Self-consistent field theory (SCFT) has been used to calculate the Helfrich bending elastic constants for monolayers of AB diblock copolymers at interfaces between immiscible A and B homopolymer liquids. We focus on the properties of saturated monolayers, with vanishing interfacial tension, considering both symmetric and asymmetric systems. In asymmetric mixtures containing homopolymers with different degrees of polymerization or statistical segment lengths, we calculate the copolymer block compositions required to create balanced surfactants, for which saturated monolayers have vanishing spontaneous curvature.