M. L. Corrin

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 Δ d 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.

Adsorption of long-chain electrolytes from aqueous solution on graphite of known area and on polystyrene☆

Abstract The adsorption isotherms of sodium dodecyl sulfate and potassium myristate on ash-free graphite of known area have been determined at 30°C. and 35°C., respectively. The experimental methods are discussed in some detail. Calculations, based on two extreme assumptions concerning the concentration of solvent in the surface region, yield values of the specific adsorption which differ by less than the experimental errors of the observations. The isotherm of sodium dodecyl sulfate exhibits a discontinuity at the critical concentration for micelle formation; it is possible that a similar discontinuity occurs in the myristate isotherm. Both isotherms pass through a maximum at equilibrium concentrations above the critical concentration. The adsorption of sodium dodecyl sulfate on polystyrene has been measured over a short concentration range; the specific area of this solid is not known with any degree of precision. This isotherm also exhibits a maximum at approximately the same equilibrium concentration at which a maximum in the isotherm of the same salt on graphite occurs. The areas per adsorbed molecule have been calculated for sodium dodecyl sulfate and potassium myristate on graphite as a function of the equilibrium concentration of electrolyte. The minimum area per molecule is 51.0 A2 for the sulfate and 36.6 A2 for the myristate.

Effect of micelle formation on the absorption spectra of decyl and dodecyl pyridinium iodides

Abstract 1. 1. Absorption spectra are presented for solutions of dodecyl pyridinium iodide in water, ethanol, and chloroform (containing 0.7% ethanol), and for decyl pyridinium iodide in water and in ethanol. 2. 2. The change of absorption spectrum with concentration in water can be related to the critical micelle concentration of these salts. 3. 3. The changes of absorption spectrum above the critical micelle concentration have been discussed in terms of the structure of the micelle.