| Summary: | The micellar properties of the anionic surfactant, sodium dodecyl sulfate (SDS) are modified by the biologically active amino acids. Amino acids (AAs) have experienced a variety of interactions and are proposed to influence SDS micelles due to their nominated hydrophobic interactions. The present study determines the critical micellar concentration (CMC) of SDS in aqueous solutions as well as in amino aqueous solutions. Three amino acids (glutamic acid, histidine, and tryptophan) are considered here. The conductometric measurements were carried out using a wide range of SDS concentrations at different temperatures. Surface tension experiments have also been applied to estimate many surface parameters including surface excess concentration (Γ<sub>max</sub>), surface occupied area per surfactant molecule (<i>A<sub>min</sub></i>), surface tension at CMC (<i>γ<sub>cmc</sub></i>), surface pressure at CMC (Π<sub>cmc</sub>) and Gibbs free energy of adsorption (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>∆</mo><msubsup><mi>G</mi><mrow><mi>a</mi><mi>d</mi><mi>s</mi></mrow><mo>°</mo></msubsup></mrow></semantics></math></inline-formula>), enthalpy <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>∆</mo><msubsup><mi>H</mi><mi>m</mi><mo>°</mo></msubsup></mrow></semantics></math></inline-formula> and the critical packing parameter (<i>CPP</i>). Interestingly, CMC values of SDS in water and in aqueous amino acids estimated by the surface tension method are comparable with the corresponding values obtained by the conductance method. The thermodynamic parameters of SDS micellization were also evaluated in both presence and absence of AAs. The additives of AAs work to reduce the CMC values, as well as the SDS thermodynamic parameters. This reduction is highly dependent on the hydrophobicity of the AA side chain. Negative values of <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>∆</mo><msubsup><mi>G</mi><mi>m</mi><mo>°</mo></msubsup></mrow></semantics></math></inline-formula>, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>∆</mo><msubsup><mi>H</mi><mi>m</mi><mo>°</mo></msubsup></mrow></semantics></math></inline-formula> elucidate that micellization of SDS in the presence of amino acids is thermodynamically spontaneous and exothermic. The outcomes here might be utilized for pharmaceutical applications to stabilize proteins and inhibit protein aggregation.
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