The Salinity-Phase-Inversion method (SPI-slope): A straightforward experimental approach to assess the hydrophilic-lipophilic-ratio and the salt-sensitivity of surfactants

HYPOTHESIS

The salinity at which the dynamic phase inversion of the reference system C₁₀E₄/n-Octane/Water occurs in the presence of increasing amounts of a test surfactant S₂ provides quantitative information on the hydrophilic/lipophilic ratio and on the sensitivity to NaCl_aq of S₂.

EXPERIENCES

The Salinities causing the Phase Inversion (SPI) of the reference system mixed with 12 ionic and 10 nonionic well-defined surfactants are determined in order to quantify the contributions of the nature of the polar head and of the alkyl chain length.

FINDINGS

The SPI varies linearly upon the addition of S₂. The slope of the straight variation with the molar fraction of S₂ is called the "SPI-slope". It quantifies the hydrophilic/lipophilic ratio of S₂ in saline environment and its salt-sensitivity with respect to the reference surfactant C₁₀E₄. The SPI-slopes of C₁₂ surfactants bearing different polar heads are found to decrease in the following order: C₁₂NMe₃Br > C₁₂E₈ > C₁₂E₇ ≥C₁₂SO₃Na ≈ C₁₂COONa ≥ C₁₂SO₄Na > C₁₂E₆ > C₁₂E₅ > C₁₂E₃. This classification is different from that obtained when the phase inversion is caused by a change in temperature (PIT-slope method) because the addition of NaCl in significant amounts (3 to 10 wt%) partially screens the ionic heads and diminishes their apparent hydrophilicities. A simple model, valid for all types of nonionic surfactants, is developed on the basis of the HLD_N equation (Normalized Hydrophilic-Lipophilic Deviation) to express the SPI-slope as a function of the hydrophilic/lipophilic ratio (PACN₂) and the salinity coefficient (δ₂) of S₂. All studied surfactants are positioned on a 2D map according to the values of their SPI-slope and their PIT-slope to graphically highlight their hydrophilic/lipophilic ratio and their salt-sensitivity. Finally, a linear model connecting the PIT-slope and the SPI-slope is derived for nonionics, emphasizing that the thermal partitioning of C₁₀E₄ towards n-octane is much greater in the PIT-slope than in the SPI-slope experiments.