SDS is increased by the presence of PVP, indicating that the two components interact cooperatively at the surface. Between the CAC and the critical micelle concentration (CMC) of the surfactant there is a slight depletion of SDS from the surface. Comparison of coverages determined by neutron reflection with those from earlier radiotracer work indicates that, in the higher concentration range, PVP is bound to the surfactant layer, creating a region from which surfactant is depleted, which is further evidence for a strong polymer/surfactant interaction at the surface. Comparison of the effect of added PVP on the surface tension with the neutron reflection measurements indicates that, even below the CAC, the surfactant complexes to the polymer to some extent in the bulk solution. There are no measurable effects of the polymer on the thickness of the surfactant layer at any concentration. There is an indication that at the surface the surfactant is slightly displaced outward from water on addition of polymer, but accurate structural determination of the mixed layer proved too difficult to be certain of this result.