Study of Brij Micelles Using Dynamic Light Scattering Spectroscopy

We studied properties of Brij-35 surfactant micelles using Dynamic Light Scattering (DLS) and Optical Probe Diffusion method. Aqueous solutions of Brij-35 with concentrations ranging from 2 to 100g/L were prepared, both with and without polystyrene latex probes of diameters 24, 50, 282, and 792nm. Solutions were studied at four temperatures of 10, 25, 40, and 70$^{o}$C with DLS to obtain micelle and probe diffusion coefficients (D$_{m}$, D$_{p})$. Using both diffusion coefficients we deduced micelle radius (a$_{m})$, micelle water content ($\delta )$, and number of surfactant molecules per micelle (N) using two different models. First, we used the hard sphere model of micelle/probe interaction to analyze the data by two methods. In this model, a$_{m}$ is obtained from Stokes-Einstein equation using the intercept of D$_{m}$(c). The first method of the model uses the slope of D$_{m}$(c) and the size of probes to determine N and $\delta $. The second method of the model uses the linear least-squares fit of D$_{p}$(c) for different probe sizes to determine N and $\delta $. Both methods reveal that with solution temperature increase, a$_{m}$ increases by 10{\%}, N increases and $\delta $ decreases by a factor of 2. Two hard sphere methods yield somewhat different trends, but overall agree with published data on Brij micelles. The second model treats micelles as core-shell particles and uses D$_{m}$(c) to determine not only a$_{m}$, $\delta $, and N, but also micelle corona radius a$_{c}$.