Waxy phases

Figure 18-4. Phase diagram for the system lithium stearate-white oil. A Crystalline lithium stearate I. B Crystalline lithium stearate II. C Waxy phase. D Liquid crystal phase. E Isotropic solution. Data by D. B. Cox [7]. ...

Broad-line nuclear magnetic resonance has been used to study melting in stearic acid and a mesomorphic crystalline to waxy) phase transition in lithium stearate. Extensive motion, liquidlike, though less extensive than that in an isotropic free-flowing liquid, takes place within the system below the melting point of stearic acid or the crystalline to waxy phase transition of lithium stearate. The amount of liquid-like character, as measured by the intensity of a narrow component in the NMR spectrum relative to the total intensity of the whole spectrum, depends on the presence of impurities in the system and even more significantly on whether and how many times the sample has been melted. 

The question arises as to what the behavior is if the lithium stearate is heated only through the crystalline to waxy phase transition and then... 

Figure 9. Line widths of waxy phases of anhydrous NaS, NaP, NaM, and NaL as function of temperature (16)...

The order of the line widths of the waxy phases deserves some comment. There is some correlation between the line widths and the lengths of the hydrocarbon portion of the molecules. With perhaps the exception of NaL, the line widths increase as the chains become shorter. At most temperatures the order of the line widths is NaM > NaL > NaP > NaS. It is expected that a longer chain will be more mobile and thus have a sharper line. The order of the line widths in a given soap is without exception subwaxy > waxy > superwaxy > subneat the neat phase is not found in the temperature range covered in this study. As expected, the line widths decrease with increasing temperature. 

In Table II representative second moments of the various waxy phases are tabulated (17) along with the temperatures at which they were determined. In most cases the determinations were made in the center of the range of existence of the particular phase. Second moments were not determined at every temperature at which a line width measurement was made. The second moments are in the same order as the line widths within a given soap, but there appears to be little correlation between the chain lengths and the magnitude of the moments. For the most part it appears that the values of the second moments depend upon the temperature. 

In Table V some experimental second moments from several neat and middle phases are shown. Although the accuracy of the second moment measurements is not great, mainly because the wide wings in the curves make it difficult to determine where to truncate, it is obvious that the moments from the neat phases are larger than those from the middle phases. This again is reasonable since the molecules in the neat phase are expected to be more closely packed and to enjoy less freedom of movement. The moments from the neat phases are nearly the same in magnitude as the moments from the waxy phases (Table II). This is consistent with their structural similarity. 

The unique super-Lorentzian line shapes found in the lyotropic neat and middle phases can be explained in terms of a distribution of correlation times of the surfactant chain protons similar to the conditions in the waxy phases (17). This explanation can be related to the structures proposed by Luzzati (22) for these phases, shown in Figure 11, if it is assumed that the chain protons near the hydrophilic groups are more restricted in their motion than those near the ends of the hydrocarbon chains. Several lines of evidence support this hypothesis. 

Amphiphilic molecules represent a vast resource for condis crystals. It is easy to establish long-range order in one of the two components of the molecule that can tolerate higher temperatures than the crystal of the other component. At higher temperature one component can then become mobile and conformationally disordered before the other, but is hindered to take on orientational and positional disorder due to attachment to the thermally more stable crystal. The waxy phases of soaj , many high temperature phases of long chain carboxylates and substituted aimnonium salts as well as lipids are representative of such condis crystals. 

A substantial amount of work has been carried out on thermotropic transitions of anhydrous soaps and other surfactants using high-temperature XRD and NMR. The mobility of the hydrocarbon chains increases with temperature, which leads to the formation of mobile solid phases (waxy phases) and liquid aystalline phases [27-29], These transitions have very limited application in cleansing products. Some of these transitions take place during processing of soap at elevated temperatures. 

Note At WPS (waxy phase sqtaration) of the total multiple emulsions the liquid state at 40°C becomes solid at 20°C. 

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