Small angle neutron scattering studies on micellar systems part 1. Ammonium octanoate,ammonium decanoate and ammonium perfluoroocatanoate

Small angle neutron scattering has been used to examine the size and shape of micelles of ammonium octanoate, ammonium decanoate and ammonium perfluoro-octanoate. Ammonium octanoate was found to form micelles with a micellar weight of 1640 and ammonium decanoate with a micellar weight of 12,576; both materials appeared to form spherical micelles. Ammonium perfluoro-octanoate formed micelles with a micellar weight of 17,610. Evidence from the scattering experiments suggested that the micelles were cylindrical and a model for the micelle is proposed.     

Structure of casein micelles I. Small angle neutron scattering and light scattering fromβ- andχ-casein

Casein is the main protein component of milk and is of remarkable colloidal stability. Under the influence of milk clotting enzymes casein shows the striking behaviour of coagulation. This clotting process has already been studied by other groups, neglecting the fact that casein is not a homogeneous protein. The purpose of the present study is focused, in this first stage, on the determination of the structure of the various casein components. In cooperation with other laboratories we have been able to obtain the well separated individual proteins. Studies have been performed so far with- and-casein. For detailed structural information we carried out small angle neutron scattering and combined static and dynamic light scattering measurements and determined the molecular weight,M _w, the radius of gyration, S ² the hydrodynamic radius,R _H, the-value and the particle scattering factor, P_z(q). The two caseins show a strikingly different behaviour. For the-casein we found a star-like structure, i. e. an aggregation pattern that is expected for a common micelle. The micelle consists of about 38 monomer chains. The aggregates of-casein appear to be composed of star-like submicelles, where each submicelle contains nine-casein chains and the total degree of aggregation is about 140.     

Small-angle neutron scattering on shear-induced micellar structures

Small-angle neutron scattering (SANS) experiments on sheared aqueous surfactant solutions of tetradecyltrimethylammoniumsalicylate (TTMA-Sal) are reported. A5-mM-solution without shear shows a weak correlation peak at a momentum transfer of 0.09 nm^–1 which has its origin in the micellar interaction. For shear rates above a threshold value of =40 s^–1 the scattering pattern shows an irregular increase in anisotropy. The analysis of the anisotropic pattern reveals the existence of two types of micelles: Small rodlike micelles which are weakly aligned and very large rodlike aggregates which are strongly aligned and which are present above the threshold value of. The two micelles are in equilibrium with each other and the equilibrium shifts with increasing shear rate to the side of the large oriented micelles.     

Physical gels from PVC: molecular structure of pregels and gels by low-angle neutron scattering

In this paper we report on investigations into the short-range molecular structure of pregels (aggregates formed in dilute solutions) and gels as a function of ageing time, temperature and solvent type. In some solvents (diethyl oxalate and bromobenzene) it is found that the structure of the pregels is cylinder-like or ellipsoid-like. The structure is seen to be the same for the gels and the pregels in the investigated range of transfer momentum which leads us to conclude that the gels are probably fiber-like, a statement in agreement with the morphology observed by Yang and Geil. The fiber section is also about the same order of magnitude as that reported by these authors. Also, further evidence is given for the existence of a second type of physical knot, different from those formed by the more syndiotactic sequences. The validity of the molecular structures deduced from these experiments is discussed in the light of macroscopic properties such as compression modulus and swelling.     

Small-angle neutron scattering of colloidal silica dispersions in a non-polar solvent

Small-angle neutron scattering studies were performed on dilute dispersions of colloidal silica spheres in mixtures of h₁₂- and d₁₂-cyclohexane. The particles consisted of a SiO₂-core and a layer of stearyl alcohol molecules terminally attached with a chemical bond (Si-O) to the particle surface. The contrast variation method was applied to reveal the internal structure of the particles. The matchpoints determined with this method were in accordance with those calculated from the mass density of the particles and the atomic composition, as determined with elemental analysis.For a detailed interpretation of the scattering curves, we assumed that the particles were spherosymmetrical and consisted of two concentric layers. With the relation we derived between the radius of gyration and the reciprocal contrast for such a model, it was possible to determine all the parameters characterizing the particle in terms of this model. The model calculations performed using these parameters fitted very well to the experimental intensities for high contrasts. For lower contrasts, the fit was somewhat less good. This is probably due to random fluctuations in the scattering length density within the particle core.The different radii as found by neutron scattering, agreed very well with those determined using other techniques, such as light scattering (static and dynamic) and electron microscopy.     

