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Lipophobicity
Lipophobicity
Lipophobicity
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Lipophobicity, also sometimes called lipophobia (from the Greek λιποφοβία from λίπος lipos "fat" and φόβος phobos "fear"), is a chemical property of chemical compounds which means "fat rejection", literally "fear of fat". Lipophobic compounds are those not soluble in lipids or other non-polar solvents. From the other point of view, they do not absorb fats.

"Oleophobic" (from the Latin oleum "oil", Greek ελαιοφοβικό eleophobico from έλαιο eleo "oil" and φόβος phobos "fear") refers to the physical property of a molecule that is seemingly repelled from oil. (Strictly speaking, there is no repulsive force involved; it is an absence of attraction.)

The most common lipophobic substance is water.

Fluorocarbons are also lipophobic/oleophobic in addition to being hydrophobic.

Uses

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A lipophobic coating has been used on the touchscreens of Apple's iPhones since the 3GS,[1] their iPads,[2] Nokia's N9 and Lumia devices,[citation needed] the HTC HD2[citation needed], the Blackberry DTEK50,[3] Hero, and Flyer[4] and many other phones to repel fingerprint oil, which aids in preventing and cleaning fingerprint marks. Most "oleophobic" coatings used on mobile devices are fluoropolymer-based solids (similar to Teflon, which was used on the HTC Hero[5]) and are both lipophobic and hydrophobic. The oleophobic coating beads up the oils left behind a user's fingers, making it easy to clean without smearing and smudging. This helps decrease the feasibility of a successful smudge attack.[6] In addition to being lipophobic or oleophobic, perfluoropolyether coatings impart exceptional lubricity to touch screens and give them a "slick feel" that eases their use.[7]

DIY products exist to restore or add an oleophobic coating to devices lacking one.[8]

See also

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Lipophobicity

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from Grokipedia
Lipophobicity is a of substances characterized by their rejection of , rendering them insoluble in fats, oils, and non-polar solvents, and is fundamentally the converse of . This "fat-fearing" quality typically arises from polar, ionic, or highly electronegative functional groups that favor interactions with or other polar media over non-polar environments. In molecular design, lipophobicity is crucial for amphiphilic compounds like , where the lipophobic (often hydrophilic) head group solubilizes in aqueous phases while the lipophilic tail anchors into oily phases, facilitating emulsification, detergency, and stability. Increasing the lipophobicity of reduces their saturation concentration in oils, enhancing their efficiency in separating dispersed phases. Applications extend to surface modifications, where lipophobic coatings—commonly incorporating fluoropolymers—repel oils and stains, providing durability in textiles, cookware, and biomedical devices due to their chemical inertness and low . In and biochemistry, lipophobicity governs drug partitioning and membrane permeability; compounds with high lipophobicity exhibit poor absorption across lipid-rich biological barriers, influencing and necessitating formulation strategies like prodrugs or lipid carriers to improve delivery. This property also underpins colloidal stability and protein- interactions, where lipophobic regions prevent unwanted aggregation in aqueous milieus.

Definition and Properties

Definition

Lipophobicity, derived from the Greek words lipo (fat) and phobos (fear), refers to the property of a substance exhibiting an aversion to or lack of affinity for , oils, fats, and non-polar solvents. This term describes the inability of certain molecules, surfaces, or materials to dissolve in or interact favorably with these non-polar substances, often manifesting as resistance to or adsorption by oily compounds. In contrast to lipophilicity, which denotes the capacity of a compound to dissolve in fats, oils, and non-polar environments due to favorable van der Waals interactions, lipophobicity arises from structural features that favor polar or ionic interactions over those with hydrocarbons. This property is particularly evident in molecules bearing polar functional groups, such as hydroxyl (-OH) or ionic heads, which render them incompatible with environments; for instance, the hydrophilic heads of amphiphilic molecules like exemplify lipophobic behavior by repelling the non-polar tails. Lipophobicity thus plays a key role in molecular within aqueous versus organic phases and in surface phenomena where materials resist oil adhesion. The concept of lipophobicity primarily operates in the domains of chemistry, where it governs the partitioning of solutes between polar and non-polar media, and , focusing on surface repulsion of non-polar liquids. It emerged in the mid-20th century, coinciding with advancements in research and the development of fluoropolymers, with initial references appearing in chemistry literature describing the oil-repelling traits of perfluorinated compounds.

