1370
Langmuir 1994,10, 1370-1376
Interphase Transfer Rates in Emulsions Studied by NMR Spectroscopy Michael J. Hey* and Fakhreia Al-Sagheer Department of Chemistry, University of Nottingham, Nottingham NG7 2RD, U.K. Received December 7,1992. In Final Form: April 12,1993@
NMR line shape analysis has been used to monitor the rate of solute transfer across interfacial layers around emulsion droplets. For this purpose, emulsionsof o-xylene in water stabilized by nonionic emulsifiers were prepared in which 1,4-dioxane was partitioned between the two phases. Because of the different magnetic susceptibilitiesof the xylene and water, two chemically shifted, exchange-broadened signals were observed from the dioxane. By simulating the spectra, the average residence time of a dioxane molecule in an oil droplet was obtained. The average time for a dioxane molecule to diffuse from within a droplet to the interface can be estimated from the droplet size distribution. Comparison of the diffusion and residence times then gives the probability that a dioxane molecule will transfer into the aqueous phase on arrival at the droplet interface. Addition of alkali-metal halides to the aqueous phase reduced the probability of transfer. For sodium salts, the magnitude of the reduction is in the order F - > C1- > I-. LiC1, NaCl, and KC1 produced similar effects. Introduction While it is recognized that adsorbed films at oil/water interfaces may play an important role in determining the kinetics of solubilization processes,l direct measurements of the properties of stabilized interfaces around small droplets are rarely made. Instead, results obtained from measurements made on macroscopic,planar interfacesare assumed to be equally valid for the corresponding layer around emulsion droplets. In order to monitor the permeability of films around emulsion droplets in situ, we have developed a method based on dynamic NMR spectroscopy. It is well known that the exchange of magnetic nuclei between two chemicallyshifted sites leads to a broadening and coalescence of the lines when the exchange rate is of the same order of magnitude as the difference in resonance frequencies a t the two sites.2 This phenomenon can also be produced in emulsions by exploiting the different bulk magnetic susceptibilities of the oil and aqueous phases. A solute which is partitioned between the two phases will give rise to two NMR signals corresponding to the two different environments. If a dynamic equilibrium exists in which solute molecules are exchanging between the two phases at an appropriate frequency, analysis of the band shape allowsthe exchange frequency to be calculated. We show below that when 1,ddioxane is dissolved in o-xylene-in-water emulsions stabilized by either Span 40 or Tween 40, these conditions are satisfied. The choice of 1,Gdioxane as a probe was dictated by the requirements that the solute should be soluble in both oil and water phases and that its proton magnetic resonance spectrum should consist of a single line. After the exchange rates are normalized with respect to the droplet size distribution, a parameter is obtained which is characteristic of the barrier properties of the interface with respect to the passage of 1,ddioxane. Data are also presented which show the effect of adding electrolytes to the aqueous phase. Materials and Methods Materials. Deionized water which had been triply distilled in an all-glass apparatus and deuterium oxide (99.9 atom %, Abstract published in Advance ACS Abstracts, May 15, 1994. (1)Ward, A. J. I. Proc. R. Ir. Acad. 1989,89B,375. (2) Harris, R. K. Nuclear Magnetic Resonance Spectroscopy, A Physicochemical View; Pitman: London, 1983.
0743-7463/94/2410-1370$04.50/0
Figure 1. Photomicrographs of emulsion droplets stabilized by Tween 40. Concentrationof Tween 40 in the aqueous phase: (a) 0.45 % , (b) 1.5%, (c) 5 % . Adrich) were used in a 7030 (v/v) ratio in the aqueous phase. span 40 (sorbitan monopalmitate,HLB = 6'7) and Tween 40 (an ethoxylated derivative of Span 40 with 18-22 ethylene oxide groups per molecule, HLB = 15.6)Were suppliedby IC1Specidties and used as received. 1,CDioxane(Fluka)and o-xylene(Aldrich) 0 1994 American Chemical Society
Interphase Transfer Rates in Emulsions
Langmuir, Vol. 10, No. 5, 1994 1371
HZ0
B f 4
2 I . . . . . .
0.1
20
10
1 p:.rtizle size / pm
TMS
7.0
1
6.0
5.0
4 .O
3.0
2.0
I .o
0.0
Ciicmical shin I ppm
Figure 2. 1H NMR spectrum (100 MHz) of a 44 vol % emulsion of o-xylene in water prepared with 0.45% Tween 40 and containing 1,Cdioxane. Inset: droplet size distribution obtained from a Malvern Instruments MasterSizer. were both spectroscopic grade. The inorganic salts NaF, NaC1, NaI, LiC1, and KC1 were of analytical grade where available. Agarose(typeV,Sigma)hadagelpointof41* 1.5OC (1.5% gel). Preparation of Emulsions. Both the xylene phase and the aqueousphase, which consisted of a 3070 (v/v) D20/H20mixture to provide a deuterium lock signal for the NMR spectrometer, contained 1,4-dioxaneat an initial concentration of 2.4 mol dmJ. Emulsions were stabilized by adding either Tween 40 to the aqueous phase or Span 40 to the xylene phase at concentrations in the range0.5-5 wt % . In order to prevent creaming of emulsion samples during spinning in NMR tubes, 0.6-1 wt % agarosewas added to the aqueous phase. A small amount (
