Definition
Hydrophilic means a substance mixes easily with water and clings to it, while hydrophobic means a substance avoids water and refuses to mix. Whether a molecule is friendly or unfriendly to water depends on how its electric charge is spread out, which is called polarity.
Water's Friends and Water's Avoiders
Put on a rain jacket on a rainy day, and the raindrops don't soak in. They bead up into little balls and roll right off. Now wear a cotton T-shirt in the same rain, and the fabric drinks up the water and gets soggy in no time.
Cotton fibers cling to water. They spread each drop wide and pull it inside. A rain jacket, on the other hand, is treated to have a water-shy hydrophobic surface. The drops can't spread out, so they ball up and bounce away.
The stuff around us falls into two camps, depending on how it reacts to water. Water spreading thin across a window, sugar melting away in a cup of tea, dew rolling like beads on a lotus leaf: these are all tricks played by hydrophilic and hydrophobic behavior.
Opposite Charges at Each End: The Secret of Polarity
A water molecule is made of one oxygen atom and two hydrogen atoms. The oxygen side carries a slightly negative charge (-), and the hydrogen side carries a slightly positive charge (+). With opposite charges at opposite ends, it looks a bit like a bar magnet, but the force here isn't magnetism. It's the pull between + and - electric charges. When charge is lopsided inside a molecule like this, we call it molecular polarity.
Just as + and - attract, other polar molecules and ions (atoms carrying an electric charge) feel a strong pull toward water molecules. Hydrophilic substances join hands with water through hydrogen bonds (strong attractions where a hydrogen atom acts as the bridge) or get surrounded by water through electric attraction, so they mix in naturally.
Oils like sesame oil or cooking oil are different. Their charge is spread evenly, so they are nonpolar. To be more precise, water doesn't really shove oil away. The pull between water molecules themselves is so strong that oil molecules can't find a gap to squeeze into, so they get pushed out. If oil barged in, the nearby water molecules would have to force their hydrogen-bond arrangement into a more rigid, orderly pattern, and that is a bad deal for water.
The Trick of the Two-Handed Molecule
After you grill bacon, the frying pan stays greasy no matter how much cold water you rinse it with. That's because water and oil are a hydrophobic mismatch. But add just one drop of dish soap, and the grease washes away like magic.
Soap and detergent contain a very special kind of molecule. Its head is hydrophilic, so it loves water. Its tail is hydrophobic, so it loves oil. It's a two-handed design. We call these molecules surfactants.
When detergent molecules meet grease, their hydrophobic tails wrap around the oil and lock it inside, while their hydrophilic heads face outward toward the water, forming a tiny ball. This capsule is called a micelle. Because its outside is dressed up as water-friendly, the grease floats along in the water and rinses cleanly away.
Amazingly, the cell membrane that wraps every cell in your body uses the same trick. Phospholipid molecules, with their hydrophilic heads and hydrophobic tails, stack into a double-layered wall that keeps the inside of the cell safe in the watery environment of your body.
Common misconceptions
Hydrophobic substances have a physical force that pushes water away (repulsion).
Hydrophobic molecules don't actively push water away. Water molecules attract each other so strongly that nonpolar oil molecules can't wedge in between them, so they get squeezed out. If oil got in the way, the water molecules would have to force their hydrogen-bond arrangement into a new pattern, and that is a bad deal for them.
Where you meet it
The tiny bumps and waxy coating on a lotus leaf are strongly hydrophobic, so raindrops roll off like beads.
Face wash and shampoo contain surfactants that have both hydrophilic and hydrophobic parts, so they can rinse oil from your skin and hair with water.
In one sentence
Hydrophilic substances are polar or charged, so they get along with water. Hydrophobic substances have evenly spread charge, so they get pushed out from between water molecules.