Shear-induced deformation of polystyrene coils in dilute solution from small angle neutron scattering 2. Variation of shear gradient,molecular mass and solvent viscosity

The technique of small angle neutron scattering (SANS) has been used to study the conformation of polystyrene chains in dilute solution under a constant shear gradient. The experiments reveal a distinct anisotropy of the molecular dimensions with regard to the directions parallel and perpendicular to the flow direction on the 2D-multidetector. The deformation ratio of the single polymer chain (R ²/R _iso ² )–1 as a function of the reduced shear gradient=([] · · M _w G)/RT shows a transition from the ideal ²-behaviour for dynamic infinitely flexible coils found at small gradients, to a behaviour with smaller increase at larger. These results are qualitatively consistent with the theory of Cerf for a polymer with finite internal viscosity in a shear gradient. At low(<1), a better agreement with the model of a free-draining coil (Rouse behaviour) than with the Zimm model is observed.     

Small angle X-ray scattering study of an extremely elongated micelle system of CTAB-p-toluidine solution

Whenp-toluidine is added to an aqueous solution of CTAB, a remarkable increase of viscosity is accompanied by a spectacular elasticity. We detected the existence of extremely elongated rod-like micelles in electron micrographs. SAXS measurements indicate a closely packed array of cylindrical rod-like micelles, brought about when solutions flow through a thin capillary. A scattering maximum ofd=160 Å almost corresponds to the distance between the nearest neighbours of the cylindrical rod-like micelles. This value agrees with the diameter measured on electron micrographs. The second broad peak (d=75 Å) is assigned to a subsidiary maximum of the shape function of the cylinder with infinite length.     

Counter ion condensation on mixed cationic/nonionic micellar systems: Bjerrum's electrostatic condition

The association of counter-ions with mixed ionic/nonionic micelles has been investigated in the case of dodecyl/tetradecyl/ and hexadecyl-trimethylammonium bromide with two nonionic surfactants: dodecylpolyoxyethylene 23 and Triton X-100. The degree of association has been measured by potentiometry using a Bromide ion-selective electrode. Previous results with sodium and copper dodecylsulfate suggesting that in the nonionic-rich composition domain, bare mixed micelles are formed without associated counter-ions have been confirmed. These results are in agreement with the prediction of Bjerrum's condition for ion association. The effect of copper dodecylsulfate on the cloud point of Triton X-100 has also been determined as a means of investigating mixed micelles with multivalent counter-ions. The dramatic cloud point increase observed, even larger than with sodium dodecylsulfate, has been discussed as evidence of the solvation of divalent ions by ether groups, a factor which complicates the analysis of multivalent counterion condensation on mixed micelles.     

Mixed micelles of tetradecyltrimethylammonium bromide and chlorhexidine digluconate in aqueous solutions of isopropyl alcohol

The influence of isopropyl alcohol (IPA) on the size and composition of the mixed micelles in mixtures of tetradecyltrimethylammonium bromide (TTAB) and chlorhexidine digluconate (CG) has been determined as a function of the composition of the systems. The addition of 0.5 M and 1.0 M IPA had little significant effect on the composition of the mixed micelles as determined both by analysis of critical micelle concentration (CMC) data using a theoretical treatment based on excess thermodynamic quantities and by an empirical treatment of conductivity data. Static and quasielastic light scattering measurements showed a progressive decrease of the aggregation number and hydrodynamic radius of TTAB micelles on addition of IPA, but minimal changes in the properties of the small CG aggregates. The results show that the micellar weight in the TTAB/CG/IPA solutions is determined by the ratio of the surfactants in the system and for each TTAB/CG ratio decreases on addition of IPA.     

Spinodal decomposition of the polymer blend deutereous polystyrene (d-PS) and polyvinylmethylether (PVME) studied with high resolution neutron small angle scattering

Decomposition of a polymer blend in the miscibility gap has been studied by small angle scattering of neutrons, using a double crystal diffractometer. It covers a range of 10^–5 to 10^–3 Å^–1, which connects the region of elastic light scattering and, at the upper end, the region of conventional neutron small angle scattering instruments. The decomposed structure in the miscibility gap can be described by a characteristic lengthR which increases with time,t ^1/3 between 7 and 25 min. At longer times, the growth follows a more linear power law. This behaviour is consistent with predictions of the domain growth for liquid mixtures and is estimated by using the self diffusion constant, viscosity, and the surface energy of the polymer.on leave ESFM-IPN Mexico     

Determination of the structure factor for concentrated silica dispersions using small angle x-ray scattering I. Simulation

