Journal of Colloid and Interface Science 299 (2006) 378–387 www.elsevier.com/locate/jcis Incorporation of substituted acrylamides to the lamellar mesophase of Aerosol OT Isabel E. Pacios a,b,∗ , Carmen S. Renamayor a,∗ , Arturo Horta a , Björn Lindman b , Krister Thuresson b a Dep. Fisicoquímica (CTFQ), Facultad de Ciencias, Universidad a Distancia (UNED), 28040 Madrid, Spain b Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, SSE-2211 00 Lund, Sweden Received 20 September 2005; accepted 31 January 2006 Available online 20 March 2006 Abstract The structure and stability of the lamellar liquid crystal formed by the surfactant sodium bis-2ethylhexyl sulfosuccinate (AOT) in water is perturbed by small amounts of the substituted acrylamides N-isopropyl, N,N-diethyl, N-acryloylmorpholine, and N,N-dimethyl methacrylamide, as revealed by small angle X-ray scattering (SAXS), deuterium NMR, and microscopy. These molecules are water soluble and stay mostly in the water layers between lamellae, but a small fraction of them (5–19%) are incorporated into the AOT bilayers, thereby producing dramatic changes. Both, the degree of anisotropy in the water molecules hydrating AOT (quadrupolar splitting in 2 H NMR) and the long period spacing between lamellae (SAXS), decrease with addition of this molecules at low concentrations, which is attributed to the lower average headgroup density at the AOT/water interface when the acrylamide is incorporated. The strength of these perturbations depends on the acrylamide, and goes in parallel with the hydrophobic character of the alkyl side groups in its molecule, which suggests that the acrylamides incorporated to the bilayer enter into contact with the lipophilic tails of the AOT molecule. An interaction with the hydrated heads of AOT is also suggested in the particular case of N-isopropylacrylamide. On increasing the molecule concentration an incipient melting of the lamellar phase towards an isotropic solution takes place, first at the microscopic level, then macroscopic. Near this phase transition, the ordered domains lose the random orientation prevailing at lower acrylamide concentrations, and adopt a preferred orientation, perpendicular to the magnetic field.  2006 Elsevier Inc. All rights reserved. Keywords: Substituted acrylamides; AOT; Lamellar mesophase; Deuterium NMR; SAXS 1. Introduction The surfactant AOT contains two tails in the lipophilic part of its molecule (Scheme 1) that can aggregate forming bilayers. In water, the ionic heads of the molecules are strongly hydrated and constitute the outer face of the bilayer, while the hydro- carbon tails give its inner part (Scheme 1). The AOT/water pair forms different phases, depending on AOT concentration [1]. At low concentrations, it gives an isotropic (micellar) phase, S. At higher concentrations, anisotropic liquid crystal phases appear, such as lamellar L α , cubic Q, and inverse hexagonal H 2 [2]. Of these liquid crystals, the one that appears first, next to the isotropic micellar, is the lamellar L α phase. The AOT concen- * Corresponding authors. E-mail addresses: ipacios@ccia.uned.es (I.E. Pacios), csanchez@ccia.uned.es (C.S. Renamayor). tration of the present study (0.25 volume fraction) is within the monophasic region where the AOT/water pair forms such lamellar liquid crystal as a single phase. Cosurfactants modify the ranges where the liquid crystal phases are stable [3], and the addition of a small amount of a third component can change the phase behaviour in the sense that the lamellar phase is partially destroyed or disappears [4,5]. This macroscopic phase instability should be the consequence of perturbations introduced by the molecules of the third com- ponent in the structure of the lamellar phase. The structure is an ordered one, with a regular array of surfactant bilayers alternat- ing with water layers (Scheme 1). Our objective here is to study the perturbations introduced in the order of such structure (and the ensuing instability of the phase) when a particular class of molecules is added as third component. The experimental techniques used are small an- gle X-ray scattering and deuterium NMR spectroscopy. These 0021-9797/$ – see front matter  2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.01.070