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Changes in sebum levels and skin surface free energy components following skin surface washing
Colloids and Surfaces B: Biointerfaces 10 (1998) 243–250
Changes in sebum levels and skin surface free energy components following skin surface washing A. Mavon a,b,*, D. Redoules a, P. Humbert b, P. Agache b, Y. Gall a a Institut de Recherche Pierre Fabre (IRPF), BP 74, 31322 Castanet Tolosan cedex, France b Laboratoire de Biophysique Cutane´e/C.R.T., Faculte´ de Me´decine and Pharmacie, Place Saint-Jacques, 25030 Besanc¸on cedex, France Received 2 April 1997; accepted 5 January 1998
Abstract The forehead skin sebum casual (or usual ) level, which is remarkably stable in a given individual, is responsible for the monopolar basic character of the skin surface. However, the significance of this finding, in terms of the physicochemical behaviour and wettability of the skin surface, remains unexplained. How does this vary during sebum casual level restoration following skin washing and what is its relationship with so-called ‘‘dry skin’’? The present work was aimed at answering these questions. Forehead surface free energy components were assessed before and during the sebum casual level restoration following skin washing by nonionic soap and water, in 12 healthy volunteers: four oligo- (OS), four normo- (NS), and four hyper-seborrheic (HS) skins. Skin contact angles of water, formamide and diiodomethane were measured at intervals, and surface free energy components (i.e. Lifshitz–van der Waals, electron acceptor and electron donor components) were calculated using the contact angle values and acid–base theory of C.J. van Oss et al. The results showed that a low sebum-level does not deprive the skin surface of a normal Lifshitz–van der Waals component. This suggests that a low sebum-level is sufficient to render the skin normally lipophilic. The very low acidic component of the skin surface seemed to be due to the masking of acidic moieties by adsorbed water. The basic component was sebum-level dependent below a threshold value and remained to be related to the sebum free fatty acids and especially to their outward carboxylic group orientation. We conclude that the wetting properties of the skin surface could be defined by a ‘‘surface free energy component balance’’ or SEB. The SEB of NS and HS skins was around 0.22, and that of OS skin was 0.07. This low SEB stems from a weak hydrophilic component, allowing OS skin to be called dry skin, or better, a dry skin surface. This approach may provide a new insight into so-called ‘‘skin hydration’’. © 1998 Elsevier Science B.V. Keywords: Surface free energy; Human skin surface; Sebum casual level; Advancing contact angle
1. Introduction At any given site in the same individual, certain lipids are present on the human skin surface [1] at a remarkably constant level. Whereas this casual * Corresponding author. 0927-7765/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0 9 2 7 -7 7 6 5 ( 9 8 ) 0 0 00 7 - 1
level is very low at some sites like the volar forearm, and is mostly made up of epidermal lipids, on the forehead it is maximal, owing to the adjunction with sebum. In all body areas and especially on the forehead, the skin surface lipid level is restored within 2 h [2,3] following washing or degreasing through a sudden and sustained flow
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of sebum from the pilosebaceous orifices [4]. The overwhelming prevalence of sebum over epidermal lipids on the forehead leads to the description of oligo-, normo-, or hyper-seborrheic skin to characterize the skin surface lipid level in this area [5]. In a previous work on the wettability of human skin [6 ], it was shown that skin surface lipids and mainly sebum increase the skin surface free energy c and especially the electron donor component s c− as described by the theoretical approach of van s Oss et al. [7–9]. However, we did not find any correlation between the sebum level and either the overall c or its c− component. s s As sebum, at its casual level, gives the skin surface a basic monopolar character, three questions arise: what does this finding mean in terms of the skin surface physicochemical behaviour and wettability; how does it change during sebum casual level restoration following skin washing; and does it have a relationship with so-called ‘‘dry skin’’? To answer these questions, we calculated the skin surface free energy components, from contact angle measurements and the acid–base theory of van Oss, Good and Chaudhury [7–9] on the forehead of oligoseborrheic (OS), normoseborrheic (NS) and hyperseborrheic (HS) subjects; before and after washing and during the spontaneous sebum-level restoration.
2. Theory Nearly thirty years ago, Lifshitz [10] showed that in the condensed phase, the three electrodynamic forces (namely London ‘‘dispersion’’, Keesom dipole–dipole and Debye dipole-induced dipole forces) decay with distance at the same rate and obey the same combining rules. Recently, van Oss et al. [7–9] pointed out that surface tension arising from these three electrodynamic forces, when put together, gives rise to a surface-tension component that is called the Lifshitz–van der Waals (LW ) component. If suitable chemical groups are present, significant contributions to interfacial energy can arise from hydrogen bonding, i.e. Lewis acid–base or electron-acceptor– electron-donor interactions. This kind of force
was, at first, called ‘‘short range’’ or ‘‘SR’’ [7], owing to the rapid decay with distance, but later the term acid–base was preferred. Fowkes [11] suggested that the surface tension, c, can be considered as the sum of independent terms, each representing a particular intermolecular force. Accordingly, c can be expressed as the sum of Lifshitz–van der Waals, cLW, and acid– base, cAB components: c=cLW+cAB
(1)
The surface tension of a solid surface can be readily calculated by measuring the contact angle of two different liquids of known surface-tension components. A more complex approach was developed by van Oss et al. [8,9] incorporating the electron-donor and the electron-acceptor parameters in the acid–base component. A substance can show both electron-donor and electron-acceptor properties, or can be overwhelmingly basic showing virtually only electron-donor behaviour; the opposite case being possible but far less common. Materials exhibiting only electron-acceptor or electron-donor capacity are called monopolar materials. It must be noted that an acid–base parameter will be manifested only if the opposite parameter is present in another molecule or in another part of the same molecule; thus the polar parameter of a monopolar material cannot, by itself, contribute to the energy of cohesion. The polar or acid–base components can be expressed as: cAB=2(c+c−)1/2
(2)
The c+ and c− components indicate the electronacceptor and the electron-donor components respectively. Finally, the three components of the interfacial tension between materials 1 and 2 can be defined by the next equation [8,9]: c =[cLW +cLW −2(cLW cLW )1/2]+2[(c+ c− )1/2 12 1 2 1 2 1 1 +(c+ c− )1/2−(c+ c− )1/2−(c− c+ )1/2] (3) 2 2 1 2 1 2 Solid surface free energy can be determined from contact angle measurements, in appropriately chosen systems using Young’s equation: c cos h=c −c −p l s sl e
(4)
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where c is the solid surface free energy, c the s sl interfacial solid–liquid free energy, c the liquid l surface tension, h the contact angle and p the film e pressure of the liquid. After combining Eq. (3) with Eq. (4) and neglecting p [11], the following e equation is obtained: (1+cos h)c =2(cLW cLW )1/2 l s l +2(c+ c− )1/2+2(c− c+ )1/2 (5) s l s l The three parameters characterizing the solid surface free energy can be determined by measuring the contact angles of at least three standard liquids, provided the relevant values of the surface tension components of these liquids are known [9,12].
