New sensitive micro-measurements of dynamic surface tension and diffusion coefficients: Validated and tested for the adsorption of 1-Octanol at a microscopic air-water interface and its dissolution into water

Abstract

Currently available dynamic surface tension (DST) measurement methods, such as Wilhelmy plate, droplet- or bubble-based methods, still have various experimental limitations such as the large size of the interface, convection in the solution, or a certain "dead time" at initial measurement. These limitations create inconsistencies for the kinetic analysis of surfactant adsorption/desorption, especially significant for ionic surfactants. Here, the "micropipette interfacial area-expansion method" was introduced and validated as a new DST measurement having a high enough sensitivity to detect diffusion controlled molecular adsorption at the air-water interfaces. To validate the new technique, the diffusion coefficient of 1-Octanol in water was investigated with existing models: the Ward Tordai model for the long time adsorption regime (1-100s), and the Langmuir and Frumkin adsorption isotherm models for surface excess concentration. We found that the measured diffusion coefficient of 1-Octanol, 7.2±0.8×10cm/s, showed excellent agreement with the result from an alternative method, "single microdroplet catching method", to measure the diffusion coefficient from diffusion-controlled microdroplet dissolution, 7.3±0.1×10cm/s. These new techniques for determining adsorption and diffusion coefficients can apply for a range of surface active molecules, especially the less-characterized ionic surfactants, and biological compounds such as lipids, peptides, and proteins.

From Single Microparticles to Microfluidic Emulsification: Fundamental Properties (Solubility, Density, Phase Separation) from Micropipette Manipulation of Solvent, Drug and Polymer Microspheres

“…First used in a non-mechanical way in 1998 to simply position micro-hydrogel beads with and without a loaded-drug (doxorubicin) in a controlled flow field of different pH [23,24], the micropipette technique was then developed by Needham et al [1][2][3][4][25][26][27][28][29][30][31][32][33][34][35][36][37], Tony Yeung et al [38], and others, in a series of papers on adsorption at interfaces and two-phase oil-aqueous systems. It has now become a highly versatile experimental setup that allows a wide variety of studies at microscopic interfaces and with single-and pairs-of microparticles.…”

“…Pressure control was achieved by a simple syringe and monitored by using a pressure transducer (Valydine Engineering Corp., Northridge, CA, USA). Real-time images were acquired using a CCD camera (DAGE-MIT, Michigan City, IN, USA), all monitored image was recorded in real time on a computer by using a home-built LabVIEW program, and analyzed with ImageJ software provided by National Institutes of Health (NIH, Bethesda, MD, USA) [1].…”

“…As shown in Figure 3a, the two pipettes, the "delivery" and the "catching" pipette [1], were held in the 3D stage micrometers and aligned inside the chamber. In order to make diffusion-controlled dissolution of the microdroplets, the chamber was then sealed with hexadecane to avoid water convection due to water evaporation at the chamber edges.…”

“…As shown by the video micrographs in Figure 3b, a single microdroplet (~40 µm diameter) was then injected by the delivery pipette and transferred to the catching pipette and held in the center of the chamber, as recently described [1]. The droplet formation and dissolution process were recorded by using a CCD camera.…”

“…In this respect, we introduce here our micropipette manipulation technique [1][2][3][4], capable of making fundamental single particle measurements and analyses, and how this information is critical to processing parameters such as in microfluidic processing, and also homogenization. Thus, in this paper, where we start to combine the two techniques, we simply wanted to introduce and show that the micropipette manipulation technique can guide certain aspects of scale-up study associated with parameters especially the use of microfluidic devices.…”