What I Think I Know: Hydrophilic and hydrophobic things repel each other. Since the cell membrane contains hydrophobic tails, it is difficult for hydrophilic molecules to pass through the cell membrane.
Question: Why don't the heads of phospholipid bilayers repel hydrophobic molecules? In other words, if the phospholipid membrane has both hydrophilic and hydrophobic portions, why do only the hydrophobic portions act as repellents?
Stipulation: I need a quotation from a textbook, university, or other credible source.
Evidence of Prior Research:
Why lipophilic molecules can pass phospholipid bilayer, in spite of 2 hydrophilic layers?: This question has a comment that links a graph from a textbook. I can't understand the graph by itself. There are no other references.
Understanding what passes through and doesn't pass through the plasma membrane: All references discuss the topic, but none answer the question.
I understand that this topic is addressed quite frequently, but the question I'm asking is never answered. Texts tend to say something like, "Ions can't get through the core because of their charge" and completely gloss over the concept of lipids getting through the heads. Please don't dismiss this as a homework question that I didn't take the time to research. This is my second try asking this question (on this site); I have spent hours trying to find an answer.