3. Experimental details 3.1. Liquids (Table 1) Formamide and diiodomethane (analytical grade, minimum 98% purity) were supplied by Fluka (St Quentin Fallavier, France). Water was distilled twice and deionized (Milli-Q reagent water system, Millipore) yielding a resistivity of 18 MV cm−1 at 25°C. 3.2. Volunteers (forehead surface) and skin surface treatments The twelve volunteers (age range 23–31 years) were selected in each of the previously defined skin types [5], namely oligoseborrheic (OS), normoseborrheic (NS ) and hyperseborrheic (HS): sebum levels were less than 80 AU, 120–160 AU and Table 1 Literature values (mJ m−2) for Lifshitz–van der Waals (cLW ) l electron-acceptor (c+ ) and electron-donor, (c− ) components l l of standard liquids (c ) used for the determination of skin surl face free energy components Liquid
c l
cLW l
c+
Water Formamide Diiodomethane
72.8 58 50.8
21.8 39 50.8
25.5 2.28 0.72
l
c−
l
25.5 39.6 0
Ref. [12] [12] [12]
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more than 200 AU, respectively. The sebum levels were assessed using a photometric method [13] giving a direct reading in arbitrary units (AU ) using the SebumeterA SM 801, (Courage and Khazaka Gmbh, Cologne, Germany). Contact angle and sebum-level measurements were carried out on the forehead before washing (baseline); on the skin kept unwashed for at least 8 h; just after washing (T 0) and during the resuming sebaceous excretion at different times: T 20 min, T 40 min, T 60 min, T 90 min and T 120 min. A previous work (A. Mavon et al., unpublished data, 1996) where we studied six subjects over a 5 h period showed that the sebum level and the surface free energy components were reconstituted in the first 2 h. The forehead was washed by scrubbing twice for 10 s with a gauze pad impregnated with ‘‘Savon Doux’’, (amphoteric washing soap purchased from Laboratoires Anios, Lille, France) followed by rinsing three times with a gauze pad soaked in water and drying by three applications of absorbent paper without rubbing.
3.3. Measurements of contact angles The method was the same as that used in a previous study [6 ]. Advancing contact angles h A were measured according to Zisman’s method [14,15]. However, where Zisman used a fine platinum wire, our procedure used a Hamilton microsyringe with a fine needle. The volume of the initial drop placed on the skin surface was increased by addition of droplets. The final volume was 5 ml. Contact angles were assessed using a surgical microscope ( Wild HeerbruggA M650), with a magnification of 25×, fitted with a slanted mirror and a video camera (CCD-IRIS, SonyA) connected to a personal computer. The computer was equipped with video capture hardware ( VideospigotA) and digital video software (MicrosoftA). This system provided the observation, acquisition, storage and computation of h . A The measurements were carried out at room temperature (21±1.5°C ). Using contact angles value, Eq. (5) was solved for the water–formamide–diiodomethane ( WFD) triplet [12,15]. Calculations were made for each time and each volunteer.
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3.4. Skin roughness and skin temperature A discussion on the influence of the skin roughness and temperature on the h measurements was A documented in a previous paper [6 ]. In this experiment, as in the previous study, whatever the errors made in our h measurements from the skin relief A and from a change in the temperature of the drop, they were similar in size in all measurements and accordingly could not strongly alter the results. These difficulties stem from the fact that the skin is a living surface. However, the absolute error in contact-angle values of the three standard liquids on the forehead surface remained low (2°–4°).
4. Results 4.1. Sebum level (Fig. 1) The volunteers were classified into three groups according to sebum casual level of the forehead at baseline 70 AU, 139 AU and 211 AU in OS, NS and HS respectively. Thus, the three skin types were clearly defined at this time. At T 0, just after washing, the sebum levels were 4 AU, 9 AU and 14 AU in OS, NS and HS respectively. This level differed from zero for two reasons. First, using a soap, degreasing was not drastic. Secondly, sebaceous excretion is continuous and had already resumed during the 2 min between degreasing and
Fig. 1. Baseline, washing and sebum-level restoration on forehead in OS (oligoseborrheic), NS (normoseborrheic) and HS (hyperseborrheic) skins.
Fig. 2. Advancing contact angles of diiodomethane on the forehead in the three skin types.
the sebum-level measurement. Afterwards, the sebum level increased and the baseline value was reached by 90 min, whatever the skin type. By 2 h it was at its maximum and slightly higher than at baseline [16 ]. 4.2. Contact angles The experimental contact angles and their interindividual variations are shown in Figs. 2–4. The standard deviation (between 6° and 12°) was in the range of contact angle values, as measured on human skin [6,17–19]. Intra-individual variations were smaller, the standard deviation ranged from
Fig. 3. Advancing contact angles of water on the forehead in the three skin types.
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Continuously over 2 h, the values decreased to baseline, keeping a final difference of about 15° between OS skin and the two other skin types.
