Mesoscopic Simulation of Surfactant Oligomers Behavior in Aqueous Solution and Interface
Advancing the chemical engineering fundamentals
Interfacial & Colloidal Phenomena - II (T2-6b)
Keywords: Surfactant, Self-assembly, Dissipative particle dynamics, Mesocale-simulation
In recent years, new classes of surfactants have been synthesized and have attracted much attention from academies and industries. One of them is the surfactant oligomers, consisting of two or more conventional single-chain surfactants with hydrophilic head connected by spacer groups. Both experimental studies and computer simulations indicate that the surfactant oligomers exhibit better performance compared to the conventional single-chain counterpart. The efficiency of surfactants is related to its structure. However, its effect is not fully understood. Dissipative particle dynamics (DPD), a coarse-grained method introduced by Hoogerbrugge and Groot, is performed in this paper to better understand the behavior of surfactant oligomers in aqueous solution and oil/water interfaces. The interfacial tension was calculated by Irving-Kirkwood method.
The critical micellar concentration (CMC) of dimeric or trimeric surfactant is much lower than that of their single-chain counterpart. All surfactants show self-assembling properties due to their amphiphilicity in an aqueous medium. They all form spherical micelles at the concentration not far above their CMC. The transition from spherical to cylindrical micelles appears with increasing surfactant concentration. Lamellar micelles will appear with further increasing the surfactant concentration. For dimeric and trimeric surfactants, cylindrical micelles transform into extremely long “wormlike” or “threadlike” micelles before the transition to lamellar micelles. These results are in qualitative agreement with experimental observations. The average aggregation numbers (AN) of micelles increase with a power law of AN∝c^α when the surfactant concentration c>>CMC. The self-diffusion coefficients will drop with a power law of D∝c^-α when wormlike micelles are formed.
The surface area A occupied by one surfactant at the interface is an important characteristic of a surfactant, which decreases slightly when the carbon number m of alkyl chain increases. In the case of surfactant oligomers, our simulation indicates that A also decreases slightly when m is small. Due to the premicellar aggregation, the relationship between A and m become unclear as the value of m larger than 16. The variation of A with the spacer length is complex. For the diametric m-s-m surfactants with a hydrophobic spacer s, A has a maximum when m is approximate 9 ~ 15. For the relative trimeric surfactants, A increase slightly while m increase.
The efficiency of surfactants is measured by the reduced surface tension at a given surfactants amount. In short, mostly the efficiency increases while the carbon number m of alkyl chain increases. For some of the diametric and trimeric surfactants with long spacer groups, the efficiency meets a maximum value when m is approximate 9 ~ 15. With the increase of spacer lenth s, the efficiency has a minimum when s is approximate 3 ~ 6.
Presented Thursday 20, 09:05 to 09:25, in session Interfacial & Colloidal Phenomena - II (T2-6b).
