The dependence of the l3C NMR chemical shifts from the n-nonylammonium bromide carbons on the amphiphile concentration have been used to estimate the aggregation number in the NAB micelles, resulting in 37 f 5 monomers/micelle. Furthermore, the observed downfield shift on micelle formation has been interpreted as caused by an increased proportion of the trans conformation of the alkyl chains in the micelles compared to the monomers.It could be expected that 13C NMR with its high resolution and large chemical shift range should be well suited for the study of the association of amphiphilic compounds, for example, ionic surface-active agents. Indeed it has been demonstrated in recent years that on micelle formation there are considerable changes in both shielding and relaxation and that it is possible to characterize the effects for a large number of carbons in an alkyl ~h a i n . l -~ In the present communication we wish to demonstrate that two types of significant information on the amphiphile association process may be obtained from I 3 C chemical shift investigations. First, it is possible by detailed studies of the concentration dependence of the chemical shift to deduce quantitative information on the amphiphile aggregation number and, secondly, the shift change on passage from the intermicellar solution to a micelle gives qualitative information on any accompanying comformational change of the alkyl chain.Assuming an idealized situation where the amphiphile may occur either as a monomer or in a single type of micelle with an aggregation number n the chemical shift can be written C m ct 6obsd = -6mHere C , and C t denote the concentration of micellized amphiphile and the total amphiphile concentration, respectively. 6&d is the observed chemical shift and 6 , the shift of micellized amphiphile, both taken relative to the chemical shift of the monomer. The monomer shift is obtained by extrapolation to zero amphiphile concentration. The concentrations of monomer and micelle in the equilibrium nX X, with K = [Xn]/[XIn may be expressed as [XI = ct -6obsd 6m and 6obsd 6 , n[X,] = CtThe expression for K may then be rewritten as In (Ctdobsd) = n In [Ct(6, -6,,bsd)] + In K + In n (1) Consequently, a plot of In (Ct6obsd) vs. In [Ct(6m -6obsd)l yields the aggregation number and the equilibrium constant if the simple model holds. 6 , may be estimated from plots of 6obsd -(n -1) In 6 , vs. the inverse amphiphile concentration.l Analogous procedures have bees used previously for lH and l9F chemical shifts.6,7To illustrate these principles we present variable concentration 13C chemical shift data for aqueous solutions of a cationic surfactant, n-nonylammonium bromide (NAB). Chemical shifts of the three methylenes in the center of the alkyl chain are presented in Figure 1. The critical micelle concentration obtained from the plots of Figure 1 (cf. ref 1) is 0.11 mol kg-l and thus in close agreement with results obtained by other methods.8,g A previously found* change in the NAB micellar structure at high concentrations is ve...