4.3. Surface free energy components The average value of the surface free energy components on the forehead surfaces ( Tables 2–4) at different times were obtained by solving Eq. (5) and using the surface tension components of the standard liquids used ( Table 1). Fig. 4. Advancing contact angles of water on the forehead in the three skin types.
2° to 4°, also in accordance with literature values [6,17,18]. 4.2.1. Diiodomethane contact angle (Fig. 2) At baseline (before washing), the contact angles of diiodomethane were the same for the three skin types (30°). At T 0, ( just after degreasing) it was high and similar (42°) in NS and HS skin, but still higher in OS skin (50°). By 20 min, contact angles in NS and HS skins had substantially decreased, and lowered down to baseline values by 1 h. Conversely, on the OS skin, the contact angles slowly decreased over 2 h and only after this time was the baseline value reestablished. 4.2.2. Water contact angle (Fig. 3) At baseline, the water contact angles were the same (57°) in NS and HS skin but much higher in OS skin (73°). At T 0 (after degreasing), regardless of skin type, they had all increased by 20°. Baseline values were reestablished within 2 h. The observed difference of 15° between OS skin and the two other types before degreasing, remained at this time. 4.2.3. Formamide contact angle (Fig. 4) As observed for water, the formamide contact angle, at baseline, in the NS and HS skin (41°) differed from the OS skin (58°). Washing increased the values by around 15° at T 0 in all skin types.
4.3.1. Lifshitz–van der Waals component, cLW s At baseline, cLW was about 37.5 mJ m−2 for the s three skin types. The degreasing induced a slight decrease (3 mJ m−2) in NS and HS skin, and a greater decrease in the OS skin (5 mJ m−2). On HS and NS skin, the baseline value was reached by 20 min although the sebum level was restored by 90 min. This was not the case in OS skin, where cLW increased in parallel to the sebum excretion s and by 90 min the baseline value was reached for both.
4.3.2. Electron acceptor or acidic component, c+ s The acidic component ranged from 0 to 1 mJ m−2 and was considered negligible, at each time, for each skin type and whether the skin had just been washed or the sebum level restored.
4.3.3. Electron donor or basic component, c− s At baseline, all skin types had a strong electron donor character, of similar value in NS and HS skin (21 mJ m−2) but much lower in OS skin (12 mJ m−2). In all skin types also, it dropped sharply immediately after degreasing, and by the same amount (11 mJ m−2). However, only the OS skin became almost apolar, because its basic component fell to about 0 mJ m−2. In OS skin, restoration started immediately and was continuous over 2 h but it only increased after 20 min in the other skin types. In all of them, restoration of c− was s achieved after sebum-level restoration.
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Table 2 Total surface free energy (c ) and Lifshitz–van der Waals (cLW), acid–base (cAB), electron acceptor (c+), electron donor, (c−) compos s s s s nents in mJ m−2, on forehead in four normoseborrheics, NS, (mean and SD)
Baseline T0 T 20∞ T 40∞ T 60∞ T 90∞ T 120∞
c s
cLW s
cAB s
c+ s
c− s
46.3±1.1 38.2±5.6 41.3±2.7 40.6±3.0 41.4±2.7 43.4±3.2 44.8±3.7
37.5±1.1 33.4±1.8 37.2±1.9 38.3±1.4 38.4±1.0 38.5±0.4 38.0±0.9
8.8±1.8 4.4±3.3 4.1±1.8 2.8±2.3 3.2±2.8 5.2±2.5 7.6±1.4
1.0±0.5 0.8±0.9 0.7±0.4 0.2±0.2 0.2±0.2 0.4±0.3 0.7±0.5
21.0±4.3 8.3±0.5 7.2±2.1 10.5±5.4 18.1±6.9 18.3±5.4 23.8±6.1
Table 3 Total surface free energy (c ) and Lifshitz–van der Waals (cLW), acid–base (cAB), electron acceptor (c+), electron donor, (c−) compos s s s s nents in mJ m−2, on forehead in four hyperseborrheics, HS, (mean and SD)
Baseline T0 T 20∞ T 40∞ T 60∞ T 90∞ T 120∞
c s
cLW s
cAB s
c+ s
c− s
44.4±2.3 37.4±3.7 42.2±2.1 41.6±3.3 40.5±1.9 42.6±4.9 42.9±4.9
36.6±2.4 34.0±2.2 38.1±0.9 37.3±1.1 37.4±1.1 37.4±1.2 37.0±1.7
8.0±2.3 3.2±2.0 3.9±1.9 4.4±2.2 3.3±1.9 6.5±3.4 7.5±2.7
0.8±0.4 0.4±0.4 0.7±0.5 0.7±0.7 0.2±0.2 0.6±0.7 0.6±0.6
20.6±3.3 8.0±4.6 6.3±2.8 10.7±4.0 13.6±4.4 17.0±2.7 20.4±1.8
Table 4 Total surface free energy (c ) and Lifshitz–van der Waals (cLW), acid–base (cAB), electron acceptor (c+), electron donor, (c−) compos s s s s nents in mJ m−2, on forehead in four oligoseborrheics, OS, (mean and SD)
Baseline T0 T 20∞ T 40∞ T 60∞ T 90∞ T 120∞
c s
cLW s
cAB s
c+ s
c− s
41.8±2.4 34.6±4.0 34.8±4.9 38.2±3.1 39.5±1.9 40.1±1.5 40.2±0.7
38.6±0.7 33.5±4.2 33.1±4.8 34.6±3.1 37.3±1.1 38.3±0.4 38.8±0.4
2.8±2.4 1.2±0.9 1.7±1.3 3.8±1.0 2.7±1.4 2.1±0.5 1.8±0.5
0.3±0.4 0.4±0.2 0.3±0.3 0.6±0.3 0.3±0.5 0.1±0.1 0.1±0.1
11.6±2.4 1.3±1.1 3.7±2.1 6.5±1.8 6.8±1.8 10.2±0.7 11.8±1.5
5. Discussion 5.1. Surface free energy components At baseline, forehead cLW was about the same s in the three skin types in spite of significant differences in sebum levels. This shows that cLW is s independent of sebum casual level. Following removal of the skin-surface lipids by washing, only cLW decreased slightly, (4 mJ m−2) indicating s that the bare (i.e. without added lipids) skin sur-
