Lifshitz-van Der Waals Research Articles

The use of physicochemical methods to detect organic food soils on stainless steel surfaces

Food processing surfaces fouled with organic material pose problems ranging from aesthetic appearance, equipment malfunction and product contamination. Despite the importance of organic soiling for subsequent product quality, little is known about the interaction between surfaces and organic soil components. A range of complex and defined food soils was applied to 304 stainless steel (SS) surfaces to determine the effect of type and concentration of soil on surface physicochemical parameters, viz surface hydrophobicity (ΔGiwi ), surface free energy (γs), Lifshitz van der Waals ( ), Lewis acid base ( ), electron acceptor () and electron donor () measurements. When compared to the control surface, changes in and were indicative of surface soiling. However, soil composition and surface coverage were heterogeneous, resulting in complex data being generated from which trends could not be discerned. These results demonstrate that the retention of food soil produces changes in the physicochemical parameters of the surface that could be used to indicate the hygienic status of a surface.

Influence of surface modifying macromolecules on the surface properties of poly(ether sulfone) ultra-filtration membranes

The separation performances and physico-chemical characteristics of poly(ether sulfone) membranes were evaluated by blending of surface modifying macromolecules (SMMs). Hydrophobic SMMs (bSMM) and charged SMMs (cSMM) were synthesized and characterized. By addition of SMMs, glass transition temperature decreased and membrane pore size increased. The membrane became more hydrophobic by blending of bSMM, and became more hydrophilic after addition of cSMM. It was also demonstrated by the result of surface free energy study that the bSMM blended membrane showed the lowest solid–liquid (water) interaction by low Lifshitz–van der Waals interaction and Lewis acid (liquid)–base (solid) interaction. In contrast, cSMM blended membrane showed the highest Lewis acid–base interaction, which resulted in the highest interaction energy between membrane surface and liquid (water). It was also found from the result of differential scanning calorimeteric measurement that bound water was formed in the cSMM blended membrane.

The wettability of polytetrafluoroethylene and polymethylmethacrylate by aqueous solutions of Triton X-100 and short chain alcohol mixtures

Measurements of advancing contact angles ( θ) were carried out for aqueous solutions of Triton X-100 (TX-100) and methanol and ethanol mixtures at constant TX-100 concentration equal to 1 × 10 −7, 1 × 10 −6, 1 × 10 −5, 1 × 10 −4, 6 × 10 −4 and 1 × 10 −3 M, respectively, on polytetrafluoroethylene (PTFE) and polymethylmethacrylate (PMMA). Using measured contact angle values the relationships between cos θ, adhesion tension and surface tension of the solutions were determined, and on their basis the critical surface tension of PTFE and PMMA wetting was calculated. The obtained average value of the critical surface tension of PTFE wetting is lying in the range of the PTFE surface tension values which can be found in the literature, while for PMMA it is even lower than the Lifshitz–van der Waals component of its surface tension. From the relationship between the adhesion and surface tension and Lucassen–Reynders equation it results that in the case of PTFE the adsorption at the PTFE–solution and solution–air interfaces is the same, which was confirmed by a linear relationship between the cos θ and 1/ γ LV and intercept on cos θ axis equal to −1. However, for PMMA the adsorption of the surface active agents at solution–air interface is higher than at PMMA–solution. Using the values of the contact angle the values of the adhesion work of solution to the PTFE and PMMA surface were also determined, which are constant for PTFE, but for PMMA decrease with alcohol concentration increase. Next, using the contact angle values in the Young equation, the PTFE(PMMA)–solution interface tension was also calculated. The obtained values of PTFE–solution interface tension were compared with those evaluated from the Szyszkowski, Connors and Fainerman and Miller equations, and good agreement between these values was observed for all series of TX-100 and alcohol mixtures at a low alcohol concentration.

