rXXXX American Chemical Society A dx.doi.org/10.1021/la202876g | Langmuir XXXX, XXX, 000–000 ARTICLE pubs.acs.org/Langmuir SolGel Transition in Dispersions of Layered Double-Hydroxide Nanosheets Vikrant V. Naik and Sukumaran Vasudevan* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India b S Supporting Information ’ INTRODUCTION Gelation occurs in a wide range of dispersed systems where particles interact via attractive forces. 1,2 Despite its ubiquity and significance, the molecular mechanisms involved in the forma- tion of a gel are far from understood. One of the most extensively investigated processes is the gelation in dispersions of clay particles in both aqueous and nonaqueous media. The process is of theoretical interest as well as industrial importance because clay gels have wide applications in drilling fluids, paints, ceramic additives, and cosmetic and pharmaceutical formulations. 39 Clay suspensions are composed of anisotropic disklike charged particles that interact via an attractive screened Coulomb poten- tial and undergo a transition from a fluidlike sol to a solidlike gel with increasing volume fraction of the clay in the dispersion. 10 Various gelation mechanisms and gel microstructures have been proposed for clay dispersions. 1115 These include the 3D “house- of-cards” aggregated structure originating from electrostatic at- traction between oppositely charged double layers at the edges and faces of the particles and tactoidal structures formed by the regular stacking of the clay particles. Gel formation in the latter is a consequence of electrical double-layer repulsion between clay particles, the so-called Wigner glass. 15 Organoclays, a closely related system, are formed by the intercalation of long-chain surfactants in the clay. 16 These are widely used to control the rheology of hydrocarbon fluids and as flow modifiers in a range of applications that include oil-field drilling fluids, paints, and lubricating greases. 59 A gel state can be induced in the organoclay dispersion by changing the volume fraction of the solvent and is also facilitated by additives such as water molecules. 4,17 The underlying mechanism of the gelling process in the organoclays, however, remains obscure. Recently, we outlined a simple strategy to delaminate layered double-hydroxide (LDH) solids to their ultimate constituent, intact single layers of nanometer thickness and micrometer size in a nonpolar solvent such as toluene. 18 The layered double hydro- xides are anionic clays derived from the Brucite (Mg(OH) 2 ) structure by the isomorphous substitution of M 2+ with M 3+ with charge neutrality being preserved by interlamellar exchangeable anions. The delamination procedure involved the ion-exchange intercalation of an ionic surfactant to form a hydrophobic anchored surfactant bilayer in the interlamellar space of the solid. Delamination was effected by simply stirring the surfactant- intercalated layered solid in the solvent. The method was rapid but at the same time gentle enough to produce exfoliated nanosheets with Received: July 25, 2011 Revised: September 17, 2011 ABSTRACT: Surfactant-intercalated layered double-hydroxide solid MgAl LDHdodecyl sulfate (DDS) undergoes rapid and facile delamination to its ultimate constituent, single sheets of nanometer thickness and micrometer size, in a nonpolar solvent such as toluene to form stable dispersions. The delami- nated nanosheets are electrically neutral because the surfactant chains remain tethered to the inorganic layer even on exfolia- tion. With increasing volume fraction of the solid, the dispersion transforms from a free-flowing sol to a solidlike gel. Here we have investigated the solgel transition in dispersions of the hydrophobically modified MgAl LDHDDS in toluene by rheology, SAXS, and 1 H NMR measurements. The rheo-SAXS measurements show that the sharp rise in the viscosity of the dispersion during gel formation is a consequence of a tactoidal microstructure formed by the stacking of the nanosheets with an intersheet separation of 3.92 nm. The origin and nature of the attractive forces that lead to the formation of the tactoidal structure were obtained from 1D and 2D 1 H NMR measurements that provided direct evidence of the association of the toluene solvent molecules with the terminal methyl of the tethered DDS surfactant chains. Gel formation is a consequence of the attractive dispersive interactions of toluene molecules with the tails of DDS chains anchored to opposing MgAl LDH sheets. The toluene solvent molecules function as molecular “glue” holding the nanosheets within the tactoidal microstructure together. Our study shows how rheology, SAXS, and NMR measurements complement each other to provide a molecular-level description of the solgel transition in dispersions of a hydrophobically modified layered double hydroxide.