face cLW component was about 33 mJ m−2, and s that sebum was only responsible for a minor change. Baseline cLW (37.5 mJ m−2) was reestabs lished 1 h before sebum casual levels in NS and HS skin, whereas in OS skin, the baseline cLW and s sebum casual levels were reached at the same time. This suggests that the lower sebum flow in OS skin is responsible for the delay and that a low sebum-level is enough to maintain a normal skin surface lipophilicity. These data confirm our previous findings [6 ], where no correlation was found
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between cLW and sebum casual level, and where s cLW was identical on forehead and on volar fores arm, although the former surface is sebum-rich and the latter a sebum-poor surface, mostly coated with epidermal lipids ( EL) [1]. Accordingly, sebum and EL are thought to give the same cLW compos nent to the skin surface. As far as the acidic component is concerned, our data show that the forehead skin surface, regardless of its sebum level, exhibited a very low acidic character (c+). Similar results were found s on the forearm [6 ] and a low acidic character is also found on other surfaces such as polymer [9,20] or psoralen surfaces [21]. Good and van Oss [9] observed that some carbohydrates are c− s monopoles, although on the basis of structure, they were expected to be bipolar because the protons of the hydroxyl groups should be electron acceptors. One of the explanations given [9] is surface hydration. The presence of water, in the form of a ‘‘water vapor coat’’ all over the body surface is a common (and measurable) parameter named ‘‘transepidermal water loss’’ [22]. Water molecules originate either from passive diffusion through the epidermis [22], or, and especially on the forehead, from inconspicuous sweating [23]. According to this hypothesis [9] and owing to the presence of adsorbed water molecules tightly bound to the surface, the Lewis acid character of the underlying groups may not be expressed and the skin surface may found to have a very low acidic character. The largest difference found in this study is for the basic component: 21 mJ m−2 for HS and NS skin and 11 mJ m−2 for OS skin. The composition of the bulk sebum mainly depends on age [24,25], but large differences were also observed in young subjects [26 ], because of the degree of hydrolysis of triglycerides to free fatty acids ( FFA). A previous study [27] showed that this was higher in HS skin than in OS skin. From our calculations, we assume that a low basic component is the result of a low sebum-level and low FFA concentration on NS and HS skins. From and above the NS sebum-level, the sebum level and the FFA concentration have no influence on the c− component. s Considering the masking of acidic moieties by water molecules, c− reflects an outward orientation s
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of the ‘‘hydrated carboxyl groups’’ of the FFA. This suggests that their amphipathic nature makes them act as surface-active agents, located at the sebum–water interface of the skin surface. Specific adsorption of amphipathic molecules was previously reported to explain the high c− values s found on cholesterol [28] or psoralen surfaces [21] treated with bile salts. After washing with soap, c− was still about 8 mJ m−2 in HS and NS, despite s very low sebum-levels (9 and 14 AU respectively). This suggests that some FFA or water molecules still remain tightly bound to the skin surface. Conversely, washing seems more efficient in OS skin, probably because hydrophilic molecules are less numerous on the surface. Finally, the c− level s was restored later than the sebum level for all skin types, possibly owing to the time needed for the hydrolysis of native sebum triglycerides into FFA [26 ]. We can conclude that a high FFA fraction in a high sebum-level is needed to provide the skin surface with a normal c− level, i.e. hydrophilicity. s This is not the case in OS skin. 5.2. The hydrophilic–lipophilic balance of the skin surface We have shown that a low sebum-level gives the skin surface a normal lipophilicity, but a normal hydrophilicity level is only reached with medium or high sebum-levels. Accordingly, the polar–apolar ratio (cAB /cLW) may be assumed, in terms of s s ‘‘surface free energy component balance’’ (SEB), to be a parameter of the normal wetting properties of the skin surface. This is in agreement with Good’s proposal [14] which suggests the re-examination of the hydrophilic–lipophilic balance (HLB) methodology in terms of surface free energy components. The ratio cAB /cLW at baseline and after sebums s level restoration was 0.23:0.20 for NS skin. In the HS and OS skins, the SEBs were 0.22:0.20 and 0.07:0.05 respectively, showing that the HS skin SEB is similar to the NS skin SEB. In contrast, the very low SEB value of the OS skin shows that this skin suffers from a deficit of hydrophilic components, possibly through a lower concentration of FFA and a weaker retention of water at the skin–air interface.
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This suggests that OS skin should be considered as a dry skin or more accurately as a dry skinsurface. This approach may provide new insights into so-called ‘‘skin hydration’’ determined from surface free energy calculations and the surface ability to bind water molecules.