Relation to Hydrophobicity and Oleophobicity

Lipophobicity and hydrophobicity represent distinct yet sometimes overlapping repulsion properties. Hydrophobicity denotes the aversion to , a polar , primarily driven by low that prevents by aqueous liquids. In contrast, lipophobicity targets non-polar and fats, such as triglycerides and hydrocarbons, enabling materials to resist or spreading of oily substances. While numerous display both traits—resulting in omniphobic or amphiphobic surfaces that repel diverse liquids—lipophobicity operates independently and can manifest in hydrophilic contexts, where surfaces attract but repel oils. For example, specially engineered hydrophilic-oleophobic surfaces, achieved through chemical modifications like zwitterionic polymers, exhibit water contact angles below 90° while maintaining oil contact angles above 90°, illustrating decoupled repulsion mechanisms. Oleophobicity is commonly employed as a for lipophobicity within surface chemistry and , emphasizing repulsion specifically toward oils and oil-like fluids. Derived from "oleo" (oil) and "phobic" (repelling), the term highlights practical resistance to substances like or mineral oils, often quantified in industrial applications. However, a nuanced difference emerges in their breadth: oleophobicity typically focuses on non-polar oils, whereas lipophobicity extends to a wider array of , including more complex biological fats. This distinction arises because encompass diverse molecular structures beyond simple oils, allowing lipophobicity to address broader scenarios. Fluorinated polymers, such as polytetrafluoroethylene (PTFE, or Teflon), exemplify high lipophobicity through surface energies as low as 18 mN/m, which inherently imparts both oil and water repellency. Overlaps between these properties are evident in amphiphobic surfaces, which integrate hydrophobicity and oleophobicity to achieve versatile liquid repellency, often via micro/nanostructured topologies combined with low-energy chemistries. In biological systems, lipophobicity facilitates the prevention of adhesion on cellular membranes or surfaces without necessitating hydrophobicity, aiding in anti-biofouling and self-cleaning functions. For instance, certain biomineralized hydrogels incorporate lipophobic layers to resist from lipophilic dyes and tapes, enhancing by minimizing unwanted interactions. These relations underscore how lipophobicity complements rather than mirrors hydrophobicity, enabling tailored material designs for specific environmental challenges.

Chemical Basis

Molecular Mechanisms

Lipophobicity arises primarily from the dominance of polar or ionic intermolecular forces over weaker der Waals interactions when interfacing with non-polar molecules. High-electronegativity elements such as and oxygen contribute to this by generating electron-rich surfaces that enhance polar repulsion, thereby reducing attractive forces with lipid chains. In contrast, non-polar rely on dispersion forces for interactions, which are minimized on lipophobic surfaces due to low . At the structural level, lipophobic molecules often feature polar head groups, such as carboxyl (-COOH) or amino (-NH₂), which promote hydrophilic and lipophobic behavior by facilitating bonding and electrostatic interactions that exclude non-polar . In polymers, chains exemplify this mechanism: the replacement of with atoms decreases dispersion forces, as the C-F bonds exhibit low and high bond strength, thereby lowering the overall affinity for hydrocarbons. From an energetic perspective, lipophobic surfaces typically possess a low surface free energy, often below 20 mJ/m², which promotes non-wetting by oils and . This behavior is quantitatively described by Young's equation, adapted for oil-solid-gas interfaces: γsg=γsl+γlgcos⁡θ\gamma_{sg} = \gamma_{sl} + \gamma_{lg} \cos \theta where γsg\gamma_{sg} is the solid-gas interfacial tension, γsl\gamma_{sl} is the solid-liquid (oil) interfacial tension, γlg\gamma_{lg} is the liquid-gas (oil) , and θ\theta is the . A θ>90∘\theta > 90^\circ indicates lipophobic repulsion, as the high θ\theta reflects unfavorable γsl\gamma_{sl} due to mismatched intermolecular forces; the equation derives from horizontal force balance at the three-phase contact line, assuming without . Representative examples illustrate these principles. In phospholipids, the polar head groups—such as those containing and choline—exhibit lipophobicity, enabling into bilayers where these heads orient outward to avoid contact with non-polar tails and surrounding . Perfluoroalkyl substances (PFAS), like , demonstrate extreme lipophobicity through their C-F bonds' low , which severely limits van der Waals interactions with chains, resulting in oil contact angles exceeding 110°.

