The adsorption of BAC at the pure air–water interface was estimated, to determine the critical micelle concentration (CMC) of the preservative (
Fig. 2A). The CMC corresponded to 0.01% BAC, at which a plateau of π at 39 mN/m was realized. The obtained value concurred with the one determined in another study.
7 Thus, the range of clinically used BAC concentrations (0.001%–0.02%) spans more than two concentration regions: below and above the CMC of the preservative.
The adsorption of BAC to the air–water interface covered with predeposited meibum film with initial surface pressure π
0 was also measured (
Fig. 2B). After BAC was injected into the subphase, if the preservative penetrated the surface film, it was registered as an increase in surface pressure (Δπ). The penetration of BAC into the meibum film decreased with the increase in π
0. This result is easy to explain, because a higher value of π
0 means a higher surface concentration of lipid and less free area available for the incorporation of BAC molecules. By the linear extrapolation of Δπ(π
0) dependence, the so-called critical surface pressure π
CR was obtained: the minimum surface pressure of the predeposited lipid film at which and above which BAC is not able to enter the interfacial film.
8 It can be seen that π
CR was identical with the surface pressure of BAC at CMC (39 mN/m). The results presented in
Figure 2B are from experiments with the meibum of one person. The value of π
CR = 39 ± 0.7 mN/m was reproduced also with meibum from each of the other three volunteers and with “mixed” meibum (an equiweight mixture of the samples collected from each volunteer) which suggests reproducibility of π
CR despite individual differences. Synonymously, π
CR exceeding 35 mN/m was registered for synthetic lipids such as DPPC or corneal cell lipid extract (data not shown). These values were significantly higher than the surface pressure of the tears in an open eye
11,26 (∼30 mN/m) and of the cellular biomembranes
8 (30–33 mN/m). Therefore, it can be expected that under physiological conditions, BAC penetrates and alters the integrity of the TFLL and of the membranes of the corneal epithelial cells.