Elucidating the effect of additives on the alkyl chain packing of a double tail cationic surfactant

Abstract

Hypothesis: Some low molecular weight additives can strongly influence the phase behavior of aqueous surfactant systems, and this offers an important handle to control the properties of surfactant solutions and thus to optimize various formulations concerning stability and performance. Experiments: The surfactant dioctadecyldimethylammonium chloride (DODAC) self-assembles into two lamellar phases in water, the gel phase (Lβ) and the liquid crystalline phase (Lα). Here, we present approaches to tune the gel-to-liquid crystalline transition temperature (Tm) with the use of additives. The effects of urea, sodium butyrate and butyric acid on the packing behavior of DODAC were determined. The surfactant phases were characterized using polarized optical microscopy (POM), differential scanning calorimetry (DSC), and small/wide angle X-ray scattering (SWAXS). Findings: Added urea, sodium butyrate and butyric acid yield a single and stable lamellar phase. Whereas urea and sodium butyrate have only minor effects on Tm, butyric acid gives a large decrease, thus stabilizing the Lα phase with respect to the Lβ phase. From the bilayer thickness of the gel phase an interdigitated or tilted packing of the surfactant molecules is indicated. Addition of sodium butyrate gives a strongly interdigitated gel structure and a transition from lamellar liquid crystal to an isotropic L3 phase.