Role in Surfactants

are amphiphilic s characterized by a lipophobic hydrophilic head group and a lipophilic hydrophobic tail, enabling them to reduce at interfaces. The lipophobicity of the head group, often conferred by polar moieties such as ions or polyoxyethylene chains, ensures that this portion of the strongly repels oil phases and prefers aqueous environments, preventing the from fully dissolving in nonpolar solvents. This selective affinity drives the of at - boundaries, where the heads orient toward the water phase while the tails extend into the oil phase. The lipophobic nature of heads plays a crucial role in enhancing emulsification processes by stabilizing oil-in-water or water-in-oil through the formation of ordered interfacial layers. By minimizing direct contact between immiscible phases, these heads promote the creation of micelles or bilayers that encapsulate dispersed droplets, thereby reducing coalescence and improving longevity. Furthermore, lipophobicity directly influences the (CMC), the threshold surfactant concentration above which micelles form; stronger lipophobic head groups, which increase the molecule's overall hydrophilicity, elevate the CMC by heightening the energy barrier for tail aggregation in water. For instance, in nonionic featuring polyoxyethylene (POE) chains as heads, increasing the POE chain length enhances lipophobicity, resulting in progressively higher CMC values due to greater by water molecules. In anionic surfactants, such as (SDS), the lipophobicity arises from the charged head group, which exhibits strong electrostatic repulsion from nonpolar oil phases, conferring inherent ionic lipophobicity that bolsters interfacial activity. This property allows SDS to achieve low at oil-water interfaces, facilitating applications in detergency and dispersion. Nonionic surfactants, by contrast, offer tunable lipophobicity through variations in head group architecture; shorter POE chains yield milder lipophobicity and lower CMC, while longer chains amplify it, tailoring the surfactant's and performance in diverse formulations. A key quantitative insight into this interfacial behavior is provided by the Gibbs adsorption isotherm, which relates the surfactant surface excess concentration (Γ) at the interface to changes in interfacial tension (γ) with bulk concentration (C): Γ=−1RTdγdln⁡C\Gamma = -\frac{1}{RT} \frac{d\gamma}{d \ln C} Here, R is the gas constant and T is the temperature; for nonionic surfactants, this form applies directly, while ionic types require adjustment for ion activity. Lipophobic heads enhance Γ at the water-oil boundary by increasing the chemical potential gradient that drives adsorption, as the heads' aversion to oil maximizes orientation and packing density at the interface, leading to steeper tension reductions and more effective stabilization. This elevated adsorption is particularly pronounced in systems with polar oils, where head group lipophobicity amplifies the partitioning toward the aqueous side.