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[11] F.M. Fowkes, Ind. Eng. Chem. 56 (1964) 40. [12] B. Janczuk, E. Chibowski, J.M. Bruque, M.L. Kerkeb, F. Gonzalez-Caballero, J. Colloid Interface Sci. 159 (1993) 421. [13] D. St-Le´ger, C. Berrebi, C. Duboz, P. Agache, Arch. Dermatol. Res. 79 (1979) 265. [14] C.O. Timmons, W.A. Zisman, J. Colloid Interface Sci. 22 (1966) 165. [15] R.J. Good, J. Adhesion Sci. Technol. 6 (12) (1992) 1269. [16 ] M.B. Jani, Ph. D. thesis, Besanc¸on, no 92-014, 1992. [17] H. Schott, J. Pharm. Sci. 60 (1971) 1893. [18] A. El-Shimi, E.D. Goddard, J. Colloid Interface Sci., 48 (1974) 242; Appl. Chem. Prot. Interf., 8 (1974) 141. [19] H.A.B. Wo¨sten, T.G. Ruardy, H.C. van der Mei, H.J. Busscher, J.G.H. Wessels, Colloids Surfaces B: Biointerfaces 5 (1995) 189. [20] F. Boury, J.E. Proust, J. Adhesion Sci. Technol. 6 (12) (1992) 1359. [21] A. Saı¨d, A. Mavon, S. Makki, P. Humbert, J. Millet, Colloids Surfaces B: Biointerfaces 8 (1997) 227. [22] D.R. Wilson, H.I. Maibach, in: J.L. Leveˆque (Ed.), Cutaneous Investigation in Health and Disease, Marcel Dekker, New York, 1989, p. 153. [23] K. Sato, W.H. Kang, K. Saga, K.T. Sato, J. Am. Acad. Dermatol. 20 (4) (1989) 537. [24] M.E. Stewart, Seminars in Dermatology 11 (2) (1992) 100. [25] M.E. Stewart, D.T. Downing, J. Invest. Dermatol. 95 (1990) 603. [26 ] D.T. Downing, J.S. Strauss, P.E. Pochi, J. Invest. Dermatol. 53 (1969) 322. [27] Y.F. Korolev, Vestn. Dermatol. Venerol. 32 (1958) 9. [28] B. Janczuk, E. Chibowski, J.M. Bruque, M.L. Kerkeb, F. Gonzalez-Caballero, J. Colloid Interface Sci. 159 (1993) 421.
Changes in sebum levels and skin surface free energy components following skin surface washing A. Mavon a,b,*, D. Redoules a, P. Humbert b, P. Agache b, Y. Gall a a Institut de Recherche Pierre Fabre (IRPF), BP 74, 31322 Castanet Tolosan cedex, France b Laboratoire de Biophysique Cutane´e/C.R.T., Faculte´ de Me´decine and Pharmacie, Place Saint-Jacques, 25030 Besanc¸on cedex, France Received 2 April 1997; accepted 5 January 1998
Abstract The forehead skin sebum casual (or usual ) level, which is remarkably stable in a given individual, is responsible for the monopolar basic character of the skin surface. However, the significance of this finding, in terms of the physicochemical behaviour and wettability of the skin surface, remains unexplained. How does this vary during sebum casual level restoration following skin washing and what is its relationship with so-called ‘‘dry skin’’? The present work was aimed at answering these questions. Forehead surface free energy components were assessed before and during the sebum casual level restoration following skin washing by nonionic soap and water, in 12 healthy volunteers: four oligo- (OS), four normo- (NS), and four hyper-seborrheic (HS) skins. Skin contact angles of water, formamide and diiodomethane were measured at intervals, and surface free energy components (i.e. Lifshitz–van der Waals, electron acceptor and electron donor components) were calculated using the contact angle values and acid–base theory of C.J. van Oss et al. The results showed that a low sebum-level does not deprive the skin surface of a normal Lifshitz–van der Waals component. This suggests that a low sebum-level is sufficient to render the skin normally lipophilic. The very low acidic component of the skin surface seemed to be due to the masking of acidic moieties by adsorbed water. The basic component was sebum-level dependent below a threshold value and remained to be related to the sebum free fatty acids and especially to their outward carboxylic group orientation. We conclude that the wetting properties of the skin surface could be defined by a ‘‘surface free energy component balance’’ or SEB. The SEB of NS and HS skins was around 0.22, and that of OS skin was 0.07. This low SEB stems from a weak hydrophilic component, allowing OS skin to be called dry skin, or better, a dry skin surface. This approach may provide a new insight into so-called ‘‘skin hydration’’. © 1998 Elsevier Science B.V. Keywords: Surface free energy; Human skin surface; Sebum casual level; Advancing contact angle
1. Introduction At any given site in the same individual, certain lipids are present on the human skin surface [1] at a remarkably constant level. Whereas this casual * Corresponding author. 0927-7765/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0 9 2 7 -7 7 6 5 ( 9 8 ) 0 0 00 7 - 1
level is very low at some sites like the volar forearm, and is mostly made up of epidermal lipids, on the forehead it is maximal, owing to the adjunction with sebum. In all body areas and especially on the forehead, the skin surface lipid level is restored within 2 h [2,3] following washing or degreasing through a sudden and sustained flow
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of sebum from the pilosebaceous orifices [4]. The overwhelming prevalence of sebum over epidermal lipids on the forehead leads to the description of oligo-, normo-, or hyper-seborrheic skin to characterize the skin surface lipid level in this area [5]. In a previous work on the wettability of human skin [6 ], it was shown that skin surface lipids and mainly sebum increase the skin surface free energy c and especially the electron donor component s c− as described by the theoretical approach of van s Oss et al. [7–9]. However, we did not find any correlation between the sebum level and either the overall c or its c− component. s s As sebum, at its casual level, gives the skin surface a basic monopolar character, three questions arise: what does this finding mean in terms of the skin surface physicochemical behaviour and wettability; how does it change during sebum casual level restoration following skin washing; and does it have a relationship with so-called ‘‘dry skin’’? To answer these questions, we calculated the skin surface free energy components, from contact angle measurements and the acid–base theory of van Oss, Good and Chaudhury [7–9] on the forehead of oligoseborrheic (OS), normoseborrheic (NS) and hyperseborrheic (HS) subjects; before and after washing and during the spontaneous sebum-level restoration.
