The microstructure of soap structured gels consists of Na-soap crystals, organized in a fiber and/or ribbon structure. The dimensions of these micro structural entities depend on the soap chain length, the presence of surfactants and solvents. C16/C18 soap gives in the absence of surfactants soap ribbons or fibers having a width, dependent on the water/solvent ratio, in the range around 20-100 nm with crystal repeating distances of 4.5 nm. So the crystal domains are 4-20 crystal layers. In the presence of a large amount of synthetic surfactant mixture (~ 60% w/w), very fine soap fibers (between 10 and 20 nm) are found with crystal repeating distances of 3.3 nm. Being so thin, it is not expected to detect an X-ray diffraction line of the soap. It is hypothesised that the ribbons are curled up to fibers of a hollow tube structure. Due to the “curling-up”, the soap molecules are orientated in the direction of the soap fibres resulting in a manifold repetition of the bi-layer structure and a diffraction signal is measured. In this study gels with different soap chain lengths, C8, C12, C16, Pristerene (mixture C16/C18), C22 and a reference without soap are made. In addition, temperature is an important parameter to obtain knowledge about the crystallisation behaviour of crystalline soap structures at a specific temperature.

The X-ray diffraction measurements are performed on the Bruker D8-Discover in a theta/theta configuration. A cupper anode is used and the K-alpha radiation with wavelength 0.15418 nm is selected. Long spacings are measured by performing X-ray diffraction in the transmission mode. The X-ray source and the detector are positioned in front of each other (Theta 1 and Theta 2 = 0 degrees). To prevent the detector being hit by the primary beam a leaden beam stopper is precisely positioned in the middle and just in front of the detector. The sample is placed in a Linkam temperature stage between X-ray Mylar film.

The X-ray diffraction pattern of the blank reference without soap is compared with the pattern of the Pristerene soap structured gel. A big overlap between the X-ray diffraction patterns of the blank system, showing a surfactant molecular arrangement (“ordered” L2-structure) and Pristerene is visible. Due to the absence of the theoretical expected d-value of 4.5 nm for Pristerene soap and knowing the fact that the soap fibers are very thin (10-20 nm measured with SEM), meaning less than 20 repeating layers, the question arose whether it is possible to measure soap fibres with X-ray diffraction. A way to prove this is to use longer or shorter soap chain lengths, which can possibly establish more separation between the diffraction lines of the soap and the surfactants.

It is found that very thin (10-20 nm) molecular soap arrangements in soap structured gels containing surfactants can be measured with X-ray diffraction. This is unusual, but measurable due to our hypothesis that the repeating units are in the length direction of the fiber. A large change in soap-crystal morphology and soap-crystal network structure between C12 and C16 and longer is detected with different analytical techniques in systems with large amounts of surfactants: a change in tilt-angle (XRD) and a change in the morphology of the soap-crystal network from coarse ribbons to fine fibers (SEM). The surfactants arrangement in the gel systems still exists at 80°C, while the soap is already dissolved.

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