Measurement Techniques

Experimental Methods

Contact angle goniometry is a primary experimental method for assessing lipophobicity by measuring the formed by oil droplets on a surface. In this technique, a low-surface-tension liquid such as n-dodecane is dispensed as a sessile drop onto the sample surface using an optical tensiometer or equipped with a high-resolution camera. The advancing and receding are determined by tilting the stage or expanding/contracting the droplet volume, with angles greater than 90° indicating lipophobic behavior due to poor by the oil. Automated systems facilitate precise measurements by fitting the droplet profile to a theoretical shape via the Young-Laplace equation. Immersion tests provide a practical evaluation of lipophobicity through direct exposure of samples to lipid solutions. Samples are submerged in oils like olive oil or hexadecane for specified durations, typically ranging from hours to days, followed by removal, rinsing, and assessment of adhesion via weight change or visual inspection for residual oil film. Minimal weight gain or absence of visible staining signifies effective lipophobicity, as the surface resists oil attachment. This method is particularly useful for testing the durability of coatings under prolonged contact. For nonionic , the cloud point method evaluates lipophobicity by observing temperature-induced in aqueous solutions. A solution, such as 1 wt% of a polyoxyethylene alkyl , is heated gradually in a bath while monitored for onset, which marks the temperature where the aggregates and separates due to reduced . Higher s correlate with greater lipophobicity, reflecting a stronger hydrophilic component that limits oil affinity. Advanced nanoscale characterization employs (AFM) to quantify force interactions between lipophobic surfaces and probes. In force mode, an AFM tip functionalized with a or oil-like approaches the surface in a environment, recording approach-retraction curves to measure and repulsion forces at the picoNewton scale. Low forces confirm lipophobicity by demonstrating weak interactions driven by mismatches. This technique reveals molecular-level repulsion underlying macroscopic lipophobic properties.

Quantitative Assessment

Lipophobicity of molecular compounds can be quantitatively assessed using the (logP), where low or negative values indicate strong lipophobicity, reflecting poor solubility in nonpolar solvents like octanol relative to , as molecules with logP < 0 preferentially partition into the aqueous phase due to dominant hydrophilic interactions. This metric is derived from the distribution equilibrium between n-octanol and phases, where logP = log(K_{ow}) and K_{ow} is the partition ratio; for example, compounds like sugars exhibit logP values around -3 to -5, signifying high lipophobicity. Surface lipophobicity is evaluated through oil contact angle measurements, often analyzed via the Zisman plot to determine the critical surface tension (CST, γ_c). The Zisman method involves plotting the cosine of the contact angle (cos θ) against the surface tension of various test liquids (γ_{lg}), yielding a linear relationship; the x-intercept where cos θ = 1 provides γ_c, which represents the surface tension below which liquids spread completely. Surfaces with γ_c < 25 dyn/cm (mN/m) exhibit high lipophobicity, as they resist wetting by low-surface-tension oils (typically 20–30 mN/m); for instance, fluorinated polymers like polytetrafluoroethylene have γ_c ≈ 18 mN/m, enabling effective oil repellency. Goniometry is briefly referenced for measuring θ, but the plot provides the interpretive scale for lipophobic character. In , lipophobicity is quantified by the hydrophile-lipophile balance (HLB) scale, ranging from 0 (fully lipophilic) to 20 (fully hydrophilic/lipophobic), where values >8 indicate lipophobic dominance due to stronger hydrophilic moieties driving oil repulsion in oil-in-water systems. For nonionic , HLB is calculated as: HLB=20×(1−SA)\text{HLB} = 20 \times \left(1 - \frac{S}{A}\right) where S is the (mg KOH/g) and A is the (mg KOH/g) of the component, allowing prediction of emulsification behavior; like Tween 80 with HLB ≈ 15 demonstrate strong lipophobicity in aqueous dispersions. Standardized oleophobic ratings for materials, such as fabrics or coatings, employ the AATCC 118 scale (0–8), assessing resistance to oil penetration by a series of hydrocarbons from n-heptane (rating 8, highest lipophobicity) to (rating 0, no resistance). This rating correlates with practical lipophobicity, where grades ≥4 indicate moderate oil repellency suitable for protective applications.

