Influence of molecular association on surface tension

Surfactant-polymer mixtures are extensively investigated for personal care applications. Until now the interactions in these systems have been not amenable to characterization by surface tension measurements. With the Kibron Delta-8 the variation of surfactant composition in a 96-well-plate with varying polymer and surfactant concentration is easily performed. Once the samples are on the plate and the equilibrium is reached the plate is read in just a few minutes.

Surface tension as a function of the content of a cationic surfactant, CPC (N-hexadecylpyridinium chloride) is illustrated below (Fig. A) . The obtained CPC-isotherm is typical for a surfactant, i.e. the surface tension decreases with an increasing concentration, leveling off upon reaching the CMC at ca. 50 μM.

Figure A

A pattern of behavior when using a non-ionic (non-surface active) polymer, PSS (sodium poly (styrenesulfonate)) and CPC is illustrated below (Fig. B). The first discontinuity seen is CAC, the critical aggregation concentration, corresponding to the association of micelles on the polymer. Increasing the CPC concentration the polymer becomes finally saturated and further addition of surfactant decreases the surface tension until it is high enough for it to form free micelles, reaching CMC.

Interestingly, the adsorption isotherms for CPC at varying concentrations of PSS reveals CAC to be  nearly independent of the polymer concentration (Figure C). 

       Figure C

The above can be understood using the fundamental mass action model. More specifically, aggregation under consideration can be written as:

nCP+ + PSS−m ⇔ [PSS−(m−n) - CPn]

where n is the number of CP+ ions involved. Thus, for a large n the equilibrium will be strongly dependent on the CPC concentration in contrast to the PSS concentration.

The saturation of the polymer chains is evident as a transition from the plateau above the CAC into decrease in surface tension with increasing total CPC concentration. The CPC concentration range in which saturation occurs depends on the polymer concentration. Further addition of CPC results in the formation of surfactant micelles.