A structure-property study of the dynamic surface tension of three acetylenic diol surfactants

Abstract

Three acetylenic diol surfactants are compared using the pendant bubble technique. The surfactants studied are the highly branched surfactant Surfynol 104, a smaller branched diol, 2,7 dimethyl-4-octyn-3,6-diol, (DMOD) and a diol with linear alkyl chains, 7-tetradecyn-6,9-diol (TDD) which is a structural isomer of Surfynol 104. The molecules differ significantly in terms of both their equilibrium and dynamic surface tension. For each surfactant, the equilibrium surface tension as a function of bulk concentration is obtained and fit to an adsorption isotherm. While the two branched diols are adequately described by a Langmuir model, the third surfactant, TDD, undergoes a transition from a surface gaseous to a surface liquid expanded phase. The TDD data was fit to a Frumkin isotherm modified by a Maxwell construction to account for the surface phase change. For Surfynol 104 and TDD, the dynamic surface tension data agree well with a numerically integrated diffusion-controlled adsorption model. For DMOD, the surface tension equilibrated faster than the first pendant bubble image was obtained (∼0.06 s). The numerical integration of the diffusion-controlled relaxation model for this molecule predicts that the interface equilibrates within this time. All three molecules have similar diffusivities in solution, yet differ significantly in terms of the rate that they reduce the surface tension. The diffusion flux to the surface is set up by adsorption to the interface, which depletes the bulk to some depth. This depth, the adsorption depth h, can be derived from a mass balance and related to the adsorption isotherm for a given surfactant. The greater is h, the longer is required for the interface to equilibrate. The characteristic time for a given surfactant to reduce the surface tension can be shown to be τ D = h 2/ D. This timescale is calculated for each solution. The surface tension equilibrates within ∼10 τ D for the solutions studied. This timescale shows the manner in which the surfactant material parameters determine the dynamic surface tension.