The gas from a free air bubble will readily dissolve in water, driven by two main factors: the concentration (undersaturation) of dissolved gas in the aqueous solution and the surface tension of the gas bubble-water interface via a Laplace overpressure in the bubble that this creates. This paper experimentally and theoretically investigates each of these effects individually. To study the effects of surface tension, single- and double-chain surfactants were utilized to control and define interfacial conditions of the microbubble in saturated solution. To study the effect of undersaturation, solid distearoylphosphocholine lipid was utilized to coat the gas microparticle with, essentially, a wax monolayer and to achieve zero tension in the surface. The experimental work was performed using a micromanipulation technique that allows one to create and micromanipulate single air microparticles (5-50 microm radius range) in infinite dilution and to accurately record the size of the particle as it loses volume due to the dissolution process. The micropipet technique has shown to be an improvement over other previous attempts to measure dissolution time with a 3.2% average experimental error in gas microparticle dissolution time. An ability to study a gas microparticle in infinite dilution in an isotropic diffusion field is in line with the theoretical assumptions and conditions of the Epstein-Plesset model. The Epstein-Plesset model on average underpredicted the experimentally determined dissolution time by 8.6%, where the effect of surface tension was considered with a range of surface tensions from 72 down to 25 mN/m. The Epstein-Plesset model on average overpredicted the dissolution time by 8.2%, where the effect of undersaturation was considered for a microparticle with zero tension in the surface (zero Laplace pressure) and a range of gas saturations from 70% to 100%. Compared to previous attempts in the literature, this paper more appropriately and accurately tests the Epstein-Plesset model for the dissolution of a single microbubble and an air-filled microparticle in aqueous solution.

W This paper contains enhanced objects available on the Internet at http://pubs.acs.org/ journals/langd5.A polycrystalline phospholipid monolayer self-assembled at the surface of an air microbubble in aqueous solution represents a novel material structure: in essence, a solid shell of wax with micrometer-scale dimensions and a thickness of only a single molecule. Micropipet manipulation of these microparticles revealed the dependence of the mechanical properties of the lipid shells, specifically, yield shear and shear viscosity, on the composition, grain microstructure, and thermal processing of the material, in particular the cooling rate of the shells from the melt. Properties were measured as a function of the (1) lipid composition at a fixed cooling rate and (2) cooling rate at a fixed lipid composition. Epifluorescent microscopy and transmission electron microscopy revealed that the morphology of the 1,2-distearoyl-sn-glycero-3-phosphatidylcholine monolayer microstructure, which develops upon freezing from the melt, is dependent on the cooling rate through the lipid transition temperature Tm, with larger micrograins being formed at slower cooling rates. Mechanical properties of the lipid shell follow micrograin size, with the coarse grain structure exhibiting a higher resistance to shear deformation than the fine grain structure does, which is behavior consistent with that of more traditional bulk crystalline materials. IntroductionThe same principles of self-assembly that describe the formation and structure of a lipid monolayer on a flat trough of water also apply to the spontaneous organization of amphiphilic material at a curved gas-liquid interface such as that posed by the surface of an air microbubble in water. 1-3 It is well-known that at sufficient concentrations the surface contaminants on a "dirty" bubble in water can effectively immobilize the air-water interface and cause the bubble to behave as a rigid particle. A relatively recent addition to the family of experimental techniques for studying monolayer properties known as axisymmetric drop shape analysis (ADSA) was employed by Kwok and co-workers as a film balance, albeit on a curved droplet surface, to obtain surface pressure-surface area (Π-A) isotherms for monolayer films of 1-octadecanol. 4,5 ADSA experiments have demonstrated the equivalence of curved monolayers and flat monolayers with respect to surface phase states and transitions. Until recently, however, the direct measurement of the mechanical properties of monolayers spread at micrometer-scale curved interfaces has eluded investigators; furthermore, extant work on monolayers, whether on flat or curved interfaces, has been mostly confined to the liquid-expanded or liquid-condensed-expanded coexistence regimes of their phase diagrams. The current study presents the first mechanical property measurements made for solid lipid monolayer shells of well-defined composition and micrograin structure formed on the surfaces of gas microbubbles, thus essentially comprising gas microparticles s...

The Epstein-Plesset model was originally derived for the dissolution of a single gas bubble in an infinite aqueous solution (Epstein, P. S.; Plesset, M. S. J. Chem. Phys. 1950, 18, 1505-1509). The micropipet manipulation technique was previously shown to test this theory on air microbubbles and air-filled lipid-coated microparticles accurately and appropriately (Duncan, P. B.; Needham, D. Langmuir 2004, 20, 2567-2578). This same theory is now tested to model liquid microdroplet dissolution in a well-defined solution environment. As presented previously for the gas-bubble system, holding a single microparticle at the end of a micropipet was not shown to affect the dissolution profile and allowed isotropic diffusion significantly, a necessary condition for the validation of the theory. Here, an aniline-water system with an initial droplet diameter of 50 microm was used as a model liquid-liquid system. A microdroplet of aniline in an aqueous solution presatureated with aniline at distinct levels was tested, as was the reverse system of a water droplet in an aniline solution. The dissolution lifetime was shown to increase with increasing medium saturation fraction according to the Epstein-Plesset time-dependent theory (including the time required to establish the stationary layer) neglecting interfacial tension. The droplet lifetime can be increased by an order of magnitude (from about 10 to 100 s) by increasing the saturation fraction from 0 to 0.9 and by another order of magnitude by increasing from 0.9 to 0.99. The technique proved to be an accurate and appropriate method to test the dissolution of single liquid microdroplets in a second liquid solution and establishes a systematic experimental and theoretical approach to the investigation of the formation of polymer and other microparticles.

While the Stokes-Einstein ͑SE͒ equation predicts that the diffusion coefficient of a solute will be inversely proportional to the viscosity of the solvent, this relation is commonly known to fail for solutes, which are the same size or smaller than the solvent. Multiple researchers have reported that for small solutes, the diffusion coefficient is inversely proportional to the viscosity to a fractional power, and that solutes actually diffuse faster than SE predicts. For other solvent systems, attractive solute-solvent interactions, such as hydrogen bonding, are known to retard the diffusion of a solute. Some researchers have interpreted the slower diffusion due to hydrogen bonding as resulting from the effective diffusion of a larger complex of a solute and solvent molecules. We have developed and used a novel micropipette technique, which can form and hold a single microdroplet of water while it dissolves in a diffusion controlled environment into the solvent. This method has been used to examine the diffusion of water in both n-alkanes and n-alcohols. It was found that the polar solute water, diffusing in a solvent with which it cannot hydrogen bond, closely resembles small nonpolar solutes such as xenon and krypton diffusing in n-alkanes, with diffusion coefficients ranging from 12.5ϫ 10 −5 cm 2 / s for water in n-pentane to 1.15ϫ 10 −5 cm 2 / s for water in hexadecane. Diffusion coefficients were found to be inversely proportional to viscosity to a fractional power, and diffusion coefficients were faster than SE predicts. For water diffusing in a solvent ͑n-alcohols͒ with which it can hydrogen bond, diffusion coefficient values ranged from 1.75ϫ 10 −5 cm 2 / s in n-methanol to 0.364ϫ 10 −5 cm 2 / s in n-octanol, and diffusion was slower than an alkane of corresponding viscosity. We find no evidence for solute-solvent complex diffusion. Rather, it is possible that the small solute water may be retarded by relatively longer residence times ͑compared to non-H-bonding solvents͒ as it moves through the liquid.