Parabolic Viscosity Behavior of NaCl-Thickened Surfactant Systems upon Temperature Change

Abstract

The viscosity of household care products plays an important role in pleasant delivery using consumer experience at home. A novel solution to mitigate the sharp rising of viscosities at low temperatures of detergents was proposed. By designing the formulation of the surfactant blend, formulators can achieve acceptable viscosity profiles in the temperature range encountered in daily life. The verification and modulation of formulas bearing parabolic viscosity-temperature behavior were systematically studied, including in single, binary, and ternary systems, based on the modulation of sodium ethoxylated alkyl sulfate (AES) by other anions, zwitterions, and nonions. The R ratio theory was used to have a better understanding of the molecular assembly of surfactants behind the parabolic behavior exhibited in rheology analyses. One of the key findings is that the parabolic viscosity-temperature phenomenon could be easily observed in the highly hydrated ethoxylated anionic systems like AES-based systems. For those anions lacking ethoxylation, especially sodium linear alkylbenzene sulfonate (LAS), the monotonic variation of hydration affinity with temperature led to the disappearance of parabola in the observed temperature window (>0 °C). Moreover, salinity played an important role in the hydration affinity of the polar group and the interaction between the hydrophilic headgroups. A balanced salinity should be optimized to modulate the hydration affinity in a desired range so that the parabola could be easily tuned within the target temperature region. These findings provide opportunities for the formulators in the household care industry to design products with better pourability through carefully selecting a combination of surfactants and fine-tuning their ratios to improve consumer use experience, especially in winter.

Figure 1

Viscosity changes of five representative commercial dishwashing detergents at different temperatures.

Scheme 1. Representative Chemical Structures of Sodium Ethoxylated Alkyl Sulfate (AES, 2EO), Lauroylamidopropyl Betaine (LAB), and Sodium Linear Alkylbenzene Sulfonate (LAS)

Figure 2

Temperature and rheological behaviors of AES (15% total active surfactant, pH 8.0, 3% NaCl). (a) Viscosity–temperature relationship. (b) Shear thinning effect at 3, 10, and 20 °C. (c–e) G′ and G″ moduli at different temperatures. (f) FF-TEM image at 10 °C showing entangled wormlike micelles.

Figure 3

Temperature and rheological behaviors of 80% AES + 20% LAB (15% total active surfactant, pH 8.0, 1% NaCl). (a) Viscosity–temperature relationship. (b) Shear thinning effect at 3, 12, and 25 °C. (c–e) G′ and G″ moduli at different temperatures. (f) Viscosity–temperature relationship of 80% AES + 20% AEO₃/AEO₇ (15% total active surfactant, 1%/2.5% NaCl).

Figure 4

Viscosity–temperature relationship of 20% LAB + 70% AES + 10% replace surfactant (15% total active surfactant, pH 8.0, 1% NaCl). (a–d) Curves in different viscosity regions.

Figure 5

(a,b) Viscosity–temperature relationship of 80% AES + 20% LAB (15% total active surfactant, pH 8.0, and 1% NaCl) with AES partially replaced by LAS. (c) Possible explanation of monotonical viscosity behavior. (d) Viscosities of 60% AES + 40% LAS (15% total active surfactant, pH 8.0) along with temperature change. Inset shows the viscosity–NaCl relationship at 25 °C.