Applications

Materials and Coatings

Fluoropolymer coatings, such as (PTFE) and (PVDF), are widely employed in non-stick and protective applications due to their inherently low , which imparts lipophobicity by minimizing interactions with oils and greases. PTFE coatings, with a surface energy of approximately 19 mJ/m², enable resistance to oil adhesion in cookware, where they form durable barriers that prevent staining and facilitate easy cleaning. Similarly, PVDF-based coatings, valued for their chemical inertness and mechanical strength, are applied to textiles to provide oil-repellent properties, enhancing stain resistance in outdoor fabrics and protective gear. These fluoropolymers achieve lipophobicity through the alignment of fluorinated chains at the surface, reducing van der Waals forces with non-polar substances like oils. Nanostructured surfaces inspired by the lotus effect utilize silica nanoparticles combined with fluorosilane modifiers to create omniphobic coatings that repel both water and oils, promoting self-cleaning in fabrics. These coatings involve spray or dip application of fluorosilane-treated silica nanoparticles onto textile substrates, forming hierarchical micro- and nanostructures that trap air pockets and lower effective surface energy, resulting in high contact angles for oils (e.g., >150° for hexadecane). For instance, multicomponent systems incorporating fluoro-containing polymers and fluorinated silica nanoparticles yield robust omniphobic fabrics suitable for self-cleaning applications, where liquids roll off without adhering. Such designs mimic natural superoleophobic surfaces, ensuring minimal oil staining on materials like cotton. In and , lipophobic surfaces on plates and prevent unwanted of oil-based during lithographic processes, ensuring clean image transfer and reduced defects. Offset lithography relies on plates with oleophobic non-image areas that repel , achieved through chemical treatments that maintain hydrophilicity while exhibiting lipophobicity to oils, thereby minimizing scumming and ghosting. Specialized lipophobic , often incorporating fluorinated additives, further enhance non-adhesion on substrates, allowing precise control over transfer in high-speed production. These properties are critical for maintaining print quality in oil-sensitive applications like . Despite their efficacy, lipophobic coatings face durability challenges, including wear from mechanical abrasion, which is assessed using tape adhesion tests such as ASTM D3359 to evaluate coating integrity after repeated stress. These tests involve applying and removing over scored surfaces to quantify loss, revealing potential in layers under everyday use. Additionally, environmental concerns surround per- and polyfluoroalkyl substances (PFAS) used in many such coatings, as they are persistent and bioaccumulative pollutants; for example, (PFOA), a common processing aid, was phased out in the U.S. by 2015 under the EPA's voluntary stewardship program to mitigate ecological and health risks. Efforts to improve wear resistance include hybrid formulations, but ongoing regulatory scrutiny of PFAS drives innovation toward sustainable alternatives.

Biomedical and Biological Uses

Lipophobicity plays a crucial role in biomedical applications by enabling the design of materials that resist lipid adsorption, protein fouling, and bacterial adhesion, thereby enhancing and device performance. Fluoropolymers, known for their inherent lipophobicity due to low and high fluorine content, are widely used in medical devices such as catheters, stents, and implants. For instance, these polymers prevent non-specific interactions with biological , reducing the risk of and in blood-contacting applications. In and systems, lipophobic fluorinated materials facilitate controlled release and improved by minimizing unwanted interactions. Fluorinated micelles and nanoparticles, leveraging their lipophobicity, encapsulate hydrophilic drugs while repelling in physiological environments, leading to enhanced stability and targeted delivery. This also supports anti-fouling surfaces on surgical instruments and vascular grafts, where lipophobicity inhibits formation and extends device longevity. Tissue engineering benefits from lipophobic surfaces that modulate and proliferation through controlled protein adsorption. (PFPE)-based elastomers, with water contact angles around 110° and strong lipophobic characteristics, promote and organization in cultures, comparable to standard . These materials enable patternable scaffolds for directing cell growth without excessive . Lipophobic coatings, such as phospholipid-based LipoCoat, mimic cell membranes to create biocompatible barriers on medical devices like contact lenses and catheters. In contact lenses, lipophobicity reduces deposits from tear fluid, allowing extended wear and improved comfort by preventing and irritation. For stents and catheters, these coatings minimize bacterial and clot formation, enhancing and reducing risks during implantation.

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