= Ion-dipole attraction
{title2=$U=-\mathbf p\cdot\mathbf E$}
An <ion-dipole attraction> couples an <ion>'s <electric field> to a <molecule>'s <electric dipole moment>. In the elementary point-<electric dipole> approximation, the interaction <potential energy> is $U=-\mathbf p\cdot\mathbf E$. An orientation with the <electric dipole moment> along the <electric field> lowers this <energy>. For <water>, the <oxygen> end preferentially faces a <cation>, and the <hydrogen> end an <anion>. At nonzero <temperature>, fluctuations and competing <hydrogen bonds> prevent perfect alignment of every <molecule>.<Chemistry 2e by Paul Flowers and others/11.2 Electrolytes>[Chemistry 2e by Paul Flowers and others; “Ionic Electrolytes”, Figure 11.7 and its explanatory paragraph]{ref}
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