2. Theory Nearly thirty years ago, Lifshitz [10] showed that in the condensed phase, the three electrodynamic forces (namely London ‘‘dispersion’’, Keesom dipole–dipole and Debye dipole-induced dipole forces) decay with distance at the same rate and obey the same combining rules. Recently, van Oss et al. [7–9] pointed out that surface tension arising from these three electrodynamic forces, when put together, gives rise to a surface-tension component that is called the Lifshitz–van der Waals (LW ) component. If suitable chemical groups are present, significant contributions to interfacial energy can arise from hydrogen bonding, i.e. Lewis acid–base or electron-acceptor– electron-donor interactions. This kind of force
was, at first, called ‘‘short range’’ or ‘‘SR’’ [7], owing to the rapid decay with distance, but later the term acid–base was preferred. Fowkes [11] suggested that the surface tension, c, can be considered as the sum of independent terms, each representing a particular intermolecular force. Accordingly, c can be expressed as the sum of Lifshitz–van der Waals, cLW, and acid– base, cAB components: c=cLW+cAB
(1)
The surface tension of a solid surface can be readily calculated by measuring the contact angle of two different liquids of known surface-tension components. A more complex approach was developed by van Oss et al. [8,9] incorporating the electron-donor and the electron-acceptor parameters in the acid–base component. A substance can show both electron-donor and electron-acceptor properties, or can be overwhelmingly basic showing virtually only electron-donor behaviour; the opposite case being possible but far less common. Materials exhibiting only electron-acceptor or electron-donor capacity are called monopolar materials. It must be noted that an acid–base parameter will be manifested only if the opposite parameter is present in another molecule or in another part of the same molecule; thus the polar parameter of a monopolar material cannot, by itself, contribute to the energy of cohesion. The polar or acid–base components can be expressed as: cAB=2(c+c−)1/2
(2)
The c+ and c− components indicate the electronacceptor and the electron-donor components respectively. Finally, the three components of the interfacial tension between materials 1 and 2 can be defined by the next equation [8,9]: c =[cLW +cLW −2(cLW cLW )1/2]+2[(c+ c− )1/2 12 1 2 1 2 1 1 +(c+ c− )1/2−(c+ c− )1/2−(c− c+ )1/2] (3) 2 2 1 2 1 2 Solid surface free energy can be determined from contact angle measurements, in appropriately chosen systems using Young’s equation: c cos h=c −c −p l s sl e
(4)
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where c is the solid surface free energy, c the s sl interfacial solid–liquid free energy, c the liquid l surface tension, h the contact angle and p the film e pressure of the liquid. After combining Eq. (3) with Eq. (4) and neglecting p [11], the following e equation is obtained: (1+cos h)c =2(cLW cLW )1/2 l s l +2(c+ c− )1/2+2(c− c+ )1/2 (5) s l s l The three parameters characterizing the solid surface free energy can be determined by measuring the contact angles of at least three standard liquids, provided the relevant values of the surface tension components of these liquids are known [9,12].
3. Experimental details 3.1. Liquids (Table 1) Formamide and diiodomethane (analytical grade, minimum 98% purity) were supplied by Fluka (St Quentin Fallavier, France). Water was distilled twice and deionized (Milli-Q reagent water system, Millipore) yielding a resistivity of 18 MV cm−1 at 25°C. 3.2. Volunteers (forehead surface) and skin surface treatments The twelve volunteers (age range 23–31 years) were selected in each of the previously defined skin types [5], namely oligoseborrheic (OS), normoseborrheic (NS ) and hyperseborrheic (HS): sebum levels were less than 80 AU, 120–160 AU and Table 1 Literature values (mJ m−2) for Lifshitz–van der Waals (cLW ) l electron-acceptor (c+ ) and electron-donor, (c− ) components l l of standard liquids (c ) used for the determination of skin surl face free energy components Liquid
c l
cLW l
c+
Water Formamide Diiodomethane
72.8 58 50.8
21.8 39 50.8
25.5 2.28 0.72
l
c−
l
25.5 39.6 0
Ref. [12] [12] [12]
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more than 200 AU, respectively. The sebum levels were assessed using a photometric method [13] giving a direct reading in arbitrary units (AU ) using the SebumeterA SM 801, (Courage and Khazaka Gmbh, Cologne, Germany). Contact angle and sebum-level measurements were carried out on the forehead before washing (baseline); on the skin kept unwashed for at least 8 h; just after washing (T 0) and during the resuming sebaceous excretion at different times: T 20 min, T 40 min, T 60 min, T 90 min and T 120 min. A previous work (A. Mavon et al., unpublished data, 1996) where we studied six subjects over a 5 h period showed that the sebum level and the surface free energy components were reconstituted in the first 2 h. The forehead was washed by scrubbing twice for 10 s with a gauze pad impregnated with ‘‘Savon Doux’’, (amphoteric washing soap purchased from Laboratoires Anios, Lille, France) followed by rinsing three times with a gauze pad soaked in water and drying by three applications of absorbent paper without rubbing.
3.3. Measurements of contact angles The method was the same as that used in a previous study [6 ]. Advancing contact angles h A were measured according to Zisman’s method [14,15]. However, where Zisman used a fine platinum wire, our procedure used a Hamilton microsyringe with a fine needle. The volume of the initial drop placed on the skin surface was increased by addition of droplets. The final volume was 5 ml. Contact angles were assessed using a surgical microscope ( Wild HeerbruggA M650), with a magnification of 25×, fitted with a slanted mirror and a video camera (CCD-IRIS, SonyA) connected to a personal computer. The computer was equipped with video capture hardware ( VideospigotA) and digital video software (MicrosoftA). This system provided the observation, acquisition, storage and computation of h . A The measurements were carried out at room temperature (21±1.5°C ). Using contact angles value, Eq. (5) was solved for the water–formamide–diiodomethane ( WFD) triplet [12,15]. Calculations were made for each time and each volunteer.
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3.4. Skin roughness and skin temperature A discussion on the influence of the skin roughness and temperature on the h measurements was A documented in a previous paper [6 ]. In this experiment, as in the previous study, whatever the errors made in our h measurements from the skin relief A and from a change in the temperature of the drop, they were similar in size in all measurements and accordingly could not strongly alter the results. These difficulties stem from the fact that the skin is a living surface. However, the absolute error in contact-angle values of the three standard liquids on the forehead surface remained low (2°–4°).
