The effect of hydrogen bonding on the diffusion of water in n-alkanes and n-alcohols measured with a novel single microdroplet method

Abstract

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.

Prediction of binary diffusion coefficients in non-ideal mixtures from NMR data: Hexane–nitrobenzene near its consolute point

“…In fact the NMR-derived molecular diffusion coefficients show no special variation close to the consolute point. This is in agreement with Su et al (2010) who, for a range of water-alcohol (C 4 -C 16 ) mixtures, found no evidence for solute-solvent cluster diffusion.…”

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

“…For microfluidics, this time has an inverse relation to the velocity of the fluid in the microchannel and is directly proportional to the length of that channel. Thus, calculations of the time for dissolution are readily obtained from the EP model for the dissolution of microparticles as done previously [2,4,27,30]. The time, tD, for a spherical microparticle of Ibp to dissolve in infinite medium is given by the following equation,…”

“…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. Among other applications, the technique has been established for studying solvent dissolution, measuring fundamental properties such as diffusion coefficients and solubilities of the dissolving liquids [4,27,30], and for evaluating the phase separation, precipitation of droplets containing different solutes upon solvent loss, such as the micro-glassification of proteins by fast removal of water into water-imbibing solvents like octanol [3,31], and drug-containing polymer microspheres by removal of the organic solvent into water [28,29]. It is this last application that has motivated and guided the current studies with poly(lactic-co-glycolic acid) (PLGA) microparticles containing Ibuprofen [34].…”

“…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.…”

“…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.…”