William D. Harkins

A General Thermodynamic Theory of the Spreading of Liquids to Form Duplex Films and of Liquids or Solids to Form Monolayers

A comprehensive thermodynamic theory is developed for the spreading of any liquid or solid b over the surface of any liquid a. Spreading is considered to occur by two types of processes: (A) duplex film or D spreading, and (B) non‐duplex or M spreading. A duplex film is always unstable, and transforms into a non‐duplex film and a lens. Every non‐duplex film of known structure is a monolayer (M). Either type of film may spread if its formation involves a decrease of free energy. The free surface energy of mercury at 20°C is 476 erg cm—2, and the spreading of water or any organic liquid on the surface as either a duplex film or a monolayer, gives a decrease of free surface energy, so all such liquids spread on mercury. On water all organic liquids spread as monolayers; an organic solid also spreads as a monolayer unless it is too non‐volatile in the two‐dimensional system. A liquid b will spread over the surface of a liquid a as a duplex film if the initial spreading coefficient Sb/a = WA — WCb is positive,...

Solubilization by Solutions of Long‐Chain Colloidal Electrolytes

Increase in the concentration of a soap or other detergent does not increase the solubility of an oil above that in water until the critical concentration for the formation of micelles (cmc) is attained. Above this the solubility, designated as solubilization, increases and, in general, more rapidly as the soap concentration increases; i.e., per mole of soap the solubilization is greater in a 25 percent than in a 5 percent soap solution. For a homologous series the volume of oil solubilized at a constant temperature is to a first approximation inversely proportional to the molar volume. The polarity and shape of the molecules solubilized also play a role. Salts increase the extent of the solubilization; at low concentrations to an extent which may be accounted for by the increase in micellar area resulting from the depression of the cmc by the salt. At higher soap concentrations the increase in solubilization is greater than can be accounted for in this way.

Contact Potentials and the Effects of Unimolecular Films on Surface Potentials. I. Films of Acids and Alcohols

Simultaneous measurements of film pressure and surface potential have been carried out by the use of an apparatus designed in such a way that the potential may be determined for any location on the surface of the film. At film pressures above that of the gaseous films, organic substances with homo‐heteropolar molecules give a single smooth curve for the relation between surface potential and molecular area. At areas sufficiently great to reduce the pressure to that of the gaseous film the surface potential becomes variable and remains variable until the area becomes so great that the continents and islands of condensed film evaporate in the two‐dimensional system to give a gaseous film alone. For example with films of myristic acid at 17° the surface potential is represented by a single curve below a molecular area of about 50 sq. A, and by any value below 170 mv at higher areas, at which islands in the film persist. The areas above which the surface potentials become variable, due to the effects of islan...

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.

A Superliquid in Two Dimensions and a First‐Order Change in a Condensed Monolayer II. Abnormal Viscosity Relations of Alcohol Monolayers in Condensed Liquid Phases

The new LS phase found in alcohol monolayers has the compressibility of a solid and, at temperatures near that of the first‐order L2⇌LS transition, the low viscosity of a very highly fluid liquid. The viscosity is almost independent of pressure, but varies in an abnormal way with temperature. For example, octadecanol exhibits a minimum viscosity at about 8.8°C. As is normal, the viscosity increases with decrease of temperature over the small range from 8.8° to 7.5°C where a transition to the S phase occurs. However, above 8.8°C an increase of temperature of 16° increases the viscosity of the LS phase by a factor of 25, and changes it from Newtonian to non‐Newtonian. At a pressure of 18 dynes cm−1 the logarithm of the viscosity above 12°C varies nearly either as T, or as 1/T. At other pressures (Fig. 5) the relations are different. At 1 dyne cm−1 the viscosity of the condensed liquid (L2) phase decreases in a normal way with temperature, but at 12 dynes cm−1 the relation is reversed and is abnormal, since ...

