Structure of soap micelles as indicated by X-rays and interpreted by the theory of molecular orientation: II. The solubilization of hydrocarbons and other oils in aqueous soap solutions

Abstract

1. 1. The lamellar micelles formed by oriented double layers of soap molecules in water (Fig. 1) “solubilize” a layer of oil between the hydrocarbon ends of soap molecules in adjacent single layers of soap. The thickness ( d l ) of a double layer of soap and a single water layer, as determined by X-rays, may be considered to be given by the relation d l = d o + K 1 log (1/c) in which c is the fraction of soap in the solution. In a solution saturated with n-dodecane in 20% potassium laurate at 25°C., d l is increased by Δ d = 9 A, while with n-heptane and triptane in 25% potassium laurate Δ d is 13.8 A and 14.7 A, respectively (triptane is the more soluble), and with n-hexane and n-pentane in 15% potassium laurate increasingly larger. Thus, at saturation with oil, Δ d increases rapidly with decrease in length of the molecule of the solubilized oil. 2. 2. For undersaturated and saturated solutions of n-heptane Δ d = 0 + 3.82 C and for its isomer, triptane, Δ d = 0.15 + 3.87 C, or the same within the limits of experimental error. 3. 3. The area per soap molecule in the layer of soap is found to be 26–28 A 2 (or constant within the limits of error) for close packing and independent of whether a solubilized hydrocarbon layer is present. However, the actual packing is shown by the X-ray diffraction to be that of a liquid, which gives a slightly larger area, which for purposes of calculation has been rounded off to 30 A 2. 4. 4. Assuming the mean area of 30 A 2 and that the total area (Σ) of the double soap layers is given by Σ = 30 × 10 −16( N/2) cm. 2, where N is the total number of soap molecules present, a quantity τ, a mean apparent thickness of the oil layer, is obtained. For n-heptane in 25% potassium laurate solution τ is 40% of Δ d, for triptane τ Δd is about 0.38, and for 2.9% ethyl benzene in 15% potassium laurate, 0.4. No explanation of this will be offered until other work now in progress is completed. 5. 5. The apparent specific volume of 0.08% n-heptane solubilized at 25°C. in 25% potassium laurate is 1.4400, while with a saturated solution it is 1.470986, or practically the same as for n-heptane in bulk. With triptane, however, the reverse is true: the thickest layer departs most from the specific volume of the liquid, while this is approached as the layer gets thinner. 6. 6. Monomer layers with Δ d as high as 16 A (75% isoprene and 25% styrene constituted the monomer layers in this particular case) were found to have this decrease to zero when polymerized by the action of a catalyst, and to exhibit a considerable decrease by the action of X-rays. This shows that the polymer molecules cannot be held by soap micelles. Thus, molecules may be too large to be held by micelles.