Adsorption of Proteins at the Solution/Air Interface Influenced by Added Nonionic Surfactants at Very Low Concentrations for Both Components. 2. Effect of Different Surfactants and Theoretical Model

Abstract

The influence of the addition of the nonionic surfactants dodecyl dimethyl phosphine oxide (C12DMPO), tetradecyl dimethyl phosphine oxide (C14DMPO), decyl alcohol (C10OH), and C10EO5 at concentrations between 10(-5) and 10(-1) mmol/L to solutions of β-casein (BCS) and β-lactoglobulin (BLG) at a fixed concentration of 10(-5) mmol/L on the surface tension is studied. It is shown that a significant decrease of the water/air surface tension occurs for all the surfactants studied at very low concentrations (10(-5)-10(-3) mmol/L). All measurements were performed with the buoyant bubble profile method. The dynamics of the surface tension was simulated using the Fick and Ward-Tordai equations. The calculation results agree well with the experimental data, indicating that the equilibration times in the system studied do not exceed 30 000 s, while the time required to attain the equilibrium on a plane surface is by one order of magnitude higher. To achieve agreement between theory and experiment for the mixtures, a supposition was made about the influence of the concentration of nonionic surfactant on the adsorption activity of the protein. The adsorption isotherm equation of the protein was modified accordingly, and this corrected model agrees well with all experimental data.

Adsorption of proteins at the solution/air interface influenced by added non-ionic surfactants at very low concentrations for both components. 1. Dodecyl dimethyl phospine oxide

“…Details of these calculations are discussed in [25]. Here we note that for BCS and a bubble radius of about 1.6 mm the optimum diffusion coefficient was several times larger than the physically realistic one, which is probably due to the presence of convection in the measuring cell.…”

“…To theoretically better describe the given experimental results we have to assume that at low concentrations the non-ionic surfactant increases the activity of the adsorbing proteins. A theoretical model which takes this effect into account will be discussed in later communication [25].…”

“…4. Equilibrium surface tension isotherms for BCS + C12DMPO mixtures at two BCS bulk concentrations: 10 −5 mmol/l ( ) and 10 −3 mmol/l ( ); (♦) the data for pure C12DMPO measured in this work by using the buoyant bubble method; ( ) the data from [25]; ( ) the data from [26]; dotted lines are values for pure BCS solutions; black solid line was calculated for C12DMPO; thin blue line was calculated for the BCS/C12DMPO mixtures using the model Eqs. (1)-(6) and the parameters in Table 1; thick blue and thin black lines are guides for the eye.…”

Adsorption of Proteins at the Solution/Air Interface Influenced by Added Nonionic Surfactants at Very Low Concentrations for Both Components. 3. Dilational Surface Rheology

“…For mixtures of non-ionic surfactants with more flexible proteins like β-casein (BCS) there is a sharp maximum in the dilational elasticity. Experimental results on the dilational rheology of protein/non-ionic surfactant mixtures were reported in 13-17 ; some of these results and the relevant theoretical models of the corresponding surface layers were discussed previously 18,19 . The influence of the addition of the non-ionic surfactants C 12 DMPO, C 14 DMPO, С 10 ОН and С 10 ЕО 5 at concentrations from 10 −5 to 10 −1 mmol/L to BCS and BLG solutions, respectively, at a fixed concentration of 10 −5 mmol/L on the surface tension were studied in 19 .…”

“…It was shown that a significant change ( The largest correction of this constant was found for the BCS+C 14 DMPO mixtures. 3 The solutions of proteins and surfactants studied here were the same as in 19 . The dilational rheology of individual solutions and their mixtures after equilibration of the system was studied using the buoyant bubble profile method at surface area oscillation frequencies in the range of 0.005 to 0.5 Hz.…”