4. Results 4.1. Sebum level (Fig. 1) The volunteers were classified into three groups according to sebum casual level of the forehead at baseline 70 AU, 139 AU and 211 AU in OS, NS and HS respectively. Thus, the three skin types were clearly defined at this time. At T 0, just after washing, the sebum levels were 4 AU, 9 AU and 14 AU in OS, NS and HS respectively. This level differed from zero for two reasons. First, using a soap, degreasing was not drastic. Secondly, sebaceous excretion is continuous and had already resumed during the 2 min between degreasing and
Fig. 1. Baseline, washing and sebum-level restoration on forehead in OS (oligoseborrheic), NS (normoseborrheic) and HS (hyperseborrheic) skins.
Fig. 2. Advancing contact angles of diiodomethane on the forehead in the three skin types.
the sebum-level measurement. Afterwards, the sebum level increased and the baseline value was reached by 90 min, whatever the skin type. By 2 h it was at its maximum and slightly higher than at baseline [16 ]. 4.2. Contact angles The experimental contact angles and their interindividual variations are shown in Figs. 2–4. The standard deviation (between 6° and 12°) was in the range of contact angle values, as measured on human skin [6,17–19]. Intra-individual variations were smaller, the standard deviation ranged from
Fig. 3. Advancing contact angles of water on the forehead in the three skin types.
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Continuously over 2 h, the values decreased to baseline, keeping a final difference of about 15° between OS skin and the two other skin types.
4.3. Surface free energy components The average value of the surface free energy components on the forehead surfaces ( Tables 2–4) at different times were obtained by solving Eq. (5) and using the surface tension components of the standard liquids used ( Table 1). Fig. 4. Advancing contact angles of water on the forehead in the three skin types.
2° to 4°, also in accordance with literature values [6,17,18]. 4.2.1. Diiodomethane contact angle (Fig. 2) At baseline (before washing), the contact angles of diiodomethane were the same for the three skin types (30°). At T 0, ( just after degreasing) it was high and similar (42°) in NS and HS skin, but still higher in OS skin (50°). By 20 min, contact angles in NS and HS skins had substantially decreased, and lowered down to baseline values by 1 h. Conversely, on the OS skin, the contact angles slowly decreased over 2 h and only after this time was the baseline value reestablished. 4.2.2. Water contact angle (Fig. 3) At baseline, the water contact angles were the same (57°) in NS and HS skin but much higher in OS skin (73°). At T 0 (after degreasing), regardless of skin type, they had all increased by 20°. Baseline values were reestablished within 2 h. The observed difference of 15° between OS skin and the two other types before degreasing, remained at this time. 4.2.3. Formamide contact angle (Fig. 4) As observed for water, the formamide contact angle, at baseline, in the NS and HS skin (41°) differed from the OS skin (58°). Washing increased the values by around 15° at T 0 in all skin types.
4.3.1. Lifshitz–van der Waals component, cLW s At baseline, cLW was about 37.5 mJ m−2 for the s three skin types. The degreasing induced a slight decrease (3 mJ m−2) in NS and HS skin, and a greater decrease in the OS skin (5 mJ m−2). On HS and NS skin, the baseline value was reached by 20 min although the sebum level was restored by 90 min. This was not the case in OS skin, where cLW increased in parallel to the sebum excretion s and by 90 min the baseline value was reached for both.
4.3.2. Electron acceptor or acidic component, c+ s The acidic component ranged from 0 to 1 mJ m−2 and was considered negligible, at each time, for each skin type and whether the skin had just been washed or the sebum level restored.
4.3.3. Electron donor or basic component, c− s At baseline, all skin types had a strong electron donor character, of similar value in NS and HS skin (21 mJ m−2) but much lower in OS skin (12 mJ m−2). In all skin types also, it dropped sharply immediately after degreasing, and by the same amount (11 mJ m−2). However, only the OS skin became almost apolar, because its basic component fell to about 0 mJ m−2. In OS skin, restoration started immediately and was continuous over 2 h but it only increased after 20 min in the other skin types. In all of them, restoration of c− was s achieved after sebum-level restoration.
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Table 2 Total surface free energy (c ) and Lifshitz–van der Waals (cLW), acid–base (cAB), electron acceptor (c+), electron donor, (c−) compos s s s s nents in mJ m−2, on forehead in four normoseborrheics, NS, (mean and SD)
Baseline T0 T 20∞ T 40∞ T 60∞ T 90∞ T 120∞
c s
cLW s
cAB s
c+ s
c− s
46.3±1.1 38.2±5.6 41.3±2.7 40.6±3.0 41.4±2.7 43.4±3.2 44.8±3.7
37.5±1.1 33.4±1.8 37.2±1.9 38.3±1.4 38.4±1.0 38.5±0.4 38.0±0.9
8.8±1.8 4.4±3.3 4.1±1.8 2.8±2.3 3.2±2.8 5.2±2.5 7.6±1.4
1.0±0.5 0.8±0.9 0.7±0.4 0.2±0.2 0.2±0.2 0.4±0.3 0.7±0.5
21.0±4.3 8.3±0.5 7.2±2.1 10.5±5.4 18.1±6.9 18.3±5.4 23.8±6.1
Table 3 Total surface free energy (c ) and Lifshitz–van der Waals (cLW), acid–base (cAB), electron acceptor (c+), electron donor, (c−) compos s s s s nents in mJ m−2, on forehead in four hyperseborrheics, HS, (mean and SD)
Baseline T0 T 20∞ T 40∞ T 60∞ T 90∞ T 120∞
c s
cLW s
cAB s
c+ s
c− s
44.4±2.3 37.4±3.7 42.2±2.1 41.6±3.3 40.5±1.9 42.6±4.9 42.9±4.9
36.6±2.4 34.0±2.2 38.1±0.9 37.3±1.1 37.4±1.1 37.4±1.2 37.0±1.7
8.0±2.3 3.2±2.0 3.9±1.9 4.4±2.2 3.3±1.9 6.5±3.4 7.5±2.7
0.8±0.4 0.4±0.4 0.7±0.5 0.7±0.7 0.2±0.2 0.6±0.7 0.6±0.6
20.6±3.3 8.0±4.6 6.3±2.8 10.7±4.0 13.6±4.4 17.0±2.7 20.4±1.8
Table 4 Total surface free energy (c ) and Lifshitz–van der Waals (cLW), acid–base (cAB), electron acceptor (c+), electron donor, (c−) compos s s s s nents in mJ m−2, on forehead in four oligoseborrheics, OS, (mean and SD)
Baseline T0 T 20∞ T 40∞ T 60∞ T 90∞ T 120∞
c s
cLW s
cAB s
c+ s
c− s
41.8±2.4 34.6±4.0 34.8±4.9 38.2±3.1 39.5±1.9 40.1±1.5 40.2±0.7