Poisson Analysis of Streptococcal Bond Strengthening on Stainless Steel with and without a Salivary Conditioning Film

Poisson analysis of retract force-distance curves in atomic force microscopy (AFM) has yielded a new dimension to the decoupling of individual bond forces into a hydrogen bonding and nonspecific force component. Accordingly, bacterial adhesion forces have been decoupled into a hydrogen bonding and nonspecific Lifshitz-Van der Waals contribution. Due to the forced nature of AFM contact, the nonspecific force contribution has hitherto turned out to be repulsive in the analysis of bacterial adhesion forces on nonconducting surfaces. In this study, we present the results of a Poisson analysis of adhesion forces for streptococci adhering to a conducting surface. Adhesion forces measured between stainless steel, both in the absence and presence of an adsorbed salivary conditioning film, increased with increasing contact time between the streptococcal AFM probe and the surface. Concurrent with the increase in adhesion force, there was an increase in the number of minor force peaks in the retract force-distance curves. Poisson analyses of the adhesion forces indicated repulsive nonspecific Lifshitz-Van der Waals forces for streptococci adhering to saliva-coated stainless steel, but interestingly and for the first time, attractive nonspecific forces were revealed on stainless steel in the absence of a salivary conditioning film. We tentatively attribute this to attraction between the negatively charged streptococci and their positive image charges in the conducting material, which cannot develop in a nonconducting material or in the presence of a nonconductive protein layer on the stainless steel surface.

Surface Thermodynamics and Extended DLVO Theory of Acidithiobacillus ferrooxidans Cells Adhesion on Pyrite and Chalcopyrite

The adhesion of Acidithiobacillus ferrooxidans bacterial cells have been assessed by following the thermody- namic and extended DLVO theoretical approaches. Surface potential, interfacial tension and contact angle parameters that are necessary for the calculation of free energy of adhesion have been determined experimentally. The Hamaker constant involved in the Lifshitz-van der Waals interaction energy has been estimated by microscopic and macroscopic methods. The free energy of adhesion found to be negative on pyrite and chalcopyrite minerals indicating the adsorption of bacterial cells on these minerals. The potential energy diagrams of total interaction energy versus separation distance curves also il- lustrate the feasibility of bacterial cells adhesion except in a pH region where the bacterial species and minerals possess similar surface charge with high magnitude. The present theoretical analysis of bacterial adhesion on mineral surfaces found to be in good agreement with the experimental results and previous findings in the literature. Thus, the bacterial ad- hesion behavior on minerals can be judged and explained by considering the physico-chemical interaction forces.

Effects of environmental conditions on fluorinated diamond-like carbon tribology

The effects of environmental conditions and fluorine content on friction of diamond-like carbon (DLC) films are determined and discussed. Relative humidity (RH) has a considerable effect on friction values, modifying them up to a factor 2. Two regions of RH are found, depending on its effect over friction. From 20% to 60% of RH, friction decreases probably due to a better substitution of superficial atoms by hydrogen. From 60% to 80% friction remains stable and equal for almost all samples. A possible explanation would be the formation of a physisorbed layer of water on the surface, which would mask the relatively small chemical and structural variations of the films. Moreover, the different components of surface free energy (SFE) are determined using Van Oss–Chaudury–Good model. The results clearly indicate the importance of Lifshitz–Van der Waals component on SFE and the small correction that supposes acid-base interactions. According to this model, the studied DLC is a monopolar basic material. The basic component is reduced when F content is increased.

Role of Interfacial Interactions in the Deposition of Colloidal Clay Particles in Porous Media

Colloidal clay particle transport under saturated conditions is believed to be controlled by its interactions with the surrounding environment. The dominating forces among these interactions are electrostatic forces that are determined by colloidal clay particle and porous medium surface charge density and Lifshitz–van der Waals forces that are determined by colloidal clay particle and porous medium surface thermodynamic properties. Electrostatic forces are greatly affected by solution chemistry in terms of solution ionic strength and pH. In this research, electrostatic and Lifshitz–van der Waals forces of natural colloidal clay particles with a model porous medium of silica sand were quantified at different ionic strength and pH conditions. At the same time, colloidal clay particle transport in the model medium of silica sand was conducted in a laboratory column. The maximum electrostatic forces, F EL (max), which occurred when the separation distance between colloidal clay particles and the porous medium was in the range of the sum of the double layer thicknesses of the colloidal clay particles and the porous medium, was found to be the determinant factor for colloidal clay particle deposition in the porous medium. Colloidal clay particle desorption in the porous media was related to the net effect of attractive Lifshitz–van der Waals forces and repulsive electrostatic forces, evaluated at the equilibrium distance where physical contact between the colloidal clay particle and silica sand actually occurred (i.e., affix force). Higher colloidal clay particle desorption was found to coincide with smaller affix force values.

