Light-Triggered Rheological Changes in a System of Cationic Wormlike Micelles Formulated with a Photoacid Generator

"Smart" fluids displaying large changes in their rheological properties in response to external stimuli have been of great interest in recent years. For example, "smart" wormlike micelles (WLMs) that respond to pH can be readily formulated by combining a cationic surfactant such as cetyltrimethylammonium bromide (CTAB) with an aromatic compound such as 1,2-dihydroxybenzene (DHB). Here, we show that a pH-responsive aqueous formulation as mentioned above can be simultaneously made responsive to ultraviolet (UV) light by incorporating a photoacid generator (PAG) into the system. A commercially available PAG, diphenyliodonium-2-carboxylate, is used here. Upon exposure to UV light, this PAG irreversibly photolyzes into iodobenzene (IB) and benzoic acid (BA), with the formation of BA, leading to a drop in pH. WLMs formed by mixtures of CTAB, DHB, and the PAG are systematically characterized before and after UV irradiation. As the PAG photolyzes, an increase in the viscosity of WLMs occurs by a factor of 1000. We show that the ratio of the zero-shear viscosity η₀ (after UV/before UV) depends on the initial pH of the sample. The UV-induced increase in η₀ can be attributed to the growth of WLMs in solution, which in turn is influenced by both the ionization state of DHB and the presence of IB and BA.

Vesicle formation by ultrashort alkyl-phosphonic acids and serine in aqueous solutions

Vesicles possess unique biofilm structures and offer biomimetic advantages for drug and gene delivery. Herein, we report the spontaneous vesicle formation from ultrashort alkyl-phosphonic acids in the presence of amino acids. The aggregation characteristics and self-assembly structures of vesicles in aqueous solution were investigated by using dynamic light scattering, zeta potential, and cryo-transmission electron microscopy. We combined low-field nuclear magnetic resonance and Fourier transform infrared spectroscopy to study the H-proton-induced multilamellar vesicle formation. When we increased the molar fraction of serine, stable and closed spherical vesicles were formed at relatively low critical micelle concentrations. This transition of the self-assembled structure indicates that vesicle formation occurs when the chain length and the magnitude of the surface charge cause a fluctuation in the volume of the vesicle. Density functional theory reveals the critical role of the mixed alkyl-phosphonic acid/amino acid-enhanced electrostatic attraction between the head groups and hydrogen bonds associated with the aggregated states.

Multistimuli-Responsive Foams Using an Anionic Surfactant

In this work, we report a novel class of a commercially available surfactant which shows a multistimuli-responsive behavior toward foam stability. It comprises three components-a hydrophobe (tristyrylphenol), a temperature-sensitive block (polypropylene oxide, PO), and a pH-sensitive moiety (carboxyl group). The hydrophobicity-hydrophilicity balance of the surfactant can be tuned by changing either the pH or temperature of the system. At or below pH 4, the carboxyl functional group is dominantly protonated, resulting in zero foamability. At higher pH, the surfactant exhibits good foamability and foam stability marked with a fine bubble texture (∼200 μm). Foam destabilization could be achieved rapidly by either lowering the pH or bubbling CO₂ gas. At a fixed pH in the presence of salt, increasing the temperature to 65 °C resulted in rapid defoaming because of the increased hydrophobicity of the PO chain. This stimuli-induced stabilization and destabilization of foam were found to be reversible. We envisage the use of such a multi-responsive foaming system in diverse applications such as foam-enhanced oil recovery and environmental remediation where spatial and temporal control over foam stability is desirable. The low-cost commercial availability of the surfactant further makes it lucrative.

Cargo Release from Polymeric Vesicles under Shear

In this paper we study the release of cargo from polymeric nano-carriers under shear. Vesicles formed by two star block polymers- A 12 B 6 C 2 ( A B C ) and A 12 B 6 A 2 ( A B A )-and one linear block copolymer- A 14 B 6 ( A B ), are investigated using dissipative particle dynamics (DPD) simulations. A - and C -blocks are solvophobic and B -block is solvophilic. The three polymers form vesicles of different structures. The vesicles are subjected to shear both in bulk and between solvophobic walls. In bulk shear, the mechanisms of cargo release are similar for all vesicles, with cargo travelling through vesicle membrane with no preferential release location. When sheared between walls, high cargo release rate is only observed with A B C vesicle after it touches the wall. For A B C vesicle, the critical condition for high cargo release rate is the formation of wall-polymersome interface after which the effect of shear rate in promoting cargo release is secondary. High release rate is achieved by the formation of solvophilic pathway allowing cargo to travel from the vesicle cavity to the vesicle exterior. The results in this paper show that well controlled target cargo release using polymersomes can be achieved with polymers of suitable design and can potentially be very useful for engineering applications. As an example, polymersomes can be used as carriers for surface active friction reducing additives which are only released at rubbing surfaces where the additives are needed most.