Krafft Points, Critical Micelle Concentrations, Surface Tension, and Solubilizing Power of Aqueous Solutions of Fluorinated Surfactants
Authors: Hironobu and Kozo Shinoba
Journal: The Journal of Physical Chemistry, Vol. 80, No. 22, 1976, p2468-2470
This paper discusses the Krafft points, critical micelle concentrations (cmc), surface tension above the cmc, and solubilizing power in aqueous solutions of perfluoroalkane carboxylates as functions of fluorocarbon chain length.
The surface tension of these types of solutions is lower than that for ordinary surfactants. Fluorinated surfactants are very stable and have special industrial uses. The longer chain surfactants are more surface active and have high solubilization at low concentrations (compared to shorter chain surfactants). However, the Krafft point increases with increasing hydrophobic chain length. The Krafft point is the point above which ionic surfactants form micelles (and therefore dissolve well). Before this paper was published, it was shown that the kinds of [gegenions] affect the Krafft point. This paper outlines some results on the effect of the kinds of gegenions and chain length of surfactant on the Krafft point, surface tension, cmc, and solubilizing power in aqueous solutions of these perfluoroalkane carboxylates.
Solutions of perfluoroalkane carboxylates with hydrocarbons of varying lengths were created.
To measure Krafft Point: Krafft point was determined by the abrupt increase in the electrical conductivity of as a function of temperature
CMC: measured by the electrical conductivity-concentration curve at constant temperature
Surface Tension Measurements: [(described in another paper)]
A drop-weight method was used to measure surface tension. This method involves obtaining an image of a drop and comparing its shape and size to theoretical profiles. Once <math>\beta</math> and b have been determined from shape and size comparisons, the surface tension is calculated from:
The reason this is a viable method is, the shape of an axisymmetric pendant or sessile drop (see figure below) is only dependent on the Bond number (a measure of the relative importance of gravity to surface tension in determining the shape of the drop). For drops with Bond numbers near zero, surface tension dominates and the drop is close to a sphere in shape. For drops with larger Bond numbers, the drop becomes increasingly deformed by gravity.
Surfactants form micelles above the Krafft point and the solubility of surfactants in water increases drastically. Therefore, solubilization occurs above the Krafft point. The longer chain surfactants have low CMC's and are much more surface active (i.e. they are more efficient surfactants). To be able to use longer chain surfactants, the effect of the kinds of gegenions on the Krafft point has been studied and is shown below:
Krafft points are affected by the nature of the bond between the surface active ion and the gegenion, or the degree of hydration. The figure above shows that the more hydrated the gegenion is, the steeper the slope of Krafft point versus chain length.
The figure below shows the solubilizing power of perfluoroalkane carboxylates. The solubilization of perfluoroalkane carboxylate of different gegenions and fluorocarbon chain length is plotted. The solubilization increases linearly with the concentration of the surfactant (above the cmc). The solubilization is larger larger when the flurocarbon chain length is longer. However, it is also found that the hydrophile-lypophile balance (HLB) of a surfactant, was also affected. Nonionic surfactants whose HLB's are well balanced form large micelles and solubilize more oil.
It is clear that the solubilization of a surfactant is dependent on chain length, temperature and the types of interactions that occur between compounds.