Faculty of Life and Medical Sciences, Doshisha University , Kyoto 610-0321, Japan.
Institut Charles Sadron (CNRS), University of Strasbourg , 23 rue du Loess, 67034 Strasbourg, France.
Langmuir. 2016 Nov 29;32(47):12461-12467. doi: 10.1021/acs.langmuir.6b01883. Epub 2016 Aug 1.
The numerous applications of microbubbles in food science and medicine call for a better understanding and control of the effects of the properties of their shells on their stability and ability to resonate at chosen frequencies when submitted to an ultrasound field. We have investigated both millimetric and micrometric bubbles stabilized by an amphiphilic block copolymer, Poloxamer 188 (e.g., Pluronic F-68). Although Pluronic F-68 is routinely being used as a dispersing and foaming agent to facilitate phospholipid-based microbubble preparation, it has never been studied as a shell component per se. First, we investigated the adsorption kinetics of Pluronic F-68 at the interface between water and air, or air saturated with vapors of perfluorohexane (F-hexane), using bubble profile tensiometry analysis. F-Hexane was found to strongly accelerate the adsorption of Pluronic F-68 (at low concentrations) and decrease the interfacial tension values at equilibrium (at all concentrations). We also found that relatively stable microbubbles could unexpectedly be prepared from Pluronic F-68 in the absence of any other surfactant, but only when F-hexane was present. These bubbles showed an only limited volume increase over ∼3 h, while a 10-fold increase in size occurred within 200 s in the absence of a fluorocarbon. Remarkably, their deflation rate decreased when the Pluronic F-68 concentration decreased, suggesting that bubbles with semidilute copolymer coverage are more stable than those more densely covered by copolymer brushes. Single-bubble experiments using laser Doppler vibratometry showed that, by contrast with other surfactant-coated microbubbles, the resonance radius of the Pluronic F-68-coated microbubbles was lower than that of naked microbubbles, meaning that they are less elastic. It was also found that the bubble's vibrational displacement amplitude decreased substantially when the microbubbles were covered with Pluronic F-68, an effect that was further amplified by F-hexane.
微泡在食品科学和医学中的众多应用要求更好地理解和控制其壳的性质对其稳定性的影响，以及在超声场中施加选定频率时共振的能力。我们研究了由两亲嵌段共聚物泊洛沙姆 188（例如，Pluronic F-68）稳定的毫米级和微米级气泡。尽管 Pluronic F-68 通常被用作分散剂和发泡剂来促进基于磷脂的微泡的制备，但它从未被作为壳成分本身进行过研究。首先，我们使用气泡轮廓张力计分析研究了 Pluronic F-68 在水和空气之间的界面或空气与全氟己烷（F-hexane）蒸气饱和的界面上的吸附动力学。发现 F-hexane 强烈加速 Pluronic F-68 的吸附（在低浓度下）并降低平衡时的界面张力值（在所有浓度下）。我们还发现，即使没有任何其他表面活性剂，也可以出乎意料地从 Pluronic F-68 制备相对稳定的微泡，但仅当存在 F-hexane 时才可以。这些气泡在大约 3 小时内仅略有增加，而在没有氟碳化合物的情况下，在 200 秒内尺寸增加了 10 倍。值得注意的是，当 Pluronic F-68 浓度降低时，它们的去气速率降低，这表明具有半浓聚合物覆盖的气泡比由聚合物刷更密集覆盖的气泡更稳定。使用激光多普勒振动计进行的单个气泡实验表明，与其他涂覆有表面活性剂的微泡相比，Pluronic F-68 涂覆的微泡的共振半径小于裸露的微泡，这意味着它们的弹性较小。还发现，当微泡被 Pluronic F-68 覆盖时，气泡的振动位移幅度大大降低，并且 F-hexane 进一步放大了该效应。
