Abstract
HypothesisAt low temperatures stability issues arise in commercial detergent products when surfactant crystallisation occurs, a process which is not currently well-understood. An understanding of the phase transition can be obtained using a simple binary SDS (sodium dodecyl sulfate) + DDAO (N,N-dimethyldodecylamine N-oxide) aqueous system. It expected that the crystallisation temperature of an SDS system can be lowered with addition of DDAO, thus providing a route to improve detergent stability. ExperimentsDetergent systems are typically comprised of anionic surfactants, non-ionic surfactants and water. This study explores the crystallisation of a three component system consisting of sodium dodecyl sulfate (SDS), N,N–dimethyldodecylamine N-oxide (DDAO), and water using wide-angle X-ray scattering (WAXS), differential scanning calorimetry (DSC) and confocal Raman microscopy. FindingsThe presence of DDAO lowered the crystallisation temperature of a 20 wt% SDS system. For all aqueous mixtures of SDS + DDAO at low temperatures, SDS hydrated crystals, SDS.1/2H2O or SDS·H2O, formed. SDS hydrates comprising of layers of SDS separated by water layers. DDAO tended to reside in the vicinity of these SDS crystals. In the absence of DDAO an additional intermediary hydrate structure, SDS.1/8H2O, formed whereas for mixed SDS + DDAO systems no such structure was detected during crystallisation.
Highlights
Home and personal care products are primarily composed of water and surfactants, with the total surfactant concentration ranging from 10 to 50 wt. % [1, 2]
There are unanswered questions surrounding the influence that dimethyldodecylamine Noxide (DDAO) has on sodium dodecyl sulfate (SDS) crystallisation in terms of the crystallisation kinetics, crystal shape and the structure of the crystals, for which this paper aims to address
Differential scanning calorimetry (DSC) thermograms for the SDS + DDAO systems were acquired upon cooling the system and inducing crystallisation
Summary
Home and personal care products are primarily composed of water and surfactants, with the total surfactant concentration ranging from 10 to 50 wt. % [1, 2]. Home and personal care products are primarily composed of water and surfactants, with the total surfactant concentration ranging from 10 to 50 wt. The decrease in Tk of an anionic surfactant system upon the addition of a second anionic or non-ionic surfactant has been demonstrated for binary surfactant solutions comprising of two different sulfates or sulfonates [11]. The addition of the non-ionic surfactant, nonylphenol ethoxylate, to a solution of SDS caused the precipitation boundary to reduce in size [12]. In these mixed surfactant systems micelle formation becomes increasingly favoured, lowering the CMC and reducing the monomer concentration that is able to form crystals [13]
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