The structure factor of a concentrated colloidal suspension is an important means in the characterization of the interaction forces between the colloidal particles. It can, in principle, be determined with small angle x-ray scattering. To avoid unacceptably long measuring times, one has to use a high power x-ray source or a slit collimation camera. The first is not readily accessible because of the very high costs. The latter is available in many laboratories, but here the fundamental information is contained in the data in a complicated way. A so-called desmearing operation is needed to reveal this information. Because of the different experimental errors and their sensitivity to the desmearing, the accuracy of the structure factor will be rather limited. In this paper we simulate the experimental errors separately to check their influence in combination with the desmearing.Although the overall accuracy is limited some important features can be determined. The value atK=0 and thus the osmotic compressibility can be calculated, and the position and the height of the first maximum in the structure factor are quite reliable too. This gives some insight in the type of interaction and the influence of polydispersity.Special attention should be given to the determination of the form factor by using extra long measuring times for the very diluted sample, this will improve the overall accurary.     

Small-angle neutron scattering study of the effect of n -alkanols on interacting micelles

Small-angle neutron scattering cross-section distributions of sodium dodecyl sulphate (SDS) and dodecyl trimethyl ammonium bromide (DTAB), each 0·3 M in D₂O were obtained in the absence and presence of 0·1 M 1-pentanol, 1-hexanol, and 1-octanol at 25°C. The Hayter-Penfold type analysis was adopted. An ellipsoidal model with semiminor axis (a=16·5 ?) and semimajor axes (b=40·7 ? and 29·8 ?) for pure SDS and DTAB micelles has produced best fits. On increasing alkanol chain lengths an increase inb values was found. Micellar parameters like effective radius (R), (a, b), fraction of counterions per micelle, and intermicellar distances were obtained. Surfactant aggregation number, additive aggregation number intermicellar interaction potentials and values of Debye screening length were obtained for SDS and DTAB in the presence of alkanols. Implications of partitioning effect, surfactant ionicity and intermicellar potentials on the microstructures are rationalised.     

Small angle neutron scattering studies on non-aqueous dispersions of calcium carbonate. Part III. Concentrated dispersions

Small angle neutron scattering has been used to examine calcium carbonate dispersions in toluene over the w/w concentration range 5 to 40 %. The particles were stabilised by a combination of a surface active agent and a linear polymeric molecule. Analysis of the experimental data gave results for the structure factorS(Q) and the pair correlation functiong(r).Theoretical computations of the structure factor were made using a hard sphere model and the mean spherical approximation approach. These analyses indicated that the interaction was essentially soft at low volume fractions but became hard at the higher volume fractions.     

Adsorption of fish oil and poly(vinylbutyral) onto aluminum oxide particles studied by light scattering

The effects of added poly(vinylbutyral) (PVB) and oxidized Menhaden fish oil (O-FO) on dilute suspensions of Al₂O₃ in the azeotropic solvent mixture 73/27 Wt.-% trichloroethylene/ethanol were studied by static (SLS) and dynamic light-scattering (DLS). The DLS correlation function consisted of four terms from: 1) interdiffusion of the two solvents, 2) diffusion of Al₂O₃, 3) diffusion of added O-FO and/or PVB, and 4) an induced velocity of particles through the scattering volume due to laser pressure. The amplitude and relaxation time of all terms could be determined. Addition of PVB to the Al₂O₃ suspension resulted in an increase in particle size corresponding to the adsorption of a monolayer of coils. Addition of O-FO to the Al₂O₃ suspension did not produce a significant increase in particle size, indicating either no adsorption or a compact conformation of O-FO molecules on the Al₂O₃ surface.     

Catalase monolayers at the air/water interface I. Reversibilities and irreversibilities

/A-isotherms of catalase monolayers established at the air/water-interface are discussed quantitatively on the basis of molecular data: A relationship between a critical value of the surface pressure, the corresponding molecular area, and the molecular dimension of the molecules at the interface is proposed. It is shown that the unfolding of molecules at the water surface is pH-dependent. For each pH-value there is a distinct degree of unfolding; the molecules keep their globular state at neutral pH. Establishment at the surface of bulk solutions corresponding to globular and partly unfolded states, respectively, catalase molecules keep their original configuration on changing the pH-value of bulk-phase. The monolayers are confirmed to show reversibility with regard to lateral changes of state as well as irreversibility with respect to desorption of molecules.A model is proposed to explain the nature of the critical/A-value occurring in the/A-isotherms: on compression beyond _c, molecular segments are transferred from the surface into the bulkphase via a subsurface layer. From the experiments it is concluded that the surface pressure is determined, not only by the surface itself, but also by this subsurface layer.     

