Experimental and theoretical investigation of zwitterionic surfactant adsorption on calcite for enhanced oil recovery.

Zwitterionic surfactants offer unique physicochemical properties for enhanced oil recovery in carbonate reservoirs; however, their adsorption mechanisms on carbonate reservoir rocks remain incompletely understood. This study presents a comprehensive experimental and theoretical investigation into the adsorption behavior of the zwitterionic surfactant 3-(N, N-Dimethylmyristylammonio) propane sulfonate (ZW4) on calcite surfaces. The effects of key variables including salinity, temperature, pH, and surfactant concentration were systematically examined. Critical micelle concentration was measured under varying salinity and temperature conditions, and a detailed thermodynamic analysis revealed that ZW4 micellization is a spontaneous, entropy-driven process, with enthalpy-entropy compensation ensuring thermodynamic favorability across a wide temperature range. Temperature influenced the CMC non-linearly: it decreased from 10 °C to 30 °C due to reduced hydrophilicity but increased above 30 °C as hydrophobic interactions disrupted micelle formation. The roles of different electrolytes (MgSO₄ and Na₂SO₄) were compared, showing that Mg²⁺ ions significantly reduced the CMC more effectively than Na⁺ ions due to stronger electrostatic double-layer compression and higher ion charge density. At 0.01 M MgSO₄, the CMC decreased from 2000 ppm to 1300 ppm, while Na₂SO₄ at the same concentration reduced it to 1700 ppm. Surface charge modifications of calcite by ZW4 were quantified using zeta potential measurements, which identified a point of zero charge near pH 6.7 and demonstrated increased surface negativity with rising surfactant concentration up to 2500 ppm. This observation is consistent with the pseudo-phase separation model. Higher pH levels inhibited adsorption due to electrostatic repulsion between ZW4's sulfonate group and the negatively charged calcite surface. Increasing salinity enhanced adsorption, transitioning from a "V-shaped" orientation (low packing density) to a more vertical "I-shaped" configuration (high packing density), with MgSO₄ demonstrating a greater effect than Na₂SO₄, attributed to its stronger ability to neutralize surface charge. Adsorption equilibrium data, evaluated using multiple models, identified the Sips model as providing the best fit, highlighting its flexibility in describing complex adsorption phenomena. These findings provide molecular-level insight into zwitterionic surfactant-calcite interactions and underline the importance of thermodynamics, solution chemistry, and mineral surface charge for optimizing EOR surfactant flooding strategies in carbonate reservoirs.