FOAMING OF ANIONIC SURFACTANT SOLUTIONS IN THE PRESENCE OF CALCIUM IONS AND TRIGLYCERIDE-BASED ANTIFOAMS

Abstract

Sodium linear alkylbenzene sulphonate (NaLAS) is the usual surfactant present in high foam laundry detergents. The foam behaviour of NaLAS is significantly dependent upon the foam generation methodology, water hardness and the antifoam action of deterged sebum soils. Here a study of the foam behaviour of NaLAS (and C12 4-phenyl SO3Na) solutions at different Ca2+ concentrations and pHs in the absence and presence of antifoam is presented. Two foam generation methodologies were used - tumbling tube rotation and cylinder shaking. It has been found that these two methodologies correlate well with a coefficient of greater than or equal to 0.95 when comparing foamabilities. The correlation coefficient however declines to ~0.82 when comparing foam stabilities. The reason of this deterioration has been attributed to the differences of antifoam effect in foam films after foam generation due to differences in bubble size distribution formed by these two methodologies.In the absence of antifoam, the foam behaviour is independent of pH and is dominated by the formation of Ca(LAS)2 (or Ca(C12 4-phenyl SO3)2 lamellar phase liquid crystals. Dynamic surface tension measurements confirm that low foamability after the micellar-precipitate boundary of the Ca2+-LAS- (or Ca2+- C12 4-phenyl SO3-) precipitation phase diagram is due to low rates of transport of surfactant to the rapidly expanded air-water surfaces.Mixtures of triolein/stearic acid and triolein/tristearin are used as models for sebum soil antifoam, as they show similar antifoam effects regardless of pH and calcium concentration. In these two systems, crystalline particles are always present provided, in the case of triolein/stearic acid, formation of soaps is suppressed at low pH. Both stearic acid and tristearin particles adopt an oil-water contact angle θOW > 90o measured through the aqueous phase. They invert the O/W emulsion behaviour shown by triolein alone to W/O by rupturing the oil-water-oil emulsion films. They will also rupture the air-water-oil pseudoemulsion films provided the conditions of θAW > 2.6o for stearic acid and θAW > 0o for tristearin are satisfied. This behaviour of particles will facilitate the emergence of triolein droplets into air-water surfaces. Foam film rupture however only occurs under dynamic conditions, where bridging coefficients for triolein are expected to be positive. However under the near-equilibrium conditions prevailing during foam stability, bridging coefficients for triolein are negative. Little or no antifoam activity is therefore observed under those conditions with these triolein-based mixtures.Oil/particle mixed antifoams probably deactivate through a splitting and coalescence process. Triolein/stearic acid antifoam deactivates more rapidly than sebum soil and triolein/tristearin. This is mainly caused by formation of large inactive agglomerates which occurs both after antifoam dispersion and after continuous foam generation.

Date of Award	1 Aug 2011
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Paul Grassia (Supervisor)