Dynamics in a photoresponsive surfactant system

Abstract

The study of surface tension and other surface properties is motivated by the large number of industrially relevant processes involving interfaces, such as coating, detergency, printing, foams, and so forth. These surface properties become increasingly important as the length scale of the system is reduced (as in microfluidic devices). Recently, much research has been focused on developing surfactants which respond to a particular stimulus (such as temperature, pH, light, etc.), so that properties such as surface tension and viscosity can be controlled using a convenient external trigger. Using light for this purpose has some advantages over other methods, as light can easily be focused and patterned (using optical lenses, filters, and masks) to give excellent precision for changing solution properties in a targeted area. A nonionic photoresponsive surfactant has been developed which incorporates the light-sensitive azobenzene group into its hydrophobic tail. Cis-trans photo-isomerization of this group causes a change in the structure of the surfactant molecule which alters its aggregation state in bulk solution and its adsorption capacity at an air-water interface.

(cont.) NMR studies indicate that a solution removed from light for an extended period of time is comprised almost entirely of the trans isomer, while samples exposed to light of fixed wavelength eventually reach a photostationary state containing significant amounts of both isomers, with UV illumination producing a mixture dominated by the cis isomer. Surface pressure measurements of adsorbed monolayers of the surfactant under various illumination conditions were made using a Langmuir film balance. The results indicate that adsorbed cis surfactant exerts a greater surface pressure than the adsorbed trans isomer, and that any cis present in a saturated layer tends to dominate the surface pressure behavior of the film. Fluorescence experiments were used to study the aggregation behavior of the surfactant in aqueous solution. The results suggest that the trans and cis isomers segregate into separate, co-existing aggregate phases and that the critical concentration associated with the onset of aggregation is very different for the two isomers. In measurements performed well above the CMC. the dynamic surface tension of surfactant solutions following the creation of a fresh interface was found to depend strongly on the illumination state of the sample, though the same equilibrium tension was reached in all cases.

(cont.) The observed dynamic behavior is consistent with a mechanism in which the cis and trans isomers present in the mixtures compete for adsorption at the air/water interface. Diffusion models were developed to estimate the time scales expected for surfactant adsorption and surface tension relaxation in these systems. These models account for the role of aggregates in the adsorption process, and consider limiting behavior for three aggregate properties: mobility, dissolution rate, and ability to incorporate into the interface. Good agreement is found between the model predictions and the experimentally observed relaxation time scales. The results suggest that trans-rich aggregates are important to the adsorption of trans surfactant, but that aggregates play little or no role in the adsorption of the cis isomer. In other experiments, high-intensity illumination focused on a surface saturated with surfactant was used to drive photoisomerization of adsorbed surfactant, resulting in rapid, substantial changes in surface tension. These changes are consistent with proposed conformations of the adsorbed surfactant, and with earlier monolayer studies.