In this work, we present the effects of salts on sodium dodecyl benzene sulfonate micellization and on the interfacial performance of a sodium dodecyl benzene sulfonate-heptane-brine system at optimum formulation, i.e., hydrophilic-lipophilic deviation (HLD) = 0. In order to do that, interfacial tension and dilational interfacial rheology properties of surfactant-heptane-water systems at optimum formulation are measured using an interfacial spinning drop tensiometer with an oscillating velocity, which can accurately measure interfacial rheology properties at both low and ultralow interfacial tensions. The brines used contain one of the following salts: MgCl 2 , CaCl 2 , NaCl, NH 4 Cl, NaNO 3 , CH 3 COONa, or Na 2 SO 4 . We performed a one-dimensional salinity scan with each of these salts to achieve an optimum formulation. In relation to the Hofmeister series, we found that, at optimum formulation, systems with chaotropic ions (NH 4 + , NO 3 − ) present interfaces with ultralow interfacial tensions, very low dilational modulus, and a low phase angle, whereas kosmotropic ions (Mg 2+ , Ca 2+ , SO 4 −2 ) generate high interfacial tension and high rigidity monolayers. Intermediate ions in the Hofmeister series (Na + , CH 3 COO − , Cl − ) present interfaces with intermediate properties. Furthermore, according to the Hofmeister series, interfaces can be respectively ordered from higher to lower rigidity for surfactant counterions Mg 2+ > Ca 2+ > Na + > NH 4 + and coions SO 4 2− > CH 3 COO -> Cl − > NO 3 −, which correspond to a saltingout (highest rigidity) and salting-in (lowest rigidity) effect. We observed that counterions have a more significant effect on surfactant-oil-water system properties than those that act as coions.Keywords Hofmeister series Á Interfacial rheology Á Microemulsion Á Micellization Á Salt effects Á Ultralow interfacial tension J Surfact Deterg (2020) 23: 603-615.

There is no consensus regarding the role of asphaltenes at the oil–water interface. In trying to solve this uncertainty, this work shows for the first time the n-C7 asphaltenes characterization as surfactants and polar oil based on the normalized hydrophilic–lipophilic deviation model (HLDN). Interfacial tension (IFT) and interfacial rheological properties of asphaltene/solvent/brine systems at equilibrium were determined. Additionally, stability and electrical conductivity measurements of emulsions obtained from those systems were performed. The results seem to indicate that when the asphaltenes concentration C A in the oil is equal to the critical nanoaggregate concentration (CNAC), the optimum formulation (HLDN = 0) of the system is reached. When C A < CNAC (HLDN < 0), the asphaltenes molecules behave similarly to a slightly hydrophilic nonionic surfactants, while when C A ≫ CNAC (HLDN > 0), asphaltenic clusters have a dual behavior as interfacially active lipophilic aggregates and polar oil.

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The field of action of surfactants encompasses the formulation of dispersed systems in the chemical and petroleum industry and, particularly, in applications such as enhanced oil recovery (EOR) and chemical dewatering (dewatering). In these applications it is necessary to use hydrophilic surfactants that are not sensitive to salinity and water hardness, minimizing their precipitation and improving their performance in terms of interfacial tension decrease. Some of the most widely used surfactants in the industry are linear alkylbenzenesulfonates (LAS-C15-18) due to their low production costs. However, the use of these surfactants in the oil industry has been limited due to problems associated with their precipitation. In this work, we evaluated the action of tetramethyl ammonium chloride (TMACl) as an additive to improve the solubility of the commercial surfactant LAS-C15-18. Solubility tests performed at T = 8, 25 and 40 °C show that the addition of TMACl significantly improves the solubility of the surfactant (Csurf > 1wt%) in moderately-hard-water (Hardness 96.2 ppm Ca, Mg). These results suggest that TMACl acts as a hydrotrope, enhancing the interactions of the LAS-C15-18 surfactant with the aqueous phase. Likewise, the interfacial tension (IFT) curves with respect to theNaCl concentration in the surfactant-heptane-brine system show that the addition of TMACl improves the formulation quality due to the decrease of the interfacial tension in the HLD = 0 condition, from 0.027 mN/m without TMACl to 0.008 mN/m with 1.5 wt% TMACl. In these systems, the addition of TMACl did not result in a change in optimum salinity (S*). Additionally, the IFT-temperature curves of the surfactant/heptane/aqueous phase (NaCl = 1wt%) and surfactant/heptane/moderately-hard-water systems show IFT minima as the TMACl concentration increases in the aqueous phase. Surfactant precipitation was not observed in these systems.

Cationic Surfactant's quantification in aqueous solution is relevant in fundamental and applied research, as well as for quality control in processes in the chemical industry. Colloidal titration is a commonly used method for surface charge determination in colloidal systems such as polyelectrolytes, macromolecules and solid particles, but it can also be applied for the detection of ionic surfactants in aqueous solution. It is a simple technique, easy to implement in a laboratory and its principle is based on the stoichiometric neutralization between two species of opposite charge, potassium polyvinylsulfonate (standard PVSK) and a cationic polyelectrolyte or cationic surfactant. When the stoichiometric ratio is exceeded, the excess polyelectrolyte reacts with a cationic indicator (crystal violet) forming an electrically neutral pair that modifies the solutions' absorbance value which is determined by spectrophotometry at 587 nm. To assess the feasibility of this technique, the colloidal titration method was normalized by titration between two standard polyelectrolytes (PDADMAC and PVSK), also with analytical grade cationic surfactants (DDBAB, CTAB and HDBAC) and finally with some commercial ones (Praepagen WK, Dodigen 1828, Empygen BAC 50). High molecular weight surfactants showed stoichiometric proportionality between their concentration and the amount of tritant added for concentration between 10-100 mN. This proportionality was not observed for low molecular weight surfactants. Finally, it was also observed that for commercial surfactants the presence of 0.01 M sodium chloride does not affect the titration results.

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