Surfactant Adsorption to Gypsum Surfaces and the Effects on Solubility in Aqueous Solutions

Vibrational sum frequency generation (VSFG) spectroscopy, conductometric titration measurements, and EDX elemental mapping were used to examine surfactant adsorption to the gypsum (010) surface and assess the effects of surfactant adsorption on gypsum solubility in aqueous solutions. Sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium chloride (DTAC) were used as anionic and cationic surfactants, respectively. Gypsum/SDS interactions result in an ordered precipitate layer on the gypsum surface after water evaporation; gypsum/DTAC interaction did not show a similar effect, despite exposure of gypsum to equivalent amounts of surfactant. VSFG spectra showed that SDS molecules adsorb with their chains parallel to the gypsum surface; spectra from gypsum surfaces treated with DTAC, however, showed no measurable response, implying that these surfactants form disorganized aggregates with no polar ordering. Vibrational data were supported by independent EDX measurements that show a uniform distribution of SDS across the gypsum surface. In contrast, element-specific EDX images showed that DTAC clustered in tightly localized patches that left most of the gypsum surface exposed. The uniform adsorption of SDS on the gypsum surface suppresses long-term dissolution up to 40% when compared to samples exposed to DTAC. Gypsum samples in DTAC-containing solutions lose approximately the same amount of material to dissolution as samples immersed in pure water.