Methods for measuring the skin-cleansing effect of surfactants in comparison with skin roughness and compatibility

Abstract

The role of adsorption of dodecylethyldimethylammonium bromide (C12(EDMAB)) and benzyldimethyldodecylammonium bromide (BDDAB) at water–air and polytetrafluoroethylene (PTFE)–water and poly(methyl methacrylate) (PMMA)–water interface, in wetting of PTFE and PMMA surface, was established from the measured values of the contact angle (θ) of aqueous C12(EDMAB) and BDDAB solutions in PTFE (PMMA)-solution drop–air system, and from the measured values of the surface tension of aqueous C12(EDMAB) and BDDAB solutions. Adsorption of C12(EDMAB) and BDDAB at water–air interface was determined earlier from the Gibbs equation. Adsorption at solid–water interface was deduced from the Lucassen-Reynders equation based on the relationship between adhesion tension (γLV cos θ) and surface tension (γLV). The slope of the γLV cos θ–γLV curve was found to be constant and equal to −1, and about −0.3 for PTFE and PMMA surface, respectively (in the case of both surfactant studied: C12(EDMAB) and BDDAB, and in the whole range of surfactants concentration in solution). It means that the amount of the surfactant adsorbed at the PTFE–water interface, ΓSL, was essentially equal to its amount adsorbed at water–air interface, ΓLV. However, ΓSL at the PMMA–water interface was about three times smaller as compared to that at water–air interface. By extrapolating the linear dependence between γLV cos θ–γLV and dependence between cos θ–γLV and cos θ = 1 we determined the value of the critical surface tension of PTFE and PMMA surface wetting, γc. The obtained values of γc for PTFE surface were equal 23.4 and 23.8 mN/m, 23.1 and 23.2 mN/m for C12(EDMAB) and BDDAB, respectively and they were higher than the surface tension of PTFE (20.24 mN/m). On the other hand, the obtained values of γc for PMMA surface were equal 31.4 and 30.9 mN/m, 31.7 and 31.3 mN/m for C12(EDMAB) and BDDAB, respectively and they were smaller than the surface tension of PMMA (39.21 mN/m). Using the values of PTFE and PMMA surface tension and the measured values of the surface tension of aqueous C12(EDMAB) and BDDAB solutions in the Young equation, the PTFE (PMMA)-solution interfacial tension, γSL, was also determined. Next, the work of adhesion (WA) was deduced, and it occurred that the dependence between the WA and the surface tension (γLV) for both studied solids was linear. However, the values of the WA for PMMA change as a function of log C (C—surfactant concentration) changed from 91.7 to 68.5 mJ/m2 and from 91.8 to 65.1 mJ/m2 for C12(EDMAB) and BDDAB, respectively. On the other hand, the work of adhesion of both studied surfactants solutions to the PTFE surface was practically constant (an average value was equal 45.8 and 45.4 mJ/m2, respectively). These values were close to the value of the work of water adhesion to PTFE surface (45.5 mJ/m2).