Surface activity of sodium salts ofalpha‐sulfo fatty esters. The oil/water interface

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Abstract
Adsorption studies at the heptane/water and the benzene/water interfaces have been conducted for four α‐sulfo fatty esters: sodium hexyl α‐sulfopelargonate, sodium heptyl α‐sulfopelar‐gonate, sodium methyl α‐sulfomyristate, and sodium methyl α‐sulfopalmitate. The results were compared with those at the air/water (A/W) interface (3). Surfactant solutions with constant counter‐ion concentrations of 0.01 and 0.04 M Na+ were used. Interfacial tensions were measured by using a drop‐volume method and were found to be in the range 1 to 25 dynes/cm.The critical micelle concentration (CMC) values of, these esters were in the range 8.1 to 0.14 mM at the A/W interface (3); they were decreased by about 10% in the presence of heptane (7.2 to 0.11 mM) and by 30–40% in the presence of benzene (5.5 to 0.08 mM). This decrease in CMC was attributed to the water solubility of the hydrocarbon; the greater the solubility, the larger was the decrease in CMC.The co‐areas at the A/W interface were in the range 44 to 59 Å2/ for all the esters (3). At the heptane/water interface they were in the range 49 to 66 Å2/ and at the benzene/water interface in the range 59 to 75 Å2/, i.e., an increase of about 10% was obtained in the presence of heptane and about 30–40% in the presence of benzene. The surface films at the O/W interface were more expanded than at the A/W interface, as expected, owing to the lack of lateral cohesion between the hydrocarbon chains at the O/W interface.