Aggregate-based sub-CMC solubilization of hexadecane by surfactants

Abstract

Solubilization of hexadecane by two surfactants, SDBS and Triton X-100, at concentrations near the critical micelle concentration (CMC) and the related aggregation behavior was investigated in this study. Solubilization was observed at surfactant concentrations lower than CMC, and the apparent solubility of hexadecane increased linearly with surfactant concentration for both surfactants. The capacity of SDBS to solubilize hexadecane is stronger at concentrations below CMC than above CMC. In contrast, Triton X-100 shows no difference. The results of dynamic light scattering (DLS) and cryogenic TEM analysis show aggregate formation at surfactant concentrations lower than CMC. DLS-based size of the aggregates (d) decreases with increasing surfactant concentration. Zeta potential of the SDBS aggregates decreases with increasing SDBS concentration, whereas it increases for Triton X-100. The surface excess (Γ) of SDBS calculated based on hexadecane solubility and aggregate size data increases rapidly with increasing bulk concentration, and then asymptotically approaches the maximum surface excess (Γmax). Conversely, there is only a minor increase in Γ for Triton X-100. Comparison of Γ and d indicates that excess of surfactant molecules at aggregate surface has great impact on surface curvature. The results of this study demonstrate formation of aggregates at surfactant concentrations below CMC for hexadecane solubilization, and indicate the potential of employing low-concentration strategy for surfactant application such as remediation of HOC contaminated sites.

Aggregate-based sub-CMC solubilization of n-alkanes by monorhamnolipid biosurfactant

“…MSR for all of the four alkanes are significantly higher at monoRL concentration below CMC than above CMC. Similar results were observed for octadecane solubilization by monoRL, 8 and hexadecane solubilization by SDBS (also an anionic surfactant) 18 .…”

“…The interfacial tension data at sub-CMC monoRL concentration are well fitted by equation (3) in ref.18 (Fig. 2b), and K , Γ max , and A m obtained are summarized in Table 1.…”

“…This indicates a relationship between alkane solubilization and interfacial partitioning of monoRL. It is worth noting that the MSR for hexadecane solubilization by the monoRL at sub-CMC concentrations (2.55) is larger than that for SDBS (0.84) and Triton X-100 (1.90) 18 , indicating higher solubilization efficiency of biosurfactant monoRL over synthetic surfactants. This is probably due to the presence of the double alkyl chains in the monoRL molecule.…”

“…Due to the extremely low water solubility and high octanol-water partition coefficient ( K ow ) of these four alkanes (Table 1), the amount of freely-dissolved alkane in bulk aqueous phase is minimal and all the solubilized alkane is assumed to be associated with the aggregates. Hence, based on the spherical aggregate assumption, the aggregate surface excess, Γ , and the bulk concentration, C w , of monoRL monomer were calculated by applying equation (2) and (5) in ref.18 using Γ max and K previously obtained.…”

“…More significant change of Γ and A is observed for the long-chain alkanes (tetradecane and hexadecane). Based on equation (2) in ref.18, Γ is more sensitive to change of C w with a smaller K . The K for four alkanes follows the order of dodecane > decane > tetradecane > hexadecane (Table 1).…”