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Knowledge of nonideal behavior of surfactant mixtures of nonionic surfactants at air/aqueous solution interface is important in order to obtain surfactant mixtures with improved properties. In this work interfacial behavior of Brij S20/P407 and Triton X-100/P407 surfactant mixtures was investigated. The nonideal behavior was elucidated by extending Clint's theory and by determination of the ideal maximum surface excess concentration values. Rosen's model for mixed monolayers was used to investigate interactions between different surfactants at the interface. Synergism was noticed in Brij S20/poloxamer 407 mixtures. However, very different interfacial behavior was observed for Triton X-100/poloxamer 407 systems.

The development and optimization of systems consist of polymers and surfactants are strongly related to the presence of interactions between them. Therefore, the interpolymer complexes between biopolymer xanthan gum and synthetic polymer Carbopol 940 were investigated before and after the addition of surfactant. The mechanism of interactions in ternary system consisted of xanthan gum, Carbopol 940 and anionic surfactant (sodium dodecylsulfate) as well as nonionic surfactant (Tween 80) was studied by the combination of different techniques such as viscometry, conductometry, tensiometry, spectrophotometry and fluorimetry. Xanthan gum and Carbopol 940 formed complexes through hydrogen bonding and hydrophobic interactions. Therefore, the calculated specific viscosities of polymer blends were much lower than experimentally determined values. Namely, both polymers participated in the association process with sodium dodecylsulfate or Tween 80. In case of anionic surfactant hydrophobic interactions between sodium dodecylsulfate and xanthan gum-Carbopol 940 complexes exist together with electrostatic repulsive forces between the same charged groups. Tween 80 interacts with xanthan gum-Carbopol 940 complexes through hydrogen bonds and hydrophobic interactions. The formed xanthan gum-Carbopol 940 complexes with sodium dodecylsulfate or Tween80 could find their application by optimizing the interactions in the system of xanthan gum and Carbopol 940 using the appropriate concentration of surfactant.

In the present study, binary mixed micelles of poloxamer 407 and a homologous series of polysorbate surfactants (polysorbate 20, polysorbate 60, polysorbate 80, polysorbate 85) were investigated. Rubingh’s model was applied to determine the physicochemical parameters of the poloxamer/polysorbate binary systems. The obtained negative values of the interaction parameter and excess Gibbs energy showed the existence of synergism. By comparing the values of the investigated parameters, and the values of micropolarity measurements, it was noticed that the inclusion of polysorbate surfactants with longer and more saturated hydrophobic chains leads to improved packing in the micellar interior and stronger interactions.

Bile acid anions are biosurfactants which form small micelles. To enhance the hydrophobic volume of the micelle, bile salts are used as mixed micelles with non-ionic surfactants. Micellar-bound bile acids (sodium-hyodeoxycholate, HD and sodium-cholate, C in binary mixed micelles with non-ionic surfactant Triton X-100) at concentration of 10 cmc have higher pKa values than bile acids from the aqueous solution. Different pKa values of micellar and non-micellar bile acids (ΔpKa) are due to the excess Gibbs energy G HA E which is a function of interaction coefficient and mole fraction of Triton X-100 in the binary surfactant mixture. In the mixed micelles ΔpKa values show synergistic interactions between different building units, similarly as interaction coefficient.

The study examines binary mixed micelles of sodium-hyodeoxycholate (HD) and polysorbates: Tween 20 (T-20), Tween 60 (T-60) from α = 0.1 to α = 0.5 molar fraction of polysorbates in the mixture of surfactants at 298.15 K. At every value of α there are synergistic reactions between different building units of mixed micelle. In the mixed micelle HD–T-20 the thermodynamic stability of the real mixed micelle compared to the ideal binary mixed micelle is greater than in the mixed micelle HD–T-60. The pKa values of the micellarly bound bile acids are higher than the pKa values of non-micellar bile acids (ΔpKa). At constant ionic strength this means that the change in Gibbs energy of protolysis of the micellar bile acid has excess Gibbs energy determined by the energy of stability of bile acid in micellar phase. The ΔpKa value of the micellar bile acid from the mixed micelle of bile salts and homologous polysorbates, similar to the interaction coefficient (βij) from regular solution theory, characterizes the thermodynamic stability of the mixed micelle and also enables its fast evaluation.

In this work mixed micelles of Na salts of bile acids and Tween 80 are studied. In the experiment, number and orientation of OH and oxo groups in the steroid skeleton of bile acid anion varies. In every investigated mixture, mole fraction of Tween 80 (α) varies from 0.05 to 0.5 of the total surfactant concentration. Critical micelle concentrations of binary surfactant mixtures are determined by tensiometric and conductometric methods (modified Philips method of data processing). By employing regular solution theory for every studied binary surfactant mixture, values of interaction coefficient (β1,2) are determined. Based on the obtained β1,2 values, investigated bile acid anions form two groups. Group I consists of bile acids having only OH groups attached to the steroid skeleton. Characteristics of mixed micelles of this group are negative values of β1,2 for α to 0.1 (synergistic effect between different types of the micelle building units), and positive values of β1,2 for α over 0.1 (antagonistic effect). This change of sign of β1,2 as a function of α can be explained by phase transformation in mixed micelle as a pseudophase, and due to rigid geometry of cis-oleic residue of Tween 80. Group II is formed of bile acid anions having one or more oxo groups in the steroid skeleton. Mixed micelles of Tween 80 and group II of bile acid anions are characterized by positive β1,2 values in the whole investigated range of α. Based on the intensity measurements at different temperatures of first (I1) and third (I3) vibrational bands of pyrene (probe molecule) emission spectrum as a function of total surfactant concentration, it can be concluded that mixed micelles having Tween 80 are less rigid than mixed micelles having Tween 40. This also indicates that in hydrophobic domain of mixed micelles, cis-oleic residues are more difficult to pack together compared to the palmitic residues of Tween 40.

Salts of bile acids are steroid amphiphilic compounds which form micelles consisting of 2–12 monomer units. In nature, the formation of mixed micelles with phospholipids (gall bladder) increases the capacity of bile salts to solubilize cholesterol. Oxo derivatives of bile salts are of special research interest because of their lower membrane toxicity compared to the corresponding OH analogues. In view of the fact that the micelles of anions of oxo derivatives have lower hydrophobicities, it was interesting to study mixed micelles of bile salts and the hydrophobic nonionic cosurfactant Tween 40. With this aim, the critical micelle concentrations were determined by conductometric and tensiometric methods. It was found that the absolute value of the interaction parameter (measure of the synergism between different constitutive units, i.e., the measure of the departure from an ideal micelle) increases with the increase in the number of equatorial OH or oxo groups in the steroid skeleton of the bile acid anion...