Variables Affecting Phase Change in Lithium Stearate–Mineral Oil

Variables Affecting Phase Change in Lithium Stearate–Mineral Oil Systems. David B. Cox. J. Phys. Chem. , 1958, 62 (10), pp 1254–1256. DOI: 10.1021...
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1254

DAVIDB. Cox

Vol. 62

VARIABLES AFFECTING PHASE CHANGE IN LITHIUM STEARATEMINERAL OIL SYSTEMS BY DAVIDB. Cox Technical Service Laboratory, Socony Mobil Oil Company, Inc., Brooklyn, N . Y . Received March 9, 1068

Equilibrium phase diagrams have been determined using thermal techniques for pure lithium stearate in six mineral oils and a paraffin wax. The new diagrams are compared to the previous phase diagram for lithium stearate-n-hexadecane. A correlation was found between equilibrium phase change temperatures and the molecular weight of the hydrocarbons. The phase change temperatures were relatively insensitive to differences in molecular shapes and types in the hydrocarbons. A new structure is postulated for the liquid crystal phase.

Two previous phase change studies of soaphydrocarbon systems have been reported in which a single soap was studied in a series of hydrocarbons. Smith and McBainl found that sodium stearate gave the same phase behavior in toluene, cyclohexane, isooctane, and related light hydrocarbons despite their varying structures. Vold and Vold2gave phase diagrams for lithium stearate in n-hexadecane and in decalin showing distinct differences between the two systems, They also compared their two model systems with a few data on technical lithium stearate in some mineral oils, and they concluded that lithium stearate-nhexadecane most nearly resembled the lithium stearate-oil behavior.

termined3 lithium stearate%-hexadecane system, for the sake of comparison. Diagrams for the lithium stearate systems containing Oils C, D, and E and the paraffin wax were closely similar to those for the systems containing Oils A and B. Since all of the hydrocarbons used in this study are mixtures, the use of two-component phase diagrams is arbitrary. It was found experimentally, however, that most of the phase transitions were as rapid and well defined as if the hydrocarbon were a single component. DTA thermograms of the soap-oil systems showed peaks of the same sharpness as those for lithium stearate-nhexadecane. In the diagrams for the oil-containing systems, however, there is a greater order of un-

TABLE I Mineral oil

Wax A B C

D E F

Type and origin

Fully-refined paraffin wax, map., 59-60' Non-aromatic white oil, mid-continent Non-aromatic white oil, coastal Solvent-refined coastal oil Acid-treated naphthenic oil Solvent-refined coastal oil Naphthenic bright stock

Viscosity, centistokes 37.8' 98.9'

... 28.7 81.3 112.0 171.4 194.7 990.3

The present study was made in order to help bridge the gap between studies of model soaphydrocarbon systems and more complex, multicomponent systems such as soap-thickened lubrieating greases. Experimental The techniques used to obtain data were differential thermal analysis (DTA) and visual observation in a microscope hot stage. The apparatus and procedures have been described.3 The lithium stearate was also the same used in that work. Physical characteristics of the six mineral oils used are given in Table I. Average molecular weights of the oils were obtained by benzene boiling point elevation (modified Menzies-Wright method). The carbon-type compositions Of these oils were obtained by the graphical method of Kurtz, et al.,4 and are shown in Table 11. The paraffin wax included in these two tables had been analyzed by a mass spectrometer, and its average molecular weight was based on the weighted average number of carbon atoms per molecule (27). It consisted mainly of straight-chain paraffins.

4.00 4.96 8.39 10.0 10.7 10.9 27.2

Av. mol. dPQi

?&POD

wt.

..

..

0.8588 .8846 ,8978 .9265 .9332 .9649

1.4722 1.4828 1.4922 1.5106 1.5113 1,5210

385 393 440 462 422 385 526

TABLE I1 Mineral oil

% CP5

% CNa

% C.4

Wax A B C

98 2 0 64 35 0 56 45 0 55 42 3 D 47 37 16 E 41 45 14 F 22 67 11 5 Percentage of carbon atoms in paraffinic structures. * Percentage of carbon atoms in naphthenic (saturated cyclic) structures. 'Percentage of carbon atoms in aromatic structures.

