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We have determined the structure of a layer of tetradecyltrimethylammonium bromide (C14TAB) adsorbed from solution at the air/water interface using neutron specular reflection data from several isotopic compositions. Two different methods of analysis of the data have been used, one fitting a single structural model to the set of reflectivity profies at a given concentration, and the other using an approximate but more direct method. The relative locations of the chain, head, and water distributions across the interface have been determined directly. For the saturated monolayer Gust below the critical micelle concentration (cmc)) the mean center-to-center distribution of chains and heads is found to be 7 f 0.5 A, that between chains and water to be 7 f 0.5 A, and that between heads and water 1 f 0.5 A. The mean thickness of the different regions is model dependent.The chain region is found to have a thickness of 17.5 f 1 A for a uniform layer model and 16 f 1 A for a Gaussian distribution (at l / e of the height), both less than the fully extended chain length. The corresponding values for the heads are 7 f 3 (uniform layer) and 6 f 3 A. At about one-third the cmc the chain region becomes thinner (Gaussian width = 12 f 2 A) and the mean separation of chains and water decreases to 6 A. Above the cmc the surfactant is more closely packed and the thickness of the head-group distribution increases to 12 f 3 A (Gaussian profile). This is attributed to a "roughening" of the head-group part of the layer. htroductionIn this paper we apply the recently developed technique of neutron reflection to the study of the structure of layer of the surfactant tetradecyltrimethylammonium bromide (C14TAB) adsorbed at the air/water interface.An important feature of neutron reflection is the use of different isotopic species to establish a structure. This often depends on the assumption that isotopic substitution does not affect the chemical and physical properties of a substance. We have already established in a previous paper that the interfacial properties of C14TAB are not affected by deuterium substitution, and we have compared some of the results obtained from neutron reflection with surface tension measurements.' We now use isotopic substitution to determine the structure of the layer, and we also analyze in some detail how best to determine structure from reflection measurements. Experimental DetailsFour isotopic species of tetradecyltrimethylammonium bromide were used in the experiments, CI4D2&J(CD3),Br, C14DB(CH3),Br, C14H29N(CD3)3Br, and C14H29N(CH3)3Brr which we refer to as dC ,,dTAB, dC I &TAB, hC ,PTAB, and hC ,hTAB respectively. They were the same samples described in ref 1, where a full description of the preparation, purification, and characterization is given. High-purity water was used throughout (Elga Ultrapure system) and the methods of cleaning the glassware and Teflon troughs for the neutron experiment are also described elsewhere?The neutron reflection measurements were made on the reflectometer CRISP at the Ruth...

The surface adsorption behavior and solution aggregate microstructure of the dichain cationic surfactant dihexadecyl dimethylammonium bromide (DHDAB) have been studied using small angle neutron scattering (SANS), light scattering, neutron reflectivity (NR), and surface tension (ST). Using a combination of surface tension and neutron reflectivity, the DHDAB equilibrium surface excess at saturation adsorption has been measured as 2.60 +/- 0.05 x 10 (-10) mol.cm (-2). The values obtained by both methods are in good agreement and are consistent with the values reported for other dialkyl chain surfactants. The critical aggregation concentration (CAC) values obtained from both methods (NR and ST) are also in good agreement, with a mean value for the CAC of 4 +/- 2 x 10 (-5) M. The surface equilibrium is relatively slow, and this is attributed to monomer depletion in the near surface region, as a consequence of the long monomer residence times in the surfactant aggregates. The solution aggregate morphology has been determined using a combination of SANS, dynamic light scattering (DLS), cryogenic transmission electron microscopy (CryoTEM), and ultrasmall angle neutron scattering (USANS). Within the concentration range 1.5-80 mM, the aggregates are in the form of bilamellar vesicles with a lamellar " d-spacing" of the order of 900 A. The vesicles are relatively polydisperse with a particle size in the range 2000-4000 A. Above 80 mM, the bilamellar vesicles coexist with an additional L beta lamellar phase.

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 X lCh8 M) down to 1 X "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, *; i.e. * is a measure of the thickness of that part of the layer. The width, *, of the alkyl chain part of the layer varied from 14 ± 1A at an area per molecule (A) of 36 A* 12 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 X 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 ______is oriented close to the normal direction when plane when A = 89 A2.

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