Monomer Concentrations in Simple Micellar Systems from NMR Relaxation Times and C-13 Chemical Shifts Michael J. Doyle, Maria K. Mullally, Trudy A. Taylor, Michael D. Wilson, and D. Gerrard Marangoni Department of Chemistry, St. Francis Xavier University, Antigonish, Nova Scotia, Canada The monomer concentrations for some simple micelle forming surfactants, octylammonium bromide (OTAB), sodium decyl sulphate, SDecS, and dodecyltrimethylammonium bromide (DTAB) were determined from the NMR-paramagnetic relaxation enhancement experiment (the NMR-PRE) on the H-1 spin lattice relaxation times for the surfactant protons as a function of the surfactant concentration above and below the critical micelle concentration (cmc region). The NMR relaxation times, determined from the inversion recovery method, decrease steadily as the concentration of the surfactant is increased above the cmc value. A second, simple method is proposed whereby the p-value of the surfactant in the micelle is deter- mined from the two-site model via extrapolation of the observed relaxation rate data to high concentrations. The p-values and the calculated monomer concentrations are compared to each other and to those obtained using the mass-action and phase separation models. In most cases, the monomer concentrations appear to remain constant at concentrations up to 2–3 times the cmc value, and then begin to decrease thereafter. These results are compared to the trends in monomer concentrations obtained from other NMR experiments in other surfactant systems. Keywords Micelles, surfactants, NMR spectroscopy, relaxation times chemical shifts, monomer concentrations INTRODUCTION The study of surfactants is vital to a wide variety of indus- trial, manufacturing and recovery processes, as well as an assortment of consumer products. Detergency is perhaps the best known application of surfactants; [1] however, surfactants find applications in paints and coatings, ink formulation, oil- well drilling, and secondary and tertiary oil recovery, due to their ability to modify the energetics of the surface or inter- face. [2] A common misconception in surfactant technology, specifically in their usefulness as detergents, is that the for- mation of micelles is one of the most important aspects of these systems; however, the ability of a given surfactant to reduce surface tension is limited by the action of micelle formation. It is clear then that the determination of monomer concentration in a micellar solution is an important parameter in understanding the efficacy of a surfactant to act as a detergent. There have been very few determinations of monomer con- centrations in micellar systems. We define the p-value of the surfactant as the concentration of the surfactant in the micelles divided by the total surfactant concentration. p ¼ c surf ;mic c surf ;t ½1 where c surf,mic is the concentration of the surfactant in micellar form, and c surf,t is the total surfactant concentration. The amount of surfactant monomer in equilibrium with micelles or bound to polymers and membranes has been determined via a number of techniques. However, the thermodynamic techniques (e.g., emf measurements [3–11] ) require a deconvolution of the mean activity into separate contributions for the monomer and micelles, [12] and a method for calculating the contribution of the micelle charge to the ionic strength of the solutions. Additionally, emf measurements require the construction of a surfactant ion specific electrode; this has only been successfully accomplished for a limited number of surfactants. Present address for M. J. D. is Industrial Products Division, 3M Canada, Bedford NS, B4B 1G7, Canada. The financial support of NSERC (research grant, D. G. M) and the StFX University Council for Research is greatly appreciated. M. J. D. acknowledges the grant of a StFX University Council for Research Award. Received 31 January 2007; Accepted 12 February 2007. Address correspondence to D. Gerrard Marangoni, Department of Chemistry, St. Francis Xavier University, P. O. Box 5000, Antigonish, Nova Scotia B2G 2W5, Canada. E-mail: gmarango@stfx.ca or g_marangoni@colloid.stfx.ca Journal of Dispersion Science and Technology, 29:366–374, 2008 Copyright # Taylor & Francis Group, LLC ISSN: 0193-2691 print /1532-2351 online DOI: 10.1080/01932690701716119 366