J. Phys. Chem. 1981, 85,2587-2589
2587
Determination of Organic Counterion Binding to Micelles through Fourier Transform NMR Self-Diffusion Measurements Peter Stilbs and Bjorn Llndman’ Institute of Physhl Chemlstry, Unlverslty of Uppsals, 8 7 5 1 2 1 Uppsals 1, Sweden, and Physlcal Chemlstry 1, Chemlcal Center, UnlverslW of Lund, S-220 07 Lund, Sweden (Recelved: May 12, 1981; In Flnal Form: July 13, 1981)
A development of the NMR pulsed field gradient spin-echomethod for determining self-diffusion coefficients is shown to conveniently give a complete picture of surfactant self-association. Experimental self-diffusion coefficientsof decylammonium dichloroacetate in aqueous solution are deduced to show that the free surfactant concentration decreases above the critical micelle concentration while the free counterion concentration increases. The ratio of counterions to surfactant in the micelles is constant over a wide concentration range, a so-called counterion condensation behavior which has been theoretically predicted.
Introduction The self-diffusion properties of a complex surfactant system contain important information on phase structure and association beha~iorl-~ but it is not an easy task to obtain the self-diffusion coefficients of the different components making up the system. Previously applied methods have required specific isotopic labeling (radioactive or suitable NMR active isotope) of the components one by one and making one diffusion experiment for each component. The large amount of work involved in synthesizing the labeled compounds and making a large number of different NMR experiments has been a barrier to self-diffusion studies of complex surfactant systems; furthermore, this procedure may create problems of interpetation if nonidentical conditions must be compared. The Fourier transform pulsed field gradient spin-echo NMR technique (FT-PGSE)$S which permits the simultaneous determination of the self-diffusion coefficients of several components, is therefore a very interesting possibility. Indeed, recent studies with ‘H or 13C NMR on microemulsion structure6J and on solubilizatione have demonstrated that this technique is particularly well suited for complex surfactant systems. For a typical ionic surfactant with inorganic counterion, the counterion generally does not contribute to the NMR spectrum used and its self-diffusion is therefore not obtainable (unless additional experiments are done). This can be overcome by choosing a counterion which contains ‘H(or 13C)nuclei. Then it is possible by the FT-PGSE method to monitor simultaneously the diffusion of amphiphile ion, counterion, solubilizate, and water. Since also the interest in the association behavior of surfactants with organic counterions has grown it was decided to start investigations on these systems. The present communication reports FT-PGSE ‘H NMR studies on DzO solutions of decylammonium dichloro(1)Lindman, B.;Wennerstrom, H. Top. Current Chem. 1980,87,1-83. (2) Wennerstrom. H.: Lindman. B. Phvs. Rem 1979.52.1. (3) Lindman, B.; K k e n k a , N.; Kathophii, +.-M.; Brun; B.; Nilsson, P.-G. J. Phys. Chem. 1980,84,2485. (4)Stilbs, P.;Moseley, M. E. Chem. Scr. 1980,15, 176. ( 5 ) Stilbs, P.; Moseley, M. E. Chem. Scr. 1980,15,215. (6)Stilbs, P.;Moseley, M. E.; Lindman, B. J.Magn. Reson. 1980,40, 401. (7)Lindman, B.;Stilbs, P.; Moseley, M. E. J. Colloid Interface Sci. In press. (8) Stilbs, P.; J. Colloid Interface Sci. 1981,80, 608. (9) Hoffmann, H.; Ulbricht, W.; Tagesson, B. 2. Phys. Chem. (Frankfurt an Main) 1978,113,17. (10)Kale, K. M.; Zana, R. J. Colloid Interface Sci. 1977,61, 312. (11)Hoffmann, H.;Niisslein, H.; Ulbricht, W. In “Micellization, Solubilization and Microemulsions”;Mittal, K. L., Ed.; Plenum Press: New York, 1977;Vol. 1, p 263. 0022-3654/81/2085-2587$01.25/0
acetate (Cl&IzlNH3+CHClzC00-)in which small amounts of tetramethylsilane were solubilized to permit the determination of the self-diffusion coefficient of the micelles.
Results and Discussion Self-diffusion coefficients of the decylammonium ion (D+),of the dichloroacetate ion ( D J , and of solubilized were determined as a function of tetramethylsilane (D,) surfactant concentration (in D20)at 40 “C. (The Krafft point is above 25 OC. Water self-diffusion changes only little in the concentration interval covered and was not studied.) The experimental results given in Table I show the expected behavior of all self-diffusion coefficients decreasing with increasing concentration and with D, C D+ < D-. The observed self-diffusion coefficients are populationweighted averages of the diffusion coefficients of the nonassociated species and of the micelles. (For the counterions,alternatives to this simple two-site model have been discussed but recent theoretical and experimental work1J2suggests it to be a good approximation.) For the solubilizate, which has a very low aqueous solubility, the observed self-diffusion coefficient should (except at the lowest concentrations) be very close to that of the micelles (Drn).l3The decrease in D, with increasing concentration should mainly be due to electrostatic micelle-micelle rep u l s i o n ~ .A ~ ~plot of l/Dmvs. the micelle concentration (12)Gunnarsson, G.;Jansson, B.; Wennerstrom, H. J. Phys. Chem. 1980,84,3114. (13)A micelle diffuses typically 1000 times ita own diameter during the time of measurement in the present work (0.2 8 ) . Self-diffusion of a solubilized molecule within the micelle is therefore unimportant, and the (apparent) self-diffusion coefficient of a completely solubilized molecule is therefore equal to the self-diffusion coefficient of the micelles. Tetramethylsilane (MedSi) is a very convenient compound for the purpcae in question, having a single, isolated ‘H NMR signal. The apparent, population-weighted self-diffusion coefficient of a partly solubilized molecule is, however, strongly influenced by even small fractions of unsolubilized molecules (Dap pD, + (1- p ) D h ;typically Dk 20Dd13. As there may be some do&; with regard to the hydrophobicity of Me& some tests were also made with two considerably more hydrophobic solubilizates (1-chlorodecane and 2,2,4-trimethylpentane).The results indicated that there could be 50.5% unsolubilized Mel% in a 0.1 M surfactant solution, corresponding to a systematic error in the apparent D, of
