Surfactant crystals

The purpose of employing various light incidents when using a microscope was to distinguish between different crystals such as mineral crystals (e.g., clays, silicons, NaOH, etc.) from surfactant crystals (30-33). It was found that under transmittant light only bituminous hydrocarbons were non-transparent (absorb light) and appeared either... 

Fig. 3 The model of equilibrium spreading from a surfactant crystal at the air/water interface. The ESP is attained when crystal, monolayer, and subphase solution are at equilibrium.

Figure 1. Photomicrograph (XlOO), with crossed polarizers, of liquid crystals produced by equilibrating dry surfactant crystals with 0.3 wt % aqueous solution of NaCl between two glass slides. The appearance is the same at NaCl concentrations from 0 to 1 wt %.

The formation of surfactant crystals (i.e. liquid crystals) at the oil-aqueous interface can be easily determined by the use of polarized light microscopy (35). 

The independence of the maximum adsorption (or the lowest possible area per molecule in the dense adsorption layer. smin = 5,) of the surfactant chain length can only be explained if we assume that as the adsorption values reach Tmax, the surfactant molecules are closely packed and oriented normal to the surface. Estimates of the limiting values of adsorption, Tmax b / R T, from the experimental a(c) dependencies, with the successive evaluation of minimum area per molecule, s, = 1/NA Tmax, for carboxylic acids, are -0.21 nm2, which agrees with the values established by other methods, e.g. by X- ray diffraction on surfactant crystals. 

Due to micelle formation the total surfactant concentration undergoes an abrupt increase. Since true (molecular) solubility of surfactants, determined by the CMC, remains essentially constant, an increased surfactant concentration in solution is caused by an increase in a number of formed micelles. Micellar solubility increases with increase in temperature, and thus a continuous transition from pure solvent and true solution to micellar solution, and further to different liquid crystalline systems and swollen surfactant crystals (see below), may take place in the vicinity of the Krafft point. 

One may also consider a reverse transition from macroscopic heterogeneous system (surfactant crystals in water) to micellar solutions existing above the Krafft point, via gel formation and its spontaneous dispersion stages. In this case, the swelling of soap upon the penetration of water between the lamella formed with polar ( strongly hydrated) groups occurs prior to the formation of colloidal solution. At sufficient dilutions separation of individual particles, e.g. lamella, from crystals occurs due to the... 

They also studied the ternary phase diagram of the divalent surfactant dipotassium dodecylphosphate (K2D0P), monovalent surfactant potassium tetradecanoate (KTD) and D2O [155]. At 25 °C K2D0P exhibits L, (0-37% surfactant), I, (37-67%), H, (67-75%) and two phase liquid crystal/ hydrated surfactant crystal (75-100%). In the ternary system I cubic is seen between 20 and 60% K2D0P. Up to 30% KTD ean be incorporated into the phase at lower eon-... 

Some care is needed to preserve the column performance for long time periods of intensive MLC use, which can be comparable or even longer than in conventional RPLC. First, since most micellar solutions are able to dissolve minute amounts of silica, the mobile phase should be saturated in silica by placing a short precolumn before the injection valve. Second, the micellar solution should never stay motionless in a chromatographic system to avoid the formation of surfactant crystals that can clog the... 

The Krafft point is the temperature at which the solubility of hydrated surfactant crystals increases sharply with increasing temperature and forming micelles. This increase is so sharp that the solid hydrate dissolution temperature is essentially independent of concentration above the critical micelle concentration (cmc) and is therefore often called the Krafft point without specifying the surfactant concentration. The steep increase in solubility above the sharp bend is caused by micelle formation. Micelles exist only at the temperature designated as the Krafft point. This is a triple point at which surfactant mole-... 

We are interested here only in phase transitions arising solely from the action of thermal energy on single- and double-chain surfactant crystals (synthetic bilayer-forming amphiphiles) and in a novel class of surfactants—catanionic surfactants, which are composed of oppositely charged ionic single-chain surfactants. We do not discuss the vast field of molecules of primarily amphiphilic character such as lipids, proteins, and copolymers. 

A lamellar structure is usually found in crystals of single-chain surfactants [20]. The unit cells of single-chain surfactant crystals typically contain either a pair of molecules or a small number of pairs. In the case of an ionic surfactant, one or more types of ion pairs are found [21]. X-ray diffraction patterns are typical, with long spacing in the ratio 1 1/2 1/3 1/4 characteristic of a lamellar structure. 

Filipovic-Vincekovic and Tomasic (Croatia) have contributed Chapter 12, Solid-State Transitions of Surfactant Crystals, which discusses the effect of surfactants on the crystallization of materials in aqueous and nonaqueous solutions. The chapter describes the crystalline structure of surfactant and its thermal... 

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