- FROM THE ARCHIVES -
SARAH: Daddy, were you in the shower?
DAD: Yes, I was in the shower.
SARAH: Why?
DAD: I was dirty. The shower gets me clean.
SARAH: Why?
DAD: Why does the shower get me clean?
SARAH: Yes.
DAD: Because the water washes the dirt away when I use soap.
SARAH: Why?
DAD: Why do I use soap?
SARAH: Yes.
DAD: Because the soap grabs the dirt and lets the water wash it
off.
SARAH: Why?
DAD: Why does the soap grab the dirt?
SARAH: Yes.
DAD: Because soap is a surfactant.
SARAH: Why?
DAD: Why is soap a surfactant?
SARAH: Yes.
DAD: That is an EXCELLENT question. Soap is a surfactant because
it forms water-soluble micelles that trap the otherwise insoluble
dirt and oil particles.
SARAH: Why?
DAD: Why does soap form micelles?
SARAH: Yes.
DAD: Soap molecules are long chains with a polar, hydrophilic
head and a non-polar, hydrophobic tail. Can you say
‘hydrophilic’?
SARAH: Aidrofawwic
DAD: And can you say ‘hydrophobic’?
SARAH: Aidrofawwic
DAD: Excellent! The word ‘hydrophobic’ means that it avoids
water.
SARAH: Why?
DAD: Why does it mean that?
SARAH: Yes.
DAD: It’s Greek! ‘Hydro’ means water and ‘phobic’ means ‘fear
of’. ‘Phobos’ is fear. So ‘hydrophobic’ means ‘afraid of
water’.
SARAH: Like a monster?
DAD: You mean, like being afraid of a monster?
SARAH: Yes.
DAD: A scary monster, sure. If you were afraid of a monster, a
Greek person would say you were gorgophobic.
(pause)
SARAH: (rolls her eyes) I thought we were talking about
soap.
DAD: We are talking about soap.
(longish pause)
SARAH: Why?
DAD: Why do the molecules have a hydrophilic head and a
hydrophobic tail?
SARAH: Yes.
DAD: Because the C-O bonds in the head are highly polar, and the
C-H bonds in the tail are effectively non-polar.
SARAH: Why?
DAD: Because while carbon and hydrogen have almost the same
electronegativity, oxygen is far more electronegative, thereby
polarizing the C-O bonds.
SARAH: Why?
DAD: Why is oxygen more electronegative than carbon and
hydrogen?
SARAH: Yes.
DAD: That’s complicated. There are different answers to that
question, depending on whether you’re talking about the Pauling or
Mulliken electronegativity scales. The Pauling scale is based on
homo- versus heteronuclear bond strength differences, while the
Mulliken scale is based on the atomic properties of electron
affinity and ionization energy. But it really all comes down to
effective nuclear charge. The valence electrons in an oxygen atom
have a lower energy than those of a carbon atom, and electrons
shared between them are held more tightly to the oxygen, because
electrons in an oxygen atom experience a greater nuclear charge and
therefore a stronger attraction to the atomic nucleus! Cool,
huh?
(pause)
SARAH: I don’t get it.
DAD: That’s OK. Neither do most of my
students.