38.6±0.7 33.5±4.2 33.1±4.8 34.6±3.1 37.3±1.1 38.3±0.4 38.8±0.4
2.8±2.4 1.2±0.9 1.7±1.3 3.8±1.0 2.7±1.4 2.1±0.5 1.8±0.5
0.3±0.4 0.4±0.2 0.3±0.3 0.6±0.3 0.3±0.5 0.1±0.1 0.1±0.1
11.6±2.4 1.3±1.1 3.7±2.1 6.5±1.8 6.8±1.8 10.2±0.7 11.8±1.5
5. Discussion 5.1. Surface free energy components At baseline, forehead cLW was about the same s in the three skin types in spite of significant differences in sebum levels. This shows that cLW is s independent of sebum casual level. Following removal of the skin-surface lipids by washing, only cLW decreased slightly, (4 mJ m−2) indicating s that the bare (i.e. without added lipids) skin sur-
face cLW component was about 33 mJ m−2, and s that sebum was only responsible for a minor change. Baseline cLW (37.5 mJ m−2) was reestabs lished 1 h before sebum casual levels in NS and HS skin, whereas in OS skin, the baseline cLW and s sebum casual levels were reached at the same time. This suggests that the lower sebum flow in OS skin is responsible for the delay and that a low sebum-level is enough to maintain a normal skin surface lipophilicity. These data confirm our previous findings [6 ], where no correlation was found
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between cLW and sebum casual level, and where s cLW was identical on forehead and on volar fores arm, although the former surface is sebum-rich and the latter a sebum-poor surface, mostly coated with epidermal lipids ( EL) [1]. Accordingly, sebum and EL are thought to give the same cLW compos nent to the skin surface. As far as the acidic component is concerned, our data show that the forehead skin surface, regardless of its sebum level, exhibited a very low acidic character (c+). Similar results were found s on the forearm [6 ] and a low acidic character is also found on other surfaces such as polymer [9,20] or psoralen surfaces [21]. Good and van Oss [9] observed that some carbohydrates are c− s monopoles, although on the basis of structure, they were expected to be bipolar because the protons of the hydroxyl groups should be electron acceptors. One of the explanations given [9] is surface hydration. The presence of water, in the form of a ‘‘water vapor coat’’ all over the body surface is a common (and measurable) parameter named ‘‘transepidermal water loss’’ [22]. Water molecules originate either from passive diffusion through the epidermis [22], or, and especially on the forehead, from inconspicuous sweating [23]. According to this hypothesis [9] and owing to the presence of adsorbed water molecules tightly bound to the surface, the Lewis acid character of the underlying groups may not be expressed and the skin surface may found to have a very low acidic character. The largest difference found in this study is for the basic component: 21 mJ m−2 for HS and NS skin and 11 mJ m−2 for OS skin. The composition of the bulk sebum mainly depends on age [24,25], but large differences were also observed in young subjects [26 ], because of the degree of hydrolysis of triglycerides to free fatty acids ( FFA). A previous study [27] showed that this was higher in HS skin than in OS skin. From our calculations, we assume that a low basic component is the result of a low sebum-level and low FFA concentration on NS and HS skins. From and above the NS sebum-level, the sebum level and the FFA concentration have no influence on the c− component. s Considering the masking of acidic moieties by water molecules, c− reflects an outward orientation s
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of the ‘‘hydrated carboxyl groups’’ of the FFA. This suggests that their amphipathic nature makes them act as surface-active agents, located at the sebum–water interface of the skin surface. Specific adsorption of amphipathic molecules was previously reported to explain the high c− values s found on cholesterol [28] or psoralen surfaces [21] treated with bile salts. After washing with soap, c− was still about 8 mJ m−2 in HS and NS, despite s very low sebum-levels (9 and 14 AU respectively). This suggests that some FFA or water molecules still remain tightly bound to the skin surface. Conversely, washing seems more efficient in OS skin, probably because hydrophilic molecules are less numerous on the surface. Finally, the c− level s was restored later than the sebum level for all skin types, possibly owing to the time needed for the hydrolysis of native sebum triglycerides into FFA [26 ]. We can conclude that a high FFA fraction in a high sebum-level is needed to provide the skin surface with a normal c− level, i.e. hydrophilicity. s This is not the case in OS skin. 5.2. The hydrophilic–lipophilic balance of the skin surface We have shown that a low sebum-level gives the skin surface a normal lipophilicity, but a normal hydrophilicity level is only reached with medium or high sebum-levels. Accordingly, the polar–apolar ratio (cAB /cLW) may be assumed, in terms of s s ‘‘surface free energy component balance’’ (SEB), to be a parameter of the normal wetting properties of the skin surface. This is in agreement with Good’s proposal [14] which suggests the re-examination of the hydrophilic–lipophilic balance (HLB) methodology in terms of surface free energy components. The ratio cAB /cLW at baseline and after sebums s level restoration was 0.23:0.20 for NS skin. In the HS and OS skins, the SEBs were 0.22:0.20 and 0.07:0.05 respectively, showing that the HS skin SEB is similar to the NS skin SEB. In contrast, the very low SEB value of the OS skin shows that this skin suffers from a deficit of hydrophilic components, possibly through a lower concentration of FFA and a weaker retention of water at the skin–air interface.
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This suggests that OS skin should be considered as a dry skin or more accurately as a dry skinsurface. This approach may provide new insights into so-called ‘‘skin hydration’’ determined from surface free energy calculations and the surface ability to bind water molecules.
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