Effect of polymerized silicic acid on mixed lipid-protein monolayers used as cell-membrane models II. Pepsin-sphingomyelin films

Mixed pepsin-sphingomyelin films exhibit behavior typical of bidimensional systems whose components are miscible in all proportions. The non-polar chains of their components are believed to establish weak hydrophobic interactions. The dynamic compression-expansion II-A curves obtained in the presence of polysilicic acid differ from those recorded in its absence, which may arise from an ionic interaction whereby silicate ions accommodate themselves among the film molecules and bind strongly to the sphingomyelin molecules to form a rigid surface network in which the protein molecules are trapped.     

The dynamik contact angle II. Investigation of the adsorption mechanism based on the measurement of the preceding contact angles

The structure of the adsorption layer at the solid/gas interface is characterized, as a function of conditioning concentration, by the measurement of preceding contact angles. The contact angles were determined tensiometrically (plate method) and cinema tographically (capillary rise method) in the system glass or mercury/n-dodecyl ammonium chloride solution/air, respectively. In the dependence of contact angle on concentration, four regions are provable. These regions correlate with the surfactants, which are bound to adsorption in a heteropolar mode or by van der Waals forces of interaction, with the formation of layer-like coverage and with bilayers. Special attention was given to the fact that loosely bound surfactants are transferred from the solid/gas interface to the liquid/gas interface and cause a reduction of the preceding contact angle.Publication No. 1077 from the Research Institut of Mineral Processing, Academy of Sciences of the GDR, Freiberg, G.D.R.     

Small-angle x-ray scattering by crystalline polymer fibers 2. Investigation of the linear paracrystalline model using various materials

The linear position-sensitive detector is well-suited to measure quantitatively the distribution parallel to the fibre axis of the intensity of small-angle x-ray scattering (SAXS) by polymer fibres, except that in the case of four-point patterns their width is greater than that of the detector window. A method is described which overcomes this problem, and which has high angular resolution. Using this method, the variation of scattered intensity with angles from 0.3° to 2.5° has been measured for fibres of poly(ethylene terephthalate), nylon, and low density poly(ethylene) (LDPE), and compared with that predicted by the linear paracrystalline model. In all cases except LDPE, when the distribution of phase lengths was given by the Reinhold function, there was no significant disagreement between the measured and predicted scattering except for a very small range of angles on the low angle side of the peak intensity. With LDPE small but significant discrepancies were found at other angles as well, and these were worse if the symmetrical Gaussian distribution function was used. The method enabled quantitative parameters describing the morphology to be obtained. It is concluded that the morphology of the linear paracrystalline stack is consistent with the SAXS intensity distribution, and that the Reinhold function is a reasonable approximation to the distribution of phase lengths. A small modification so that this decays more rapidly at long lengths might be necessary to explain the scattering for all materials over the entire angular range and other small changes might be needed with LDPE, although the asymmetrical nature of the distribution must be retained.On leave from Department of Physics, University of Technology Malaysia, 81300 Sekudai, Malaysia.     

Formamide,a water substitute XIV (1) waterless microemulsions 8. Structural analysis by X-ray scattering of CTAB aggregates in formamide and in the microemulsion system (formamide,CTAB, isooctane, 1-butanol)

X-ray scattering was used to analyse the structures of aggregates of CTAB in various formamide-containing media.Firstly, CTAB micelles were detected in formamide solutions. A spherical micelle model was proposed; at values close to the CMC, smaller micelles were observed in formamide than in water.Secondly, the structure of microemulsions in the isooctane-rich zone of the system (formamide/CTAB=2, isooctane, 1-butanol) was studied by X-ray scattering. The structure was found to be quite different in this zone from that normally observed in aqueous microemulsions. Structures compatible with small filaments of formamide were detected. The results were related to previous measurements of electrical conductivity.     

The dynamic contact angle I. Dependence of the receding contact angle on velocity in the surfactant-containing three-phase system

Spontaneous three-phase contact (tpc) motion is investigated in order to determine the dependence of the static contact angle on tpc velocity in surfactant-containing systems after recession. To interpret the experimental results, the molecular-kinetic sitechanging theory and the hydrodynamic theory were considered. It is shown that, especially at very high tpc velocities, the experimental results are not thoroughly described by these theories. The deviations are explained as a surfactant transfer from the liquid/gas to the solid/gas interface which, under insufficient afterdiffusion, leads to an increase in surface tension and to a changed surface rheology. This mechanism could be governed by a model.