The use of contact angle measurements to estimate the adhesion propensity of calcium carbonate to solid substrates in water

We have studied a series of solids using contact angle measurements; stainless steel, gold, aluminium, titanium nitride and PTFE that are frequently used in domestic water environments. It was found the influence of electron-donor ( γ −) and electron-acceptor ( γ +) free energies on material scaling rate was dominated by water wetting angles, providing materials exhibit an average roughness below 100 nm. The γ − component had the greatest influence on theoretical adhesion, while γ LW, (Lifshitz-van der Waals) γ + and γ AB (acid-base) had little effect. From the materials analysed, amorphous carbon coatings were least adhesive, while ‘kettle coating’ and highly roughened steel the most adhesive. The size and distribution of asperities also influenced the polar free energies and subsequent adhesion due to fluctuations in the wetting angle. The results obtained indicate works of adhesion can be used as a complementary technique with Lewis acid–base theory to deliver useful information about the propensity of scale to deposit on solids.

Surface characterization of biopolyurethanes based on cellulose derivatives

Surface tension parameters and surface morphology of biopolyurethanes based on cellulose derivatives thin films, before and after HF cold plasma treatment has been investigated. Calculations are based on the geometric mean approach of Owens and Wendt, Rabel and Kälble, on the Lifshitz-van der Waals acid/base approach of van Oss and co-workers and on the theoretical methods involving quantitative structure-property relationship. For all the investigated samples the polar component contributes significantly to the total surface tensions, as due to the large electron donor interactions. HF cold plasma treatment modifies the surface energy of biopolyurethanes by changing their surface polarity and hydrophilicity. The hydrophilic/hydrophobic balance was studied by means of the free energy of hydration between the biomaterial film and water. The protein adsorption tests of fibrinogen were effected to evaluate the applicability of these biopolyurethanes as biomedical thromboresistant devices.

Ultraviolet Bonding of PFPE With Carbon Overcoat: Surface Energy and Spreading

Ultraviolet (UV) irradiation effects on the functional perfluoropolyether (PFPE), i.e., Ztetraol films, were investigated by studying the bonded film thickness as well as the surface energy of PFPE lubricated media using Lifshitz-van der Waals and Lewis acid/base (LW/AB) theory. It was found that short-time UV irradiation enhanced the bonding of Ztetraol film with ion beam deposited carbon overcoat, which is attributed to the direct photodissociation of PFPE molecules. Long-time UV irradiation degraded PFPE chain and caused the loss of lubricant by evaporation. L-t plot was further employed to analyze the spreading behaviors of Ztetraol films, where L represents the distance of the leading edge of the lubricant film travels in the time t. With UV irradiation, the spreading of Ztetraol films was retarded possibly due to the enhancement of bonding of Ztetraol with overcoat and cross-linking between Ztetraol molecules.

Interfacial Energetics of Bacterial Silicification

Silica nanoparticle size distributions were measured from transmission electron micrographs of bacterial cells in thin-sectioned hot spring sediment samples, as well as specimens from laboratory experiments on bacterial silicification. Diameters of silica nanoparticles on bacterial cells were smaller than corresponding values of those occurring away from bacteria in the same fields of view. Regression analysis of the particle size data and application of the Lifshitz-van der Waals Acid-Base approach to evaluations of solid surface energy established that the size difference extends from a nearly 40% decrease in the mean interfacial energy of silica nanoparticles on bacteria (0.9 mJ/m 2 ) as opposed to free in aqueous suspension (1.2 mJ/m 2 ). In thermodynamic terms, the lower interfacial energy serves to reduce equilibrium solubility values, and enhance nucleation rates, of silica nanoparticles on bacteria cells. The well documented culmination of these events in hot spring sediments and experimental studies is rapid preferential silicification and structural preservation of bacterial cells.

Residence time dependent desorption of Staphylococcus epidermidis from hydrophobic and hydrophilic substrata

Adhesion and desorption are simultaneous events during bacterial adhesion to surfaces, although desorption is far less studied than adhesion. Here, desorption of Staphylococcus epidermidis from substratum surfaces is demonstrated to be residence time dependent. Initial desorption rate coefficients were similar for hydrophilic and hydrophobic dimethyldichlorosilane (DDS)-coated glass, likely because initial desorption is controlled by attractive Lifshitz–Van der Waals interactions, which are comparable on both substratum surfaces. However, significantly slower decay times of the desorption rate coefficients are found for hydrophilic glass than for hydrophobic DDS-coated glass. This difference is suggested to be due to the acid–base interactions between staphylococci and these surfaces, which are repulsive on hydrophilic glass and attractive on hydrophobic DDS-coated glass. Final desorption rate coefficients are higher on hydrophilic glass than on hydrophobic DDS-coated glass, due to the so called hydrophobic effect, facilitating a closer contact on hydrophobic DDS-coated glass.