certainty about the position of boundaries like those indicated by broken lines in Fig. 1. These are not indicated and have been Omitted for the most part. In Some experimental samples, two transitions occurred tooclose together for resolution by DTA, Results on heating. I n such cases, the number and temEquilibrium phase diagrams for Some of the sys- peratures of the transitions was determined G S U tems studied are given in Figs. 2 to 4. Figure 1 ally withthemicrosCoPehotstage. gives the corresponding data for the previously deDiscussion of Results (1) G. Ha Smith and J. W. McBain, THISJOURNAL, 51,1189 (1947). Equilibrium Data.-In each of the lithium &,ea(2) M. J. Vold and R. D. Vold, J . Colloid Sci., 5, 1 (1950). ratehydrocarbon systems studied, the number and (3) D. B. COXand J. F. McGlynn, A n d . C h s n . , 29,960 (1957). nature of the phases found is remarkably similar. (4) Kurtz, et at., {bid., 28. 1928 (1956).

PHASE CHANGE IN LITHIUM STEARATE-MINERAL OIL SYSTEMS

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1256

DAVIDB. Cox

Variables Affecting Phase Change. Despite the genera1 similarity between the various lithium stearate-oil diagrams, there are small quantitative differences between them. The most obvious characteristics of the oil which might be expected to influence soap-oil inter-solubility are molecular shape, type and weight. The compositions found by the Kurtz method can be given only in terms of per cent. carbon atoms in paraffinic, naphthenic (saturated cyclic), or aromatic structures. Such an analysis does not completely describe the assortment of molecules present in an oil, of course, but it gives a useful approximation. The molecular weights, likewise, are an average only) but are useful. Attempts were made to correlate the transition temperatures of the waxy, liquid crystal, and solution phases with molecular weight of the oils and with each of the three carbon-type compositions. The phase change temperatures for each system were taken at 10 and 30 mole yo soap. Results were closely similar at either concentration. As shown by Fig. 6, the liquid crystal transition temperatures are a function of the molecular weight of the oil. The point taken from the lithium stearate-Oil F system is actually the solution temperature, there being no liquid crystal phase in that system at 10 mole yosoap concentration, The point taken from the wax-containing system deviates from the correlation, but the reason is presently obscure. The temperatures of the waxy and solution transitions appear to depend slightly on the paraffinic content of the oils. However, the waxy transition temperatures fall within the range of 184 i 5", and the solution temperatures within the range of 207 5'. These ranges include the data from the n-hexadecane and paraffin wax systems. If points from these two systems are excluded, the range narrows to *3" for the waxy transition and *2" for the solution temperatures. It is apparent that these two phase changes are relatively insensitive to the composition of the oils used.

*

Colloid Structure of the Liquid Crystal Phase.-

Vold and Vold2 have discussed the structure of the mesomorphic phases of lithium steara te-n-hexadecane in terms of the earlier postulates of Doscher and Vold.6 The "non-aqueous middle soap" of the Volds was said to be made up of sheet-like aggregates of clusters of commonly oriented soap and solvent molecules. However, the experimental findings of the work reported here suggest that the liquid crystal phase is more probably made up of long, chain-like aggregates of soap molecules. I n the waxy phase of the lithium stearate oil systems, the sheet-like layers of the soap crystal are (5)

T.M. Doscher and R. D. Vold, THISJOURNAL,52.

97 (1948).

Vol. 62

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M O L E C U L A R WT. OF OIL. Fig. 6.-Liquid crystal transition temperatures at 10 mole % soap.

riddled with hydrocarbon molecules, but the ionic and coordination forces in the lithium-carboxylate planes hold the soap lattice together enough to give some rigidity to the whole phase. The transition to the liquid crystal phase requires about 2 kcal. per mole of soap (estimated from peak areas in differential thermograms) and the liquid crystal phase has the stringy, rubber-like elasticity of a polymer solution. The liquid crystal phase in these systems is birefringent to plane polarized light, but it has no focal conics or other formal structures associated with the smectic phase. These facts suggest that there are still polar forces holding the soap molecules together, but only in a chain-like manner. The transition from liquid crystal phase to solution again requires about 2 kcal. per mole of soap, which shows that the bonds broken on solution are similar t o those broken in the transition from waxy phase to liquid crystal phase. Acknowledgment.-The author wishes to thank Mr. E. L. Armstrong, Dr. R. T. Edwards and Dr. Henry Ra,ich for suggestions and discussion during this work.