Lamellar Micelles Research Articles

A test of the interpretation of X-ray patterns of micelles.

CLEAR transparent solutions of soaps and detergents give characteristic X-ray patterns, due to the colloidal micelles formed by association of ions or molecules or both. There is still, however, no agreement about the number and kinds of micelles which can be present1. Hess and Gunderman2 were the first to explain the X-ray patterns upon the basis of lamellar micelles consisting of double layers of molecules separated by equally spaced layers of water. Harkins3 recently directed attention to a so-called 'M-band' in the pattern and attributed it to diffraction from the double-leaflet alone.

Organised Structure in Soap Solutions

MY simple picture, fully described elsewhere1–3, of the aggregates in the clear mobile solutions of paraffin-chain salts is that they are essentially liquid in nature and spherical in form. The discovery4, by X-ray diffraction, of a characteristic long spacing, decreasing with increase of concentration, appeared to favour a different picture—that of a lamellar micelle in which the chains lay parallel to one another and the outer faces of arrayed ionic groups were separated by a definite thickness of water. One of the protagonists of the lamellar micelle has recently5 come to modify his picture considerably, and his collaborator6 to consider that the X-ray data can be explained by the spherical micelle.

Lamellar and Other Micelles, and Solubilzation by Soaps and Detergents.

Lamellar and Other Micelles, and Solubilzation by Soaps and Detergents.

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

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.

Amphipathic Character of Proteins and Certain Lyophile Colloids as Indicated by Absorption Spectra of Dyes

A characteristic change of spectral absorption of certain dyes as between water and organic solvents is shown to appear in aqueous solutions of gelatin or of the colloidal detergent, cetyl pyridinium chloride. This behavior is attributed to a bilateral character of lamellar micelles of such bodies, the elementary lamellae being polar and hydrophile on one side, non-polar and organophile on the reverse side. The possibility is indicated of diagnosing such amphipathic characteristics by comparative spectrophotometric measurements with suitable dyes.

Points from Foregoing Letters

J. W. MCBAIN surveys present knowledge of the so-called colloidal electrolytes. He believes that ionic micelles begin to form in very dilute solution and increase steadily with the concentration of simple ions; lamellar micelles arise from ions-pairs and higher aggregates, and produce X-ray patterns in the solution as soon as they have grown sufficiently large.