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

Abstract

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 the viscosity increases very rapidly as the temperature rises, to correspond with that of the LS phase. This makes the pressure relations extremely abnormal, since the LS phase at 25°C and the S phase at 5°C do not differ greatly in their behavior from normal liquids, which follow the relation log η=log η0+kπ. However, at the intermediate temperatures in the range of 8.85 to 10° the viscosity of this phase decreases only slowly with pressure at low pressure, but more and more rapidly as the pressure increases, until the extremely rapid decrease is stopped by a transition to the LS phase. The viscosity relations indicate that the transition S⇌LS occurs very close to a molecular area of 19.98A². While the area for the transition is almost constant, its temperature increases slowly with pressure. At 16 dynes cm⁻¹ an increase of 0.07A in mean molecular distance increased the viscosity by a factor of 55. It should be noted that this effect is in the abnormal direction. The S phase is the only one of the three condensed phases which exhibits normal viscosity relations. In both the S and the LS phases the molecules may be assumed to be oriented perpendicular to the surface. In three‐dimensional crystals the hydrocarbon chains occupy an area of 18.5A², in the S alcohol films from 19.5 to 20.0A², in the LS films from 20 to 20.75, and in the L₂ films from 19.8 to 22.7. The highest temperature employed was 35°C. An increase above this temperature would increase the upper limits of area for the LS and L₂ monolayers. It is obvious that the possibility exists for hydrogen bonding between the —OH groups of the alcohols themselves, or between these groups and the water molecules. What is needed for the explanation of the abnormal relations of the L₂ and LS phases of the alcohols is to determine what type of binding will increase in energy as the molecular distance is increased, either by increase of temperature or decrease of pressure. The normal behavior of the solid surface phase of the alcohols is similar to that of the acids, which has been discussed in earlier papers.