Plasma treatment of heat treated beech wood – investigation on surface free energy

With an increased interest in finding alternatives for tropical timbers and preservative-treated wood, several new wood treatments have recently been commercialised, e.g., acetylation, furfurylation and heat treatment. For the latter, diverse techniques protected by trademarks have been developed, such as the Plato process, retification process, oil-heat treatment, thermo wood and Stellac treatment, whereby the chemical and physical properties of the treated wood are altered (Boonstra et al. 2006a,b). Beneficial changes are the improved dimensional stability and durability which, among other reasons, are caused by the loss of hydroxyl groups which results in reduced hygroscopicity and increased hydrophobicity of surfaces (Tjeerdsma and Militz 2005; Petric et al. 2007). On the other hand, wettability is a prerequisite for good adhesion, but it is reduced by less hydrophilic surfaces after heat treatment. Accordingly, the deposition of waterborne finishes is generally more difficult on heat treated wood (Petrissans et al. 2003; Geradin et al. 2007). Polymer processing industries are using plasma treatment techniques to improve wettability and adhesion on hydrophobic polymer surfaces (Kogelschatz 2003). By plasma treatment, the O/C ratio is increased by the formation of hydroxyl and carboxyl groups which brings about an increase in surface polarity (Jie-Rong et al. 1999; Drnovska et al. 2003) and may be able to compensate partly the loss of functional groups by heat treatment. Several publications deal with plasma treatment of wood or wooden materials for hydrophilicity increment: elevated surface free energy and an ameliorated wetting behaviour with water were found (Rehn and Viol 2003; Rehn et al. 2003; Evans et al. 2007; Toth et al. 2007; Wolkenhauer et al. 2007a,b,c). In general, air, nitrogen or oxygen are the gaseous media to increase hydrophilicity; however, plasma techniques can also be applied for this purpose, i.e., to decrease hydrophilicity. For example, in the presence of special chemicals or reactive gases (e.g., HMDSO, silane) thin layers can be deposited on wood surfaces for creating water repellent characteristics (Denes et al. 1999; Bente et al. 2004; Odraskova et al. 2007). Podgorski et al. (2000) investigated the wetting behaviour of water on heat treated wood after application of oxygen plasma treatment at low pressure (0.08 mbar) and found a significant decrease of water contact angles. To explore the surface modification by plasma treatment in more detail, the surface free energy and the work of adhesion based on the Lifshitz-van der Waals/acidbase approach were determined (Van Oss et al. 1988). In this approach, the total surface energy is divided into (1) a polar or acid-base component, and (2) a dispersive or Lifshitz-van der Waals component. The polar part is further subdivided into an acid part (electron acceptor) and a basic part (electron donor) with regard to the Lewis acid-base model. The latter includes hydrogen bonding, whereas the disperse part or the Lifshitz-van der Waals part comprises London, Keesom and Debye interactions. In the present study, a dielectric barrier discharge (DBD) was used for plasma treatment of heat treated wood. DBD was performed at atmospheric pressure and ambient air to avoid the impact of low pressure.

Staphylococcus epidermidis adhesion to He, He/O 2 plasma treated PET films and aged materials: Contributions of surface free energy and shear rate

