ARTICLE Albena Jordanova Zdravko Lalchev Boris Tenchov Formation of monolayers and bilayer foam films from lamellar, inverted hexagonal and cubic lipid phases Received: 6 June 2002 / Accepted: 3 October 2002 / Published online: 1 November 2002 Ó EBSA 2002 Abstract This study revealed large distinctions between the lamellar and non-lamellar liquid crystalline lipid phases in their spreading at the air/water interface and propensity to form bilayer foam films. Comparative measurements were made for the lamellar L a , the in- verted hexagonal H II and the bicontinuous cubic Pn3m phases of the phospholipid dipalmitoleoylphosphatidy- lethanolamine (DPoPE). With regard to monolayer formation, followed as the decrease of surface tension with time, the best spreading (lowest surface tension) was observed for the L a phase, and poorest spreading (highest surface tension) was recorded for the H II phase. The cubic Pn3m phase of DPoPE, induced by temper- ature cycling, retained an intermediate position between the L a and H II phases. According to their ability to lower surface tension and disintegrate at the air/water interface, the three phases thus order as L a >Pn3m> H II . Clearly expressed threshold (minimum) bulk lipid concentrations, C t , required for formation of stable foam bilayers from these phases, were determined and their values were found to correlate well with the bulk lipid phase behaviour. The C t values for L a and H II substantially increase with the temperature. Their Arrhenius plots, lnC t versus 1/T, are linear and intersect at 36–37 °C, coinciding with the onset of the bulk L a fiH II phase transition, as determined by differential scanning calorimetry. However, the C t value for the Pn3m phase, equal to 30 lg/mL, was found to be con- stant over the whole range investigated between 20 °C and 50 °C. The horizontal C t versus T plot for the Pn3m phase crosses the respective plot for the L a phase at the temperature bounding from below the hysteretic loop of the L a MH II transition (26 °C), thus providing a certain insight about the thermodynamic stability of the Pn3m phase relative to the L a phase. The established strong effect of the particular lipid phase on the formation of monolayers and stable black foam films should be of importance in various in vitro and in vivo systems, where lipid structures are in contact with interfaces and disintegrate there to different extents. Keywords Foam film Monolayer Cubic phase Inverted hexagonal phase Surface tension Introduction As a result of their amphiphilic structure, membrane lipids are able to spread at interfaces and to form vari- ous kinds of monolayers and foam films. The lipid be- haviour at the air/water interface is of certain physiological relevance, at least for the reason that it determines the functioning of the lung surfactant. Studies carried out with vesicle dispersions have shown that spreading at interfaces, as well as the formation of lipid foam films, strongly depend on the vesicle phase state (Cohen et al. 1991; Exerowa and Nikolova 1992; Lalchev et al. 1994; Nikolova et al. 1994, 1996; Panaiotov et al. 1995; Vassilieff et al. 1996; Lalchev 1997). All parameters of importance, such as film thickness and homogeneity, threshold bulk lipid con- centration required for formation of a stable foam film, film lifetimes, lateral lipid diffusion in foam films, rate of monolayer spreading, equilibrium surface tension, etc., were found to strongly differ between the lamellar gel (L b ) and the lamellar liquid crystalline (L a ) states of the vesicle dispersions. The observed effects have often been considered in terms of adhesion of whole vesicles to the interface, followed by their rupture and spreading, and have been interpreted as associated with surface phase transitions, which correspond to the melting transition in the bulk lipid phase. These findings clearly demon- strated that the phase state of the vesicle dispersion is an essential determinant of the lipid interfacial behaviour. Eur Biophys J (2003) 31: 626–632 DOI 10.1007/s00249-002-0263-x A. Jordanova B. Tenchov (&) Institute of Biophysics, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria E-mail: tenchov@obzor.bio21.bas.bg Z. Lalchev Biological Faculty, Sofia University ‘‘St. Kliment Ohridski’’, 8 Dragan Tsankov Street, Sofia 1164, Bulgaria