Evaporation and Hydrocarbon Chain Conformation of Surface Lipid Films

Abstract

Purpose: The inhibition of the rate of evaporation (R_evap) by surface lipids is relevant to reservoirs and dry eye. Our aim was to test the idea that lipid surface films inhibit R_evap.

Methods: R_evap were determined gravimetrically. Hydrocarbon chain conformation and structure were measured using a Raman microscope. Six 1-hydroxyl hydrocarbons (11-24 carbons in length) and human meibum were studied. Reflex tears were obtained from a 62-year-old male.

Results: The Raman scattering intensity of the lipid film deviated by about 7 % for hydroxyl lipids and varied by 21 % for meibum films across the entire film at a resolution of 5 μm². All of the surface lipids were ordered. R_evap of the shorter chain hydroxyl lipids were slightly (7%) but significantly lower compared with the longer chain hydroxyl lipids. R_evap of both groups was essentially similar to that of buffer. A hydroxyl lipid film did not influence R_evap over an estimated average thickness range of 0.69 to >6.9 μm. R_evap of human tears and buffer with and without human meibum (34.4 μm thick) was not significantly different. R_evap of human tears was not significantly different from buffer.

Conclusions: Human meibum and hydroxyl lipids, regardless of their fluidity, chain length, or thickness did not inhibit R_evap of buffer or tears even though they completely covered the surface. It is unlikely that hydroxyl lipids can be used to inhibit R_evap of reservoirs. Our data do not support the widely accepted (yet unconfirmed) idea that the tear film lipid layer inhibits R_evap of tears.

Keywords: dry eye; hydroxyl lipids; meibum; tear evaporation; tear film lipid layer.

Figure 1

Typical pictures taken using a Raman spectrometer showing the surface of synthetic lipid alcohol films under white light on the surface of physiologically buffered saline. The 5 µm² box is the area sampled by the Raman laser. Lipids on the surface were in motion and slowly moved in and out of the field of view. A and B: 1-tetracosanol. C and D: 1-undecanol. The estimated average thickness of the lipid layer was 0.69 µm, about 7 times thicker than the tear film lipid layer.

Figure 2

A: Raman CH stretching region. B: Raman Fingerprint region. Typical Raman spectra of i) liquid 1-undecanol and ii) 1-tetracosanol. C: Raman spectra of aqueous samples i) 1-undecanol 0.69 µm thick on buffer, ii) 1-tetracecanol 0.69 µm thick on buffer, iii) human tears, iv) buffer.

Figure 3

Schematic of ordered and disordered wax conformations. Lipid order is related to viscosity and indirectly related to fluidity. Gauche rotamers cause kinks in the hydrocarbon chain that disrupts tight packing. The more trans rotamers and the less gauche rotamers, the more ordered the lipid.

Figure 4

Typical pictures taken under white light using a Raman spectrometer showing human meibum applied to the surface of (top) human reflex tears or (bottom) PBS, at a lipid thickness of 35.5 µm. The 5 µm² box is the area sampled by the Raman laser. Scale bar is 10 µm² in top A-D and bottom figures and 100 µm² in top E–F. Lipids on the surface were in motion and slowly moved in and out of the field of view.

Figure 5

Typical Raman spectra. A: The surface of human reflex tears in vitro. B: Human reflex tears in a capillary tube. C: Physiologically buffered saline. D: Water.

Figure 6

Typical Raman spectra of i) meibum; ii) human reflex tear surface; iii) human reflex tears plus meibum. A: The CH stretching region. B: The ‘fingerprint’ region. C: C-C acoustic mode region. Numbers correspond with the band assignments in Table 3.

Figure 7

The relative rate of evaporation of buffer at 22°C with 0.69 µm thick 1-hydroxyl n-hydrocarbons films on the surface. There was no difference in R_evap of the shorter chain alcohols (11–13 carbons) or between the longer chain alcohols (16–24 carbons), P>.05, so the shorter chain alcohols and the longer chain alcohols were averaged separately. The evaporation rate ratio, buffer plus lipid/buffer, of the shorter chain alcohols was 00.99 ± 0.10 slightly but significantly lower compared with the longer chain alcohols 1.07 ± 0.15, P=.04. Values in parenthesis are the number of trials.

Figure 8

The relative rate of evaporation of buffer at 22°C with the synthetic lipids listed in Table 1 on the surface. Hydrocarbon chain length had a minimal influence of Revap so data from all the lipids for each estimated thickness were averaged. The thickness of the lipid film did not influence the evaporation rate ratio (P>.05). Values in parenthesis are the number of trials.

Figure 9

The evaporation rates of buffer and human tears with a film of human meibum 34.4 µm thick was measured at 34°C. The evaporation rate of human tears and buffer with and without human meibum was not significantly different, P>.05. Results from the three pools of meibum in Table 1 were averaged. Bars are ± standard error of the mean. Values in parenthesis are the number of trials.