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    Spontaneous vesicle formation and vesicle-to-alpha-gel transition in aqueous mixtures of sodium monododecylphosphate and guanidinium salts

    Year: 2021

    Journal: Soft Matter, Volume 17, MAY 7, page 4604–4614

    Authors: Gao, Meihua; Du, Na; Yao, Zhiyin; Li, Ying; Chen, Nan; Hou, Wanguo

    Organizations: National Natural Science Foundation of China [21872082]; Natural Science Foundation of Shandong Province, China [ZR2019MB025]; Key Research and Development Plan of Shandong Province, China [2018GGX102014]

    Monoalkyl phosphates (MAPs) are one kind of important single-chain weak acid/salt type surfactants, but the understanding of their aggregation behavior in water is very limited due to their insolubility at room temperature. In the current work, the effect of guanidinium salts (GuSalts) on the solubility of sodium monododecylphosphate (SDP), a typical MAP, in water was determined at 25.0 degrees C, and the aggregation behavior of SDP in the GuSalt/water mixtures was investigated. The solubility of SDP is significantly improved by GuSalts including GuCl, GuSO(4), GuSO(3), GuPO(4), and GuCO(3) at 25.0 degrees C, resulting in an isotropic phase. SDP vesicles are spontaneously formed in the isotropic phase, with a critical vesicle concentration of similar to 1.0 mM independent of the type of GuSalts. A bridging dimer mechanism is proposed to explain the formation of SDP vesicles. The SDP vesicles have a unilamellar structure with a size of similar to 80 nm and an alkyl interdigitated degree of similar to 25%, and exhibit size-selective permeability. Interestingly, a temperature-induced reversible transition between vesicles and alpha-gels was observed for the SDP/GuSalt/H2O systems when the SDP content is higher than 20 mM. The alpha-gels obtained are composed of vesicles and bilayer sheets, showing similar viscoelasticity to conventional gels, although their water content is as high as similar to 98 wt%. The microviscosity of SDP vesicle membranes (ca. 35.79-49.34 mPa s at 25.0 degrees C) and the transition temperature between vesicles and alpha-gels (ca. 21.0-22.8 degrees C) are all dependent of the type of GuSalts. This work deepens the understanding of the aggregation behavior of MAPs and also provides valuable information for their practical applications.
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