Energy Relations of the Surface of Solids I. Surface Energy of the Diamond

From the known structure of the diamond the total surface energy of the crystal has been calculated in terms of the energy of the carbon‐carbon bond, and is found to be: 1.50×10−9EB erg cm−2 for the 111 face, and 2.10×10−9EB erg cm−2 for the 100 face, where EB is the energy in ergs per bond. If the bond energy is assumed to be 90 kcal. mole−1 the values become 5650 erg cm−2 for the 111 face, and 9820 erg cm−2 for the 100 face. The corresponding free surface energies are found to be: 111 face at 25°=5400 erg cm−2 100 face at 25°=9400 erg cm−2. One uncertain feature in the calculation is that involved in the calculation of the decrease in energy caused by the long range binding of the valence bonds in the surfaces. In the 111 face the bonds are 2.517A apart, and are perpendicular to the surface. Thus the bond directions are parallel, while inside the diamond the carbon‐carbon distance is only 1.54A, and the bonds meet head on. While in the 111 face there is only one bond per carbon atom, in the 100 face the...

Structure of Micelles of Colloidal Electrolytes. III. A New Long‐Spacing X‐Ray Diffraction Band, and the Relations of Other Bands

A newly found x‐ray diffraction band, obtained from aqueous solutions of soaps and detergents, gives a Bragg spacing which is independent of concentration and is close to the double‐length of the molecule. This is designated as the micelle thickness or M‐band. The already known long‐spacing band, interpreted here as related to the inter‐micellar distance, is designated as the I‐band. Its spacing dI increases with decreasing soap concentration. If a hydrocarbon is solubilized in the micelles, these spacings increase by ΔdM and ΔdI. Formerly, ΔdI was supposed to give the mean thickness of the layer of oil dissolved between the ends of the hydrocarbon chains of the soap molecules as a middle layer in the micelle. However, ΔdM seems to be more directly related to this thickness. As measured by dM, the thickness of the micelle remains constant with increasing soap concentration: e.g., the values are 40.8, 41.5, 40.7, 40.2, 39.5, 39.6, 40.2A for potassium myristate at 25°C at the following respective concentrat...

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.

Energy Relations of the Surfaces of Solids II. Spreading Pressure as Related to the Work of Adhesion Between a Solid and a Liquid

This is the second paper in a series which gives theoretical values for the free and total surface energy of the diamond, and experimental values for the total and free energy, the latent heat, and the entropy of adhesion between various crystalline solids and liquids. This second paper deals with the oldest part of the subject, but a part which has always been treated incorrectly. The total energy (eA(SL)) required to separate water from solids of the general type of BaSO4, TiO2, and ZrSiO4, has been found by Harkins and Boyd to vary from 600 to 1000 erg cm−1, while to separate octane from these solids requires only from 150 to 250 erg cm−2. The free energy of separation, designated by WA(SL), is smaller. It is generally believed that WA(SL)=γL(1+cos θSL), which gives values, for solids thus far investigated, from 47.5 to 144 erg cm−2 at 25°C if the liquid is water, and much smaller values if organic liquids are used. Indeed the maximum value of the work of adhesion between water and any solid at this te...

Surfaces of Solids XV First‐Order Phase Changes of Adsorbed Films on the Surfaces of Solids: The Film of n‐Heptane on Ferric Oxide

Two‐dimensional first‐order changes, in which a gaseous film of normal heptane is transformed into another phase of lower molecular area with evolution of heat, have been discovered on subphases of ferric oxide, silver, and graphite. All of the critical phenomena observed in three‐dimensional systems are found to be duplicated. For n‐heptane on ferric oxide the critical constants are: σc (area) 900A2 per molecule; π (film pressure) 0.45 dyne cm−1; and Tc, 29°C. The critical constants are found to depend on the nature of the solid as well as on that of the vapor. The heat of transformation at 25°C is estimated to be 12,000±5000 cal. mole−1. This value appears to be considerably higher than the 6150 cal. mole−1 required for the formation of three‐dimensional liquid n‐heptane from its vapor at the same temperature. The volume‐pressure relations are considered for the adsorption isotherm in the case in which a second‐ or third‐order phase change occurs.