Adhesion studies of bacteria ( Staphylococcus epidermidis) to plasma modified PET films were conducted in order to determine the role of the surface free energy under static and dynamic conditions. In particular, we investigated the effect of the ageing time on the physicochemical surface properties of helium (He) and 20% of oxygen in helium (He/O 2) plasma treated polyethylene terephthalate (PET) as well as on the bacterial adhesion. Treatment conditions especially known to result in ageing sensitive hydrophilicity (hydrophobic recovery) were intentionally chosen in an effort to obtain the widest possible range of surface energy specimens and also to avoid strong changes in the morphological properties of the surface. Both plasma treatments are shown to significantly reduce bacterial adhesion in comparison to the untreated PET. However, the ageing effect and the subsequent decrease in the surface free energy of the substratum surfaces with time – especially in the case of He treated samples – seem to favor bacterial adhesion and aggregation. The dispersion-polar and the Lifshitz–van der Waals (LW) acid–base (AB) thermodynamic approaches were applied to calculate the Gibbs free energy changes of adhesion (Δ G adh) of S. epidermidis interacting with the substrates. There was a strong correlation between the thermodynamic predictions and the measured values of bacterial adhesion, when adhesion was performed under static conditions. By decoupling the (Δ G adh) values into their components, we observed that polar/acid–base interactions dominated the interactions of bacteria with the substrates in aqueous media. However, under flow conditions, the increase in the shear rate restricted the predictability of the thermodynamic models.

The initial single yeast cell adhesion on glass via optical trapping and Derjaguin–Landau–Verwey–Overbeek predictions

We used an optical tweezer to investigate the adhesion of yeast Saccharomyces cerevisiae onto a glass substrate at the initial contact. Micromanipulation of free-living objects with single-beam gradient optical trap enabled to highlight mechanisms involved in this initial contact. As a function of the ionic strength and with a displacement parallel to the glass surface, the yeast adheres following different successive ways: (i) Slipping and rolling at 1.5 mM NaCl, (ii) slipping, rolling, and sticking at 15 mM NaCl, and (iii) only sticking at 150 mM. These observations were numerous and reproducible. A kinetic evolution of these adhesion phenomena during yeast movement was clearly established. The nature, range, and relative intensity of forces involved in these different adhesion mechanisms have been worked out as a quantitative analysis from Derjaguin-Landau-Verwey-Overbeek (DLVO) and extended DLVO theories. Calculations show that the adhesion mechanisms observed and their affinity with ionic strength were mainly governed by the Lifshitz-van der Waals interaction forces and the electrical double-layer repulsion to which are added specific contact forces linked to "sticky" glycoprotein secretion, considered to be the main forces capable of overcoming the short-range Lewis acid-base repulsions.

Surface Treatment of Indium Tin Oxide Using Radio Frequency Atmospheric and Low Pressure Plasma for OLEDs

We investigated the effect of atmospheric-pressure plasma (APP) and low-pressure plasma (LPP) treatments on the performance of organic light emitting diodes (OLEDs) with an indium tin oxide (ITO) layer. The Owens–Wendt and Lifshitz–van der Waals acid-base methods revealed that the increase of surface energy was mainly attributed to polar component and Lewis base interactions, respectively, independent of either APP or LPP treatments. Unlike APP treatment, LPP treatment more plentifully produced reactive oxygen species in the plasma. Therefore, the LPP-treated ITO surfaces slowly proceeded with reorientation compared to APP-treated ITO. The carbon content in untreated ITO was approximately 0.045%, while those of Ar APP-, APP-, and LPP-treated ITO were 0.014, 0.011, and 0.010%, respectively, mostly containing incorporated reactive oxygen. The LPP-treated surface showed more uniform roughness than Ar or APP-treated surfaces. The highest work-function value was obtained from LPP-treated ITO, intermediate values from Ar and APP-treated ITOs, and the smallest value from untreated ITO . Thus, OLED fabricated on the surface of LPP-treated ITO substrate exhibited superior performance among all plasma-treated samples.

Interaction forces between waterborne bacteria and activated carbon particles

Activated carbons remove waterborne bacteria from potable water systems through attractive Lifshitz–van der Waals forces despite electrostatic repulsion between negatively charged cells and carbon surfaces. In this paper we quantify the interaction forces between bacteria with negatively and positively charged, mesoporous wood-based carbons, as well as with a microporous coconut carbon. To this end, we glued carbon particles to the cantilever of an atomic force microscope and measured the interaction forces upon approach and retraction of thus made tips. Waterborne Raoultella terrigena and Escherichia coli adhered weakly (1–2 nN) to different activated carbon particles, and the main difference between the activated carbons was the percentage of curves with attractive sites revealed upon traversing of a carbon particle through the bacterial EPS layer. The percentage of curves showing adhesion forces upon retraction varied between 21% and 69%, and was highest for R. terrigena with positively charged carbon (66%) and a coconut carbon (69%). Macroscopic bacterial removal by the mesoporous carbon particles increased with increasing percentages of attractive sites revealed upon traversing a carbon particle through the outer bacterial surface layer.

