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Dependence of the equilibrium thickness and double layer potential of foam films on the surfactant concentration D . E x e r o w a , M. Z a c h a r i e v a , R. Cohen, and D . P l a t i k a n o v With 8 figures and 2 tables (

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Aims:  To isolate a biosurfactant‐producing bacterial strain and to identify and characterize the chemical structure and properties of its biosurfactants. Methods and Results:  The bacterium Rhodococcus wratislaviensis BN38, isolated from soil, was found to produce glycolipid biosurfactants when grown on 2%n‐hexadecane. The glycolipids were isolated by chromatography on silica gel columns and their structures elucidated using a combination of multidimensional NMR and ESI‐MS/MS techniques. The main product was identified as 2,3,4,2′‐trehalose tetraester with molecular mass of 876 g mol−1. It was also noted that the biosurfactant was produced under nitrogen‐limiting conditions and could not be synthesized from water‐soluble substrates. The purified product showed extremely high surface‐active properties. Conclusions:  The glycolipid biosurfactant produced by the alkanothrophic strain R. wratislaviensis BN38 was characterized to be 2,3,4,2′‐trehalose tetraester which exhibited high surfactant activities. Significance and Impact of the Study:  Strain BN38 of R. wratislaviensis is a potential candidate for use in bioremediation applications or in biosurfactant exploration.

The core thicknesses of macroscopic, vertical foam films have been measured by FT-IR technique for the equilibrium films formed from palmitoyllysophosphatidylcholine (lysoPC) solutions in varying concentrations of NaCl and CaCl2. The addition of NaCl exhibits no effect on the core thicknesses of equilibrium films; Newton black films (NBF) with constant core thickness of 2.5 nm are observed in the range of the measured concentrations up to 1M NaCl. Whereas, in CaCl2 the core thicknesses in equilibrium state strongly depend on CaCl2 concentrations. Consequently, the equilibrium films are NBF at below about 1.1 X KH M, silver films (SF) at about 1-4 X 1(H M, common black films (CBF) at about 5 X 1CH to 0.2 M and again NBF at higher concentrations than about 0.2 M. These transitions of film thickness are thought to be due to specific adsorption of Ca2+ on the zwitterionic head group. So, using the measured thickness of the film core and the Lifshitz theory applied for a triple-layer model, van der Waals dispersion forces ( ) were calculated and from the the effective Hamaker constants, A (h, 298 K), for a foam water film were simultaneously calculated. The values obtained vary from 2 X KH°J to 4.5 X Kb20 J in the range 5nm -30nm of film thicknesses. Next, for the equilibrium SF and CBF, surface potential^), surface charge density (a), and the binding constant (K) of Ca2+ to the equilibrium foam films were evaluated using the charge-regulation model taking account of the DLVO theory of colloid stability. At the thickness-transition concentration of 1.1 X 1(H M CaCl2 0 and are 11 mV and 1.3 X 1(H C nr* 12 345, respectively. Further, K is almost independent of the CaCl2 concentration in the range 3 X 1(H M to 0.1 M, i.e., the binding constant of Ca2+ to the lysoPC foam film is given as 15 ± 5 M-1 assuming a 2:1 stoichiometry for the PC/Ca2+.

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