Cranberry changes the physicochemical surface properties of E. coli and adhesion with uroepithelial cells

Cranberries have been suggested to decrease the attachment of bacteria to uroepithelial cells (UC), thus preventing urinary tract infections, although the mechanisms are not well understood. A thermodynamic approach was used to calculate the Gibbs free energy of adhesion changes (Δ G adh) for bacteria–UC interactions, based on measuring contact angles with three probe liquids. Interfacial tensions and Δ G adh values were calculated for Escherichia coli HB101pDC1 (P-fimbriated) and HB101 (non-fimbriated) exposed to cranberry juice (0–27 wt.%). HB101pDC1 can form strong bonds with the Gal–Gal disaccharide receptor on uroepithelial cells, while HB101–UC interactions are only non-specific. For HB101 interacting with UC, Δ G adh was always negative, suggesting favorable adhesion, and the values were insensitive to cranberry juice concentration. For the HB101pDC1-UC system, Δ G adh became positive at 27 wt.% cranberry juice, suggesting that adhesion was unfavorable. Acid–base (AB) interactions dominated the interfacial tensions, compared to Lifshitz–van der Waals (LW) interactions. Exposure to cranberry juice increased the AB component of the interfacial tension of HB101pDC1. LW interactions were small and insensitive to cranberry juice concentration. The number of bacteria attached to UC was quantified in batch adhesion assays and quantitatively correlated with Δ G adh. Since the thermodynamic approach should not agree with the experimental results when specific interactions are present, such as HB101pDC-UC ligand–receptor bonds, our results may suggest that cranberry juice disrupts bacterial ligand-UC receptor binding. These results help form the mechanistic explanation of how cranberry products can be used to prevent bacterial attachment to host tissue, and may lead to the development of better therapies based on natural products.

Increased adhesion of Enterococcus faecalis strains with bimodal electrophoretic mobility distributions

Initial adhesion is a determinant in the development of microbial biofilms. It is influenced, amongst others, by the surface hydrophobicity and the electrostatic characteristics of the substratum and adhering organisms. Enterococcus faecalis strains, grown in pure cultures, generally display subpopulations with different electrokinetic features, reflected in a bimodal electrophoretic mobility distribution. Here, the initial adhesion kinetics of five heterogeneous and five homogeneous E. faecalis strains were followed in a parallel-plate flow chamber. After 4 h of flow, heterogeneous strains adhered in significantly higher numbers than homogeneous strains (7.3 × 10 6 and 1.9 × 10 6 cm −2, respectively), but the initial deposition rates were not significantly influenced (740 and 600 cm −2 s −1, respectively). Apparently, initial deposition of bacteria is mainly governed by attractive Lifshitz–Van der Waals forces that overwhelm the electrostatic repulsion energy barrier, thus resulting in similar initial deposition rates for the various bacterial populations investigated. In contrast, during later stages of adhesion, bacteria in heterogeneous cultures likely experience a lower electrostatic repulsion from already adhering bacteria than bacteria in homogeneous cultures, thus allowing a closer proximity of the bacteria with respect to each other, which ultimately leads to increased adhesion after 4 h.

Surface energy and hybridization studies of amorphous carbon surfaces

Surface properties of a large number of amorphous carbon (a-C) films have been investigated using contact angle measurements and X-ray photoelectron spectroscopy (XPS). Dense a-C surfaces with variable sp 3/(sp 2 + sp 3) average hybridization were grown using sputtering or pulsed laser deposition (PLD) and were further chemically modified by thermal annealing, ion bombardment or covalent grafting of organic monolayers. The average carbon hybridization, impurity level and mass density, were deduced from XPS and photoelectron energy loss spectroscopy (PEELS). The depth sensitivity of the dispersive (Lifshitz–van der Waals) interaction, estimated at 1–2 nm from the dependence of γ LW on the grafted perflorodecene molecule coverage, is much better than XPS which probes a 3–5 nm depth. The observation of a non-monotonic behavior in the correlation between surface hybridization and electron donor component of surface energy reveals that the average carbon hybridization alone does not describe the entire surface energy physics. The role of π bond clustering in the polar interactions is thus considered and some implications on surface reactivity and mutual interactions with molecular or biomolecular species are discussed.